The First International Conference on Dense Z-Pinches for Fusion was held in Alexandria, Virginia on March 29 and 30, 1984. The meeting was hosted by the US Naval Research Laboratory (NRL). Thirty people, representing seven different countries attended that first conference. Fourteen papers were presented from scientists from four different countries. The conference proceedings were published by NRL. This was the birth a long series of International Dense Z-Pinch conferences that continues to this day. While subsequent conferences expanded the scope to all aspects of dense z-pinch research, that first conference concentrated on the fusion energy applications.

One of the primary themes of the conference was to achieve fusion conditions by starting the pinch at a small diameter and high density, and maintaining those conditions as the current is ramped up to achieve fusion temperatures. The conference also marked one of the first times modern high voltage pulsed power machines were used to power the dense z-pinch. Research on several types of dense z-pinches was reported at the meeting: High pressure (2-4 atm) gas embedded pinches initiated with field enhancement, x-rays, or lasers. High density pinches formed inside gas filled quartz capillaries. Pinches formed with a collapsing gas shell. More esoteric concepts were presented, including the use of a compact torus to drive the pinch. The concept of forming the pinch from a frozen deuterium fiber was first proposed at this meeting. There were also papers on dense z-pinch simulation, modeling, diagnostics, and of course, a concept for a dense z-pinch based fusion reactor.

The introduction to the Conference Proceedings stated that one of the goals of the meeting was to “simulate new ideas..that would lead to significant advances..” Considering this conference series has been continuing for 39 years, it is safe to say that goal has been met.

We present results from a novel experimental platform [1] which utilizes the radiation pulse form a wire-array Z-pinch to drive radiative ablation from solid targets positioned ~ 3 cm from the axis of the pinch. The experiments were performed on the MAGPIE generator at Imperial College (1.4 MA, 240 ns) [2]. They used critically massed aluminium or tungsten wire arrays to generate driving radiation with a typical on target brightness temperature of 10 eV. Experiments were diagnosed with a state-of-the-art diagnostic suite [3, 4] including laser interferometry; collective optical Thomson scattering; optical fast-frame imaging; Faraday rotation imaging; B-dot (inductive) probes; and X-Ray spectroscopy. The experimental measurements were compared with simulations performed using the codes Chimera [5], Helios-CR, and PrismSPECT [6].

This experimental platform is advantageous in the sense that plasmas are produced with well posed initial conditions, and have a simple overall morphology. To date experiments have been performed in double and single target configurations with a variety of low-Z and mid-Z target materials. Most of the work done to date induced plasma ablation in a background magnetic field (B~5 T, supported by the Z-pinch current pulse). We have, however, recently demonstrated the possibility of reducing this field to ~0.1 T by adjusting the return current path, with a negligible change to the character of the driving radiation pulse.

Experiments performed with a single target demonstrated a simple (quasi-1D) morphology, making it possible to study the atomic properties of the ablated plasma flows. Our initial results indicate that photoionization plays an important role in determining the charge-state distribution of ablated plasma flows. In this talk we will argue that the experimental platform is relevant to studies of atomic-kinetics in accretion-disk environments.

In experiments where a pair of ablating driven targets were fielded, we were able to study the collisions between two radiatively driven flows. We were also able to study ablated plasma jets using conical or wedge-shaped target geometries. The properties of the stagnated plasma layers, generated in collisions, were observed to depend strongly on both the ion-species in the plasma and the presence/absence of an ambient magnetic field. In this talk we argue that these observations imply the experiments are relevant to the study of radiative cooling instabilities in young stellar objects (YSOs) [7].

Supported by the US DOE under Award Nos. DE-SC0020434 and DE-NA0003764, and by the US DTRA under Award No. HDTRA1-20-1-0001

[1] J. Halliday et al. Phys, Plasmas; 29 042107 (2022). https://doi.org/10.1063/5.0084550

[2] I. Mitchell et al. Rev. Sci. Instrum. 67, 1533 (1996). https://doi.org/10.1063/1.1146884

[3] G. Swadling et al. Rev. Sci. Instrum. 85, 11E502 (2014). https://doi.org/10.1063/1.4890564

[4] L. Suttle et al. Rev. Sci. Instrum. 92, 033542 (2021). https://doi.org/10.1063/5.0041118

[5] K. McGlinchey et al. Phys. Plasmas 25, 122705 (2018). https://doi.org/10.1063/1.5064504

[6] MacFarlane et al. J. Quant. Spectrosc. Radiative Transfer 99, 381 (2006). https://doi.org/10.1016/j.jqsrt.2005.05.031

[7] R. Markwick et al. Phys. Plasmas 29, 102901 (2022). https://doi.org/10.1063/5.0095166

Magnetic reconnection –the abrupt change in magnetic field topology accompanied by the explosive release of heat and kinetic energy –is an important process in astrophysical plasmas. In high-energy-density astrophysical environments, strong radiative cooling can modify the reconnection process, by rapidly removing the magnetic energy dissipated in the current sheet. The MARZ (Magnetically Ablated Reconnection on Z) collaboration investigates radiatively-cooled reconnection in the laboratory on the Z pulsed-power machine, as part of the Z Fundamental Science Program.

We perform 2D and 3D resistive magnetohydrodynamic simulations of reconnection in a pulsed-power-driven dual exploding wire array using the code GORGON. We model the radiation using tabulated emissivity and opacity calculated using the SpK atomic model code, and model the radiation transport with either a probability-of-escape model or diffusive multi-group model.

The simulated wire arrays generate magnetized supersonic (M_s 4−5) and super-Alfvénic (M_A∼1.5) flows which collide to form a current sheet with an initial Lundquist number∼400. The simulations show that radiative cooling leads to faster reconnection rates compared to the non-radiative case, consistent with the colder temperature and the strong radiatively-driven compression of the current sheet. Plasmoids, which originate within the sheet earlier in time, are also quenched by radiative cooling. Finally, there is reduced magnetic flux pile-up outside the sheet, resulting in lower magnetic field and density of the inflows into the layer.

SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

We present results from the first experimental investigation of pulsed-power-driven magnetic reconnection in a strongly-radiatively cooled regime. The experiment is designed to provide cooling rates much faster than the hydrodynamic transit rate, which is required to observe the radiative collapse of the reconnection layer. A dual inverse wire array load driven by the Z machine (20 MA, 300 ns rise time) generates oppositely-directed highly-collisional, super-Alfvénic plasma flows with anti-parallel magnetic fields. The flows interact in the mid-plane to generate an elongated current sheet, that exhibits strong XUV and X-Ray emission. Inductive probes measure peak magnetic fields of 25-30 T in the inflow to the current sheet, which is roughly 10x larger than in previous pulsed-power-driven reconnection experiments. Visible spectroscopy, which exhibits well-defined Al-II and Al-III emission lines, is used to characterize the inflows to the reconnection layer. Experimental spectra are compared to the output of collisional-radiative models and radiation transport simulations to estimate the density and temperature of the inflows.

A filtered X-ray diode, which collects >1 keV photons from the reconnection layer, exhibits a sharp peak in intensity (50 ns FWHM), indicating radiative collapse of the layer. X-ray spectroscopy of the reconnection layer shows well-defined He-like and Li-like k-shell emission lines, consistent with temperatures > 100 eV. Radiation transport calculations further indicate that the X-ray spectrum corresponds to localized hotspots of enhanced temperature embedded within a relatively colder layer. Time-resolved X-ray images confirm this picture, and show an elongated current sheet with brightly-emitting fast-moving hotspots (up to 35 km/s near the center of the reconnection layer). Resistive MHD simulations in 2D and 3D indicate that hotspots are consistent with plasmoids, which appear as localized regions of enhanced emission.

* SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525

Magnetic reconnection is a process whereby antiparallel magnetic field lines undergo a rapid change in topology, releasing significant energy in the process. This occurs in the interaction region between two adjacent exploding wire arrays, driven by strong currents from Z facility at Sandia National Laboratories. Ablated surface plasmas in each array carry opposingly directed magnetic fields, which can undergo a change in magnetic topology when the flows collide and compress each other. At Z’s high currents the reconnection layer can reach high enough densities and temperatures to radiatively cool and ultimately collapse to a very small region.

To investigate the above system using simulation, it is critical to resolve the reconnection layer (of spatial order 100um) while also capturing dynamics of the wire arrays (of order 10cm across). Resolving features with at least six orders of magnitude difference is intractable for uniform grid simulations in all three dimensions. This talk describes a static mesh refinement capability implemented in the 3D extended-MHD code Gorgon, to address such resolution-limited systems.

Spectroscopy is a formative utility for noninvasive investigation of high-energy-density (HED) plasma. X-ray spectroscopy is notably valuable for diagnosing HED plasma regimes, where thermal and nonthermal emission features describe “hot” and “cold” plasmas, respectively. Nonthermal effects, in general, describe the interactions of non-Maxwellian electrons or photons with plasma ions of lower internal energy relative to the incident particle distribution. The effects of these kinetic interactions manifest in radiative signatures that are used to probe cooler exteriors of ICF plasmas and bright spot x-ray sources in X-pinches. In this talk, recent work will be presented that focuses on identifying and characterizing nonthermal effects in HED Z-pinches using either wire X-pinch or magnetized liner inertial fusion (MagLIF) implosions. Work was previously presented on noninvasive x-ray line polarization measurements of molybdenum (Mo) X-pinches performed on the UNR Zebra generator, representing the first polarization measurements performed on L-shell Mo X-pinch HED plasmas. We expand on this with new analysis of the relative influence of Maxwellian and non-Maxwellian contributions to the measured line polarization, which enables characterization of the energies of nonthermal electron beams (~4 – 30 keV) driving the polarization. As a continuation of this X-pinch work, a comprehensive study of stainless steel X-pinches was completed to examine the role of the experimental load configuration on nonthermal effects in the plasma source. In this investigation, analysis of x-ray diode signals (> 3 keV) and source size evolution is performed concurrently to showcase the influence of load geometry on the x-ray radiative properties of stainless steel X-pinches, with an emphasis on nonthermal electron beam generation. Non-LTE spectroscopic modeling is applied to K-shell iron (Fe), chromium (Cr), and nickel spectra to infer plasma parameters, while analysis of intensity ratios of analogous hot and cold Fe and Cr line emission provides insight into the plasma opacity. Notable results include production of hotter, thermal K-shell plasmas and intense cold K-shell line emission, with enhanced satellite line emission and nonthermal electron abundance for small-angle X-pinches. Lastly, we examine the role of non-Maxwellian photons on the production of nonthermal K-shell Fe fluorescence in a MagLIF plasma produced on the Sandia National Laboratories’ Z-machine. This is performed with a novel Monte Carlo Radiation Transport code, which employs a screened-hydrogenic atomic data package to self-consistently calculate radiative transfer processes. Numerical radiation transport modeling is performed to investigate the spatial origins of Fe fluorescence, revealing nonthermal line production over a broad region from the pinch axis in the MagLIF liner plasma shell. Summarizing results and application to future work will be discussed.

_______________________________________________________

*We thank Drs. David Ampleford and Stephanie Hansen of Sandia National Laboratories for their many fruitful discussions and valuable contribution to this work. This research was supported by the NNSA through the Krell Institute Laboratory Residency Graduate Fellowship under DE-NA0003960 and by the NNSA under DE-NA0003877, DE-NA0004133, DE-NA0002954 and DE-NA0003047.

The Talbot Numerical Tool (TNT) code was originally designed to postprocess and analyze Talbot x-ray interferometry images. TNT can separate periodic and non-periodic features in the Fourier space by detecting the main peaks of the interferogram spectra. Simultaneous and independent density information can be generated from the phase-shift, corresponding to the second term in the Fourier series expansion, and attenuation, corresponding to the first term in the Fourier series expansion. Additionally, 2D maps of dark-field can be obtained from the third term in the Fourier series expansion and 2D maps of atomic composition (Z-effective) can be derived in post-processing from attenuation and phase-shift contributions. TNT data analysis can be tailored by the user to fit specific experimental configurations. Alternatively, image processing can be automated for fast and efficient real-time analysis. X-ray Moire images of plasma ablation fronts from laser-irradiated plastic and metallic foils have been obtained with TNT using laser-produced x-ray backlighters and coherent x-ray sources. TNT data analysis has demonstrated versatility and robustness at different levels of noise. To further explore its capabilities, the TNT code has been used with visible and IR interferometry probing pulsed-power driven plasmas. We present TNT as an efficient user-friendly fringe analysis tool for the plasma community. The code has been tested against other freely distributed software and high-accuracy was obtained when probing the evolution of laser-produced plasmas, dense plasma focus, magnetically-driven plasma flows and shocks, and single-wire core expansion.

Data-driven model identification methods, like Weak Sparse Identification of Nonlinear Dynamics (WSINDy), can learn the expected governing dynamics from data under the right conditions. This is potentially a powerful tool when exploring plasma systems, where such methods could be used to bridge physical scales or modeling regimes in a physically-consistent way. Successful PDE identification (i.e., correctly determining equation terms and their numerical coefficients) depends on the types of interactions that occur for a given set of initial conditions as well as the level of noise relative to the problem dynamics. For some initial conditions, the governing dynamics can be recovered at a much higher noise level than others. For ideal magnetohydrodynamics (MHD) test problems there are regions of problem parameter space where WSINDy consistently recovers the expected governing equations, while in other regions unexpected PDEs are recovered. Tools from information theory – in particular, the Shannon information entropy – can capture the changes in data properties that separate these regions. By analyzing how and when WSINDy fails for different regions of parameter space, meaningful and theoretically-backed patterns can be identified. In addition, looking at ensembles of identified PDEs provides insight into alternative equation forms where a single data instance may not. For example, with the Brio-Wu Shock Tube (a low-information test problem) different groupings of terms are identified in a way that clearly corresponds to the plasma properties of the system. Some of these correlations suggest in certain regions of problem space the ideal MHD equations cannot be recovered in their standard form, but can in linearized-like forms.

Understanding the dynamics of magnetized turbulence is crucial to interpreting measurements of, and making theoretical predictions for a variety of terrestrial and extraterrestrial plasma systems. This understanding is challenged by the complex nature of the plasma dynamics, which often feature extremely high dynamic ranges and the need to model plasmas using different approximations across those physical scales. In addition, the interactions between magnetic fields, turbulence, and compressibility allow for complex and non-local exchanges of energy that can only be elucidated through careful modeling and analysis.

In this talk we present recent results from our collaboration's efforts to understand magnetized plasma turbulence using large-scale finite volume simulations, with a particular emphasis on our attempts to understand the transfer of energy across physical scales and between energy reservoirs (e.g., from magnetic to kinetic energy and vice versa). We also describe our development of an open source, community exascale plasma simulation tool, Athena-PK, and our near-future plans to use it to perform 3D calculations of terrestrial plasma devices such as the dense plasma focus and Hall-effect thrusters.

