  The formation of unmagnetized electrostatic shock-like structures with a high
Mach number is examined with one- and two-dimensional particle-in-cell (PIC)
simulations. The structures are generated through the collision of two
identical plasma clouds, which consist of equally hot electrons and ions with a
mass ratio of 250. The Mach number of the collision speed with respect to the
initial ion acoustic speed of the plasma is set to 4.6. This high Mach number
delays the formation of such structures by tens of inverse ion plasma
frequencies. A pair of stable shock-like structures is observed after this time
in the 1D simulation, which gradually evolve into electrostatic shocks. The ion
acoustic instability, which can develop in the 2D simulation but not in the 1D
one, competes with the nonlinear process that gives rise to these structures.
The oblique ion acoustic waves fragment their electric field. The transition
layer, across which the bulk of the ions change their speed, widens and their
speed change is reduced. Double layer-shock hybrid structures develop.
