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These experiments studied the Raman instability occurring at densities below the quartercritical density of the incident 0.53-ju,m laser light. Solid gold targets were irradiated by up to 4 kJ of energy in a 1-ns pulse with focal spots of 150 to 1880 /xm. The angular distribution of the Raman scattering shows efficient Raman sidescatter. The energy in electron plasma waves accounts for the hot-electron production over almost three orders of magnitude. We report the results and discuss the implications for laser fusion.

Particle simulations and solutions of coupled mode equations are used to analyze the energy transfer between two equal-frequency, opposed laser beams resonantly interacting with ion acoustic waves in a plasma drifting at the sound speed. The simulations and analysis illustrate the dependence of the energy transfer and the ion wave dynamics on laser intensities and detuning, and the time dependence of the phenomena. The simulation results are in qualitative agreement with experimental observations in the NOVA laser facility [E. M. Campbell et al., Rev. Sci. Instrum. 57, 2101 (1986)] at the Lawrence Livermore National Laboratory. This work is part of a continuing examination of possible resonant crossed-beam interactions in flowing plasmas and their potential effects on the fusion performance of current and future laser-fusion experiments with multiple crossing beams, e.g., proposed experiments in the National Ignition Facility [National Tech. Info. Service Document Nos. DE95017671-DE95017673 and DE95017676-DE95017700 (J. A. Paisner, E. M. Campbell, and W. J. Hogan, The Natl. Ignition Facility Project, UCRL-JC-117397 and UCRL-PROP-117093, May, 1994)].

The laser and plasma conditions expected in ignition experiments using indirect drive inertial confinement have been studied experimentally. It has been shown that there are at least three ways in which ion waves can be stimulated in these plasmas and have a significant effect on the energy balance and distribution in the target. First ion waves can be stimulated by a single laser beam by the process of stimulated Brillouin scattering (SBS) in which an ion acoustic and a scattered electromagnetic wave grow from noise. Second, in a plasma where more than one beam intersects, ion waves can be excited at the “beat” frequency and wave number of the intersecting beams, causing the sidescatter instability to be seeded, and substantial energy to be transferred between the beams [R. K. Kirkwood et al., Phys. Rev. Lett. 76, 2065 (1996)]. And third, ion waves may be stimulated by the decay of electron plasma waves produced by stimulated Raman scattering (SRS), thereby inhibiting the SRS process [R. K. Kirkwood et al., Phys. Rev. Lett. 77, 2706 (1996)].