The structure of shocks and contact discontinuities in unmagnetized plasma as a function of the electron-to-proton temperature ratio

The expansion of a dense plasma into an ambient plasma with a step-like distribution is studied with a series of one-dimensional particle-in-cell (PIC) simulations. Several electron-to-proton temperature ratios Te/Tp in the ambient plasma and density ratios nr between the dense and ambient plasmas are examined. The collisionless plasma is unmagnetized. If nr≥10, hybrid structures form, which are composed of an electrostatic shock for the ambient plasma and a double layer for the expanding dense plasma. If nr=5, the electrostatic shocks become separated from the rarefaction waves by a contact discontinuity. If Te/Tp=5, the steepening of the electrostatic shock is halted by dissipation of wave energy by Landau damping. Its transition layer is wide, it propagates at the ion acoustic speed in the downstream frame of reference, and it hardly compresses the ambient plasma that crosses its transition layer. The contact discontinuity is wide, and the total density varies across its layer, creating a thermoelectric field. With a temperature ratio of Te/Tp=100, shock steepening is halted by wave dispersion. The shock transition layer is thin; it doubles the density of the ambient plasma that crosses it as expected, and the contact discontinuity is thin and has a uniform density across its layer. PIC simulations with nr=5 and several ratios Te/Tp between 5 and 100 show how the electrostatic shock changes from being dissipative to dispersive. Non-linear Landau damping allows the dispersive shock collapse, changing it into an ensemble of waves resembling solitary ion acoustic waves.