Ion Species Stratification Within Strong Shocks in Two-Ion Plasmas

TL;DR

This paper investigates ion species stratification within strong shocks in two-ion plasmas, revealing universal temperature separation structures and a shock front enrichment of lighter ions that scales with the Mach number to the fourth power.

Contribution

It provides a detailed analysis of ion-species stratification effects in strong shocks using advanced simulations, addressing gaps in previous kinetic shock studies.

Findings

Ion temperature separation has a universal structure across ion ratios.

Shock fronts are enriched with lighter ions, scaling as M^4 for high Mach numbers.

Results help explain discrepancies in ICF experiments.

Abstract

Strong collisional shocks in multi-ion plasmas are featured in many environments, with Inertial Confinement Fusion (ICF) experiments being one prominent example. Recent work [Keenan ${\it et \ al.}$, PRE ${\bf 96}$, 053203 (2017)] answered in detail a number of outstanding questions concerning the kinetic structure of steady-state, planar plasma shocks, e.g., the shock width scaling by Mach number, $M$. However, it did not discuss shock-driven ion-species stratification (e.g., relative concentration modification, and temperature separation). These are important effects, since many recent ICF experiments have evaded explanation by standard, single-fluid, radiation-hydrodynamic (rad-hydro) numerical simulations, and shock-driven fuel stratification likely contributes to this discrepancy. Employing the state-of-the-art Vlasov-Fokker-Planck code, iFP, along with multi-ion hydro simulations and semi-analytics, we quantify the ion stratification by planar shocks with arbitrary Mach number and relative species concentration for two-ion plasmas in terms of ion mass and charge ratios. In particular, for strong shocks, we find that the structure of the ion temperature separation has a nearly universal character across ion mass and charge ratios. Additionally, we find that the shock fronts are enriched with the lighter ion species, and the enrichment scales as $M^4$ for $M \gg 1$.