In warm dense matter and high energy density plasmas, the traditional Boltzmann description of a plasma begins to break down. In this regime, collisions are not determined by binary Coulomb collisions, but instead by many body Coulomb collisions. The Mean Force Kinetic Theory [S. D. Baalrud and J. Daligault, Phys. Plasmas 26, 082106 (2019)] provides an alternate closure to the BBGKY hierarchy based on expanding about a perturbation from equilibrium rather than about strength of correlations. One property of the Mean Force Kinetic Theory is it produces the same fluid equations, with altered transport coefficients, thus existing fluid codes would only need to update the transport coefficients. A code is presented to solve the Chapman-Enskog expansion for the Mean Force Kinetic Theory. Results show the self diffusion coefficient calculated for a Hydrogen plasma in
the warm dense matter regime. These plasmas contain degenerate electrons, whose screening effect are modeled by the potential of mean force. This potential is obtained using the Quantum Hyper Netted Chain Model (QHNC) [C. E. Starrett and D. Saumon, High Energy Density Phys. 10, 35 (2014)] developed by Starrett and Saumon. Future work intends to develop an independent version of the QHNC code to calculate ion-ion potentials of mean force necessary for the existing code, a module to expand transport coefficients to an arbitrary order in the Chapman-Enskog expansion, and a model for electron transport properties.

This material is based upon work supported by the US Department of Energy, National Nuclear Security Administration, under award No. DE-NA0003868.

The MJOLNIR (MegaJOuLe Neutron Imaging Radiography) dense plasma focus (DPF) at LLNL is a prototype source for performing neutron radiography of dynamic events. MJOLNIR’s driver can store up to 2 MJ stored energy and produce currents up to 4.5 MA (so far commissioned to >3 MA). The DPF consists of two coaxial electrodes, which generate a plasma sheath by ionizing deuterium gas. The sheath implodes on the axis in a z-pinch geometry. When the pinch breaks apart, it produces a beam of ions that impacts the “target”, a region of the sheath assembled on axis past the pinch (n_e ~1e19/cm^3). The “beam-target” interaction produces a neutron burst lasting on the order of tens of nanoseconds, which we use for radiography. We present our design of a laser interferometry diagnostic to measure the electron density of the pinch and target regions. It allows us to infer the ion density in the target region, study dynamics during implosion to optimize beam generation in the pinch region, and compare with hybrid fluid-kinetic particle-in-cell stimulations to benchmark models. The interferometer builds on an existing Schlieren imaging system, showing density gradients in the plasma and bounding the density. We compare the dynamics, instabilities, and structure of undoped deuterium, and argon- and neon-doped deuterium shots, using laser interferometry, visible self-emission imaging, neutron time-of-flight detectors, and EM probes. LLNL-ABS-848005

*Prepared by LLNL under Contract DE-AC52-07NA27344.

Axially ejected plasma shocks and plasma jets have been observed in dense plasma focus devices, leaving the electrodes at high velocity. For instance, in a plasma focus operating at 400J a plasma of ~ 10^-10 kg is ejected from the pinch with a velocity > 10⁵ m/s. These plasma conditions appear promising to be used as the base of a pulsed plasma thruster (PPT), particularly to develop a miniaturized propulsion device for orientation, capable of being integrated to a small-standardized satellite, such as the CubeSat. According to theoretical and scaling estimations, it is expected that for a pulsed plasma thruster operating with a stored energy of 1 J, a bit impulse in the range of fractions of μNs to some μNs per pulse would be obtained. It is important to take into consideration that a plasma focus works at milli bar pressures, and a PPT works in a space environment, i. e. vacuum, thus in the PPT the plasma will be generated from the ablation of the insulator material, PTFE for example. Thus, different experimental electrodes and insulators arrays must be studied in order to reproduce the plasma dynamics observed in a plasma focus.

This work presents the status of the pulsed micro energy propulsion research for nano satellites orientation developed by the P²mc Research Center of CCHEN and by SPEL of University of Chile. The report includes: a) design, construction and characterization of a miniaturized fast capacitor (2 kV, 2mF, ~ 40 nH, ~20 kA), b) design and construction of plasma guns with submillimeter internal and external radius, and PTFE insulator, c) electrical characterization of the miniaturized capacitor connected to the plasma gun operated at 1 to 2 J, d) measurements of the capacitor temperature operating at different repetition rate e) discussion and design of possible experimental arrays to measure the force produced by the miniaturized pulsed plasma thruster: optical measurements of the velocity and mass, torsional pendulum and thrust stand based on a single point load cell.

Supported by ANID FONDECYT Regular 1211695.

Commissioning MJOLNIR above 3.5 MA requires operational modifications to take advantage of the higher current to produce higher yields.

INTERLEAVING: So far, shots above 3.5 MA change the machine in a way that precludes back-to-back shots, requiring a low current (2.7 MA) “recondition shot” to reset the machine. Intermediate current shots at 3.2 MA did not recondition the machine. The yield on the highest current shots was independent of the yield of the recondition shot and the number of reconditioning shots performed. The 3.5 MA high yield shots, although interleaved, still exhibit suitable consistency in pinch time, yield, and neutron shape for time-gated flash neutron imaging.

FILL PRESSURE: The highest yield shots did not follow a constant drive parameter scaling for fixed anode. Fill-pressures above 12 Torr were associated with much lower dI/dt and yield. Measurements of rundown velocity and breakdown time for different fill pressures will be compared.

For interleaved shots below the critical pressure, the yield continues a favorable scaling (yet below I^4) from 2.7 MA above 3.5 MA. Without these changes, the yield diminishes or disappears above 3.5 MA. The interleaved shots are investigated to determine clues about what is different and how it reconditions the machine.

Prepared by LLNL under Contract DE-AC52-07NA27344.

A Dense Plasma Focus (DPF) is a coaxial plasma accelerator with a z-pinch as its final phase. An m = 0 instability leads to the break-down of the pinch column generating electric fields on the order of megavolts. These electric fields accelerate the ions and electrons in the pinch region in opposite directions. The ions are accelerated into plasma on axis resulting in beam-target fusion and neutron generation. The electrons are accelerated into the anode where they generate hard x rays via Bremsstrahlung radiation. A so-called step-wedge filter [1] is used to infer the x-ray temperature from the transmission of x rays through different tantalum thicknesses in the filter. This work will present the measurements of the high energy x-ray spectrum (higher than 62 keV) and corresponding neutron yields across various plasma conditions. Probing x rays provides a potential path to studying the electric fields and the underlying physics of ion beam generation in DPFs. These measures are taken on MJOLNIR which is a DPF located at LLNL being developed as a neutron source for flash neutron radiography.

This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory (LLNL) under Contract DE-AC52-07NA27344; LNL-ABS-848278

1. G. J. Williams et al, Rev. Sci. Instrum. 89, 10F116 (2018); https://doi.org/10.1063/1.5039383

This work describes the behavior of a plasma focus discharge of low energy and current, in an optimized regimen in X-ray and neutron production to high pressure of D₂ and H₂ (>10 mbar). The study was carried out in the multipurpose generator MPG (1.2 μF, 42 nH, 22-24 kV, ~100 kA, 290-345 J) under design and working conditions that extend to regimes with operating parameters out of the efficiency limits imposed by the similarities observed in PF discharges. In this direction, we considered the parameter space p-Z_eff, with Z_eff >> 2a (where Z_eff is the is the effective anode length, p is the fill pressure and a is the anode radius) and smaller anode radius, thus reducing the initial volume of discharge region and increasing the rise-rate of initial current density. The evolution of plasma sheath was characterized by Shadowgraphy technique at different discharge evolution times. For the characterization of the plasma density in the radial phase, digital interferometry was used. Both techniques were implemented, using picosecond laser source with a pulse duration of 170 ps Additionally, the electrical behavior of the discharge, as well as its performance, were monitored with conventional electrical diagnostics and neutron and X-ray detectors, respectively. From the refractive optical records, the appearance of plasma filaments is observed from an early stage of the discharge. During the evolution of the plasma sheath, the filaments remain confined in a region of the sheath, such as a toroidal plasma belt, without reaching the top of the anode nor participating in the radial compression phase [1]. On the other hand, from the records in the radial phase, the appearance of Rayleigh-Taylor type instabilities is observed. The measured total neutron yield exceeds the values predicted by scaling laws by a factor of five or higher at the same energy in the capacitor bank and pinch current. Additionally, we observe an increase in X-ray production and compression dynamics, as evidenced by a large amplitude dip in the current derivative signal, high compression velocity, and formation of instabilities.

Acknowledgments: C. Pavez acknowledges the financial support of the ANID-FONDECYT project N°1211885. G. Avaria acknowledges the financial support of the ANID-FONDECYT project No. 1211131.

References

[1] C. Pavez, M. Zorondo, J. Pedreros, A. Sepúlveda, L. Soto, G. Avaria, J. Moreno, S. Davis, B. Bora, and J.Jain. New evidence about the nature of plasma filaments in plasma accelerators of type plasma-focus. Plasma Physics and Controlled Fusion, 65(1), 015003 (2022).

LLNL’s Megajoule Neutron Imaging Radiography Dense Plasma Focus (MJOLNIR DPF) uses a plasma discharge in a deuterium gas to produce neutrons for the purpose of imaging. We have substantial interest in fine control of the intensity and the temporal and spatial distributions of the resulting neutron source. We have investigated the introduction of noble gas dopants (neon, argon, and krypton) to the deuterium gas with the intention of exercising such control. We present both experimental results and simulations of the effects of the dopants.
This work was prepared by LLNL under Contract DE-AC52-07NA27344.

Current measurements in Plasma Focus discharges are usually made with inductive probes such as Rogowskii coils, which present the disadvantage that it cannot determine the current circulating specifically through the plasma column. This indetermination makes it more difficult to estimate plasma characteristics such as the temperature inside the column through the Bennett relation.
Zeeman splitting, based on the spectral separation of optical emission lines, enables the estimation of the magnetic field in the plasma column when a high current is present. The emitted photons have a distinct polarization identified as σ⁺ and σ^-, possible to be separated by a λ/4 polarizing plate.

This work presents preliminary measurements of the magnetic field present at the plasma column of the PF-400J discharge in a high current density configuration (d_anode = 4.5 - 6.0 mm and z_eff = 10 - 20 mm), by using the Zeeman splitting spectroscopic technique of the Ar III emission at 330.18 nm. The measurements are spatially resolved in the radial direction, with the use of the combination of a polarizing crystal and a λ/4 plate, and a bifurcated fiber optic bundle focused on the entrance of a 0.5 m spectrometer with a 2400 l/mm grating.

With this experimental configuration a magnetic field of around 2 T is estimated at the pinch volume, when the maximum current (~100 kA) is achieved.

Authors acknowledge the finantial support from grants ANID FONDECYT Regular 1211131,

FONDECYT Regular 1211885 and FONDECYT Regular 1220533

A generalized Ohm’s law is derived to treat strongly magnetized plasmas in which the electron gyrofrequency significantly exceeds the electron plasma frequency. Strong magnetization of electrons causes the frictional drag between electrons and ions due to Coulomb collisions to shift, producing an additional transverse resistivity term in the generalized Ohm’s law that is perpendicular to both the current (J) and the Hall (JxB) direction. In the limit of very strong magnetization, the parallel resistivity is found to increase by a factor of 3/2 and the perpendicular resistivity by a logarithmic factor of the ion-to-electron mass ratio. These results suggest that strong magnetization significantly changes the magnetohydrodynamic evolution of a plasma. Regions of dense z-pinch plasmas can reach such strongly magnetized regimes, and the associated influence of strong magnetization may influence plasma dynamics in ways that are unexpected based on conventional transport theory of magnetized plasmas.

Strong magnetic fields in multi-MA accelerators can magnetize the electrons to a limit that their gyroradius becomes much smaller than the Debye length. These strongly magnetized plasmas exhibit novel transport properties that need to be better understood. Traditional theories are limited to weakly magnetized transport regimes where the gyroradius is much larger than the Debye length. Here, we develop a generalized kinetic theory that can treat Coulomb collisions in the strongly magnetized transport regime and which asymptotes to the traditional Boltzmann kinetic theory in the weakly magnetized limit. The theory also spans the weak to strong Coulomb coupling regimes by incorporating the mean force kinetic theory concept. To demonstrate the utility of the generalized theory, it is used to compute the friction force on a massive test charge moving through a strongly magnetized one-component plasma. It is shown that when the plasma is strongly magnetized, the friction force on a test charge shifts, obtaining components perpendicular to its velocity in addition to the typical stopping power component antiparallel to its velocity. Strong magnetization is also found to break the fundamental symmetry of independence of the sign of the charges of the interacting particles on the collision rate, commonly known as the “Barkas effect”. Strong magnetization in combination with oppositely charged interaction is found to increase the perpendicular resistivity and conductivity by an order of magnitude, which might have implications in the reduction of accelerator efficiency by diverting current away from the load.

Regions of the plasmas formed in multi-MA pinch implosions fall into the
very strongly magnetized regime, meaning that the electron gyrofrequency ex-
ceeds the electron plasma frequency and in the weak to moderate Coulomb
coupling regime denoting that the potential energy of interaction is on the or-
der of the kinetic energy. These regimes modify the Coulomb collision frequency
making traditional kinetic theories invalid.
Using a recently developed generalized Boltzmann kinetic theory for strongly
magnetized plasmas, the electron-ion temperature relaxation rates in both paral-
lel and perpendicular directions are calculated. It is shown that during the tem-
perature evolution electron-ion collisions can lead to a temperature anisotropy.
This work also studies how this developed temperature anisotropy relaxes via
Coulomb collisions in a strong magnetic field. These results have particular
relevance to the magnetically insulated transmission line where the plasma is
expected to be very strongly magnetized, and the current shunted in this region
is potentially a significant loss. The improved understanding of temperature
relaxation presented here will contribute to a more complete fluid model in
this novel plasma regime, which can in turn be used to inform improvements
in experimental Z-pinch techniques such as improving the efficiency of current
delivered to the load.

Axial magnetization of plasma is required to reduce electron thermal conduction in current-driven magnetized liner inertial fusion (MagLIF) and has also been shown to increase plasma temperature and fusion yield in laser-driven ICF experiments. We explored an extension to this concept in which we modify the axial field profile to form a dynamic magnetic mirror. We hypothesized such a pulsed mirror configuration can reduce axial plasma outflow, increasing energy coupled to the fusion fuel. To study the effect of a dynamic magnetic mirror, we developed an auto-magnetizing wire-array platform that can be imploded on a 1-MA pulsed power machine. Electrodes consisting of twisted tubes introduce an azimuthal component to the current path to produce an axial magnetic mirror that increases and decreases in strength with the main machine current. Thin aluminum wires are threaded through the tubes to form a cylindrical wire array that generates the imploding z-pinch. By twisting the wires and electrodes separately we modify the mirror ratio and the overall magnetic field strength to explore a series of mirror configurations with different field profiles. A 3-axis Thomson scattering diagnostics and laser interferometers are used to measure the temperature, velocity, and density of the precursor plasma during wire-array ablation while radiation detectors and cameras are used to monitor the plasma condition throughout the whole implosion. A reduction in axial outflow is observed with the mirror configuration when compared to standard Z-pinch without any external axial magnetic field. I will present results from the study on the influence of mirror-ratio and overall mirror field strength on plasma temperature and radiation yield.

