Short-time-scale self-focusing of electromagnetic beams in a magnetoplasma

A Gaussian electromagnetic beam propagating in a plasma in the direction of the static magnetic field gives rise to (i) a ponderomotive force transverse to the magnetic field and (ii) nonuniform heating of electrons. These effects eventually modify the local magnetic field and thus produce a nonlinearity relevant to self-focusing. The critical power for self-focusing on a time scale shorter than the diffusion time (τD≳t) is vth/c times shorter than that on the longer time scale (t≳τD), where vth is the thermal speed of electrons.