Mitigation of stimulated Brillouin scattering in inhomogeneous plasmas by broadband lasers

A theoretical model of stimulated Brillouin scattering (SBS) developed by a broadband laser in inhomogeneous plasmas is proposed and further investigated through a series of numerical simulations using the low-coherence laser model of the Kunwu facility. Polychromatic beamlets are coupled to develop a scattering mode, when all the poles of the beamlets are on the integration path of SBS dispersion relations. The coupled beamlets can be regarded as a monochromatic beam with the same incident energy, owing to their nearly identical amplification coefficients. Therefore, a broadband laser behaves as several decoupled beams with a frequency difference around the coupling threshold of the beamlets. In the direct-drive scheme, the overall saturation level of scattering light can be reduced by the broadband laser, if the bandwidth is larger than c_{s}/c∼10^{-3}, where c_{s} and c are the velocities of ion acoustic wave and light, respectively. Two-dimensional simulations demonstrate the robust mitigation of SBS by the low-coherence laser with a moderate bandwidth ∼0.6%, even in the ignition regime. The laser plasma parameters have a minor impact on the mitigation of SBS with broadband lasers. The speckle structures vary rapidly over time at the focal plane due to the introduction of the broadband, which results in the further reduction of SBS and filamentation compared with the monochromatic laser.