Propagation of laser beams smoothed by spectral dispersion in long-scale-length plasmas

The effects of one-dimensional smoothing by spectral dispersion (1D SSD) on the propagation of laser beams in long-scale-length plasmas are studied. The properties of spatiotemporal smoothing that can lead to the enhancement of the parametric instabilities due to laser-plasma interactions (LPI) such as forward stimulated Brillouin scattering (FSBS) and filamentation are identified. The evolution of these instabilities is studied analytically and formulas for the linear gain rates are derived.; To understand how the interplay of SBS and filamentation affect the nonuniformity of SSD beams propagating in plasma, a two-dimensional LPI code is developed. Depending on the interaction parameters various levels of filamentation and FSBS are observed for narrow (0.2THz) and broad (1THz) SSD bandwidths. The scaling of the FSBS gain is found to be in agreement with the predictions of linear theory. The development of the filamentation instability is controlled by the statistics of the spatial smoothing and is limited by the laser coherence time.; The impact of these processes on the nonuniformity of the laser light is complicated. The narrow-bandwidth (0.2THz) simulations show that the uniformity of lower-intensity beams decreases significantly due to filament formation whereas the uniformity of the higher-intensity beams increases due to plasma-induced smoothing, which is made possible by FSBS. The broad-bandwidth (1THz) simulations show considerably higher level of beam uniformity because of LPI reduction caused by short laser coherence time. The suppression of FSBS leads to the propagation regime, which is qualitatively similar to that of the low-intensity-narrow-bandwidth laser beams and in which nonuniformity grows slightly with increasing intensity. However even at the intensities relevant to NIF, the uniformity of 1THz 1D SSD beams stays within acceptable limits.