  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.
