Relativistic and ponderomotive self-focusing of a laser beam in a radially inhomogeneous plasma. II. Beyond the paraxial approximation

Abstract

The propagation in a plasma of a high-intensity electromagnetic wave inducing both relativistic mass increase and ponderomotive expulsion of electrons is analyzed via two-dimensional simulations. The time/space evolution of the wave is modeled by an axisymmetric scalar wave equation in which the plasma frequency is an instantaneous and local function of the wave energy; the incident irradiance is assumed to be constant in time. The specific features of relativistic focusing are first discussed. The ponderomotive effect enforces the focusing process by expelling the plasma electrons, creating density bumps and sharp density gradient on the edge of the light beam; the nonlinear focusing is faster and stronger confirming the paraxial/Gaussian beam core analysis presented in Part I [Phys. Fluids B 5, 3539 (1993)]. In contrast to Part I, the light is guided in a sharp-edged density channel. The influence of the radial density inhomogeneity is then examined by using both convex (basin shape) and concave (bump shape) profiles. The self-focusing threshold power is increased for concave profiles. For convex profiles, the natural refraction helps the self-focusing observation but weakens the light-guiding trend previously observed. Finally, new features characterizing wave self-focusing, such as self-steepening and light reflection, are shown.