We develop a statistical theory of stimulated Brillouin backscatter (BSBS) of
a spatially and temporally partially incoherent laser beam for laser fusion
relevant plasma. We find a new collective regime of BSBS (CBSBS) with intensity
threshold controlled by diffraction, an insensitive function of the laser
coherence time, Tcβ, once light travel time during Tcβ exceeds a laser
speckle length. The BSBS spatial gain rate is approximately the sum of that due
to CBSBS, and a part which is independent of diffraction and varies linearly
with Tcβ. We find that the bandwidth of KrF-laser-based fusion systems would
be large enough to allow additional suppression of BSBS.Comment: 8 pages, 5 figures. arXiv admin note: substantial text overlap with
arXiv:1105.209