Experimental Observation of Shock-front Separation in Multi-ion-species Collisional Plasma Shocks*

In the context of inertial-confinement-fusion and high-energy-density experiments, shock-driven ion species separation in the fuel potentially leads to neutron yield degradation [1], and this has in part motivated the recent strong interest in studying multi-species plasma transport and shocks. In this work, we directly observe shock-front separation and species-dependent shock widths in collisional plasma shocks. The shock-front is produced by obliquely merging plasma jets with initial 97\%-He and 3\%-Ar atomic concentrations on the Plasma Liner Experiment [2]. Cameras with narrow bandpass filters directly observe line emission from the distinct ion species. We experimentally infer both shock-front separation and individual shock widths (for the He and Ar) to be of order several tens of post-shock thermal ion-ion mean free paths. These observations agree reasonably with results from 1D multi-fluid simulations using the Chicago code [3]. Moreover, the experimental and simulation results are consistent with first-principles theoretical predictions [4] that the lighter He ions diffuse farther ahead (toward the pre-shock region) within the overall shock-front than the heavier Ar ions. Our fundamental experimental data can be used to benchmark first-principles-based multi-fluid or kinetic simulations of multi-ion-species collisional plasma shocks, for which there have been recent known discrepancies between models [5].\\ Work performed in collaboration with Sam Langendorf, Scott Hsu, and Carsten Thoma, and supported by FES and ARPA-E of the DOE. LA-UR-19-26018. [1] P. Amendt et al., PRL 105, 115005 (2010) [2] S. C. Hsu et al., IEEE Trans. Plasma Sci. 46, 1951 (2018) [3] C. Thoma et al., Phys. Plasmas 18, 103507 (2011) [4] G. Kagan and X.-Z. Tang, Phys. Plasmas 19, 082709 (2012) [5] B. D. Keenan et al., Phys. Rev. E 96, 053203 (2017)