Boundary conditions for a Gaussian wave beam

The beam tracing technique allows the description of short-wavelength wave beams by means of a set of ordinary differential equations, in which the ef fects of diffraction (neglected by the standard geometrical-optics procedur e) are taken into account. The propagation of the beam is expressed in term s of a central ray and a set of parameters connected with the curvature of the wave front and the amplitude profile. Two crucial issues for the applic ability of this technique to situations of experimental interest are the be am reflection from a mirror and the possibility of following the beam in it s vacuum-plasma crossing. These problems are investigated in the paper. The parameters of the incident beam are regarded as known quantities and their values after reflection or inside the plasma are obtained. (C) 2001 Americ an Institute of Physics.