Concerning ion temperature separation in multi-ion collisional plasma shocks

Polar-direct-drive exploding-pusher (PDXP) experiments are well suited to probe ion temperature separation effects. These campaigns show that — when inter-species thermalization time-scales are long¹ — the ratios of inferred deuterium and tritium T_ion values scale as the ratio of their respective ion masses. The origin of this mass-scaling is typically attributed to collisional shock physics. It is often (erroneously) claimed that Zel'dovich and Raizer³ (Z&R) predict a post-shock temperature that scales with species bulk (post-shock) kinetic energy; and therefore, is ∝ species mass. However, Z&R only claimed that the contribution to the total temperature jump across a shock, from viscous heating alone, scales with species kinetic energy — which we confirm here. Additionally, we show that the ion temperature is a complicated function of the total plasma viscosity, other ion temperatures, etc., and not ∝ ion mass. Finally, we propose an alternative (kinetic) explanation for the ion mass-scaling relationship observed in the PDXP experiments.

¹N. V. Kabadi et al., Phys. Rev. E 104, L013201 (2021).

²H. G. Rinderknecht et al., PRL 114, 025001 (2015).

³Z & R, Physics of shock waves and high-temperature hydrodynamic phenomena (Academic Press, New York, 1967), Vol. 2, p. 516.