Observation of Shock-Front Separation in Multi-Ion-Species Collisional Plasma Shocks

We observe shock-front separation and species-dependent shock widths in multi-ion-species collisional plasma shocks, which are produced by obliquely merging plasma jets of a He/Ar mixture (97% He and 3% Ar by initial number density) on the Plasma Liner Experiment [S. C. Hsu et al., IEEE Trans. Plasma Sci. 46, 1951 (2018)]. Visible plasma emission near the He-I 587.6 nm and Ar-II 476.5-514.5 nm lines are simultaneously recorded by splitting a single visible image of the shock into two different fast-framing cameras with different narrow bandpass filters (589 +/- 5 nm for observing the He-I line and 500 +/- 25 nm for the Ar-II lines). For conditions in these experiments (pre-shock ion and electron densities ~5*10^14 cm^-3, ion and electron temperatures of ~2.2 eV, and relative plasma-merging speed of 22 km/s), the observationally inferred magnitude of He/Ar shock-front separation and the shock widths themselves are < 1 cm, which correspond to ~50 post-shock thermal ion-ion mean free paths. These experimental lengths scales are in reasonable qualitative and quantitative agreement with results from 1D multi-fluid simulations using the Chicago code. However, there are differences between the experimentally-inferred and simulation-predicted ionization states and line emission intensities, particularly in the post-shock region. Overall, the experimental and simulation results are consistent with theoretical predictions that the lighter He ions diffuse farther ahead within the overall shock front than the heavier Ar ions.