| Experiments and researches about laser-driven inertial confinement fusion (ICF) flied into the sight of human soon after the first ruby-laser’s coming out. In ICF, laser is used as direct or indirect driven-source, to compress the fuel mixed with deuterium(D) and tritium(T) to a extremely high density. Then some hot spots appear with high temperature and high density which meet the conditions for ignition. Finally, fusion starts and release a large amount fusion energy.Laser plasma interaction is a very complicated physical process in ICF, moreover, stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) are two very important kinds of parameter instabilities. First, the backscatter light from SBS and SRS take a large portion of energy away, leading to a lower energy utilization, and then the energy left in target can not meet the need for ignition. Second, SRS also produces the super hot electrons with high energy, and the super hot electrons will preheat target pills and make it difficult to be compressed. For indirect ICF, the incident laser should pass through a long distance (a few millimeters) in the target filling with low-density gas, so SBS and SRS receive significant attention. Early studies show that SBS and SRS are sensitive to laser power density, and it is effective to curb their growth by improving laser irradiation uniformity. This paper mainly discusses the relationship between the coherence in incident laser beams and the backscatter of SBS/SRS, expounds the principle and mechanism of SBS and SRS. Experiments were carried out on "Shenguang-Ⅱ" high power laser device, and the experimental results obtained were well treated and analysed, the specific work content as follows:1. Two laser beams (beam1#and3#,～1ns,～250J,527nm in each one) from "Shenguang-Ⅱ" were employed to irradiate Au plate targets(thickness of6.17μm) and CRF aerogel targets(p=475mg/cm3). The polarize directions of incident laser beams could be changed by rotating the frequency doubling crystal, so different laser irradiation conditions with different interference degrees were achieved. The backscatter light of SRS and SBS was obtained by sampling in small-aperture with mirrors in the experiment, and the measurement system of backscatter light was set up. The system separated SBS and SRS light successfully, the energy of backscatter light was measured by energy meters, the spectrum of SRS was measured by a spectrometer. 2. The experimental data obtained was well treated and analysed, and some regular physical phenomenon was found. According to the theory of SBS and SRS, we came to a conclusion:the driven-laser beams strongly interfered, and there was usually the more intense backscatter the higher coherent degree between the incident beams.3. The conclusion above suggests a possible way to understand the increase of SBS and SRS backscatter in recent related experiments on NIF. In other words, although each laser beam is smoothed by kinds of technologies, serious interference may appear again when more laser beams stack together. For example, the work efficiency of polarize smoothing(PS) under more laser beams may decline. So it is necessary to consider the interference relationship between beams, while each laser beam has been well smoothed. |
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