Observation of ionization-mediated transition from collisionless   interpenetration to collisional stagnation during merging of two supersonic   plasmas
  We present space- and time-resolved experimental data of head-on-merging, supersonic plasma jets (of an argon/impurity mixture) in an initially collisionless regime for counter-streaming ions. The merging begins with collisionless interpenetration followed by a transition to collisional stagnation. The transition occurs due to an experimentally inferred rising mean-ionization level, which rapidly reduces the counter-streaming ion-ion mean free path. The measurements demonstrate a specific mechanism by which a collisionless interaction transitions to a collisional one and constrain collisionality and ionization models for plasmas with complex equation of state. 
Auna L. Moser and Scott C. Hsu
physics.plasm-ph