Broadening and distribution of laser spectrum to control laser-plasma interaction

The propagation of the laser beam in the plasma generates plasma waves and the interaction of the laser beam with these waves induces loss of energy and deteriorates the irradiation symmetry on the target. Taking into account all the phenomena involved in this interaction is not so far possible, this study focuses on the stimulated Brillouin scattering that back-scatters part of the light in the axis of the beam which wastes energy and can damage the laser optics. The best method to assure the control of the laser-plasma interaction is to make the laser beam as incoherent as possible by spreading its spectrum but at the expense of the optical performance. A complementary method has been investigated, based on the distribution of the laser spectrum on several beams that allows for a complete spectrum when the beams mix on the target. As the damage on the optics is dependent on the spectral width, the spectral distribution on several beams allows a reduction of damage on optics. This method has been successfully tested on the Mega-Joule laser. (A.C.)