Experimental study of multiple-beam effects on the stimulated Brillouin scattering instability

The laser facilities designed to realize laser inertial confinement thermonuclear fusion experiments use numerous laser beams in order to meet irradiation symmetry constraints and to deposit enough energy in matter. Unfortunately, the crossing of laser beams in plasmas modifies their propagations. When a beam is propagating in plasmas, it can interact with ion acoustic waves and scatter its energy by stimulated Brillouin scattering. Not only the directions but also the levels of these scatterings are modified when beams are crossing one another in plasmas. This manuscript is about the experimental study of two kinds of multiple-beam modification: - the flexibility of the LULI2000 laser facility (Laboratoire d'Utilisation de Lasers Intenses, Palaiseau, France) enabled us to study crossed-beam energy transfer between two and three beams; - the sixty laser beams available on the OMEGA facility (Laboratory for Laser Energetics, Rochester, United-States) enabled us to observe scattering of collective Brillouin instabilities produced in planar geometries and at the entrance hole of cavities of fusion experiments. These two mechanisms can highly modify the initial laser irradiation by transferring up to 30% of the laser energy between beams and by scattering more than 10% of the laser energy in unusual directions. (author)