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.
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