The Influence Of Near-field Properties Of The Laser Beam On SRS In The Process Of Transmission

Inertial confinement fusion(ICF) project is a new direction to find a new clean energy to solve the increasing energy needs. The laser from the ICF driver must irradiate the target after a long air transport, which leads to a nonlinear optical effect named Stimulated Raman scattering(SRS), reduces the beam energy. The SRS effect is a harmful nonlinear optical effect, which leads to a series of problem, such as the main beam energy transfer, near-field beam quality uniformity degrading, the divergence angle expanding on far-field. Currently, we need to understand the generating principle of SRS to suppress it. In this thesis, the impact of threshold for the SRS effect has been explored based on the characteristics of the beam near-filed which eatablish the theory and numerical foundation.Firstly, the numerical three-dimension spatical model has been built. The Raman gain coefficient of H2, O2 and N2 had been calculated. The influences of the threshold on SRS with the change of phase distortion in theory using the model, and the basic characteristics of SRRS have been presented.Secondly, the SRS experimental platform with the gas medium of H2 has been built based on the hundreds joule level laser system, and the experiment of SRS has been seted under the condition of large diameter, high gain and high flux. The Stokes conversion efficiency is measured from two aspects. On the one hand, keep the air pressure unchanged while the pump energy gets changed. On the other hand, the two variables would be exchanging. The law of SRS are compared with the numerical simulation results. The numerical model has been checked according to this data.Finally, several sets of data with a same pump energy has been contrasted with each other which has a great different on stokes conversion efficiency. The actual beam intensity distribution field was put into the numerical model to find the law between the near-field beam quality and the Stokes conversion efficiency. At the last of all, a special pump laser intensity distribution would be set to make sure the calculation model exact.