FILAMENTATION INSTABILITY OF AN ELECTROMAGNETIC-WAVE IN AN EXPANDING PLASMA

A laser beam propagating at an angle to the density gradient in an expanding plasma is susceptible to filamentation instability. For a linear density profile and self consistent temperature and expansion velocity profiles, the amplitude of the perturbation grows as an Airy function with the distance of propagation. The characteristic growth length decreases with the size of perturbation and the ratio of expansion velocity to sound velocity. It increases the angle laser k vector makes with the density gradient. Moreover, the growth length does not always increase with theta-0: it depends on nu(b)(0). The spatial growth of perturbation of larger size starts occurring deeper into the plasma.