  The expansion of a dense plasma through a more rarefied ionised medium is a
phenomenon of interest in various physics environments ranging from
astrophysics to high energy density laser- matter laboratory experiments. Here
this situation is modeled via a 1D Particle-In-Cell simulation; a jump in the
plasma density of a factor of 100 is introduced in the middle of an otherwise
equally dense electron-proton plasma with an uniform proton and electron
temperature of 10eV and 1keV respectively. The diffusion of the dense plasma,
through the rarified one, triggers the onset of different nonlinear phenomena
such as a strong ion-acoustic shock wave and a rarefaction wave. Secondary
structures are detected, some of which are driven by a drift instability of the
rarefaction wave. Efficient proton acceleration occurs ahead of the shock,
bringing the maximum proton velocity up to 60 times the initial ion thermal
speed.
