神光Ⅱ升级装置激光加速质子照相初步实验研究

摘要

在神光Ⅱ升级装置上开展了首轮激光加速质子对间接驱动快点火靶内爆过程的照相实验研究.通过激光与靶参数的优化,获得了能量高于18 MeV的质子束.通过静态客体的照相,获得了优于20 μm的高空间分辨网格图像,为开展时间分辨的啁啾质子照相奠定了基础.开展了质子动态照相实验,获得了内爆压缩晚期的质子照相图像.实验发现内爆区域质子照相图像存在大量排空现象.内爆压缩区域不足以阻挡如此大范围质子束,证明了其中存在电磁场使得质子向外排开.动态照相的质子能量较低,分析是ns激光打靶过程产生的X射线及等离子体对质子加速存在影响.后续实验中需要进一步优化靶的屏蔽设计.%A preliminary experimental study of the indirect-driven implosion by means of proton radiography has been carried out on the SGⅡ-U laser facility.In the experiment, the probing protons were generated by a picosecond laser irradiating on a thin Au target via the target normal sheath acceleration scheme.Maximum proton energy up to 18 MeV was achieved as a result of optimizing the laser and target parameters.In the static imaging of objects, the spatial resolution of proton radiography was demonstrated to be better than 20 μm.In the time-resolved imaging of the implosion, the proton radiographs of the capsule at various stages of compression were obtained.It is found that the probing protons were expelled from the center region, which, covered an area much larger than the size of the compressed capsule.The detailed analysis reveals that the protons are expelled by the surrounding electric-magnetic fields as well as by the compressed capsule itself.It is also found that the energy of probing protons decreased when the nanosecond laser was present in the experiment.This is because the X-ray, generated from the nanosecond laser irradiation on targets, would partially destroy the rear side of proton target, thus leading to the less efficient proton acceleration.To improve the proton energy, the shielding design of proton target should be further considered in the next experiments.