Investigation of the Langdon effect on the nonlinear evolution of SBS in Au plasmas

In laser-irradiated plasmas, the Langdon effect modifies the electron energy distribution function (EEDF) from a Maxwellian to a super-Gaussian form, further influencing backward stimulated Brillouin scattering (SBS). We investigate the impact of the Langdon effect on the nonlinear evolution of SBS in Au plasmas using Vlasov–Maxwell simulations, with a particular focus on electron–ion collisions. In the absence of electron–ion collisions, the higher ion acoustic wave (IAW) frequency associated with a super-Gaussian EEDF leads to a slower initial SBS growth rate compared to the Maxwellian case. As the IAW grows to higher amplitudes, higher-order harmonics and ion trapping occur, resulting in nonlinear frequency shifts and spectral broadening, which saturate SBS and maintain it at a quasi-steady state with relatively low reflectivity. Under a super-Gaussian EEDF, the reduced IAW nonlinearity and Landau damping contribute to a higher SBS saturation level. When electron–ion collisions are present, they weaken the rapid growth of the IAW and inhibit frequency shifts and spectral broadening, leading to a higher average reflectivity for the same initial EEDF. In this scenario, the Langdon effect further enhances SBS by reducing IAW nonlinearity and inverse bremsstrahlung (IB) absorption, in which reduced IAW nonlinearity leads to relatively weaker harmonics, and reduced IB absorption allows the pump wave to propagate with a relatively higher amplitude, significantly increasing SBS reflectivity.