  Ab initio simulations of the propagation in a plasma of a soon to be
available relativistic electron-positron beam or fireball beam provide an
effective mean for the study of microphysics relevant to astrophysical
scenarios. We show that the current filamentation instability associated with
some of these scenarios reaches saturation after only 10 cm of propagation in a
typical laboratory plasma with a density 10^17/cc. The different regimes of the
instability, from the purely transverse to the mixed mode filamentation, can be
accessed by varying the background plasma density. The instability generates
large local plasma gradients, intense transverse magnetic fields, and enhanced
emission of radiation. We suggest that these effects may be observed
experimentally for the first time.
