Speaker
Description
Shock waves are ubiquitous in astrophysical, space and laboratory plasmas and often include an embedded, dynamically significant magnetic field. This magnetic field modifies the RH shock jump conditions, and allows dissipation mechanisms specific to magneto-hydrodynamics, such as Ohmic heating, to contribute to shock shaping. In fact, low Mach number shocks can be shaped exclusively by resistive heating. Such shocks are referred to as subcritical, having a magnetosonic Mach number M_MS = 1 - 2.76. Subcritical shocks have been studied experimentally in collisionless plasmas [1] but have not yet been measured in a collisional regime.
We present experimental and numerical investigations of perpendicular subcritical shock structure in a highly collisional plasma [2]. A supersonic (M_S ~ 2.5), super-Alfvénic (M_A ~ 3) plasma flow is produced by the current driven ablation of an inverse wire array z-pinch at the MAGPIE pulsed power facility (1.4 MA, 500 ns). Shocks are studied by placing obstacles into the flow and are measured with a suite of laser probing diagnostics. We find that hydrodynamic variables are continuous across the shock, as predicted by theory, and that the shock width is equal to the classical (Spitzer) resistive diffusion length.
We discuss links between these experiments and recent in situ measurements of the interstellar medium [3]. Furthermore, we show that MHD simulations using the Gorgon and AstroBEAR codes are consistent with most features of the experiment but do not reproduce features which we attribute to two-fluid effects [4].
This work was supported by First Light Fusion Ltd. and by the US Department of Energy (DoE) including awards no. DE-NA0003764 and DE-SC0020434.
[1] Schaeffer D B et al, Physics of Plasmas. 2015; 22(11): 113101
[2] Russell D R et al. Physical Review Letters. 2022; 129(22): 225001
[3] Burlaga, L F et al. The Astrophysical Journal Letters. 2013, 778(1): L3
[4] Russell D R et al, Journal of Plasma Physics. 2023; 89(4): 915890401
