Non-linear Weibel instability in the interaction of an underdense plasma with a “relativistic” laser pulse

Particle in cell numerical simulations of high-intensity laser pulses propagating in underdense plasmas show, in the wake of the laser pulse, a current layer structure characterized by one “fast” central current and two “slow” return currents, coupled to a quasi-static magnetic field. Using a relativistic two-fluid description, we show that the observed dynamics and the magnetic fields can be explained by the development of the “inhomogeneous” Weibel instability coupled to the two-stream instability.