EVIDENCE OF PLASMA FLUCTUATIONS AND THEIR EFFECT ON THE GROWTH OF STIMULATED BRILLOUIN AND STIMULATED RAMAN-SCATTERING IN LASER PLASMAS

The reflectivity levels of stimulated Brillouin scattering (SBS) in re cent large scale length laser plasma experiments is much lower than ex pected for conditions where the convective gain exponent is expected t o be large [J. C. Fernandez et al., Phys. Plasmas 4, 1849 (1997)]. Lon g-wavelength velocity fluctuations caused during the plasma formation process, or by parametric instabilities themselves, have been proposed as a mechanism to detune SBS in these experiments and reduce its gain [W. L. Kruer et al., Phys. Plasmas 3, 382 (1996); H. A. Rose, Phys. P lasmas 4, 437 (1997)]. Evidence of large-velocity fluctuation levels i s found in the time-resolved SBS spectra from these experiments, and c orrelates with observed changes in the reflectivity of both SBS and st imulated Raman scattering (SRS). Evidence of fluctuations that increas e with increasing plasma density is : presented, and their effect on t he growth of parametric instabilities is discussed. (C) 1998 American Institute of Physics.