Theory of the ultra-intense short-pulse laser interaction with under-dense plasma

Abstract

A comprehensive theory is proposed to describe the propagation and absorption of ultra-intense, short laser pulse through the under-dense plasma. The kinetic aspects of plasma are fully incorporated using extensive particle-in-cell (PIC) simulations. It is turned out that the plasma behavior is characterized by both its density and the ratio of the pulse length to the plasma wavelength. According to exact analyses and direct simulation evidences, at ultra-low densities the laser pulse is adiabatically depleted (absorbed) by the wake excitation. And the depletion is accompanied by the overall radiation red-shift. At these densities, for pulse lengths larger than the plasma wavelength the Raman type scatterings also occur without causing instability. When the plasma density grows toward the critical density, a completely new regime appears with the main character of highly unsteady light propagation. Here, based on analyses and simulations, the radiation pressure induced wave breaking (RPIWB) heavily destroys the oscillatory structure of the electron wave behind the first plasma period. On the other hand, the electron density profile induced by the ponderomotive force ahead of the pulse begins to steepen and eventually acts as a flying mirror. As a result, the pulse becomes bake scattered and its penetration becomes limited. The radiation pressure of reflecting pulse sustains a longitudinal electric field which accelerates electrons produced via RPIWB causing the plasma to undergo volumetric heating. Mostly important, based on a newly proposed model it is shown that the interaction becomes saturated at a definite time and that the overall absorption and plasma heating are directly related to this time. Also it is found that the saturation time decreases by a factor of ( , and are initial electron density, critical density and initial laser gamma factor)...