Theory and Simulation of CBET Mitigation Through Increased Laser Bandwidth

The cross-beam energy transfer (CBET) instability is currently a major obstacle to ignition in direct laser drive inertial confinement fusion (ICF). Technologies have recently been proposed that could enable a significant enhancement in the bandwidth available on fusion laser systems, and therefore improve laser coupling through mitigation of laser-plasma instabilities including CBET. In this work we present fluid and kinetic simulations of CBET driven by lasers with bandwidths in the range 2.5-10THz (exceeding the ion-acoustic wave frequency), and introduce a simple model for CBET in the presence of bandwidth. We discuss the key behaviour of the theoretical bandwidth scaling, and use it to provide an intuitive understanding of the mechanisms through which bandwidth mitigates CBET. The theory is then compared with linearized fluid simulations, performed with the LPSE code, and fully kinetic simulations performed with the VPIC particle-in-cell code. LPSE simulations exhibit good agreement with the theory due to the absence of nonlinear ion-acoustic wave dynamics. In contrast, ion-trapping induced nonlinear effects, two-ion wave decay, and stimulated Brillouin scattering cause VPIC simulations to exhibit more significant departures from linear theory.