Tungsten (W) has important applications in Z-pinch physics and ICF: wire arrays that consist of hundreds of micron-diameter W wires can be imploded at multi-MA currents and generate the highest radiation yield out of all other wire materials. Not only high current but also 1 MA university-scale pulsed power generators are able to produce multiply-ionized high-Z plasma, which is illustrated in this talk for W Z-pinches. We have previously presented and analyzed the results of experiments with W Double Planar Wire Arrays at the University of Michigan’s low-impedance Linear Transformer Driver (LTD) MAIZE generator (0.1 W, 0.5–1 MA, and 100–250 ns)^1,2 and with compact W wire arrays of various geometry and wire composition at the University of Nevada, Reno’s high-impedance Marx bank Zebra generator (1.9 W, 1-1.8 MA, and 100 ns)^3,4. In this talk, we focus on the comparative analysis of the x-ray spectroscopy results (i.e., x-ray pinholes and soft and hard x-ray spectra) with the main objective of identifying and characterizing thermal and non-thermal W Z-pinch plasmas produced in W wire load experiments on both university-scale Z-pinch facilities. In particular, M-shell W spectra in a spectral region between 4 and 8 Å that manifests “hot” keV-plasmas will be analyzed and compared between different types of W wire loads and Z-pinch generators. In addition, characteristic L-shell W spectra in a spectral range between 1 and 1.7 Å that manifest “cold” non-thermal plasmas with electron beams produced on both devices will be presented. The conditions of production of thermal and non-thermal W plasmas in one shot will be discussed as well as future work.

¹ V.L. Kantsyrev, A.S. Safronova, V.V. Shlyaptseva, I.K. Shrestha, M.T. Schmidt-Petersen, C.J. Butcher, A. Stafford, K.A. Schultz, M.C. Cooper, A.M. Steiner, D.A. Yager-Elorriaga, P.C. Campbell, S.M. Miller, N.M. Jordan, R. McBride, R.M. Gilgenbach, IEEE Transactions on Plasma Science, 7^th Special Issue on Z-pinch Plasma 46, 3778-3788 (2018).

² C.J. Butcher V.L. Kantsyrev, A.S. Safronova, V.V. Shlyaptseva, I.K. Shrestha, A. Stafford, A.M. Steiner, P.C. Campbell, S.M. Miller, D.A. Yager-Elorriaga, N.M. Jordan, R.D. McBride, R.M. Gilgenbach, Phys. Plasmas 28, 082702 (2021).

³ A.S. Safronova, V.L. Kantsyrev, R.R. Childers, A. Stafford, C.J. Butcher, V.V. Shlyaptseva, Bulletin of APS, 64th Annual Meeting of the APS Division of Plasma Physics (DPP2022), YO04.00003 (Spokane, WA, Oct. 17-21, 2022).

⁴ A.S. Safronova, V.L. Kantsyrev, R.R. Childers, C.J. Butcher, A. Stafford, V.V. Shlyaptseva, E.E. Petkov, Invited talk. 14^th International Conference on Plasma Science and Applications (ICPSA 2021), Virtual, Dec. 28-30, 2021.

This research was supported by NNSA under the DOE grants DE-NA0004133 and DE-NA0003047 and in part by DE-NA0002075 and through the Krell Institute LRGF under DE-NA0003864.

By embedding metallic wires in dielectric insulators, the energy of a pulsed power generator can be efficiently transferred into the wire material (>80%) and the subsequent explosion of the wires drives strong, multi-kms^-1 shockwaves through the insulator. Approximately 2 decades ago the use of cylindrical arrays of wires in water baths was demonstrated to produce imploding cylindrical shockwaves in the water, that were expected to result in Mbar pressures on axis of the array even from relatively small, 300-500kA, drivers. However diagnostic limitations at the time prevented detailed exploration and exploitation of the technique.

More recently the use of high speed, multi-frame synchrotron radiography has enabled direct comparison of the production and dynamics of cylindrical shockwaves with hydrodynamic simulations; and explored the use of different array configurations to drive shockwaves of arbitrary geometries. Here we describe the latest results from our work on the ESRF synchrotron, where up to 256 high resolution radiographs, spaced 176 – 704ns apart, are utilised to study phenomena including the development of the Electrothermal instability in an exploding wires down to few µm scale lengths; the production and use of planar shockwaves to drive Richtmyer Meshkov and Kelvin Helmholtz instabilities over 1000s of ns in warm dense materials and precisely shaped low density solids; and the production of jets and spherical implosions for reaching extreme pressures.

Finally we will present extensions of our research to significantly higher currents, utilising the Cepage and M3 facilities at First Light Fusion where energies ~1MJ, are deposited into exploding foils and array loads, 2 orders of magnitude higher than our previous investigations. Laser probing measurements along the axis allows us to compare shockwave velocities to previous experiments and explore how this scales.

This work was sponsored by First Light Fusion, Sandia National Laboratories, EPSRC and NNSA under DOE Cooperative Agreement Nos. DE-NA0003764 and DE-SC0018088 and the Israeli Science Foundation.

Despite extensive theoretical, computational, and experimental efforts over many years, the dynamics of the implosion of gas-puff z-pinches still needs to be fully understood and confidently modeled. Spectroscopists at the Weizmann Institute of Science have obtained a wealth of data from detailed diagnostics of the imploding plasma, including direct measurements of the magnetic field [1], density, electron and ion temperatures. Additionally, new phenomena such as axial magnetic flux amplification [2, 3], formation of almost force-free flows, and self-generated plasma rotation [3] were also observed. After successfully validating the 2D RMHD code MACH2-TCRE against high-current argon gas-puff shots on the Z machine [4], NRL now seeks to advance it against finer details of Weizmann experiments at medium current for ensuring the accuracy and reliability of simulation results.

We report recent progress in gas-puff z-pinch implosions that include new ab-initio simulations of oxygen pinches investigating the effect of the load chamber shape with electrode recesses. We have simulated the flow of neutral diatomic oxygen from a plenum into the chamber through the nozzle and successfully compared the resulting simulated gas density profiles with in-situ measurements. The measurements of the initial density profile were axially limited because the nozzle at both the cathode and the anode mesh were recessed within electrode sleeves. We used the computed neutral flow profile as the initial condition for the Radiation-MHD simulations of the implosion. Taking into account the details of the nozzle and chamber geometry significantly improves the agreement with the measured current profile, including the inductive notch, temperature, and spectroscopic data. Our predictions of unexpectedly high radiation yield from the anode recess area were confirmed in experiments with an expanded observation window. We compare our predictions of circular charged rings with experiments. We also discuss the path to modeling other features, including the radial profiles of the azimuthal magnetic field and the self-induced rotation.

1 Rosenzweig, G. et. al., Phys. Plasmas 27, 022705 (2020).

2 Mikitchuk, D. et. al., Phys. Rev. Lett. 122, 045001 (2019).

3 Cvejić, M. et. al., Phys. Rev. Lett. 128, 015001 (2022).

4 Tangri, V. et. al., IEEE TPS, 46, 3871 (2018).

We present an energy-inventory analysis for tiple-nozzle argon gas-puff z-pinch implosions driven by the 1 MA, 230 ns rise time COBRA generator at Cornell University. Implosions with and without an applied axial magnetic field are examined. The total energy coupled the plasma is inferred from current and voltage traces while spatially resolved plasma parameters such as flow velocity, temperature, and density are measured at different times and positions across highly repeatable implosions using Thomson Scattering and laser interferometry. Radial, azimuthal, and turbulent kinetic energy components are isolated through the use of two Thomson scattering collection angles, and by discriminating between thermal and non-thermal broadening of the scattering spectra. This non-thermal broadening has been demonstrated to be consistent with hydrodynamic turbulence which appears to mediate dissipation in the collisionless shock. Evidence of shock reflected ions in the Thomson scattering signal is also presented. Directed kinetic energy is also estimated from the radial implosion trajectory and initial mass distribution by assuming the mass is accreted as in a snowplow. Radiated energy is measured using a calibrated, filtered photoconducting diamond detector (PCD) and a bolometer. For the cases considered, the total energy coupled to the pinch at the end of stagnation is inferred to be approximately 4-5% of the stored electrical energy. Radiation yields for the axially magnetized implosions are reduced despite the improved stability. Reasonable agreement between the coupled and measured energy is observed until just prior to stagnation, but subsequently diverges.

Research supported by NNSA stewardship science academic programs under DOE Cooperative agreement No. DE-NA0003746

The Z machine at Sandia National Laboratories has produced bright x-ray sources with photon energies in the 1-10 keV range by imploding gas puff or wire array loads. In particular, argon gas puff implosions on the Z machine have produced argon K-shell >300 kJ [1]. The 2-D MHD Mach2+TCRE code reproduced the measured K-shell powers, yields, and emission region. Also, the ratio of the Lyα to the Heα plus IC lines from the simulation had good agreement to the experiment after peak power. Yet, those simulation had higher line ratios prior to the peak. The authors attributed this difference to 3-D effects or on the implicit assumption of steady-state kinetics [2]. This presentation will explore the effect of time-dependent atomic level kinetics using the NRL DZAPP code. DZAPP is a coupled 1-D MHD, non-LTE atomic kinetics, and radiation transport code that incorporates a transmission line to drive the load. Simulations using steady-state and time-dependent non-LTE level kinetics will be presented and compared with experimental line ratios.

1. B. Jones et al. Physics of Plasmas 22, 020706 (2015)

2. J.W. Thornhill et al. IEEE Tran. Plasma Sci. 43, 2480 (2015)

________________________________

*Work supported by the U.S. DOE/NNSA. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525.

DISTRIBUTION STATEMENT A: Approved for public release. Distribution is unlimited.

Deuterium gas-puff z-pinches are primarily studied as efficient sources of DD fusion neutrons. The first experiment with a deuterium gas jet was performed in 1978 [1]. Since then, several D₂ gas-puff experiments have been performed on various pulsed-power generators, including Angara-5, Saturn, Speed-2, Z-machine, S-300, GIT-12, Zebra, Hawk, MAIZE, and CESZAR. The highest DD neutron yields published to date were 4×10¹³ and were generated on the Z-machine at Sandia National Laboratories around 2005 [2].

More recently, z-pinch experiments with a plasma shell on a deuterium gas puff have been carried out on the GIT-12 high-impedance pulsed-power generators at 3 MA currents. These experiments produced unique results with high neutron and ion energies approaching 60 MeV [3]. The comparison of the deuterium gas-puff experiments on different high- and low-impedance generators allows the identification of the parameters that are essential for optimization of ion acceleration and neutron production. These parameters include the optimal mass, pre-ionization, short deuterium-gas injection time, zippering towards the cathode, etc.

The conclusions regarding the optimal conditions were confirmed on the Hawk generator (NRL, Washington, DC). At a current of 0.7 MA, HAWK accelerated deuterons up to 10 MeV producing one neutron pulse with a yield of the order of 10¹⁰ [4]. Such high-energy ions can be used to measure the distribution of magnetic fields in z-pinches [5]. However, the wider use of z-pinch-driven ion deflectometry is limited by the need for a specific load and higher currents. From this perspective, it is worth investigating other z-pinch configurations as point-like sources of DD fusion protons.
[1] J. Shiloh, A. Fisher, and N. Rostoker, Phys. Rev. Lett. 40, 515518 (1978).
[2] C. A. Coverdale, C. Deeney, A. L. Velikovich, et al. Phys. Plasmas 14, 022706 (2007).
[3] D. Klir, et al. New J. Phys. 22, 103036 (2020).
[4] D. Klir, et al. Matter and Radiation at Extremes 5, 026401 (2020).
[5] V. Munzar, et al. Phys. Plasmas 28, 062702 (2021).

Dense plasma focus (DPF) Z-pinches are compact pulse power driven devices consisting of two coaxial electrodes, separated by an insulator, and filled with a low-density gas. The discharge of DPF consists of three distinct phases: first generation of a plasma sheath, plasma rail gun phase where the sheath is accelerated down the electrodes and finally an implosion phase where the plasma stagnates into a z-pinch geometry. A DPF is similar in nature to a traditional gas puff z-pinch, with the rail gun phase serving as an opening switch for a fast current rise into an imploding load. Stagnation conditions are a strongly affected by the shape of the anode tip and the size of the final radius before the plasma enters freefall.

MJOLNIR is a dense plasma focus (DPF) located at LLNL being developed to produce a neutron source for flash neutron radiography. Producing a neutron source for radiography requires both a bright neutron pulse as well as the neutrons emanating from a small volume. Simulation and experimental results will be presented on neutron yield, and stagnation characteristics for anodes with a variety of implosion radii, the radius at which the implosion starts for both the 100 kA and 3 MA DPFs at LLNL.

This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory (LLNL) under Contract DE-AC52-07NA27344. Computing support for this work came from the LLNL Institutional Computing Grand Challenge program. LLNL-ABS- 848009

The FF-2B dense plasma focus facility has been upgraded with dual switches on each capacitor. This has reduced inductance and substantially increased peak current. We here report on the performance of the upgraded device, including comparisons with theory of fusion yield, plasmoid density, ion and electron energies, and ion beam characteristics. We describe the effect of the new switches, with reduced oscillations in early current, on the formation and evolution of filaments in the current sheath as observed in a series of ICCD images. This experimental series tested predicted improvements in yield with optimization of preionization current and the imposed axial magnetic field. We also report on preparations for the upcoming series of experiments with pB11 fuel.

The MegaJOuLe Neutron Imaging Radiography experiment relies on a dense plasma focus (DPF) as a source for flash neutron radiography. In a DPF, a high voltage is pulsed across a low-pressure gas between coaxial cylindrical electrodes. The ionized gas forms a current sheath that lifts off and runs down the electrode because of the jxB force. When the sheath reaches the tip of the electrode, it magnetically compresses to form a high-density plasma, called the pinch, at the tip of the central electrode. During the pinch, magnetic instabilities generate electric fields that can accelerate ions up to several MeV and produce neutrons via beam-target interaction with the dense plasma present on-axis. We present the first neutron radiographs obtained on MJOLNIR as well as our characterization of the neutron source using the suite of diagnostics implemented on MJOLNIR such as real-time neutron activation detectors, neutron time-of-flight detectors and 2D time-gated images of the neutron source. We also discuss how the neutron source size could be tailored. This work was performed by LLNL under Contract DE-AC52-07NA27344, LLNL-ABS-847854.

