Study On Stimulated Raman Scattering Of High-power Laser Driver For Inertial Confinement Fusion

In inertial confinement fusion (ICF), stimulated Raman scattering (SRS) is an extremely harmful nonlinear effect, which will produce significantly in the frequency conversion system and long air path propagation of high-power laser driver. For the frequency conversion crystals of large aperture, the transverse stimulated Raman scattering (TSRS) is the most damaging nonlinear effect, which will lead to the loss of laser energy and the degradation of the frequency conversion efficiency. When the peak Stokes fluence exceed the damage threshold of frequency conversion crystals, the fatal damage on the crystals are the most serious. Furthermore, owing to the effect of stimulated rotational Raman scattering (SRRS) on 3ω high-power ultraviolet laser beams in the process of long air path propagation after the frequency conversion system, not only laser energy losses, but also the beam quality degrades, leading to the effect on target irradiation. For improving the reliability, stability of laser driver and realizing laser fusion successfully, it is essential to study the SRS effect of high-power laser driver for ICF.In the thesis, the effect of SRS on the crystals and beam quality in the frequency conversion system and long air path propagation has been discussed in detail. The main results obtained can be summarized as follows: (1) The physical model of SRS in different mediums has been built up.Based on quantum-mechanical viewpoints, the physical model of SRS effect has been built up for two different mediums: frequency conversion crystals and air in ICF, which considered the diffraction effects of Stokes and pump lasers, Langevin noise sources, reflection at the faces and the edges of crystals, the nonlinear polarization of the medium, the depletion of pumped laser, the nonlinearamplification of the Stokes field and the Raman bandwidth, ect.(2) The characteristics of TSRS effect in frequency conversion crystals have been studied.For different operating conditions of laser driver: long and short pulse, TSRS effect in KDP and KD*P crystals has been studied. The effect of the temporal phase modulation on peak Stokes intensity, peak Stokes fluence and space-time characteristic of TSRS has been calculated and analyzed.(3) The SRRS effect in the process of high-power ultraviolet laser propagationin long air path has been studied.The SRRS effect in the process of high-power ultraviolet laser propagation in long air path has been studied quantitatively. The effect of the factors including the spatial phase perturbation and vacuum on the SRRS threshold, Stokes temporal characteristic, SRRS conversion efficiency and laser beam quality has been calculated and analyzed.