In the research of direct-drive laser fusion, laser irradiation of a target pellet can stimulate 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 the implosion quality to degrade. In the double-cone ignition (DCI) scheme of laser fusion scheme, the diagnosis of SBS and CBET is important owing to the different target configurations and oblique incident laser irradiation from the traditional spherically symmetric direct-drive 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 wavelength 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 process and CBET process in DCI.

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 result of the spherically symmetric irradiation direct-drive central ignition scheme.

By analyzing the correlation between the backscattered signal and the laser irradiation conditions and combining the results of a set of comparative experiments, we determine that the backscattered signal contains both CBET and SBS. There is a significant difference in the CBET fraction between the backscattered signal of the #5 laser and the backscattered signal of the #7 laser. By combining 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 designing subsequent large-scale laser fusion devices.