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.