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.