诱导空间非相干和消衍射透镜阵列联用方式束匀滑方案的腔壁辐照特性分析及优化

Analysis and Optimization of Irradiation Characteristics of Laser Quads on Hohlraum Wall Based on Broadband Laser Beams Smoothed Using Induced Spatial Incoherence and De-Diffraction Lens Array

针对惯性约束聚变(ICF)装置中的光路排布和柱形靶腔结构,建立了基于诱导空间非相干(ISI)和消衍射透镜阵列(DLA)联用的宽带激光束匀滑方案的靶腔内光传输模型,进而开展了腔壁辐照特性的分析和优化。在基于ISI与DLA联用的宽带激光束匀滑方案中,主透镜焦距与柱形靶腔参数的匹配至关重要,否则将对不同入射角度集束在腔壁处的光斑造成不同程度的破坏,导致腔壁光斑交叉重叠,从而严重破坏腔壁辐照的均匀性。本文通过分析主透镜焦距和DLA参数等对腔壁辐照特性的影响规律,开展了宽带激光束匀滑方案的参数设计及优化,以改善腔壁辐照的均匀性。结果表明:增大主透镜焦距,可有效保持不同入射角度集束在腔壁处光斑的包络,从而减轻腔壁光斑交叉重叠的现象;合理选取DLA的子透镜数目和子透镜长短轴比例,可以提高腔壁光斑的占空比,并有效减少束匀滑所需的时间;优化内环集束的入射角度,可以避免内环与外环集束在腔壁上交叉重叠。

Objective In the indirectly driven inertial confinement fusion facility,the high X-ray irradiation uniformity on the target pellet is the key to the success of the implosion.The uniformity of X-ray irradiation on the target pellet is primarily determined using the uniformity of laser irradiation on the hohlraum wall in the initial stage of laser irradiation on the hohlraum wall;the uniformity of irradiation on the hohlraum wall is determined using the symmetry of the distribution of laser spots on the hohlraum wall and the uniformity of the intensity of a single laser spot on the hohlraum,Therefore,it is necessary to analyze the irradiation uniformity of the laser quads on the hohlraum wall.Many researchers are currently studying the irradiation characteristics of the laser beam on the hohlraum wall,but the majority of them are discussing the beam smoothing scheme of the SSD,CPP,and PCP combination.Recently,the Shanghai Laser Plasma Research Institute proposed a broadband laser beam smoothing scheme based on induced spatial incoherence(ISI)and de-diffraction lens array(DLA).However,no research has been conducted on the irradiation characteristics of the laser beam on the hohlraum wall using this smoothing scheme.Therefore,this study analyzes and optimizes the irradiation characteristics of broadband laser quads on the hohlraum wall using DLA combined with ISI.Methods Aiming at laser quads configuration and cylindrical hohlraum structure in inertial confinement fusion(ICF)facility,the propagation model for the laser quads in the hohlraum based on broadband laser beam smoothed by ISI and de-DLA has been built up.The parameters of the broadband laser beam smoothing scheme have been optimized in this study via analyzing the influence of the focal length of the principal lens and DLA parameters on the irradiation characteristics on the hohlraum wall.Results and Discussions In the broadband laser beam smoothing scheme based on the combination of ISI and DLA,the mismatch between the focal length of the primary lens and the parameters of the hohlraum will deform the intensity envelope of laser spots on the hohlraum wall(Fig.4),resulting in laser spot overlap and irradiation nonuniformity(Fig.6).Increasing the focal length of the principal lens can effectively suppress the envelope distortion of laser spots on the hohlraum wall(Fig.7),thereby reducing the overlap of the laser spots and improving the irradiation uniformity on the hohlraum wall(Fig.12).Furthermore,by appropriately adjusting the number of DLA sub-lens and the long and short axes ratio of the sub-lens,the duty ratio of laser spots on the hohlraum wall increases,and irradiation uniformity of the laser quads on the hohlraum wall improves(Fig.14 and Table 2).Conclusions The propagation model for the laser quads in the hohlraum based on broadband laser beam smoothed using ISI and de-DLA has been built in this study,aiming at the optical path arrangement scheme and cylindrical target hohlraum structure of the NIF,and then the irradiation characteristics of the laser quads on the hohlraum wall have been analyzed and optimized.The results show that the mismatch between the focal length of the primary lens and parameters of the cylindrical target hohlraum will cause varying degrees of damage to the envelope of the laser spots on the hohlraum wall for beams with different incident angles,as a result of the laser spots overlapping on the hohlraum wall,the irradiation uniformity on the hohlraum wall greatly diminishes.The effect of the principal lens focal length,the number of DLA sub-lens,and the long and short axes ratio of the sub-lens on the intensity distribution of the laser quads on the hohlraum wall are discussed on this basis.The optimization of these parameters,such as the principal lens focal length,number of DLA sub・lens,the long and short axes ratio of the sublens,and incident angle of the laser quads,achieves the improvement of the irradiation uniformity on the hohlraum wall.The results show that increasing the focal length of the primary lens appropriately can effectively retain the envelope of the laser spots on the hohlraum wall for beams with different incident angles,minimizing the overlap of the laser spots on the hohlraum wall.Besides,the duty ratio of laser spots on the hohlraum wall increases,and the time required for beam smoothing reduces when the number of DLA sub-lens and the long and short axes ratio of the sub-lens are optimized.Furthermore,after optimizing the incident angle of the inner cone laser quads,the overlap of the inner and outer cones laser quads on the hohlraum wall is eliminated.