Nuclear diagnostics have been used to characterize deuteron acceleration and neutron production in a unique configuration of dense plasma focus (DPF) driven by the Hawk pulsed-power generator at the U.S. Naval Research Laboratory. The high inductance (607 nH) and associated high voltage (640 kV) and fast rise time (1.2 µs) of Hawk were unusual for a DPF driver, as was the initialization of the DPF using local mass injection rather than a conventional neutral gas fill. The local mass injection involved injecting transient neutral gas and plasma into a vacuum chamber at prescribed locations just prior to application of the main current pulse that drove the DPF. Rhodium foil activation counters, bubble detectors, and neutron time-of-flight detectors were used to characterize the production of neutrons from this local mass injection dense plasma focus (LMIDPF). A neutron yield with deuterium on the order of 10¹⁰ at a current of 0.67 MA was measured, significantly above the yield expected at this current based on scaling from conventional low-inductance, neutral-gas-fill DPFs. Evidence of high-energy neutron production from deuterons accelerated to energies over an order of magnitude higher than the applied generator voltage was observed in ion multi-pinhole images obtained with absorbers of varying thickness, as well as in the neutron time-of-flight detector signals. Radioactivity induced in discs of material placed in vacuum for irradiation by accelerated deuterons escaping the plasma during the DPF pulse was also characterized. The spatial distribution of this induced radioactivity was recorded using an image plate placed against each disc shortly after its removal from the vacuum chamber. The decay rate and gamma-ray energy spectrum were measured using a NaI(Tl) detector. These measurements showed evidence of reactions induced by high-energy deuterons in aluminum and polyethylene located near the axis of the machine.

This work was supported by the Naval Research Laboratory Base Program, the Defense Threat Reduction Agency, and the Ministry of Education, Youth, and Sports of the Czech Republic under grant No. LTAUSA-17084.

Pulsed plasma devices are sources of high intensity radiofrequency emissions. These emissions are comprised of a wide range of frequencies, from the High Frequency to the Ultra High Frequency, with a great complexity of the registered signals at different stages of the current pulse evolution.

These radiated signals carry information related to plasma evolution, that can be related to phenomena such as pulsed X-ray emission from the plasma device. To measure the UHF emission, different antenna designs optimized to measure a specific range of frequencies can be used.

In this work we present three different experiments that shed some light on the use of UHF signals to remotely characterize a Plasma Focus device, with the use of different antenna designs as well as machine learning algorithms. The first experiment uses a Vivaldi antenna to measure the UHF signal emitted from the PF-400J device, in parallel with a scintillator/photomultiplier tube detector to acquire pulses of X-ray emission. These signals were fed to a Convolutional Neural Network algorithm that enabled the classification of the signals and the estimation of the Hard X-ray pulse intensity from the information contained in the UHF signal with a 85% accuracy.
The second experiment consisted of the comparison of different antenna designs (Monopole, helical and Vivaldi) with the inductive sensors already available in the PF-400J device. From the comparison of time and frequency characteristics it was possible to identify different phases of the current pulse evolution (breakdown and pinch), finding that the Vivaldi design captured most of the features seen in the inductive sensor connected to the device. This opens the possibility to use these remote sensors to characterize the evolution of pulsed devices with a simple and economical sensor such as the UHF antenna.
Finally, a neural network classifier was used to find similarities between the characteristic parameters of the electrical signals from a plasma focus device (voltage divider, Rogowskii coil) and the UHF emission measured with a Vivaldi antenna. This showed that the use of the complex UHF signal fed into a machine learning algorithm rendered no observable difference with the hard X-ray pulse emission estimation from the characteristic parameters of the electrical signals.

These results show a promising area of research based on the remote sensing of the UHF emission from pulsed plasma devices.

The authors acknowledge the financial support from grants ANID FONDECYT Regular 1211131 and FONDEF IDeA ID22I10153.

Among the various pulsed plasma configurations used in nuclear fusion research, the Plasma Focus discharge (PF) is highly efficient in producing both pulsed fusion neutrons and X-rays. This unique characteristic has driven research and development of new projects in laboratories worldwide, with applications ranging from basic science to defense and material science. The PF research program at the Chilean Nuclear Energy Commission (CCHEN) has focused on the development of low-energy PF generators for fundamental physics research related to fusion and for the generation of pulsed X-ray fields and neutrons. These efforts are targeted towards materials studies, radiobiology, and dosimetry applications. The experimental evidence has shown that the production of X-rays and neutron in Z-pinch and PF discharges is proportional to the energy stored in the capacitor bank, and therefore, the typical size of the device, following a power law relationship. This condition imposes a constraint on field applications due to the compromise in production yield of X-rays and neutrons for low-energy portable and transportable devices (E < 1 kJ). This study investigates the behavior of three small low-energy and low-current plasma focus (PF) devices operating in high-pressure regimes (> 10 mbar) to enhance performance in total neutron output. The experiments have been driven by using (i) the Multipurpose Generator (MPG, 1.2 μF, 42 nH, 22-24 kV, ~100 kA, 290-345 J), (ii) PF-400J (880 nF, 38 nH, 25-29 kV, ~120 kA, 290-345 J), and (iii) PF-50J (660 nF, 40 nH, 25-29 kV, 30-70 kA, 50-72 J).The experiments are designed based on classical scaling laws and similarity [1], with a focus on achieving high input power density to increase the initial rise rate of plasma current density. This is achieved by combining a small initial discharge region volume with the operating voltage and the parameter space of p-L (where L >> 2a, L is the effective anode length, p is the gas fill pressure, and a is the anode radius), which influence the dynamical stages of the PF discharge. Various diagnostic techniques are used to monitor the experiment, including electrical monitors, X-ray and neutron detectors, TLD dosimeters, and refractive optical techniques. The neutron yield measurement includes an evaluation of the backscattered source neutrons in the laboratory walls, floor, and ceiling. The measured total neutron yield exceeds the values predicted by scaling laws by a factor of five or higher at the same energy in the capacitor bank and pinch current [2]. Additionally, we observe an increase in X-ray production and compression dynamics, as evidenced by a large amplitude dip in the current derivative signal, high compression velocity, and formation of instabilities.

Acknowledgments: The authors appreciate the financial support of the ANID-FONDECYT projects N°1211885; No. 1211131; N°1221364; ANID-Millennium Science Initiative Program- ICN2019_044 and grant PID2019-104714GB-C21, from Spanish Ministerio de Ciencia e Innovación.

References

[1] L. Soto, C. Pavez, A. Tarifeño, J. Moreno and F. Veloso. Plasma Sources Sci. Technol 19, 055017 (2010).

[2] A. Tarifeño-Saldivia, L. Soto. Physics of Plasmas, 19(9), 2012.

The electrothermal instability (ETI) is a Joule-heating-driven instability that promotes runaway temperature in conductors driven to high current density, altering the 3D evolution of the expansion and phase state of the metal. Most metals include complex distributions of imperfections (voids, resistive inclusions) which seed ETI. To simplify comparison with modeling and theory, experiments examined growth of ETI from relatively void/inclusion free, 99.999% pure, diamond-turned, 1.00 mm-diameter aluminum rods. Surfaces included a variety of deliberately machined and well-characterized perturbations, including 10-micron-scale quasi-hemispherical voids, or “engineered” defects (ED), and sinusoidal patterns of varying wavelength and amplitude. Such perturbations were studied in isolation and colocation to evaluate which defect type drove more rapid heating. First, the emission evolution of axially vs. azimuthally oriented ED pairs of identical size and spacing was studied; individual defects initially evolve independent of pair orientation, but at higher current density, pair orientation dictates global plasma evolution. Next, heating similarity, the hypothesis that heating is independent of ED size, was evaluated in experiments with scaled 12, 24, and 48-micron diameter ED. Data from 12 and 24-micron ED demonstrate similarity, exhibiting brightest emission regions above and below the ED, which later source plasma filaments. Surprisingly, the evolution of 48-micron diameter ED is qualitatively different, with brightest emissions forming on both sides of the ED and plasma filaments sourced from the divot center. This may be consistent with similar temperature evolution driving identical expansion velocity, with the ejected material therefore traversing a smaller extent of the larger defects. Next, epoxy coatings were added to select loads to evaluate the effect of hydrodynamic tamping. Reduced expansion of the metal delays surface plasma formation, thus elongating the stage where the azimuthally correlated version of ETI is expected to grow. Indeed, strata formation between azimuthally separated ED is dominant for dielectric coated surfaces. Finally, 24-micron diameter ED have been machined into sinusoidally-perturbed surfaces of varying amplitudes (0.191 – 3.05 microns) and wavelengths (24-48 microns). Maximum theoretical current density amplification (j/j₀) for the sinusoids ranges from 1.05 to 1.5, while the maximum ED-driven j/j₀ is 1.5. ED alter the periodic heating of sinusoidal emissions more than 100 microns from the divot center. Experimental data and comparisons with 3D simulation will be discussed.

Increased interest in z-pinch rotation resulted from the recent observations of such rotation appearing spontaneously in z pinches with the imposed Bz field.¹ It motivated our theoretical and numerical stability analysis of supersonic implosions of rotating magnetized z-pinches. The conservation of angular momentum density ρΩr in the imploding plasma flow will naturally create dΩ/dr <0, which is a condition for magnetorotational instability (MRI). This instability is well known in astrophysics,² particularly as a major cause of turbulent transport in astrophysical Keplerian disks. Our two-dimensional simulations using Athena code demonstrated that in the imploding rotating z-pinch plasmas, in addition to classical Magneto-Rayleigh-Taylor instability, there is also MRI. We use our exact Mag Noh shocked rotating solutions³ as a convenient vehicle for studying MRI in the imploding plasma flow and apply various perturbations to study the instabilities numerically, including following them to the nonlinear regime. The nonlinear development of instabilities results in turbulence, enhanced mixing, and other transport.

(Supported by the DOE/NNSA)

References

¹M. Cvejić, M. et. al., Phys. Rev. Lett. 128, 015001 (2022).

²S. A. Balbus and J. F. Hawley, Ap. J. 376, 214 (1991).

³A. Beresnyak et. al., J. Fluid Mech. 936, A35 (2021).

Field-reversed configurations (FRCs) are often the plasma target of choice for magnetized target fusion (MTF) efforts of multi-microsecond timescale, which require closed-field lines. While Z is fast enough to operate in a hybrid inertial MTF regime (i.e. traditional MagLIF with open-field lines), compression of a high- energy-density (HED) FRC within a MagLIF-like liner may offer many advantages – including a greater flexibility in liner size and implosion time. This offers a world of possibilities in using the pulse-shaping and self-crowbar capabilities of Z for greater energy coupling.

Analytic estimates and 2D LASNEX simulations by Slutz et al. [1] suggest very interesting fusion gains (QDTsci~1) with FRC compression at the centimeter-sized, sub-microsecond hydrodynamic scale of Z. In an advantage over FRC liner compression efforts utilizing translation [2], it is proposed these FRCs be formed in- situ to the liner by using external bias coils in addition to a helical AutoMag-type liner [3].

To investigate this novel formation method, a high-field 𝜃-pinch and FRC formation platform was developed on the MAIZE LTD (1-MA, 100-ns) at UM. Fast-framing images and magnetic probe data reveal interesting dynamics to the plasma columns & annuli formed within deuterium-filled quartz discharge tubes, perhaps suggesting successful FRC production under the proper conditions. Effects of pre-ionization, bias field, and D2 fill pressure are explored and compared to expectations from the literature.

2D simulations in GORGON/Kraken of FRCs initialized with parameters relevant for MTF on Z predict anomalous trapped flux lifetimes consistent with theory and empirical scaling laws [4]. In fact resistive lifetimes are ample for compression; formation robustness and stability to tilt and rotational modes remain the primary concern at high-density. The effects of field gradients inherent to the AutoMag concept are explored, and it is shown that FRCs would be quite robust to axial ejection during the compression/burn. Synthetic images are generated from simulation data for comparison with experimental images.

In summary, prospects for HED FRC liner compression on Z are bright – but a great deal of work remains recommended, particularly in simulation. Optimization of energy coupling to a liner on Z with pulse- shaping remains an open question which may influence the desired FRC parameters. Once such parameters have been identified, the proper formation hardware should be designed and tested at-scale on a high- impedance 1-MA machine like the Mykonos LTD. If a proper FRC starting target can be produced and diagnosed on Mykonos, the path to the first HED FRC shot on Z should become clear.

[1] S. A. Slutz & M. R. Gomez, Phys. of Plasmas 28, 042707 (2021); https://doi.org/10.1063/5.0044919 [2] J. H. Degnan et al., Nucl. Fusion 53 093003 (2013); https://doi.org/10.1088/0029-5515/53/9/093003 [3] S. A. Slutz et al., Physics of Plasmas 24.1 (2017); https://doi.org/10.1063/1.4973551
[4] Alan L. Hoffman et al., Fusion Technology 23:2 (1993); https://doi.org/10.13182/FST93-A30147

This work was supported in part by the NNSA Laboratory Residency Graduate Fellowship program under DOE Contract No. DE-NA0003960, and by the NNSA Stewardship Sciences Academic Programs under DOE Cooperative Agreement DE-NA0003764. SNL is managed and operated by NTESS under DOE NNSA contract DE- NA0003525.

Nuclear fusion is a potential source of carbon-free electricity with many concepts in development. The Fusion Z-pinch Experiment (FuZE) is a sheared-flow stabilized Z-pinch device where the Portable and Adaptable Neutron Diagnostics for ARPA-E (PANDA) suite has been deployed since 2021 at the Zap Energy facility. The FuZE capacitor banks are discharged to ionize neutral deuterium gas and induce pinch currents that cause fusion in deuterium plasmas. Neutrons with mean energies of 2.45 MeV can be detected outside of the device chamber. There are two types of neutron detectors in PANDA: lanthanum bromide (LaBr₃) activation detectors and fast plastic scintillators. The activation detectors measure the integrated neutron yield for each shot, while the plastic scintillators record time resolved signals. The 16 gain matched plastic scintillators can be positioned along the Z-pinch device to measure the neutron source spatial and temporal characteristics. FuZE is designed to produce a long lived (few µs) fusing plasma in a column. Calculating the time resolved axial profile measured by the plastic scintillators shows where and when the plasma is hot and dense enough to produce neutrons.

This work is supported by a US DOE ARPA-E award 18/CJ000/05/05. Prepared by LLNL under Contract DE-AC52-07NA27344.

The Magneto Rayleigh-Taylor instability (MRTI) is a concern for magnetized, imploding z-pinch configurations such as the magnetized liner inertial fusion (MagLIF) concept being studied on the Z facility at Sandia National Laboratories [1]. In MagLIF, MRTI degrades the assembly and confinement of thermonuclear fuel. A key component to MagLIF experiments is the pre-embedded axial magnetic field, which reduces thermal conduction losses during the implosion and traps charged fusion products at stagnation to enhance the fusion yield in the fuel. This axial field modifies MRTI, generating helical plasma striations on the liner surface. The observed pitch angle of the helices is significantly larger than what was originally anticipated. It is hypothesized that the amplification of axial magnetic field, from flux compression occurring in a low-density plasma (LDP) surrounding the liner, could explain the large pitch angle [2]. The LDP is suspected to be generated from the high current densities in the transmission lines leading up to the liner.

To investigate this hypothesis, we are developing a platform to study axial flux compression in LDP on the University of Michigan’s MAIZE facility, a 1-MA class linear transformer driver. This platform uses MAIZE to implode a low-density gas-puff plasma onto a thin foil liner. The resulting MRTI structure, with and without a pre-applied axial magnetic field, will be observed with a 12-frame optical self-emission camera as well as laser-based diagnostics. The two laser diagnostics under development are a laser schlieren refractometer [3] and a laser interferometer system, both with a 532-nm probe beam from a Nd:YAG laser. Ongoing efforts and challenges will be presented.

