Research On Precise Control Technologies Of Intensity Distribution On Target Of High Power Laser Facilities

In the research of Inertial Confinement Fusion (ICF), precise physical experiments place strict requirements on target illumination uniformity. While, wave front aberrations in high-power laser facilities for ICF produce nonuniformities in the intensity distribution of focal spot which can significantly degrade the coupling of energy into a fusion target, driving various plasma instabilities. Therefore, laser-driven ignition requires a strategy to precisely control the intensity distribution on target and vary the interference pattern on a time scale that is short compared to the characteristic hydrodynamic response time of target. Besides, in the research of laser drivers, the propagation, amplification and control of laser beam has also been the research focus of high-power laser physics and technologies. The dissertation mainly focused on the control strategy over target uniform illumination and the influence of beam smoothing technologies on laser facilities.The main research works and conclusions are as follows:1. The propagation theory of smoothing by spectral dispersion (SSD) was analyzed. Generation of frequency modulation to amplitude modulation (FM-to-AM) conversion effect when using SSD was discussed in detail. Restraining ways were proposed. The research founds theoretical basis for the research and the development of beam smoothing technologies.2. As traditional SSD using sinusoidal phase modulation has the shortcoming of nonuniformly distributed side bands and smoothing only specific spatial frequencies in the far field, a new SSD route using linear modulation was proposed. The spectrum of the stacked chirped pulse is a frequency comb under Gaussian envelope. The smoothing time with one color cycle is short and mainly determined by the pulse width of the seed pulse and grating parameters. The focal spot is distributed in Gaussian envelope, in accordance with the spectrum. The difficulty of the technique is the generation of periodic linear modulation. Here, periodic linear modulation was realized by stacking a series of chirped pulses in fiber delay lines. Smoother focal spot was obtained using linear modulation, compared with sinusoidal modulation.3. To solve the problem of one-dimensional (1-D) smoothing using linear grating, star grating (or spiral grating) dispersion was proposed. Simulation results indicate that star grating has the best smoothing effect compared with linear grating and circular grating. Insertion of star grating could realize color variation across the beam in the azimuthal direction and make the far-field pattern rotate with time. Therefore, the time-integrated focal spot is smoothed in a more comprehensive way. This smoothing method bears the advantage of simple configuration, two-dimensional beam smoothing and not enlarging the far field too much.4. Aimed at indirect drive laser fusion, key techniques of using SSD on high-power laser facilities were investigated, including generation of frequency-modulated pulse, optimization of SSD parameters, suppression of FM-to-AM conversion, optimization of spatial filter pinholes after using SSD and evaluation of the smoothed focal spot. The research founds solid basis for the application of SSD on high-power laser facilities.5. Using SSD with critical dispersion (Nc=l) and CPP machined by Magnetorheological Finishing Techniques (MFT), integrated experiments were carried out on high-power laser facilities. The contrast of the focal spot with95%energy included with CPP dropped to0.72compared with1.71without CPP, and further improved to0.47when using SSD and CPP in the same time. The experiments solve some key technical problems in implementing SSD and CPP on high-power laser facilities, and provide a flexible platform for laser-plasma interaction experiments. Using SSD and CPP, some laser plasma interaction experiments have been done. The plane target experiments indicate the X-ray distribution has been greatly improved after using SSD and CPP. Hohlraum experiments indicate backscattered light from the laser entrance hole dropped to below5%after using SSD and CPP, compared with30%-40%without SSD and CPP. To obtain smoother focal spot and suppress transverse SBS, Multi-FM SSD was studied in high fluence combined with CPP. Results indicate intermediate and high frequency modulations on the focal spot are smoothed in a more comprehensive way compared with single modulation frequency SSD.6. Aimed at suppression of laser-plasma instabilities in indirect drive laser fusion, several polarization smoothing techniques were studied, including birefringent wedge, ditributed polarization rotators and cylindrical vector beams. Considering future laser drivers with hundreds of laser beams, quad beam smoothing was studied. Simulation results indicate through adjusting dispersion directions of1-D SSD beams in a quad, two-dimensional beam smoothing could be obtained. A square waveform pulse generation method for high-power laser facilities was studied, which bear the advantages of fast rise edge and easy to be reshaped precisely, providing a new way to suppress FM-to-AM effect in the source.Highlights of the dissertation are as follows:1. A new SSD route using linear modulation was proposed. Better beam smoothing effect of the linear modulation compared with sinusoidal modulation was experimentally demonstrated. Compared with traditional SSD, without phase modulator, optical pulse shaping, and precise synchronization between different types of pulses in the front end of a multifunctional laser driver are the advantages.2. Using SSD and CPP, integrated experiments were carried out on high-power laser facilities. Some key technical problems were solved. Using SSD and CPP, laser plasma interaction characteristics were studied. Experiments indicate backscattered light from the laser entrance hole dropped notablely after using SSD and CPP.3. To solve the problem of one-dimensional (1-D) smoothing using linear grating, star grating (or spiral grating) dispersion was proposed. Simulation results indicate insertion of star grating could realize color variation across the beam in the azimuthal direction and make the far-fie Id pattern rotate with time. Therefore, the time-integrated focal spot could be smoothed in a more comprehensive way.4. Quad beam smoothing was proposed and studied for laser drivers with hundreds of laser beams. Simulation results indicate through adjusting dispersion directions of the1-D SSD beams in a quad, two-dimensional beam smoothing could be obtained.