Stimulated Brillouin scattering spectral profiles and gain coefficients for laser fusion optical drivers

Several architectural concepts of petawatt optical drivers required for the commercialization of electrical power generated by laser fusion rely on the temporal compression of ultraviolet laser pulses by stimulated Brillouin scattering (SBS). For the design of SBS pulse compressors, precise measurements of the SBS gain spectrum and the strength of coupling between the pump and seed laser pulses are critical but not available. To that end, high-resolution measurements of the SBS gain spectral profiles and absolute coupling coefficients for Ar, Kr, and N2 in the ultraviolet are reported here. The SBS spectra recorded at 266 nm with <90 MHz resolution are in general agreement with the predictions of kinetic theory (Boltzmann–BGK model) over the entire 1–6 atm pressure range investigated, whereas hydrodynamic theory (Navier–Stokes model) describes neither the observed spectral profiles nor the coupling coefficients. At one atmosphere, the peak coupling coefficients predicted by the hydrodynamic theory for the rare gases Kr, Ar, and Ne are factors of 5–13 lower than those predicted by the kinetic theory. The success of kinetic theory models in accurately predicting SBS gain spectra opens a new parameter space for the design of high-energy Brillouin amplifiers for laser fusion drivers.