  We study with a two-dimensional particle-in-cell simulation the stability of
a discontinuity or piston, which separates an electron-positron cloud from a
cooler electron-proton plasma. Such a piston might be present in the
relativistic jets of accreting black holes separating the jet material from the
surrounding ambient plasma and when pair clouds form during an X-ray flare and
expand into the plasma of the accretion disk corona. We inject a pair plasma at
a simulation boundary with a mildly relativistic temperature and mean speed. It
flows across a spatially uniform electron-proton plasma, which is permeated by
a background magnetic field. The magnetic field is aligned with one simulation
direction and oriented orthogonally to the mean velocity vector of the pair
cloud. The expanding pair cloud expels the magnetic field and piles it up at
its front. It is amplified to a value large enough to trap ambient electrons.
The current of the trapped electrons, which are carried with the expanding
cloud front, drives an electric field that accelerates protons. A solitary wave
grows and changes into a piston after it saturated. Our simulations show that
this piston undergoes a collision-less instability similar to a Rayleigh-Taylor
instability. The undular mode grows and we observe fingers in the proton
density distribution. The effect of the instability is to deform the piston but
it cannot destroy it.
