SPOT Size Effects on the Laser Plasma Interaction Features

A generalization of the paraxial approximation based on stationary transverse distribution of the laser pulse in laser-plasma interaction is addressed. The laser beam is assumed to have a nonuniform intensity distribution (along its wave front) in a plane transverse to the direction of propagation. The conditions are considered in such a way that the pulse variation in the perpendicular direction is negligible. Accordingly, the study is based on an approach that accounts for the transversal effects in an average manner. The formalism of the electron parametric instability is presented in this framework. The resulting equation is apparently similar to the equation that is obtained from the paraxial approximation but with averaged source term.