The Design And Research Of Backscatter Measurements On Shenguang-â…¢ Facility

In the research of inertial confinement fusion (ICF) backscatter caused bylaser-plasma interactions not only reduces absorbability of target with laser energy butalso reduces temperature of radiation field, which affect the coupling efficiency oflaser-plasma interaction. It is important to diagnose backscattered light of power andtemporally resolved spectral measurements precisely for the research of laser-plasmasinteractions. The paper is based on request of physics diagnostic on Shenguang-Ⅲfacility and a backscatter diagnostic system is developed. The system consists of fullaperture backscatter system (FABS) and near backscatter imaging system (NBI),which can measure backscattered lights of scattering into and near the final focusinglens respectively.At first the paper introduces generation mechanism and measurement principleof backscatter. According to design index of Shenguang-Ⅲ facility backscatterdiagnostic system, NBI and FABS are designed. The main optics elements andadjustable facility are analyzed in detail from the point of optics design and structuraldesign.The diagnostic optical system utilizes dichroic splitter to split backscatter intostimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS).Calorimeters, streak camera and fast photodiodes are used to measure power andspectra information of backscatter, the measurement path is symmetric. Due to largespatial distribution of backscattered light, the size of NBI scattered plate is as large asφ1300mm, we design four plates to stitch a full plate. The adjustable facility utilizesfour expansion links, three of them constitute an orthometric adjustable function andthe other is used to fix position, which can realize X、Y、Z and tilt directions adjustof ellipsoidal mirror. FABS utilize off-axis parabolic mirror to collect backscatteredlight, tilt placed of final optics assembly and use of spatial filter can split stray lightand backscattered light.In order to obtain precise measurement data calibration is used to gettransmittance of system.532nm and351nm are chosen to characteristic wavelengthsto simulate SRS and SBS. The beam simulated lens simulate real backscattered lightdistribution, the direction is changed by adjustable facility of turning mirror.CCD monitor system with lens view location of spot, which realizes repeat adjust of spotlocation.At last power and temporally resolved spectral data are obtained from shotexperiments. According to experiment result, the power of backscattered light is thesame essentially of each shot, which prove the system can work stably. From thetemporally resolved spectral information obtained by streak camera, the system canget accurate diagnostic of backscatter. The system provides reasonable physicsdiagnostic method for monitoring coupling of laser-plasma interactions.