Theoretical And Experimental Study On The Stimulated Scattering In Laser Indirect-Drive Inertial Confinement Fusion

The stimulated scattering processes of a laser beam in plasmas, including the stim-ulated Brillouin scattering (SBS) and the stimulated Raman scattering (SRS), are two of the most important issues in the research field of indirect drive inertial confinement fu-sion (ICF). These processes will do harm to the ignition from three aspects:a) wasting the laser energy so as to decrease the radiation temperature inside the hohlraum; b) var-ing the spatial distribution of the laser intensity on the internal surface of the hohlraum so as to destroy the symmetry of the radiation field; c) generating energetic electrons which will preheat the fusion materials so as to degrade the implosion performance. As a consequence, comprehending the stimulated scattering processes and finding a way to suppress them become urgent concerns in indirect drive ICF research. Since the stimulated scattering processes are complicated and sensitive to the plasma parameters, methods such as analytic theory, numerical simulation, and experimental diagnosis are all needed to realize a systematic research.This thesis focuses on the theoretical and experimental study on the stimulated scattering processes in the research field of laser indirect drive ICF. A robust ID planar model and a 2D cylindrically symmetric model are developed based on the linear the-ory. Some basic mechanisms in stimulated scattering processes are analyzed with these models. To perform experiments for the stimulated scattering processes, we developed a Thomson scattering diagnostic system on Shenguang-Ⅲ prototype laser facility, which is the primary platform for ICF research at present in China. This diagnostic system is capable of providing accurate plasma parameters with a high temporal and spatial resolution. With this diagnostic system, we conducted a series of gas-filled hohlraum experiments. Several kinds of quatities relative to the stimulated scattering processes were diagnosed. The abundant data obtained from the experiments can not only improve our understanding of the physical processes inside the hohlraum, but also contribute to the benchmark of the radiation hydrodynamic code.Followed are the specific contents in this thesis.i) A robust ID theoretical model is developed to deal with the stimulated scattering processes. This ID model takes into account:a) the pump depletion; b) the collisional absorption of both laser and scattered light; c) scattered light with different frequencies; and d) the potential boundary noise sources as well as the volume noise sources, con-tributed from both bremsstrahlung radiation and Thomson scattering of the pump laser. According to this model, the bremsstrahlung radiation from the high-Z wall plasmas and the reflected light of the pump laser from the critical surface will play a significant role in calculating the reflectivities of the scattered light for gas-filled hohlraums. The com-petition between SBS and SRS through pump depletion might be the primary reason for the anti-correlation of their reflectivities observed in experiments. The modification of the plasma parameters caused by the longitudinal imhomogeneity of the intensity of both laser and scattered light does hardly affect the reflectivities. Applying an external magnetic field to the plasmas will significantly suppress the stimulated scattering pro-cesses. A magnetic field of about 10 T might be enough to realize an obvious mitigation of the reflectivities for those ignition-relevant experiments.ⅱ) A 2D cylindrically symmetric model is developed to study the effect of diffrac-tion and self-focus of the beams on the stimulated scattering processes, and to analyze the trasverse distribution of the scattered light. Introduced in detail include the density fluctuation of electrostatic waves, the modification of the background density caused by ponderomotive force, the formula used to calculate the far-field distribution of the scattered light, and the numerical method for this 2D model. Based on this 2D model, some basic properties that cannot be given by the 1D model are discussed. The results indicate that it is the Rayleigh length of the laser beam instead of the longitudinal size of the plasmas that plays a significant role in the calculation of the reflectivities of the scattered light. Besides, the self-focus effect will converge the scattered light in the far field, while the collision absorption can hardly affect the transverse distribution of the scattered light. Nevertheless, this transverse distribution can be influenced by the longitudinal inhomogeneity of the plasmas through the mismatch of the coupling.ⅲ) A Thomson scattering diagnostic system is developed on Shenguang-Ⅲ proto-type laser facility to simultaneously measure the ion feature spectra and the red-shifted electron feature spectra from a 4ω probe beam. To satisfy the experimental require-ments, the optics of the diagnostic system are designed and calibrated delicately, which shows that the peak of the red-shifted electron feature spectra can be clearly detected for the plasma parameters on Shenguang-Ⅲ prototype laser facility. The spatial resolution of the imaging system is calibrated by a grid, which indicates that the resolution is better than 5μm in each branch. With this diagnostic system, we measured the plasma pa-rameters at different positions from different targets. These experimental data not only demonstrate the reliability of the diagnostic system but also contribute to the analysis of some physical mechanisms.ⅳ) A series of experiments for stimulated scattering processes are conducted on Shenguang-Ⅲ prototype laser facility. Seven types of diagnostic device are applied to detect the interaction of laser beams with the gas-filled hohlraums, and fruitful exper-imental data are obtained. Resutls given by Thomson scattering system show that the temperatures inside the heater beams in the CH region are 1.0 keV and 0.9 keV, re- spectively, for the standard hohlraums and for the square hohlraums. The evolutions of plasmas inside and outside the heater beams are quite different because of different heating mechanisms. Results given by backscattering diagnostic system indicate that SBS occurs in the Au plasmas near the wall and that the inhomogeneity of the flow in the Au plasmas may play an important role in the transverse distribution of the scattered light. The rupture of the SBS streak spectra and the red shift of the SRS streak spectra result from the propagation of a high-density plateau across the region where SRS oc-curs. The detected energetic electrons are probably generated from the electron plasma wave in the SRS process.