Nonlinear Propagation of a Short-Pulse Laser in a Plasma with Density Ripple

In recent experiments, density ripples have been used to achieve phase-matched harmonic generation from plasmas. We report a numerical study of nonlinear propagation of an intense short-pulse laser in a plasma containing a large-amplitude density ripple. The laser with a Gaussian distribution of the intensity along its wavefront undergoes periodic self focusing due to relativistic and ponderomotive nonlinearity. In the region of high plasma density, the self-focusing effect dominates over the diffraction effect and the laser converges. As the beam enters the low-density region, the diffraction effect supercedes the self-focusing effect and diverges the beam. For ripple periods shorter than the Rayleigh diffraction length, the beam width parameter has a minor wiggle as a function of the distance of propagation. When the ripple period is long, the wiggle in the beam width parameter becomes large.