Cavitation and enhanced particle acceleration in relativistic shocks

Relativistic collisionless shocks associated with gamma-ray bursts (GRBs) can accelerate charged particles to high energies. Central to this acceleration process is the formation of magnetic turbulence by plasma streaming instabilities such as the Weibel instability. We show that under certain conditions, a plasma cavitation instability can grow in the shock precursor, strongly amplifying both the strength and length scale of magnetic fields far beyond that produced via the Weibel instability. The cavities are driven by the shock-accelerated electron beam, which is charge- but not current-neutralized by the cold upstream ions. The growth rate and saturation size of these magnetic cavities are well described by analytical theory in both electron-ion and pair-loaded shocks. Cavitation is accompanied by strongly enhanced particle acceleration.