Experimental Research On Large-Scale Laser-Plasma Interactions

Indirect-drive scheme of inertial confinement fusion (ICF) is one of the most promising ways to realize the goal of ignition. In this scheme, intense laser beams have to propagate a distance of several millimeters in uniform underdense plasmas before they reach the high-Z wall of the hohlraum and convert their energy into thermal x-rays. Abundant laser-plasma interactions (LPIs) can occur in this process. Various parametric instabilities, especially the stimulated Raman scattering (SRS) and the stimulated Brilliouin scattering (SBS), can be driven with the intense laser beams and profoundly affect the propagations of the laser and the laser-hohlraum coupling efficiency. It is now well recognized that LPIs in large-scale plasmas must be carefully controlled in order to realize the ignition goal.Laser-plasma interactions in large-scale plasmas relevant to the ignition condition are very complicated. One important characteristic is that LPIs are very sensitive to the plasma conditions. Generally the LPI research should be operated in three steps. First, large-scale laser-plasma platform should be set up and well characterized; the second is that adequate data relevant to the parametric instability processes, which are the foundation of the code benchmarking, should be obtained using various high-accuracy diagnostics in the platform and be analyzed carefully for the relationship between the LPI processes and the plasma parameters; finally, all kinds of methods based upon the LPI data should be experimentally tested for the mitigation of the parametric instability processes.The main results in this thesis are presented in the follows:1 A Full Aperture Backscatter System (FABS) is designed and set up for the ninth laser beam of the SG-Ⅱfacility. The Backscattering energy in the aperture can be diagnosed by the FABS, giving the uncertainty of SRS~60% and SBS~70%.2 A method for the calibration of the FABS and near backscattering imaging system (NBI) is brought forward and is validated perfectly in XG-ⅡI laser facility. Besides, a technique for the detailed diagnosis of the reflected energy is developed. Both the techniques give positive effect to lower the uncertainty of the backscattering energy diagnosis.3 A large-scale laser-plasma platform is set up and characterized on SG-Ⅱusing gasbag targets. Two kinds of plasma components Xe and CH are used on this platform. Both the X-ray pinhole images and X-ray framing images inferred that millimeter-scale plasmas are produced. Time-resolved electron/ion temperatures and hydrodynamic velocity are diagnosed through collective Thomson scattering. The electron density is inferred from the SRS streak spectrum. The time-resolved experimental data imply a temporal window of about 600-1100ps, when the large-scale laser plasmas exist.4 Large-scale LPI experiments are operated on the platform. A one-dimensional linear-gain code is developed and the simulations reproduce the experimental SRS streak spectrum, which prove the reliability of plasma parameters provided by the experimental diagnosis mentioned above. The simulations also reproduced the disruption phenomenon in normal SRS spectrum, which implies that the SRS produces and develop in local region. The FABS results give a bit lower reflect fraction, which may be due to the only 1-mm-scale plasma through the analysis of a one-dimensional simplified model and the latest NIF experimental results.5 LPI processes in the hohraum using several kinds of targets are also experimentally addressed on SG-Ⅱ. The data through all eight FABS on the general laser beams show that the total backscattering fraction is low in the hohlraum environment; besides, coating CH in the hohlraum surely mitigate the inward movement of the Au plasmas, but it can also lower the hohraum coupling efficiency, the lower amplitude of the radiation temperature can be as much as 14%.6 A novel flat-response x-ray detector (FXRD) is developed for the purpose of integrated diagnosis in the gas-filled hohlraum experiments. The pure Au construction of this novel FXRD leads to a flat response in the photon energy range of 0.1-4keV and the further improvement on the compound filter largely relax the requirement of the calibration x-ray beam. The calibration on BSRF shows that the FXRD has a response flatness about 12% in the photon energy range of 0.1-4keV.7 Detailed researches on the angular distribution of the radiation flux from a hohlraum are taken on SG-Ⅱfacility, using the novel FXRD. Various types of targets are used in the experiments. It is found that the angular distribution of the radiation flux from a hohlraum does not follow the linearity with cosθ, but has a peak value atθ=25°. It is also found that there can be obvious discrepancy in the radiation flux even in the sameθangle. Analysis shows that the laser-hohlraum beat spot seriously affect the angular distribution of the radiation flux, both in peak value and temporal activity.