In direct-drive laser fusion studies, laser irradiation of the target pellet can excite various laser plasma instabilities such as stimulated Brillouin scattering(SBS) and cross beam energy transfer(CBET), which significantly reduce the energy coupling efficiency between the laser and target pellet as well as the laser irradiation uniformity, leading to the degradation of the implosion quality. In the double-cone ignition (DCI) scheme of laser fusion scheme,, the diagnosis of SBS and CBET is important due to the different target configuration and oblique incident laser irradiation from the conventional spherically symmetric direct-driven central ignition scheme. In this paper, a simple and reliable backscattering diagnostic system is developed and applied to the diagnosis of the time-resolved backscattering spectrum at wavelengths near 351 nm in a DCI experiment on the Shenguang-II upgrade (SG-IIU) facility. We use the system to carry out an experimental study of the SBS and CBET processes in DCI.</p> <p>The backscattering diagnostic system collects the backscattered light signal through the scattered light by reflector mirror via an optical fiber. The signal is dispersed by a spectrometer and then recorded by a streak camera. The signal contains both the laser reference signal from the frequency doubling crystal and the backscattered light. With the help of the reference signal, the diagnostic system can reliably give the energy fraction of backscattered light. The experimental results show that the energy fraction of backscattered light around 351 nm is not higher than 3%, which is significantly lower than the experimental results of the spherically symmetric irradiation direct-drive central ignition scheme.</p> <p>By analyzing the correlation between the backscattered signal and the laser irradiation conditions, combined with the results of a set of comparative experiments, we determined that the backscattered signal contains both CBET and SBS. There is a significant difference between the CBET fraction in the backscattered signals of the #5 and #7 lasers. Combined with the polarisation state of the laser beams, we confirm that this phenomenon is related to the polarisation angle between the laser beams. This finding provides a reference for the design of subsequent large-scale laser fusion devices.
