Spectral anomalies in high intensity stimulated Raman backscattering in laser plasmas

Summary form only given, as follows. A large amount of effort is being put into studies of the new physics that is observed as one moves into a regime for collective effects with ultra short pulse high intensity laser plasmas. Recently, a Livermore-UCLA collaboration has observed a sub-psec version of classic laser plasma stimulated Raman backscattering (SRBS) instability. The backscattered light displays novel spectral signatures that depend on laser intensity. Broad and modulated frequency spectrum that spreads to the blue side of the incident wavelength; that is obviously different from classic SRBS which downshifts the incident frequency by approximately the electron plasma frequency (EPW). We study anomalous SRBS signatures in our 1-D model of underdense uniform weakly collisional plasma layer. The set of three coupled equations, that account for pump depletion and relativistic detuning of EPW is simulated in space-time. By increasing a laser pump, there is a generic route via steady state: periodic and quasi-periodic regimes with an intermittent transition to spatio-temporal chaos is discovered. This type of intermittency in which quasi-periodic oscillations are interrupted by chaotic bursts displays modulated spectra with many peaks immersed in a broad band chaotic background. The picture reveals patches of turbulence inside the coherent state; the continuous transition amounts to a progressive increase of turbulence through the increase of the pump strength.