Experimental Measurements of Ion Heating in Collisional Plasma Shocks and Interpenetrating Supersonic Plasma Flows

On the Plasma Liner Experiment (PLX) at LANL, two coaxial-gun-formed, highly supersonic plasma jets (with initial n ∼ 10¹⁶ cm^-3, T_e ∼ T_i ∼ 1.5 eV, v = 25 – 80 km/s, diameter = 8.5 cm, length ≈ 20 cm) are merged obliquely to form either a strong collisional plasma shock or semi-collisional interpenetration without shock formation, depending on the merging angle. Earlier work presented detailed diagnostic measurements consistent with plasma-shock formation [2]. This work [3] presents new and comprehensive measurements of ion heating due to the colliding flows, for N, Ar, Kr, and Xe.
When the jet interpenetration length is much smaller than the jet scale size (∼20 cm), strong ion heating (inferred via high-resolution Doppler spectroscopy of ionized emission lines) to well above the electron temperature T_e is observed, followed by cooling consistent with the classical equilibration rate. The peak observed ion temperature T_i (many tens of eV) is consistent with the post-shock prediction of collisional two-fluid plasma shock theory. When the jet interpenetration distance is comparable to the jet scale size, a diffuse plasma results with peak T_i well above T_e but well below the predicted value of shock theory. This dataset presents an opportunity to validate first-principles multi-fluid and kinetic models of plasma shocks being implemented into a variety of modern codes; comparisons with multi-fluid simulations will also be presented.

²E. C. Merritt et al., Phys. Rev. Lett. 111, 085003 (2013).
³S. J. Langendorf et al., submitted (2018); https://arxiv.org/abs/1805.09933.

*In collaboration with S. Hsu, K. Yates, C. Thoma. We acknowledge HyperV Technologies for advice on plasma-gun operation. LA-UR-18-25692.