Experimental characterization of a transition from collisionless to collisional interaction between head-on-merging supersonic plasma jets

TL;DR

This study experimentally investigates the transition from collisionless to collisional interactions during the head-on merging of supersonic plasma jets, revealing how ionization affects plasma collisionality and shock formation.

Contribution

It provides detailed experimental evidence of the collisional transition in plasma jet merging, aiding validation of plasma models with complex equations of state.

Findings

Jets initially interpenetrate in a collisionless regime

Rising mean-charge state reduces ion collision length

Jets eventually form collisional shocks

Abstract

We present results from experiments on the head-on merging of two supersonic plasma jets in an initially collisionless regime for the counter-streaming ions. The plasma jets are of either an argon/impurity or hydrogen/impurity mixture and are produced by pulsed-power-driven railguns. Based on time- and space-resolved fast-imaging, multi-chord interferometry, and survey-spectroscopy measurements of the overlapping region between the merging jets, we observe that the jets initially interpenetrate, consistent with calculated inter-jet ion collision lengths, which are long. As the jets interpenetrate, a rising mean-charge state causes a rapid decrease in the inter-jet ion collision length. Finally, the interaction becomes collisional and the jets stagnate, eventually producing structures consistent with collisional shocks. These experimental observations can aid in the validation of plasma collisionality and ionization models for plasmas with complex equations of state.