Stimulated Brillouin backscattering from underdense expanding plasmas in aregime of strong filamentation

Stimulated Brillouin backscattering (B-SBS) from delayed interaction with a preformed, long scale-length expanding plasma was experimentally investiga ted in a regime favorable to strong self-focusing (SF) and filamentation. A n extremely low backscattering reflectivity (of the order of 10(-4)) was me asured and anomalous time-resolved spectra were observed. Spectral and temp oral features of the reflectivity suggest a strong effect of SF on B-SBS: I n the early phase of the interaction SF lowers the B-SBS threshold and poss ibly leads to rapid saturation, while in the late phase there is a substant ial decoupling of the SBS active regions from the plasma bulk due to their motion towards the laser beam. This motion is attributed to a moving focus effect driven by ponderomotive SF. [S1063-651X(99)00601-7].