Effects of longitudinal magnetic fields on the quasi-equilibrium propagation and focusing of a laser beam in a plasma channel

Abstract

The propagation of a laser beam in a parabolic plasma channel in a longitudinal magnetic field is investigated, taking into account the effects of relativistic self-focusing (RSF), channel focusing (CF), and ponderomotive self-channeling (PSC). A variational method is used to derive the equation describing the evolution of the laser spot size. The critical powers for the quasi-equilibrium solution and for constant spot size are obtained and the evolution of the spot size is categorized into three regimes: periodic-defocusing oscillations, periodic-focusing oscillations, and catastrophic focusing. The external magnetic field enhances RSF more than CF or PSC. Increasing the external magnetic field significantly reduces the two critical powers, allowing the focused laser beam to propagate farther in quasi-equilibrium in a wide channel created by a laser beam with a lower power threshold.