RELATIVISTIC AND PONDEROMOTIVE SELF-FOCUSING OF A LASER-BEAM IN A RADIALLY INHOMOGENEOUS-PLASMA .2. BEYOND THE PARAXIAL APPROXIMATION

The propagation in a plasma of a high-intensity electromagnetic wave i nducing 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 equat ion in which the plasma frequency is an instantaneous and local functi on of the wave energy; the incident irradiance is assumed to be consta nt in time. The specific features of relativistic focusing are first d iscussed. The ponderomotive effect enforces the focusing process by ex pelling the plasma electrons, creating density bumps and sharp density gradient on the edge of the light beam; the nonlinear focusing is fas ter and stronger confirming the paraxial/Gaussian beam core analysis p resented in Part I [Phys. Fluids B 5, 3539 (1993)]. In contrast to Par t I, the light is guided in a sharp-edged density channel. The influen ce of the radial density inhomogeneity is then examined by using both convex (basin shape) and concave (bump shape) profiles. The self-focus ing threshold power is increased for concave profiles. For convex prof iles, the natural refraction helps the self-focusing observation but w eakens the light-guiding trend previously observed. Finally, new featu res characterizing wave self-focusing, such as self-steepening and lig ht reflection, are shown.