Greatly enhanced acoustic noise and the onset of stimulated Brillouin scattering

Experiments using near-infrared to ultraviolet lasers offer the potential to study the acoustic noise in plasmas. As the onset of stimulated Brillouin scattering (SBS) has come to be closely examined, the evidence indicates that the acoustic noise may often or always be far above thermal levels. Evidence regarding the noise is reported here, from two recent experiments which confirmed the theoretically anticipated onset behavior for SBS. In one case, the noise appears to be greatly enhanced above thermal levels. In the other case, the data place an upper limit on the noise level. There is physical grounds to believe that enhanced acoustic noise may be ubiquitous in plasmas, even in the absence of plasma instabilities which drive turbulence. copyright 1997 American Institute of Physics