Description
Spectral smoothing of laser pulses is considered a promising approach for controlling parametric instabilities and improving laser plasma coupling in inertial fusion research. A limited number of experiments with broadband lasers and contradicting theoretical results motivate a more general assessment of the pump bandwidth on the excitation of parametric instabilities in a spatially inhomogeneous plasma. Here, we report on the studies of the forward and backward scattering in inhomogeneous plasma by comparing a monochromatic pump, a phase-modulated pump, and a pump with random phase modulations. In the time domain, phase-modulated and random pumps produce a qualitatively similar effect, reducing the temporal growth rate approximately inversely proportional to the bandwidth, in agreement with theoretical predictions following from the random phase approximation. Significant differences are observed, however, in the case of convective amplification in a spatially limited pump and in an inhomogeneous plasma. The level of saturation of the scattered light is enhanced and is associated with large temporal and statistical fluctuations if the pump correlation time is larger or comparable to the instability growth time. These fluctuations are associated with high-intensity spikes or groups of spikes in the pump field. The one-dimensional analysis of parametric instabilities is extended to speckled laser beams, assuming that each speckle generates scattered light independently. The laser bandwidth can suppress scattering from low-intensity speckles, but the high-intensity speckles may still dominate the response. A criterion for the bandwidth suppression of stimulated scattering in a speckled beam is proposed.
References:
V. Tikhonchuk, D. Blackman, P. Loiseau, and C. Ruyer, Effect of spectral bandwidth on the stimulated scattering of laser beams in plasma, Phys. Plasmas 33, 012107 (2026)