Research On Effect And Suppression Methods Of SRS In Inertial Confinement Fusion Laser Drivers

Currently, marshalling dozens even hundreds of large aperture (~40cm) andhigh-power laser beams to irradiate the pellet uniformly is the main design strategyfor inertial confinement fusion (ICF) laser drivers. However, many new physicalproblems have turned up with the increasing of laser intensity, which bring greatchallenges to the safe operation and heighten the output level of the laser driver.Stimulated Raman scattering (SRS) is the most representative one of the problems inthe transmission of the high-energy and high-power laser, which includes thestimulated rotational Raman scattering (SRRS) in long air paths of switchyard andthe transverse stimulated Raman scattering (TSRS) in large-aperture KDP crystalsfor third harmonic conversion. SRRS and TSRS will be amplified dramatically onceproduced, and cause great harms to the laser drivers, such as losing laser energy,degenerating beam quality, increasing divergence angle in far-field even damagingthe KDP crystal and so on, which will limit the overall output capability of the laserdriver, consequently frustrate the ICF ignition. Therefore, how to suppress the SRRSand the TSRS processes turn to be one of the key problems to be solved for the ICFlaser drivers.The nonlinear effects of SRRS and TSRS have been studied systematically inthis thesis, the relevant mechanism and variation laws have been clarified. Severalsuppression methods for SRRS and TSRS have been proposed according to theoptical structure and the laser parameters of the ICF laser driver. These studies willoffer theoretical and technical supports for evaluating the risk and avoiding thedamage of SRRS and TSRS, and designing the laser drivers. Meanwhile, the SRRSand the TSRS with broadband laser of PPM (Pure Phase Modulation), SSD(Smoothing by Spectral Dispersion) and LC (Linear Chirp) are the focus of thisstudy due to the rapid development of ICF driver towards broadband laser.Firstly, the numerical computing models of SRRS and TSRS for bothnarrowband and broadband are established based on SRS coupling wave equations.The theories of SRRS and TSRS are enriched and improved, according to thepresent design requirements and the future development trends towards broadbandof ICF laser drivers.Secondly, the SRRS in long air paths and the TSRS in large-aperture KDP ofnarrowband and broadband laser pulses have been studied. The results show that theSRRS and the TSRS have severely limited the laser pulses intensity and fluence incase of narrowband, which cannot satisfy the operation requirements of ICF drivers. The gains of SRRS and TSRS will be decreased if the bandwidth of laser bebroadened, which are mainly attributed to the damage of “phase locking” and (or)the decrease of laser fluence modulation degree in the SRRS and the TSRSprocesses. And the gains will decrease further with the increasing of modulation andchirp parameters of laser pulses, although it’s not always monotonously in somecases.Lastly, the SRRS and TSRS suppression methods of de-coherence, dichroic filmfiltering, polarization splitting and pulses stacking are proposed according to thecharacteristics such as coherence, spectrum, polarization, phase, and so on in SRRSand TSRS processes. The basic principles of these methods are described, and theirsuppression effects are analyzed and compared. The results show that the methodscan suppress the SRRS and (or) the TSRS dramatically, especially, the thresholdintensities of the laser pulses in air and (or) KDP crystals can be increased to threetimes larger than before and up to4GW/cm2by some methods, which totallysatisfied the design requirements of ICF driver at the present stage.