  The plasma dynamics resulting from the simultaneous impact, of two equal,
ultra-intense laser pulses, in two spatially separated spots, onto a dense
target is studied via particle-in-cell (PIC) simulations. The simulations show
that electrons accelerated to relativistic speeds, cross the target and exit at
its rear surface. Most energetic electrons are bound to the rear surface by the
ambipolar electric field and expand along it. Their current is closed by a
return current in the target, and this current configuration generates strong
surface magnetic fields. The two electron sheaths collide at the midplane
between the laser impact points. The magnetic repulsion between the
counter-streaming electron beams separates them along the surface normal
direction, before they can thermalize through other beam instabilities. This
magnetic repulsion is also the driving mechanism for the beam-Weibel
(filamentation) instability, which is thought to be responsible for magnetic
field growth close to the internal shocks of gamma-ray burst (GRB) jets. The
relative strength of this repulsion compared to the competing electrostatic
interactions, which is evidenced by the simulations, suggests that the
filamentation instability can be examined in an experimental setting.