Understanding the impact of hydrodynamic perturbations transmitted through thin, dense layers is important for inertial confinement fusion ignition schemes, particularly double- or multi-shell systems. Experiments to validate our understanding are challenging as they necessarily involve multiple interfaces, materials, transmitted and reflected shocks, etc. We present experimental data for a Z Machine platform at Sandia National Laboratories investigating the Richtmyer-Mehskov process and interfacial feedthrough, and a comparison to a similar laser-driven design fielded at the National Ignition Facility. The pulsed-power driven platform is a cylindrical liner filled with liquid deuterium and is magnetically imploded with >20 MA of current, driving a converging shock that propagates towards the central axis and generating a high plasma-beta system suitable for investigating HED hydrodynamical processes. These experiments test and validate aspects of transmitted instability theory and feedthrough, including the qualitative difference in behavior between long and short wavelength modes: the buckling of the layer by long modes into imprinted shapes, and the cumulative impact of short modes leading to mix. Recent improvements in analysis have demonstrated how, beyond integral measures such as dominant wavelength and mix width, higher-order metrics such as material variances may also be extracted from the diagnostics and compared with theory and simulation.

*This work conducted under the auspices of the U.S. DOE by Los Alamos National Laboratory under contract 89233218CNA000001 and by Sandia National Laboratories under contract DE-NA0003525

The Magnetized Liner Inertial Fusion (MagLIF) platform on Sandia’s Z-Machine consists of a beryllium liner filled with deuterium (D2) gas that is pre-ionized and heated by a short-pulse laser and magnetized by a set of high magnetic field coils before being directly-drive by the high-current pulse from the Z-machine. Recently, an idea to add tritium (T2) gas on the MagLIF is getting more attention because DT fusion signature can add valuable additional information on the nuclear phase of MagLIF. In this work, DT nuclear reaction history diagnostics previously developed for Inertial Confinement Fusion (ICF) experiments on the National Ignition Facility (NIF) are being extended to the MagLIF platform. The nuclear diagnostics planned to be fielded include the Gamma-Reaction History (GRH) diagnostic which consists of four independently thresholded gas Cherenkov detector cells which enables measurements of the 16-MeV gamma rays emitted from the D(T,.)5He reaction. Also in development were sets of Aerogel Cerenkov Detectors (ACDs) for measuring the 0.1 - 5 MeV gamma ray continuum from the 9Be(n,.)9mBe inelastic scattering reactions from the beryllium liner, enabling time-resolved liner areal density measurements at high temporal resolution. Also in development is a new proton-recoil telescope (PRT) which enable time-resolved neutron spectroscopy measurements for fuel areal density measurements. This suite of diagnostics will enable high temporal resolution measurements of the reaction history and liner dynamics which will enable constraints on the energy balance of theoretical models that were not previously accessible with DD implosions on MagLIF. In anticipation for future DT shots, forward modelling in MCNP6.3 was conducted to estimate the detector response functions for low tritium percentage implosions. The pulsed power environment poses a different set of challenges such as high-flux bremsstrahlung backgrounds that must be accounted for not present in laser-driven systems. Detector design will be presented along with the predicted response functions and anticipated challenges of diagnostic operation with the pulsed power environment.

The Energetics and Other Physics of a Fusion-grade 20 MA Liner-on-Plasma Implosion without Preheating or Premagnetization, Irvin R. Lindemuth, retired, formerly Los Alamos National Laboratory—magnetically driven z-pinch implosions of cylindrical and quasi-spherical liners have a wide range of applications, including controlled fusion and creating high-energy-density environments for material studies. A priori a variety of phenomena might be expected in a liner-target configuration. The axial current can Ohmically heat the liner causing the liner to expand in a manner similar to an exploding wire. The magnetic forces generated by the current can implode the liner and the liner becomes a z-pinch, subject to all of the stability concerns of a classical z-pinch. An inward moving inner liner surface, whether generated by Ohmic heating or magnetic forces, can compress the target, i.e., the liner becomes a hydrodynamic piston. If the “jump-off” velocity of the liner is high enough and rises rapidly enough, shocks can be generated in the target. Depending upon the liner’s resistivity, the magnetic field can potentially diffuse through the liner and induce an axial current in the target. The current can Ohmically heat the target and, as with the liner, cause magnetic forces that can compress the fuel. If the magnetic forces on the target are strong enough, the inward moving fuel may separate from the liner, leaving a vacuum behind.¹ As with the liner, the magnetic field itself can become a piston that can potentially create shocks in the target. If the target separates from the liner, only the magnetic piston compresses the fuel; the fuel, too, becomes subject to all of the stability concerns of the classical z-pinch. In fusion applications, simultaneous heating mechanisms and cooling mechanisms compete to determine whether or not the fusion fuel can reach fusion temperatures. Radiation and thermal conduction can potentially be either cooling or heating mechanisms. This paper reports one-dimensional magnetohydrodynamic (MHD) computations that examine the energetics and other physical aspects of liner-on-plasma implosions. The MHD model includes the continuity equation, an equation of motion that includes the Lorentz force, and Faraday’s law using a simple Ohm’s law. The model also includes two energy equations with Ohmic heating, radiation, and diffusive radiation transport. The MagLIF concept uses laser-preheat the fuel and premagnetization to achieve significant fusion yield on a 20-MA-class machine, e.g., Z, because the implosion velocity is limited. In contrast, the so-called “staged z-pinch” (SZP) is claimed to be able to achieve fusion in the same velocity regime with a direct drive z-pinch without external fuel preheating and premagnetization, although published SZP calculations have been shown to be inaccurate.² However, we have identified a parameter space, not necessarily practical, where a simple 20-MA z-pinch liner-on-plasma can achieve fusion temperatures at possibly achievable convergence (e.g., 40-50). “Cold-start” (solid liner) calculations are reported. One interesting result is that the liner is essentially low-beta during its acceleration whereas the fusion fuel is high-beta.

¹I. Lindemuth, J. Comp. Phys. 25, 104 (1977).

²I. Lindemuth, M. Weis, W. Atchison, Phys. Plasmas 25, 102707 (2018).

Zap Energy is actively developing the sheared-flow-stabilized (SFS) Z-pinch concept for fusion energy applications with its currently operational FuZE and FuZE-Q devices. Sheared-flow stabilization offers a pathway toward long duration (~ 100 microseconds) Z-pinch plasmas that are stable to the m = 0 ‘sausage’ and m = 1 ‘kink’ MHD modes characteristic of static configurations. Sheared-flow stabilization provides for a more attractive trajectory towards a commercial Z-pinch fusion energy system by avoiding the need to embed an axial magnetic field with external coils or employ a close-fitting conducting wall around the plasma column. An experimental suite of diagnostics has been deployed on FuZE, and FuZE-Q, a next-generation SFS Z-pinch device designed to produce deuterium plasma conditions consistent with deuterium-tritium (DT) scientific breakeven Q = 1. These diagnostics include magnetic probes and current Rogowski coils, spectroscopic systems (e.g., Broadband visible, Ion Doppler, Extreme Ultraviolet (EUV)), optical Thomson scattering (OTS), 633 nm and digital holographic interferometry (DHI), high-speed cameras, neutronics (e.g. plastic scintillators and LaBr₃ activation detectors), X-ray detectors, among others. Experimental results from the recently commissioned and now operational FuZE-Q device will be provided from various diagnostic systems. The methodology for the calculation and measurement of scientific gain in a SFS Z-pinch with fielded diagnostics will be provided. Lastly, plans for potential additional diagnostics such as Faraday polarimetry, Zeeman spectroscopy, internal magnetic probes, X-ray crystal spectroscopy, and bolometry will be presented as well.

We present a newly developed x-ray spectrometer known as the Multi-Optic Novel Spherical Spectrometer with Time Resolution, or MONSSTR. The MONSSTR relies on two spherically-bent crystal optics to image the spectra onto an hCMOS-based Ultra-Fast X-ray Imaging camera known as Icarus [1]. With a minimum gate time of 2 ns per frame and a maximum of 8 frames, 16 ns of continuous temporal coverage is achieved. MONSSTR was designed to detect trace dopants (e.g., Fe and Co) that are added or naturally occurring in the Magnetized Liner Inertial Fusion (MagLIF) experiments. To enable a high-collection efficiency, large aperture (36 mm wide) crystals were deployed. By changing the crystal plane (e.g., Quartz (203) vs. Ge (220)), the spectral range can easily be modified to accommodate a large variety of experiments. Good performance at x-ray energies as high as 13 keV have been demonstrated. Ray tracing models from SHADOW3 and initial data from Z-experiments will be presented.

[1] Q. Looker et. al, Rev. Sci. Instrum. 91, 043502 (2020); doi: 10.1063/5.0004711

SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525. This work was performed under the auspices of the U.S. Department of Energy by LLNL under Contract DE-AC52-07NA27344.

An open question important to a next generation pulsed power (NGPP) facility is the formation of plasma in the anode cathode (AK) gap of the power feed. Understanding this plasma formation and evolution is important as it can lead to current losses across the AK gap, which will affect a larger surface area on a NGPP facility. To help understand this plasma formation, it is important to measure the density of the plasma. We present the design and initial results of a photonic Doppler velocimetry (PDV) system that will enable the measurement of the electron density at 4 different points across 1 mm of the AK gap. PDV records the rate of change of the refractive index as a change in beat frequency and is capable of measuring densities down to 10¹⁶ cm^-3 over a 1 mm path. This diagnostic was deployed on the Mykonos pulsed power machine at Sandia, which has a peak current of 680 kA. It was tested on two different loads designed to study and diagnose plasmas in the AK region in planar and parallel plate loads. The current density on these platforms can reach values that are equivalent to 1.6 cm radius in the Z machine’s inner magnetically-insulated transmission line (MITL) making them useful platforms for studying power flow in high-stress inner MITLs. We present initial results from measurements with a single fiber probe, which shows the density rising about halfway to peak current. We will also discuss the challenges and steps for extending this measurement to 4 points.

This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories, a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

The one-dimensional imager of neutrons (ODIN) has been used to image neutrons emitted from the source of Magnetized Liner Inertial Fusion (MagLIF) experiments on the Z facility. These experiments produce DD-neutrons which pass through a 100-mm thick tungsten rolled edge slit to then be imaged on CR-39 detectors. A piece of high-density polyethylene is used as a neutron to proton convertor to improve the CR-39 detection efficiency. The latent tracks within the CR-39 are exposed using a chemical etching process and recorded using an optical scanning system. The data, consisting of track diameter, eccentricity, position, and contrast, is down selected for primary DD-neutrons expected to have traveled directly from the through the slit assemble to the detector. The selected data is binned and integrated along the resolving axis to produce an axial detector response. A mathematical model of the instrument response function has been developed which is used with existing ODIN data to perform neutron source profile recovery via a modified generalized expectation maximization algorithm. The modification uses basis functions to remove a background of intrinsic noise from the CR-39.

SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

Analyzing radiographs of pulsed-power experiments is key to understanding target behavior in ICF and HED platforms. In these platforms, the radiograph quality is heavily influenced by the Magneto Raleigh-Taylor (MRT) instability which may partially or completely obscure key features like spike growth and target density. By using machine learning postprocessing techniques on these radiography images, we aim to lessen and possibly eliminate the impacts of these obscured regions.

Here, we develop a method by which MRT darkened regions in radiography images are removed from radiographs of experiments on the Z-Machine of Sandia National Laboratories. This is achieved using a convolution neural network (CNN) that leverages existing segmentation functions^1,2 with CUDA³ additions. We employ a six-layer CNN to suppress the darkened regions by 1) treating the darkened regions as noise via a mixed loss function and 2) using end-to-end frameworks that minimize noise in each subsequent layer while maintaining sharpness. With the limited number of experimental data, training is done using synthetic target radiography supplied by 3D magnetohydrodynamics simulations at different perturbation modes, that is supplemented with experimental Z-Target data. The sparse datasets are further enlarged through transformation routines.

¹ M. Gusarev, R. Kuleev, A. Khan, A. Ramirez Rivera and A. M. Khattak, "Deep learning models for bone suppression in chest radiographs," 2017 IEEE Conference on Computational Intelligence in Bioinformatics and Computational Biology (CIBCB), Manchester, UK, 2017, pp. 1-7, doi: 10.1109/CIBCB.2017.8058543.

² H. Zhao, O. Gallo, I. Frosio and J. Kautz, "Loss Functions for Image Restoration With Neural Networks," in IEEE Transactions on Computational Imaging, vol. 3, no. 1, pp. 47-57, March 2017, doi: 10.1109/TCI.2016.2644865.

³ NVIDIA, Vingelmann, P., & Fitzek, F. H. P. (2020). CUDA, release: 10.2.89. Retrieved from https://developer.nvidia.com/cuda-toolkit

Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

a) Swcorda@sandia.gov

Modern experiments in inertial confinement fusion (ICF) are often susceptible to contamination of the hotspot region by high-Z material during compression that impedes thermonuclear burn. Hotspot mix can be quantified using the ion temperature distribution of the implosion given the predicted deviation between the plasma and contaminant [1], but current neutron time-of-flight (nToF) diagnostics can only measure the spatially integrated temperature. This project probes the feasibility of using time multiplexed optical fibers to record nToF spectra from segmented thin scintillators, which could be used to collect a spatially resolved temperature measurement on a single photomultiplier tube (PMT). A prototype detector was constructed in which 20 optical fibers of increasing length were coupled to an EJ-262 plastic scintillator at one end and a Photek PMT210 at the other. The detector was tested on the OMEGA laser and an nToF pulse was successfully measured through all 20 channels at the expected time separations. Design methodology, material selection, experimental results, and preliminary data analysis are discussed. Next steps will evaluate the feasibility of this technique for 2D ion temperature imaging based on previous work [2,3], as well as potential diagnostic development using the Michigan Accelerator for Inductive Z-Pinch Experiments (MAIZE), which could be useful to magnetically driven fusion experiments on the Z-machine.

The PuFF (Pulsed Fission Fusion) project aims to revolutionize space travel through nuclear propulsion. PuFF will produce both high specific impulse (Isp 5,000-10,000 sec) and high thrust (10-100 kN), enabling quick (~1 month) transit times to Mars, the outer planets and exiting the solar system (~5 years).

PuFF creates thrust by imploding a fission-fusion target using a Z-pinch. The process involves using the Lorentz (j×B) force to create gigapascal to terapascal pressures. Liquid lithium is injected in both a cylinder and cone. When the two connect, a circuit with a network of capacitors (referred to here as the pulser) is completed, and a 10-25 MA pulse flows down the lithium. The Z-pinch slams the lithium onto a fission-fusion target. The resulting compression (5-10 by volume) reaches super-criticality and explodes. The expanding plasma is then directed out of the back of the engine by a magnetic nozzle.

