Spectral characteristics of laser-plasma instabilities with a broadband laser

Recent experimental progresses regarding broadband laser-plasma instabilities (LPIs) show that a 0.6% laser bandwidth can reduce backscatters of the stimulated Brillouin scattering (SBS) and the stimulated Raman scattering (SRS) at normal incidence [Phys. Rev. Lett. 132 035102 (2024)]. In this paper, we present a further discussion of the spectral distributions of the scatters developed by broadband LPIs, in addition to a brief validation of the effectiveness of bandwidth on LPIs mitigation at oblique incidence. SBS backscatter has a small redshift in the broadband case contrary to the blueshift with narrowband laser, which may be explained by the self-cross beam energy transfer between the various frequency components within the bandwidth. SRS backscatter spectrum presents a peak at a longer wavelength in the broadband case compared to the short one in the narrowband case, which is possibly attributed to the mitigation effect of bandwidth on filaments at underdense plasmas. The three-halves harmonic emission (3 omega/2) has a one-peak spectral distribution under the broadband condition, which is different from the two-peak distribution under the narrowband condition, and may be related to the spectral mixing of different frequency components within the bandwidth if the main sources of the two are both two-plasmon decays.