神光-Ⅱ U装置的SBS背向散射角分辨诊断系统
摘要
Objective Stimulated Brillouin scattering (SBS) is an important topic in the field of laser -drive inertial confinement fusion (ICF).SBS can reduce the laser -target energy coupling efficiency, spoil implosion symmetry, and degrade target gain.The full -aperture stimulated Brillouin backscattering energy fraction diagnosis systems on the main laser fusion devices do not measure the spatial distribution of stimulated Brillouin backscattering signals, which simplifies the diagnosis system but loses the detailed features of stimulated Brillouin backscattering signals.An angle -resolved full -aperture backscattering diagnosis system is developed for the Shenguang - Upgrade (SG - U) facility to study the SBS process in the integration experiment of the double -cone ignition (DCI) scheme.The evaluation reveals that the angular resolution of the system is about 2 , and the diagnostic accuracy of energy fraction is 15%. MethodsThe design of the system is guided by the temporal characteristics of the backward reflection signals on the SG - U facility.It is found that both SBS signals from the target and the third harmonic reflected light from the triple - frequency -conversion crystal (called laser signals in this work) propagate along the same direction but with a fixed time delay dependent on the geometry of the final optical assembly.The level of laser signals is proportional to the incident laser energy and is stable so that it can be used as a reference to infer the SBS energy fraction.An optical fiber array is designed to collect the angle resolved signals reflected from the servo mirror of the SG - U facility.Then, the signals are split into two beams and recorded with two intensified charge -coupled devices (ICCD) separately.The time delay between the two ICCDs is determined by that between SBS signals and laser signals.To make sure the ICCDs record accurate signal energy and verify the timing and intensity ratio between the SBS signals and laser signals, the tempora