The current concept endeavors to fit the PuFF engine into a single Space Launch System (SLS) launch, with the expected 8-m fairing. This nacelle would contain all the propulsive elements for a mission. A second SLS or other launcher would lift the required propellant and payload for a variety of missions in deep space.

Development of the PuFF propulsion system has focused on the implosion process. Data on the actual compression of a nuclear target is not discussed here; however in addition to the vehicle design we discuss the implosion process, connecting the lithium medium to a Linear Transformer Driver (LTD) system. Also discussed is a regenerative magnetic nozzle and recharge system to enable pulse rates of about 1 Hz.

A dense plasma focus (DPF) is a co-axial plasma railgun whose discharge ends in a z-pinch phase. The MegaJOuLe Neutron Imaging Radiography (MJOLNIR) DPF is an LLNL experiment to demonstrate feasibility of flash neutron radiography using the DPF as a neutron source and a neutron camera to record an image. The MJOLNIR team is in the process of commissioning the DPF to full stored energy and current, 2 MJ/4.5 MA, with 1.3 MJ stored energy and 3.7 MA peak currents achieved as of spring 2023. A recent redesign/rebuild of the MJOLNIR DPF has enabled robust operations, supporting >450 high voltage/plasma shots on a single hardware setup over the course of 6 months. Neutron yields in excess of 1×10¹² have been achieved on the highest current shots with deuterium gas fill. Successful high current operation requires interleaving of high current shots with lower current (~2.8 MA peak) shots. Neutron spot sizes have been measured as a function of voltage, pressure, anode implosion radius, and gas dopant levels. An overview of the MJOLNIR redesign and high-current experimental results will be presented. This work was prepared by LLNL under Contract DE-AC52-07NA27344.

Hydrodynamic instabilities are ubiquitous in pinched implosion scenarios, leading to loss of energy for compression and to mix of dissimilar materials. In order to study them and assess their impact on more integrated experiments, dedicated instability experiments are performed. We present experimental results from a suite of platforms investigating the Richtmyer-Meshkov process and interfacial feedthrough on the Z Machine at Sandia National Laboratories. Cylindrical liners filled with liquid deuterium are magnetically imploded with >20 MA of current, driving a converging shock toward the central axis and creating a magnetically isolated region suitable for studying hydrodynamic processes. The first platform investigates the interaction of this shock with a solid beryllium rod machined with sinusoidal perturbations that then grow under the Richtmyer-Meshkov process. The second platform replaces the on-axis rod with a cylindrical liner, enabling investigation of the feedthrough of these instabilities to the inner liner surface. Simulations of the liner implosion and subsequent instabilities are presented from the xRAGE, GORGON, and ALEGRA radhydro codes. Finally, future experimental platforms will be discussed, including an exploding cylindrical liner system to study the Rayleigh-Taylor instability driven for >100 ns to a highly nonlinear regime.

This work conducted under the auspices of the U.S. DOE by Los Alamos National Laboratory under contract 89233218CNA000001 and by Sandia National Laboratories under contract DE-NA0003525. LA-UR-23-24109

Magnetized Liner Inertial Fusion (MagLIF) is a magneto-inertial-fusion concept that is studied at Sandia National Laboratories on the 20-MA, 100-ns rise-time Z Pulsed Power Facility at Sandia National Laboratories. Given the relative success of the platform, there is a wide interest in studying the scaled performance of this concept at a Next Generation Pulsed Power (NGPP) facility that may produce peak currents upwards of 60 MA. An important aspect that requires more research is the instability dynamics of the imploding MagLIF liner, specifically how instabilities are initially seeded. It has been shown in magnetized 1-MA thin-foil liner Z-pinch implosion simulations that a Hall interchange instability (HII) effect¹ can provide an independent seeding mechanism for helical Magneto-Rayleigh-Taylor instabilities. In this presentation, we study the dynamics of the HII in MagLIF driven at higher peak currents. In this study, we use the 2D Discontinuous Galerkin PERSEUS code, an extended magneto-hydrodynamics code,² which includes Hall physics. Our simulations of scaled MagLIF loads show that the Hall interchange instability remains an important effect at high currents.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

1) J. M. Woolstrum, C. E. Seyler, and R. D. McBride, “Hall instability driven seeding of helical magneto-rayleigh–taylor instabilities in axially premagnetized thinfoil liner z-pinch implosions,” Physics of Plasmas 29, 122701 (2022), https://doi.org/10.1063/5.0103651, URL https://doi.org/10.1063/5.0103651.

2) C. E. Seyler and M. R. Martin, “Relaxation model for extended magnetohydrodynamics: Comparison to magnetohydrodynamics for dense Z-pinches,” Physics of Plasmas 18, 012703 (2011), ISSN 1070-664X, URL http://aip.scitation.org/doi/10.1063/
1.3543799.

Zap Energy is developing the sheared-flow-stabilized Z pinch concept for fusion energy applications. Z pinches offer many advantages as a fusion platform, including intrinsically high beta, absence of external coils, and simple geometry particularly well-suited to tackling the engineering challenges for commercialization. Despite these advantages, the Z pinch path to fusion has not been explored with as much vigor as MCF and ICF concepts, in part, because the classic Z pinch is MHD unstable to m=0 ‘sausage’ and m=1 ‘kink’ modes. These fast instabilities destroy plasma confinement on Alfvenic timescales, hindering prospects for thermonuclear gain. Sheared-flow stabilization offers a path to overcoming these instabilities to produce an MHD-stable column of flowing Z pinch plasma that can reach thermonuclear conditions through adiabatic compression. Energy confinement is defined by the flow-through time of about 10 μs, making it possible to produce thermonuclear gain.

Tracing their lineage to early experiments at the University of Washington, the FuZE and FuZE-Q devices currently operating at Zap Energy consist of a coaxial plasma accelerator, where flowing plasma is generated, coupled to an assembly region, where Z pinches are formed. Recently, record pinch currents, > 600 kA, electron temperature > 2 keV, ion temperature > 2.5 keV and DD neutron yield > 2e8/pulse have been achieved. In parallel with experiments, we perform full device MHD simulations. These simulations produce synthetic diagnostic signals that show remarkable agreement with experimental measurements and reveal complex details of the pinch formation and neutron generation processes. The goal of the combined experimental/modeling program is to reach plasma parameters consistent with scientific breakeven Q=1 conditions. The status of this effort and its future outlook will be presented.

Magneto-inertial fusion (MIF) concepts, such as the Magnetized Liner Inertial Fusion (MagLIF) platform, constitute a promising and scalable technological path to reach high fusion yields (>100 MJ) and gains (G>100) in the laboratory. However, scaling MagLIF is not entirely straightforward: the space of experimental input parameters defining a MagLIF load is highly multi-dimensional, and the implosions themselves are complex events involving many physical processes. In this talk, we present a simplified analytical model that identifies the main physical processes at play during a MagLIF implosion [1]. Using non-dimensional analysis, we determine the most important dimensionless parameters characterizing MagLIF implosions and provide estimates of such parameters. We show that MagLIF targets can be “incompletely” similarity scaled, meaning that the experimental input parameters of MagLIF can be varied such that many (but not all) of the dimensionless quantities are conserved. Based on similarity scaling, we can explore the parameter space of MagLIF targets and estimate the performance of scaled targets. We test the similarity scaling theory against simulations for two different scaling “vectors”: current scaling [2] and rise-time scaling [3]. Good agreement is obtained.

* SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

[1] D. E. Ruiz, P. F. Schmit, D. A. Yager-Elorriaga, C. A. Jennings, and K. Beckwith, Phys. Plasmas 30, 032707 (2023).

[2] D. E. Ruiz, P. F. Schmit, D. A. Yager-Elorriaga, et al., Phys. Plasmas 30, 032708 (2023).

[3] D. E. Ruiz, P. F. Schmit, M. R. Weis, K. J. Peterson, and M. K. Matzen, Phys. Plasmas 30, 032709 (2023).

The Magnetized Liner Inertial Fusion (MagLIF) platform, at Sandia National Laboratories, requires the successful compression and confinement of pre-heated and pre-magnetized fusion fuel. Over recent years, the experimental platform has been substantially improved by increasing electrical current delivery to the target (reduced power feed losses) and increased laser energy coupled to the fuel (reduced backscatter and laser entrance foil losses) alongside more constraining measurements of these inputs [1,2]. Using the LLNL code HYDRA, this talk will review post-shot 3D modeling of a number of these experiments with a focus on the impact of MHD instabilities on fusion output and experimental observables. For a fixed set of perturbations and over a range of inputs (current and laser preheat), these simulations typically overpredict neutron yields by a factor of 2 but show agreement with Tion inferred from neutron time-of-flight diagnostics. Additionally, the experimental pressure and convergence ratio inferred using a Bayesian analysis are significantly higher than the burn averaged values calculated from simulations. Although it should be noted, the Bayesian inferences also calculate a significant mix fraction not included in the simulations. Resolving these discrepancies is a focus of ongoing work, but these simulations suggest improvements to stability offer a deceptively simple path to higher yield and motivate future research directions.

[1] Gomez et al., Phys. Rev. Lett. 125, 155002 (2020).

[2] Harvey-Thompson et al., RSI, under review.

SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525. This work was performed under the auspices of the U.S. Department of Energy by LLNL under Contract DE-AC52-07NA27344.

Metal liners rapidly imploded by a fast rising (<100ns) current to compress a magnetized, preheated fuel offer the potential to efficiently reach fusion conditions [S.A. Slutz et al. Phys. Plasmas 17, 056303 (2010)]. Liner implosion research on Z in support of the MagLIF scheme has typically focused on acceleration instabilities during the implosion phase. However, the growth of inner surface instabilities during deceleration as the liner stagnates on compressed fuel on axis have the potential to be very detrimental to performance. During the deceleration stage azimuthally asymmetric, short wavelength "flute" modes grow aggressively and can penetrate the fuel volume, mixing in liner material and degrading fuel confinement. We discuss the implications of these instabilities on MagLIF performance and how their effect may vary as experiment input parameters change. We then discuss a series of Z experiments that have been proposed to purposefully introduce azimuthally asymmetric structures into MagLIF relevant experiments to study their impact on stagnation performance and assess simulation predictions of these processes.

* SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

We present results from the Rotating Plasma Experiment (RPX) [1,2], a novel laboratory platform developed on the MAGPIE pulsed-power generator (1.4 MA, 240 ns rise-time), designed to probe physics relevant to astrophysical accretion disks and jets. RPX drives differentially rotating high-energy-density plasma flows using the slightly off-radial inward-convergence of 8 magnetized plasma jets [3], from an ablating aluminium wire array Z pinch.

The data show that rotating plasmas have a hollow density structure and are radially confined by the ram pressure of the ablation flows. A combination of axial thermal and magnetic pressure launches an axial, highly collimated, supersonic jet with a velocity ~ 100 km/s (M > 5). The axial jet also rotates, transporting angular momentum, as it remains collimated by a hot (T_i ~ 250 eV) surrounding plasma halo. The flow velocity stratification is such that angular frequency decreases with radius, as the opposite happens to specific angular momentum. The calculated squared epicyclic frequency (Rayleigh determinant) of the flow is estimated to be k² ~ r^-2.8 > 0. This implies that the flows at RPX are quasi-Keplerian and share stability properties of gravitationally-driven accretion disks in astrophysics, opening a new frontier to laboratory modelling of these objects.

References

[1] V. Valenzuela-Villaseca, et. al., Accepted at Phys. Rev. Lett. (2023). Preprint: arXiv 2201.10339v1

[2] M. Bocchi, et al., ApJ 767, 84 (2013)

[3] D. D. Ryutov, Astrophys. Space Sci. 336, 21 (2011)

Using our new tilted wire array platform, we present results from our experiments on the MAIZE facility (~500 kA peak current, 150 ns rise time) to study pulsed-power driven magnetic reconnection with an embedded guide field. Taking an existing dual exploding wire array load, we rotate the two arrays in opposite directions, such that when the oppositely directed plasma flows collide there is both an anti-parallel reconnecting component of the magnetic field, and an out-of-plane “guide” field component. Between these arrays, a current sheet is formed from the interaction of oppositely-directed magnetic fields (~2 T) advected by carbon plasma flows moving at ~50-100 km/s. We study three tilt angles: 0, 22.5, and 45 degrees, with corresponding guide field to reconnecting field ratios of 0, 0.4, and 1. Line-integrated electron density measurements of the reconnection layer in these configurations were made using a simultaneous end-on and side-on Mach-Zehnder interferometry system (1064 nm, 2 ns, 40 uJ), which measured peak line-integrated densities of ~6e17 cm^-2 inside the reconnection layer (in the absence of a guide field). Measurements were taken at different times after current start on different shots to study the evolution of the layer. An optical fast-framing camera and a four-frame XUV MCP detector observed the plasma dynamics. Oppositely-wound B-dot probe pairs were fielded at different radial distances from the wire arrays, to measure both the advected magnetic field (~2 T) and the ablation flow velocity (~50-100 km/s) via a time-of-flight technique.

This work is supported by the NSF and the DOE NNSA through grant PHY-2108050. MAIZE facility support was provided by the NNSA Stewardship Sciences Academic Programs under DOE Cooperative Agreement DE-NA0003764.

The Pulsed Power Plasmas (P3) group at UC San Diego develops experimental and diagnostic platforms for a range of HEDP and related plasma studies including inertial fusion, laboratory astrophysics and basic plasma physics. These studies are supported by simulation work carried out in collaboration with academic, national laboratory and private partners. Pulsed power drivers focus on 1-us timescale devices, where the relatively slow varying plasma parameter can be studied in detail over large (mm³) volumes. Shocks generated by supersonic and magneto-supersonic flows can be designed to be stationary in the laboratory frame, and jet and flow systems are driven for timescale many times the hydrodynamic scale.

We will present data from a new Faraday rotation system examining plasmas driven by our 1us, 200kA device, Bertha. The laser system operates at 1064ns with a 1J, 6ns pulse allowing discrimination of the Faraday signal against the load emission using high extinction polarizers. Data are compared to calculations from both short circuit and exploding wire loads. We will also present new data on the development of our 1.3us, 750kA driver, Rama. This uses a novel solid-state triggering system developed at Imperial College London, allowing all 6 of the device’s capacitors to be triggered independently. Pulse-shaping capabilities may be of interest for a variety of plasma experiments, particularly in plasma jets produced from an unsteady, episodic source.

A photoionized plasma experimental platform has been established for Zebra in which a supersonic neon gas jet is driven and backlit by the broadband x-ray flux from the collapse of a wire-array implosion and characterized by a suite of optical and x-ray diagnostics. A mm-scale cylindrical volume of photoionized plasma can be produced during the 20-30 ns time interval of the x-ray flux emission, in which the gas jet is effectively “frozen” in space. Supersonic gas flows are collimated and do not require sealing windows or tamping layers, which removes unnecessary x-ray flux attenuation and enables close proximity to the pinch ~6 mm, thus optimizing plasma x-ray heating and ionization. The plethora of diagnostic measurements are used to inform, constrain, and test theory models. The time-history of the x-ray flux is measured with PCD’s while its spectral distribution is determined over an order-of-magnitude wavelength range with a soft x-ray spectrometer equipped with a spherical diffraction grating. Atomic density radial spatial profiles of the neutral gas jet are extracted from Mach-Zehnder interferometry. The radiation drive characteristics and atomic density of the neutral gas jet inform simulations of the experiment. Simulation results are then benchmarked with electron density maps extracted from dual color air-wedge shearing laser interferometry. Analysis of the transmission x-ray spectroscopy data provides the charged state distribution and electron temperature of the plasma which further test and constrain modeling results. We will discuss the experimental platform and measurements, the modeling and data analysis, and the astrophysical impact¹.

