Hydrodynamics of thermal self-focusing in laser plasmas

Two-dimensional Eulerian hydrodynamics simulations have been carried out to investigate the thermal self-focusing of laser beams in plasmas with long scale lengths. Realistic modeling of the plasma dynamics and the time dependence of the plasma heating is found to be necessary to follow the fully nonlinear development of self-focusing. The laser propagation is treated using a self-consistent ray-tracing model. The simulations presented illustrate the dependence of thermal self-focusing on beam size, plasma scale length and initial temperature, laser wavelength and intensity, and flux limiter. For the interaction of a finite-sized beam with a preformed long-scale-length plasma, whole-beam self-focusing is found to occur when the beam diameter is smaller than the plasma scale length. For beams with a hot spot imposed on a uniform background, self-focusing occurs only for hot spots of sufficient amplitude (> or approx. =a few percent in typical cases). Self-focusing is less likely to occur in an initially hot plasma. Low-density self-focusing channels formed in the underdense plasma by the expulsion of plasma from laser-heated regions may collimate the reflected light back towards the laser. Small-scale self-focusing has also been simulated, along with a subsequent evolution into whole-beam self-focusing