A Study On Uniform Irradiation On Target With High-power Laser

In the sophisticated physical experimental researches, such as inertial confinement fusion (ICF), there are the harsh demands for the irradiation uniformity on the target surface with high-power laser. Achieving uniform irradiation of laser intensity on the target is a critical factor in experiments and also the forefront of current issues in the field, which has important academic value and practical significance. In this thesis, array uniform irradiation optical systems are studied, and the following main results are listed:1.An orthogonal cylindrical lens array (OCLA) optical system is presented for uniform irradiation on target surface.Based on the paraxial matrix optics and scalar diffraction integral theory, we put forward the designing principle of the OCLA system and the analytical expression of the intensity distribution; simultaneously, the OCLA system both in far field and in semi-far field has also been considered. Through numerical simulation, the smoothing characteristics and optimized designing for this system are obtained in details.The elimination of large-scale non-uniformity with the defocused irradiation and diffraction-weakened methods are proposed.2.Two kinds of point-focusing array systems with controllable focal spot are proposed:(a) lens arrays optical system with vari-focal-spot, and (b) orthogonal cylindrical lens arrays (OCLA) system with controllable focus length. Based on the paraxial matrix optics and generalized diffraction theory, the principles of the two vari-focus array optical systems, and the optimization of these optical systems parameters are analyzed in details. The smoothing performances of the two vari-focal array systems are confirmed by the numerical simulation and the theoretical calculation with Zemax optics design software.3. The variation in small-scale non-uniformity (SSINU) of intensity in crossed segmented wedge array (CSWA) focus system is studied theoretically and experimentally. Based on the physical optics, the separation between the major interference maxima and secondary maximum of interference fringes for the CSWA focus system are derived firstly. The variation of the SSINU has been confirmed by the numerical calculations on the basis of the Collins formula and ABCD propagation matrix theory.From the theory and experiment, we have studied the two methods of reducing the SSRNU, named after the deviation of wedge angles (DWA) and the off-focal laser radiation on target (OFLRT), and results demonstrated that they have a quite good agreement. The DWA, the OFLRT, and their optimized parameters are presented.4. Suppression of small-scale non-uniformity is realized, using the combination of the orthogonal cylindrical lens array and polarization control plate.The smoothing performance of the scheme is also analyzed by the detailed three-dimensional simulations with GLAD physical optics software. Moving the target slightly away from the exact focal plane of the principal focusing lens, a well-irradiating laser spot with small non-uniformity and great energy efficiency can be obtained accordingly.5.Based on propagation law of partially coherent light (PCL) and generalized diffraction theory, a PCL model is established, in which a quasi-monochromatic spatially partial coherent light is described by Gaussian Schell-Model (GSM) beams through the lens array system. Detailed three-dimensional numerical calculations show that the partial coherent light is advantageous in smoothening effects and reducing the small-scale non-uniformity over the complete coherent light. Moving the target slightly from the exact focal plane of the principal focus, and taking advantage of the effect of lateral thermal conduction smoothing, the high spatial-frequency intensity fluctuation can be further eliminated, which indicates the best possible irradiation uniformity and energy efficiency on the target.