Laser-plasma interaction physics in the context of fusion

Of vital importance for Inertial Confinement Fusion (ICF) are the understan ding and control of the nonlinear processes which can occur during the prop agation of the laser pulses through the underdense plasma surrounding the f usion capsule. The control of parametric instabilities has been studied exp erimentally, using the LULI six-beam laser facility, and also theoretically and numerically. New results based on the direct observation of plasma wav es with Thomson scattering of a short wavelength probe beam have revealed t he occurence of the Langmuir decay instability. This secondary instability may play an important role in the saturation of stimulated Raman scattering . Another mechanism for reducing the growth of the scattering instabilities is the so-called 'plasma-induced incoherence'. Namely, recent theoretical studies have shown that the propagation of laser beams through the underden se plasma can increase their spatial and temporal incoherence. This plasma- induced beam smoothing can reduce the levels of parametric instabilities. O ne signature of this process is a large increase of the spectral width of t he laser light after propagation through the plasma. Comparison of the expe rimental results with numerical simulations shows an excellent agreement be tween the observed and calculated time-resolved spectra of the transmitted laser light at various laser intensities. (C) 2000 Academie des sciences/Ed itions scientifiques et medicales Elsevier SAS.