The influence of the focus position of broadband pump laser on stimulated Raman scattering initiation process for ICF study

This article deals with the physical process when a high intensity broadband KrF laser beam passes through a gaseous Raman medium. We discuss the parameter dependence of the threshold power and conversion efficiency of SRS. Based on the analysis of SRS initiation and calculation of Stokes laser modes formation, as well as the spatial propagation and amplification, we focus on the influence of focus position on the physical process, especially the Stokes output characteristic. Experiments are conducted for various active mediums, medium pressures, medium lengths as well as different focusing lenses and different focus positions. By comparison between experimental results and theoretical expectation, we clarify some ambiguous viewpoints that exist in the literature of SRS, so as to obtain a clearer physical image of SRS initiation.