We study with a 3D PIC simulation discontinuities between an
electron-positron pair plasma and magnetized electrons and protons. A pair
plasma is injected at one simulation boundary with a speed 0.6c along its
normal. It expands into an electron-proton plasma and a magnetic field that
points orthogonally to the injection direction. Diamagnetic currents expel the
magnetic field from within the pair plasma and pile it up in front of it. It
pushes electrons, which induces an electric field pulse ahead of the magnetic
one. This initial electromagnetic pulse (EMP) confines the pair plasma
magnetically and accelerates protons electrically. The fast flow of the
injected pair plasma across the protons behind the initial EMP triggers the
filamentation instability. Some electrons and positrons cross the injection
boundary and build up a second EMP. Electron-cyclotron drift instabilities
perturb the plasma ahead of both EMPs seeding a Rayleigh-Taylor-type
instability. Despite equally strong perturbations ahead of both EMPs, the
second EMP is much more stable than the initial one. We attribute the rapid
collapse of the initial EMP to the filamentation instability, which perturbed
the plasma behind it. The Rayleigh-Taylor-type instability transforms the
planar EMPs into transition layers, in which magnetic flux ropes and
electrostatic forces due to uneven numbers of electrons and positrons slow down
and compress the pair plasma and accelerate protons. In our simulation, the
expansion speed of the pair cloud decreased by about an order of magnitude and
its density increased by the same factor. Its small thickness implies that it
is capable of separating a relativistic pair outflow from an electron-proton
plasma, which is essential for collimating relativistic jets of pair plasma in
collisionless astrophysical plasma.Comment: 25 pages, 12 figures, provisionally accepted for publication by the
New Journal of Physic