Influence of spectral bandwidth on the nonlinear kinetic regime of stimulated Raman scattering and cross-beam energy transfer for spatially smoothed laser beams

The main challenge for efficient laser energy deposition in inertial confinement fusion (ICF) is stimulated laser scattering on plasma waves. Various optical laser beam smoothing techniques used over the years, such as random phase plates (RPP) and smoothing by spectral dispersion (SSD), appear to be inadequate for mitigating parametric instabilities. Following the successful ignition at the National Ignition Facility, there is renewed interest in finding improved smoothing techniques for developing a new laser facility for Inertial Fusion Energy. Consequently, techniques such as broadband lasers and Induced Spatial Incoherence (ISI) are being revisited.

To study these optical smoothing techniques in the context of laser-plasma instabilities, particularly in the nonlinear kinetic regime of Stimulated Raman Scattering, 2D Particle-In-Cell (PIC) simulations are necessary. As a result, all these techniques have been implemented in the PIC code SMILEi.

In the talk, we will present the initial results of these simulations, where different bandwidths for an RPP-broadband laser, ranging from 1% to 10%, are simulated in 2D and compared to a broadband laser and ISI with the same bandwidth. We will demonstrate that the nonlinear kinetic regime can be mitigated at certain intensities when ISI is used with a broadband laser. Comparisons will also be presented when these techniques are combined with polarization smoothing. Finally, results from 2D PIC simulations will be presented using the same optical smoothing techniques on cross-beam energy transfer.