诱导空间非相干束匀滑技术的近区特性及改善技术

诱导空间非相干技术是面向激光驱动惯性约束核聚变的一种具有自身独特优势的束匀滑方法.然而直接使用诱导空间非相干方法将引起强烈的近区强度空间调制,这将威胁装置的运行安全,并严重限制装置的最大输出能力.这也是该方法应用于聚变级高功率激光装置的主要技术障碍之一.本文介绍了一种通过双透镜滤波系统对诱导空间非相干束匀滑技术导致的近区空间强度调制进行匀滑的技术.利用该技术可以在保留诱导空间非相干束匀滑方法的先天优势(更好的远区匀滑特性)的前提下,获得均匀、稳定的近区强度分布,从而避免高功率激光系统在使用诱导空间非相干束匀滑技术时,因为近区强度不均匀、不稳定导致的器件损伤及输出能力受限.在理论建模和数值分析的基础上,以近区调制度、软化因子和透过率为主要评价指标,对比分析了方形、圆形、高斯型等3种滤波孔在不同尺寸下的近区输出效果,最终给出了一种典型的优化结果:16×16诱导空间非相干分割数、0.8倍衍射极限宽度、方形小孔.此时近区强度分布均匀,同时保证了较好的远区匀滑效果和高的能量利用率.在此基础上,针对装置的实际应用情况,进一步分析了准直误差对近区强度分布的影响,结果表明准直误差小于0.1倍衍射极限便不会影响输出的近区质量.对诱导空间非相干束匀滑方法所得焦斑的模拟分析表明,滤波系统的加入能进一步改善焦斑的低频不均匀性.

Near-field character and improvement technology of induced spatial incoherence

Induced spatial incoherence technology is a beam-smoothing method with its own unique advantages for laser driven inertial confinement fusion. However, simply using the induced spatial incoherent method will induce a strong near-field intensity spatial modulation, which will threaten the safety of the operation and severely limit the maximum output capability of the device. This is also one of the main technical obstacles to applying induced spatial incoherence to a high-power laser device used for fusion. In this paper, a technique of smoothing the near-field spatial intensity modulation caused by induced spatial incoherence is introduced. By using a two-lens filter system, a homogeneous and stable near-field intensity distribution can be obtained on the premise of reserving the innate advantages of induced spatial incoherence (better far-field smoothing characteristics), thereby avoiding the damage to devices and limitation to output capacity in high power laser system using induced spatial incoherence. Based on the theoretical modeling and numerical analysis, using modulation degree, softening factor, and transmittance as evaluation parameters, the near-field light characters with three kinds of filter apertures, such as square, round, and Gaussian, are compared and analyzed. Finally, in a typical optimization result there are used 16×16 induced spatial incoherent divisions and a square aperture with 0.8 times diffraction limit width. In this case, the near-field intensity distribution is uniform, and at the same time, good smoothing effect on far-field and a high energy utilization rate are ensured. On this basis, according to the actual application of the device, the influence of the collimation error on the near-field intensity distribution is further analyzed. The results show that as long as the collimation error is less than 0.1 times the diffraction limit, the near-field quality will not be affected. The simulation analysis of the focal spot obtained by induced spatial incoherence shows that the addition of the filtering system can further improve the low frequency uniformity of the focal spot.