  Using a three dimensional relativistic particle-in-cell code we have
performed numerical experiments of plasma shells colliding at relativistic
velocities. Such scenarios are found in many astrophysical objects e.g. the
relativistic outflow from gamma ray bursts, active galactic nuclei jets and
supernova remnants. We show how a Weibel-like two-stream instability is capable
of generating small-scale magnetic filaments with strength up to percents of
equipartition. Such field topology is ideal for the generation of jitter
radiation as opposed to synchrotron radiation. We also explain how the field
generating mechanism involves acceleration of electrons to power law
distributions (N(E)~E^(-p))through a non-Fermi acceleration mechanism. The
results add to our understanding of collisionless shocks.
