Conference Agenda

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)

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*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.

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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).