¹ R. C. Mancini et al Phys. Rev. E 101, 051201 (2020)

This work is supported by DOE NNSA HEDLP Grant DE-NA0004038.

We present an in-depth analysis of micropinch formation dynamics in copper (Cu) and nickel (Ni) hybrid X-pinches using an X-ray streak camera with a ~18 ps temporal resolution. The experiments were carried out on a 400-kA peak current, 50 ns rise time pulsed-power machine. Our focus was on characterizing the L-shell radiation with energies below 1 keV, preceding the X-pinch continuum burst, which signifies micropinch formation. This enabled us to assess imploding plasma conditions just before stagnation. Through the analysis of Ne-like copper lines prior to the continuum, we determined an average electron temperature of ~200 eV and an electron density of 4.5e28 /m3. Interestingly, our data suggests that the electron temperature jumps to ~1 keV as inferred from the continuum and the post-continuum line emission. Contrary to expectations, we did not observe any rapid temperature change or significant surge in radiation emission during the 200 ps pre-continuum X-ray burst, which would have supported the radiative collapse process as a major contributor to micropinch formation. To further explore alternative hypotheses, we employed two-dimensional extended magnetohydrodynamic (MHD) simulations coupled with a collisional-radiative spectral analysis code. These simulations revealed several key factors that drive micropinch formation, including the rapid radial implosion and compression of high-temperature low-density plasma, the axial outflow of the cold wire core, the dynamo term in the generalized Ohm's law, and the dynamic pressure of the imploding Cu plasma during the final phase of the process. Our findings challenge the conventional understanding of micropinch formation and pave the way for future research on this fundamental plasma phenomenon.

X-pinches, formed by driving intense current through the crossing of 2 or more wires, provide an excellent platform for the study of “micro-pinches” due to their propensity to generate a single micro-pinch at a predetermined location in space (i.e., where the wires cross) [1,2]. Ideally, micro-pinches are areas of run-away compression to very small radii (~1 µm) leNickading to pressures on the order of ~1 Gbar for currents on the order of ~0.1 MA. However, the fraction of the total current that is driven through the dense micro-pinch plasma at small radii versus that being shunted through the surrounding coronal plasma at larger radii is not well known. To allow for the study of X-pinches and their current distribution on the 1-MA MAIZE facility, a Faraday rotation imaging diagnostic (1064 nm), as well as a corresponding modular load hardware, were developed. Presented is the status of these developments including preliminary experimental results characterizing deuterated-polyethylene-fiber X-pinches on the MAIZE LTD.

[1] S.A. Pikuz et al., Plasma Phys. Rep., 41, 291 (2015);
[2] S.A. Pikuz et al., Plasma Phys. Rep., 41, 445 (2015);

*This work was supported by the DOE Early Career Research Program under Grant DE-SC0020239 and by the NNSA SSAP under Cooperative Agreement DE-NA0003764.

Radiative collapse is a runaway process whereby a plasma reaches sufficiently high densities and temperatures such that radiative cooling drastically drops its thermal pressure, collapsing the plasma to very small regions. Excluding the effect of macro-instablities disrupting the convergence, collapse can be terminated by a transition to the optically thick regimes, or electron degeneracy pressure under stronger conditions.

This talk presents high-fidelity 2D MHD simulations of single wire explosion using fibres with varying Z. This system has been shown to go m=0 unstable and produce brightly emitting hot-spots at neck regions before they bifurcate and disrupt collapse. We revisit this system using upgraded models compared to previous simulations. These include tabulated equation of state and transport coefficients, improved radiative loss and transport models including opacities generated from an in-house atomics code, and a mesh refinement capability allowing high resolution on a large domain size. We find improved qualitative agreement between these simulations and experiments conducted at Imperial College London on the IMP generator.

Gas-puff Z-pinches have been extensively studied for over three decades as a means of generating high energy density plasmas (HEDP) using pulsed power, with primary applications in radiation sources and neutron production. Understanding the heating and pressure balance mechanisms in the stagnation stage is essential to maximize the amount of energy produced. In this way, knowing the amount of current coupled in the plasma axis and the magnetic field distribution near stagnation is particularly interesting.

In this study, we experimentally determined the plasma-current coupling for various cathode geometries (round and knife edge) in an oxygen gas-puff Z-pinch experiment using Zeeman spectroscopy. The Llampüdkeñ generator was used (~400 kA peak current and ~300 ns rise-time), with both round and knife-edge cathode geometries. Our spectroscopic technique is based on the polarization properties of the Zeeman σ components, which enables us to obtain space and time-resolved measurements of the azimuthal magnetic field (Bθ).

We used Ampere's Law to calculate the axial current (IZ) within a specific radius and compared it with the total current (IT) measured using a Rogowski coil to obtain the "plasma-current coupling" (IZ/IT). This allowed us to quantify the current flow through the plasma relative to the total current produced by the system. Then we make an extensive comparison of the results obtained for the two cathode geometries.

Authors acknowledge the finantial support from grants ANID FONDECYT Regular 1220533 and 1211131.

The Thomson scattering technique is a powerful diagnostic tool for high-energy-density plasma. This technique makes it possible to estimate the temperatures and densities of ions and electrons as well plasma velocity and ionization state. However, a large number of parameters and the complexity of the mathematical model make it difficult to estimate simultaneously all parameters with its associated uncertainty. To deal with this problem, we implement Bayesian inference, which gives us the formalism to find the most likely configuration of parameters that best fit the experimental data and its probability distribution, the associated uncertainty, and the correlation between parameters. In this work, we present preliminary results found by implementing the technique to study the dynamics of a single-liner argon gas-puff near stagnation time. The experiments were carried out on the Llampudken current generator, which provides a current pulse of ~400kA amplitude and 200ns rise time (10%-90%). For the TS setup we used a 532nm Nd-YAG laser which can produce up to 1J energy pulse with 4ns full width half maximum (FWHM) focused to a ~50µm diameter. The spatial and spectral resolution was ~400µm and 0.1nm at 532nm, respectively. We implemented complementary diagnostics to set the prior distribution in the Bayes analysis. Such as extreme ultraviolet (XUV) self-emission imaging to provide information about dynamics, Mach-Zehnder interferometry to estimate the electron density at early times, and 25µm Be filtered diode was used to measure the stagnation time and X-ray yield (>1kev). Preliminary analysis shows that plasma velocity and density agree with the observed from the MCP imaging and interferometry measurements, respectively. Also, analysis of the parameters and spectra reveals that incoming flows are able to interpenetrate partially counterpropagating flows on axis.

The University of New Mexico has begun conducting wire array experiments on our twelve brick 960 J LOBO linear transformer driver. Data has been taken on 25-micron tungsten wire arrays ranging from one to eight wires including x pinch configurations. Hand built ragowski coils on both anode and cathode sides are used to give information about current and pulse shape, where visible light, twelve frame gated nanosecond images along with open shutter images are shown. A Mach Zehnder interferometry system is used to measure time resolved densities and a fifteen-fiber array fed into a spectrometer is used to measure spatially resolved stark broadening. A pinhole camera images the array onto x ray imaging plates, while diamond XRD and pin diode measurements give time resolved x rays.

To study interaction of plasmas in high energy density (HED) environments, we have developed an experimental platform to generate and characterize shocks under controlled conditions. The interacting shock region results after the production of a laser-produced plasma plume in the path of the jet-like outflow emitted by a conical wire array. The laser plasma plume (LPP) is produced by focusing a 10¹⁰ W/cm² laser pulse onto an aluminum target, whereas the conical wire array uses 16 aluminum wires (40μm diameter each) acting as load of the Llampudken generator (~400kA with risetime of ~250ns). Primary diagnostics are time-resolved laser-probing and XUV imaging. In order to observe the shock region properly, the diagnostics are aligned perpendicular to both the propagation of the plasma plume and the z-axis of the array. Our results indicate that in the presence of both plasma sources, a shock structure appears at the interaction region, with a thickness comparable to the ion-ion mean free path calculated for a wide range of parameters for each plasma. This result suggests the presence of a collisionally-mediated shock layer created after the interaction. Further details and potential applications will be shown and discussed

Production of a shock region in jets emitted by conical wire array z-pinches.

20 years of research on fast and high-energy exploding thin metal wires allows us to propose this object as a target for high-current magnetic compression for fusion, X-ray and neutron pulse production. A strong influence of the current rate [1], dielectric coating [2], and electrode geometry [3] on the energy input and uniformity of the exploding wire in vacuum was demonstrated. In 100 ns time scale, a thin coated metal wire can be transformed into an ideal plasma cylinder 1–2 mm in diameter using a rapidly rising current of ~1 kA/ns [2]. The appropriate geometry of the cathode-anode electrodes makes it possible to realize a perfect cylindrical explosion, free from RT instabilities [3]. Any MA-current installations have a current pre-pulse with a duration of 100-200 ns, which pre-heats and pre-exploded the wires in the array before the main current. This pre-explosion is uncontrollable and mostly occurs at low currents of ~1-10A/ns per wire. The wires pass into a two-phase state with a low density and low mass of the hot corona and a solid-liquid core [4]. In our approach, we transform a single fast exploding wire into a single-phase expanding plasma cylinder without RT instability [2] and after applying the main MA current to magnetically compress this perfect plasma cylinder. Delay times and current rates for optimizing magnetic compression and radiation/neutron yields are the subject of research. Shortening the current prepulse and increasing the prepulse current rate for the wire array leads to a significant improvement in the axial symmetry of the plasma cylinder and the X-ray yield [5]. In the latest publication [6], a significant improvement in high current compression of a pre-exploded single metal wire has been demonstrated. The wire array loads a relatively difficult target for magnetic compression due to the physics of the discreet ablative flows and the challenges of creating a uniform on-axis plasma cylinder. A single high-speed, high-energy exploding wire creates a perfectly stable and uniform plasma cylinder for the next high-current compression. To obtain neutrons, one can use the diffusion of deuterium into a metal wire and a dielectric coating.

1. G.S. Sarkisov, K. Struve, D.H. McDaniel, Phys.Plasma 11, 4573 (2004).
2. G.S. Sarkisov, S. Rosenthal, K.W. Struve, Phys.Rev.E 77, 056406 (2008).
3. G.S. Sarkisov, A. Hamilton, V.I. Sotnikov, Phys.Rev.E 98, 053203 (2018).
4. G.S. Sarkisov, B. Etlicher, et al, JETP Lett. 61(7), 555 (1995).
5. G.S. Sarkisov, et all, Phys.Plasma 14, 052704 (2007); Phys.Plasma 14, 112701 (2007).
6. Z. Jiang, J. Wu, Z. Chen, et all, Phys.Rev.E 107, 055201 (2023).

Astrophysical jets are collimated, high-speed outflows observed to be natural features of objects that spin and accrete matter. Developing over a vast range of scale lengths and source energies, common features suggest that universal mechanisms may be responsible for jet formation, collimation, and stability. Currently, no single model of jet formation is universally accepted to account for the extreme collimation and stability observed in many jets; however, theory, astrophysical observations, and recent laboratory experiments suggest that some jets may represent magnetically driven configurations that form self-organized equilibria with strong stabilizing shear flows. To test this hypothesis, an experimental platform has been developed for the 1-MA, 220-ns rise time COBRA generator at Cornell University. In contrast to previous high-energy-density (HED) laboratory plasma jet experiments that use radial/conical wire arrays or foils, this experiment uses azimuthally symmetric gas-puff injection. This provides a continuous mass source and allows for free rotation of the jet foot-points. Because there is no ablation phase from a dense solid target, the magnetically driven jets develop earlier in the current pulse and can be driven longer without depleting their mass source and disrupting. Flexibility in load design permits the generation of a poloidal dipole field (mimicking a magnetized star or black hole) using permanent magnets or dynamically through a helically twisted cathode. A polarity convolute allows for the reversal of the applied electric field to investigate extended MHD (XMHD) effects. Detailed measurements of flow velocities, temperatures, densities, and magnetic fields will be obtained using optical spectroscopy, laser interferometry, Thomson scattering, magnetic probes, and Faraday rotation imaging. Results will be interpreted using the framework of generalized or canonical helicity, which extends the physics of magnetic flux tubes to canonical flux tubes (a weighted sum of flow vorticity flux and magnetic flux). Here we present the design of the experiment, preliminary experimental observations, and 3D modeling using the PERSUES XMHD code.

This work was supported by the DOE Office of Science grant No. DE-SC0023238

Microscale surface roughness on the outside of a pulsed-power--driven conductor can lead to the development of several undesirable magnetohydrodynamic (MHD) instabilities, including the electrothermal instability (ETI), electrochoric instability (ECI), and magneto-Rayleigh-Taylor instability (MRTI). In magneto-inertial fusion concepts, these instabilities lead to turbulence and material mixing that are detrimental to achieving fusion ignition. This work studies the formation and evolution of these instabilities using dimensionality reduction techniques based on dynamic mode decomposition (DMD) and Koopman theory. These techniques are applied to 2D MHD simulations of electrical wire explosions on the Mykonos current driver, resulting in the extraction of dynamically growing modes from the data and quantification of growth rates associated with the modes.

Magnetically insulated transmission lines(MITLs) can deliver 10’s of mega Amperes to Z-pinch loads on the Z-machine experiment at Sandia. The high electric fields and current densities in the MITL electrode surfaces lead to the formation of MITL plasmas which can divert current from the load. This plasma is swept along the power flow with the ExB velocity towards the load where it deposits mass and energy causing undesirable perturbations to the load. This presentation describes fully-kinetic simulations using the PIC code Chicago that models these plasmas and the flux of mass and energy that are delivered to Z-pinch loads. Analytical theory is also presented which can be implemented in fluid codes to improve the characterization of near-load vacuum conditions in fluid simulations. The vacuum treatment is known to influence target performance in fluid simulations and can be constrained by high-fidelity PIC calculations.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Magnetohydrodynamic (MHD) simulations of pulsed power experiments result in unphysical runaway heating when extended to low values of mass density. Traditionally, this has been addressed by the use of conductivity floors below which the plasma is given an arbitrary, low conductivity value. Here, a low-density treatment is presented that allows for low-density material to carry current while maintaining a reasonable temperature. This treatment is implemented in the Ares multiphysics code. Modifications to the conventional vacuum treatment include an energy-conserving density floor, an anomalous resistivity model, a modified averaging procedure to determine the thermal conductivity at the edge of adjacent zones, and a modified averaging procedure for determining the thermal conductivity in mixed zones that contain material from differing regions. Using certain settings, this approach can approximately capture low density inflowing plasma that is present in Sandia’s Z machine and allows for low density conducting plasma in rough agreement with those seen in kinetic Chicago simulations. Further, these improvements enable the eventual coupling of a mass flux source from kinetic simulations as a boundary condition for Ares simulations.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore Na- tional Laboratory under Contract DE-AC52-07NA27344.

