  We present time-resolved measurements of ion heating due to collisional
plasma shocks and interpenetrating supersonic plasma flows, which are formed by
the oblique merging of two coaxial-gun-formed plasma jets. Our study was
repeated using four jet species: N, Ar, Kr, and Xe. In conditions with small
interpenetration between jets, the observed peak ion temperature Ti is
consistent with the predictions of collisional plasma-shock theory, showing a
substantial elevation of Ti above the electron temperature Te and also the
subsequent decrease of Ti on the classical ion-electron
temperature-equilibration time scale. In conditions of significant
interpenetration between jets, such that shocks do not apparently form, the
observed peak Ti is still appreciable and greater than Te, but much lower than
that predicted by collisional plasma-shock theory. Experimental results are
compared with multi-fluid plasma simulations.