Sheared-flow-stabilized Z-pinches [Zhang et al. PRL 2019] are formed in the laboratory by magnetohydrodynamic channel flow downstream from a coaxial plasma accelerator. Such flow pinches are stable only with sufficient flow shear, so this work considers prediction of the radial plasma profiles, i.e., pressure, density, and axial velocity, as a function of the accelerator parameters. To begin, we predict the plasma pressure and density profiles using ideal flow conditions, such as adiabaticity, frozen-in flux, and conservation of mass, assuming Kadomtsev-stable axisymmetric profiles. We then obtain the flow velocity profile as a TdS-type thermodynamic integral of the plasma density, temperature, pressure, and magnetic flux. We eliminate the need for either magnetic flux or density and temperature by recasting the m=0 stability condition in entropy variables and assuming the existence of a relaxed, stable profile. Finally, we compare the theory with axisymmetric MHD simulations of flow Z-pinches formed by coaxial accelerators.

We report the observation and characterization of the emission spectrum of
Al wire-array z-pinch plasmas in the XUV range from 40–700Å. Cylindrical
wire-arrays 6 mm in diameter and 20 mm tall composed of 8 wires 15μm in
diameter were imploded using the 1MA Zebra pulsed power generator at
the University of Nevada, Reno. Zebra produces z-pinch plasmas that
radiate up to terawatts of broadband x-ray power at the collapse of the
implosion. This high x-ray output makes them attractive sources for
radiation driven experiments. The intense broadband spectrum was
recorded using a grazing incidence diffraction grating spectrograph
retrofitted from an Acton monochromator with a 1.5 m radius 600 lines/mm
aluminum spherical grating. This space and time integrated observation
shows a quasi-thermalized radiation intensity distribution comprised of line
emissions and an overall Planckian-type of envelope. Fits of the Planckian
envelope provide a space- and time-averaged estimate of the radiation
temperature of approximately 40 eV.

This work was sponsored in part by DOE NNSA HEDLP grant DE-NA0004038

In recent times, the knowledge of the biological effects of pulsed radiation (charged particles, X-rays and neutrons) coming from a pulsed plasma source, such as a plasma focus device, has aroused great interest due to its potential applications in areas such as radio medicine and radiobiology. In this study, a monolayer of isolated lymphocytes was irradiated using 5, 10, 20, 40 and 60 X -ray pulses (FWHM ~ 90 ns, dose rate ~ 10⁷ Gy/sec) emitted from a kilojoule plasma focus device, PF-2kJ, and unstable chromosome aberrations (UCA) frequencies were estimated for each sample and, in order to have the highest cellular yield, mitotic index (MI) was evaluated to establish optimal culture conditions. The results obtained evidence a different behavior of pulsed radiation compared to radiation from a continuous source [1]. Therefore, the incorporation of cytogenetic markers may contribute to the characterization of the pulsed X-ray radiation generated by plasma focus devices, since they allow direct evidence and quantification of the effects of radiation in biological systems. On the other hand, low dose hyper-radiosensitivity (LDHRS) effects have been explored in various cancer cell lines using conventional x-ray irradiation. Cell death was evaluated in human colorectal (DLD-1 and HCT-116) and breast (MCF-7) cancer cell lines (monolayer cell cultures) irradiated with 10, 20, and 40 pulses. The cell death in the DLD-1 cell line irradiated with pulsed x-ray is three times higher than the reported for a conventional continuous x-ray source at two times higher doses [2]. LDHRS was also observed in HCT-116 and MCF-7 cells exposed to 10 and 20 x-ray pulses, respectively, which are reported not to exhibit LDHRS when conventional continuous x-ray sources are used [3].

This work was supported by National Grant ANID-FONDECYT Regular grant 1190677, ANID-FONDECYT Iniciación N°11230594, ANID-FONDECYT Postdoctoral grant N°3190184, and the ANID PIA/Anillo ACT-172101 grant.

[1] Valentina Verdejo et al. Use of a plasma focus device to study pulsed x-ray effects on peripheral blood lymphocytes: Analysis of chromosome aberrations. J. Appl. Phys. 133, 163302 (2023).

[2] Andaur, R. et al. Differential miRNA expression profiling reveals miR-205-3p to be a potential radiosensitizer for low-dose ionizing radiation in DLD-1 cells. Oncotarget, 9(41), 26387 (2018).

[3] Jain, J. et al. Hyper-radiosensitivity in tumor cells following exposure to low dose pulsed x-rays emitted from a kilojoule plasma focus device. Journal of Applied Physics, 130(16), 164902 (2021).

Uncovering 3D structure of the stagnated fusion fuel and liner mix in Magnetized Liner Inertial Fusion (MagLIF) experiments is critical to understanding target performance and scaling designs to higher neutron yields. However, accurate diagnosis of 3D structure in MagLIF experiments on the Z Machine has been limited by a small number of available diagnostic views. While 2D x-ray projection images of stagnation self-emission¹ at two orthogonal views have recently been obtained, tomographic reconstruction of an emission volume from this sparse data set is still a highly ill-posed inverse problem.

Here we present a basis function-expansion approach to reconstruct MagLIF stagnation emission volumes from a sparse set of projection data.² A set of basis functions is learned from training volumes containing quasi-helical structures similar to those that are expected in MagLIF stagnation columns. Reconstructions from two orthogonal projections with learned basis functions show more accurate morphology compared to when using other bases and forms of regularization. In addition, the learned basis provides accurate estimates of fuel volume, which are necessary for inferences of stagnation pressures. In addition to validation studies with emission volumes from radiation magnetohydrodynamic simulations, we present the first 3D reconstructions of experimental MagLIF stagnation plasmas. The approach is applicable to sparse-view 3D reconstruction of other Z-pinch and HED plasmas.

1. E. C. Harding, et al., “X-ray Self-Emission Imaging with Spherically Bent Bragg Crystals on the Z-machine.” Review of Scientific Instruments. (Submitted)
2. J. R. Fein, et al., “Three-dimensional reconstruction of MagLIF stagnation columns from sparse projection data using learned basis functions.” (In preparation)

* Sandia National Laboratories is a multimission laboratory managed and operated by NTESS, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE's NNSA under contract DE-NA-0003525.

The Brillouin flow is widely considered as the equilibrium state in many crossed-field devices including magnetrons, crossed-field amplifiers (CFAs), magnetically insulated line oscillators (MILOs) and magnetically insulated transmission lines (MITLs). Explicit, closed form solutions, together with scaling laws have been found for the electron flows in terms of the gap voltage and the magnetic flux [1], whether the magnetic insulation is provided by the wall currents, as in a MITL and MILO, or by an external magnet, as in a magnetron and CFA, or by some combination of the two. These scaling laws, together with our recent MILO experiments, revealed some unexpected features, and pointed to the importance of the transition from magnetic insulation to nonmagnetic insulation. Also addressed are the implications on these crossed-field flows from recent extensions of the Ramo-Shockley theorem and of the Child-Langmuir law.

1. Y. Y. Lau, D. A. Packard, C. J. Swenson, J. W. Luginsland, D. Li, A. Jassem, N. M. Jordan, R. D. McBride, and R. M. Gilgenbach, “Explicit Brillouin flow solutions in magnetrons, magnetically insulated line oscillators, and radial magnetically insulated transmission lines,” IEEE Trans. Plasma Sci., vol. 49, no. 11, pp. 3418–3437, Nov. 2021.

________________________________

* Research supported by AFOSR MURI grant No. FA9550-20-1-0409 through the University of New Mexico, and by AFOSR grant Nos. FA9550-21-1-0184, and FA9550-21-1-0367.

Power flow studies on the 30-MA, 100-ns Z facility
at Sandia National Labs (SNL) have shown that plasmas in the
facility’s magnetically insulated transmission lines (MITLs)
can result in a loss of current delivered to the load. 1 During the
current pulse, thermal energy deposition into the electrodes
(ohmic heating, charged particle bombardment, etc.) causes
neutral surface contaminants layers (water, hydrogen,
hydrocarbons, etc.) to desorb, ionize, and form plasmas in the
anode-cathode (AK) gap. 2 Shrinking typical electrode
thicknesses (~1 cm) down to that of thin foils (5−200 µm)
produces observable amounts of plasma on smaller pulsed
power drivers (≤1 MA). 3 We suspect that as the electrode
material bulk thickness decreases relative to the skin depth of
the current pulse (50−100 µm for a 100−500-ns pulse in
aluminum), the thermal energy delivered to the neutral surface
contaminant layers increases, and thus surface contaminants
desorb faster from the current carrying surface.

In this talk, we review recent experimental results
using thin foil loads on the Mykonos facility at Sandia
National Labs to characterize the electrode contaminant layer
with the plasma temperature, density, and determination of the
species and their ionized states during surface desorption.
These results are collected using streaked visible spectroscopy
and a vacuum ultraviolet (VUV) spectroscopy system
developed to measure the atomic hydrogen Lyman-α line
(121.6 nm) and molecular hydrogen bands from wires and
foils with varying thicknesses (5−200 µm) and materials.
We use the VUV measurements to compare hydrogen
inventories to those predicted to be released via thermal
processes in the electrodes. The contaminant desorption,
thermal diffusion, hydrogen diffusion in the solid bulk, and
desorbed density, temperature and ionization profiles are
modeled in a new 1-D semi-analytic solver. The results from
the model are then used to run PrismSPECT simulations to
compare against experiment.

1. W.A. Stygar et al., Phys. Rev. S.T.-A.B. 12, 120401 (2009)
2. N. Bennett et al., Phys. Rev. A.B. 22, 120401 (2019)
3. T.J. Smith et al., Rev. Sci. Inst. 92, 053550 (2021)

*This work was supported by SNL and by the NNSA SSAP
under DOE Cooperative Agreement DENA0003764. SNL is a
multi-mission laboratory managed and operated by National
Technology and Engineering Solutions of Sandia, LLC., a
wholly owned subsidiary of Honeywell International, Inc., for
the U.S. D.O.E. NNSA under contract DE-NA-0003525.

Current losses in the magnetically insulated region of high-power pulsed power machines represents a major engineering challenge in the design of next-generation drivers. However, due to the difficulty of diagnostic access in the magnetically insulated transmission lines (MITLs) of drivers like the Z Machine at Sandia National Laboratories, it is very difficult to study these losses. In order to develop a better understanding of these phenomena, it is crucial to be able to conduct experiments at scale in smaller facilities. The typical power flow scaling experiment, known as the “stripline”, uses two parallel planar conductors connected in a loop such that current flows up one conductor and down the other, thus producing an environment nominally similar to a MITL. However, on university-scale pulsed power drivers (of typically ~1 MA peak current), it is difficult to produce stripline experiments with current densities and electric fields that match expected peak fields in next generation pulsed power machines, while also capturing the magnetic diffusion and other solid-state physics that may be relevant as well. In this presentation, we discuss a novel experiment that has been tested on the MAGPIE driver at Imperial College. Called the "exploding hairpin", this geometry uses wires of circular cross-section bent into slightly inductive loops. Since it is far easier to work with small-diameter wires than very thin planar foils, manufacturing costs are dramatically reduced compared to similar stripline experiments. In addition, the curvature of the wire cross-section provides field enhancement at the apex of the curve, making even higher magnetic and electric field strength attainable. Lastly, the inductance of the loop can easily be varied, providing precise tuning of the electric and magnetic field strengths in the experiment. Simulation results indicate that magnetic fields ranging from 70 T to 300 T and electric fields ranging from 30 MV/m to 650 MV/m can be produced with this geometry on MAGPIE. Several simulations of different hairpin designs are presented using COMSOL and GORGON; these simulations are then compared to the experimental results, and possible explanations for current losses in the inner MITL are then discussed in light of the experimental results.

SAND2023-02545C
This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.
Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525.

The concept of a variable-impedance, magnetically insulated transmission line (variable-impedance MITL) has been recently developed at the University of Rochester, LLE. [1] This approach allows us to significantly reduce the total inductance of the MITLs while preserving MITL power-flow performance. With newly developed algorithms, which combine Screamer circuit-simulations and MATLAB postprocessing procedures, we can quickly estimate the vacuum electron current flow inside MITL without running computationally expensive, particle-in-cell (PIC) simulations.

This paper focuses on the practical applications of the variable-impedance MITL for imploding loads at Z and for future, high-current drivers. Z today uses constant impedance MITLs. [2] Our simulations demonstrate that by replacing the 3.3-Ω, D-level MITL with a safe, variable-impedance MITL, we can increase the peak current up to 0.5 MA at Z. Furthermore, by implementing variable-impedance MITLs for all four, Z MITL levels, an additional up 1.2 MA of current can be gained. This increase in current happens with no increase in the level of vacuum electron flow immediately outside of the post-hole convolute (PHC). This is essentially free added current that can be made available at Z for all shots.

Regarding future, higher-current drivers, our simulations predict a significant decrease in the stack voltage and an increase in the load current compared to the constant-impedance MITL approach. These MITL innovations could lead to a reduction in overall pulsed-power risk and a substantial decrease in the cost of the proposed, future high-current facilities.

[1] R. B. Spielman & D. B. Reisman, Matter Radiat. Extremes 4, 027402 (2019).

[2] M. E. Savage, et al., in Proceedings of the 16th IEEE International Pulsed Power Conference, edited by E. Schamiloglu and F. Peterkin (IEEE, Piscataway, NJ, 2007), p. 979.

The Z facility at Sandia National Laboratories is home to the world’s largest pulsed power facility, supporting research in inertial confinement fusion (ICF) and high energy density (HED) physics for over 35 years. An ICF approach called MagLIF, in which cylindrical liners containing laser-heated and axially magnetized deuterium gas are imploded, has achieved >10¹³ DD-produced neutrons and magnetic trapping of charged fusion products. These results, along with scaling relationships and the high efficiencies achievable with pulsed power coupling to an inertial target, suggest that a larger-scale (~60 MA) driver could enable high-yield fusion in laboratory experiments.

The National Nuclear Security Administration (NNSA) is currently considering a Next-Generation Pulsed Power (NGPP) facility to replace the aging Z facility and extend the parameter space available to perform HED research. To achieve high-yield fusion on MagLIF targets and accomplish other HED research objectives, we anticipate NGPP would need to deliver 6 to 10 times the electrical power and energy of today’s Z Pulsed Power Facility.

In this talk, we will discuss conceptual architectures for a 60-MA NGPP, projected timelines, and anticipated technical and operational challenges associated with such a facility. We will present an overview of the capabilities and achievements of present and planned component and system testing facilities, which are used to advance the technology readiness level (TRL) of subsystems required to operate in new regimes on an NGPP facility. We will also highlight results from component development and improvement efforts necessary to enable the next generation of pulsed power research.

*Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.