激光尾波场驱动准连续小角度电子束研究进展

Generation of ultrafast broadband small angle hundreds Me V electron bunches from laser wakefield acceleration

报道了在北京大学新建成的5 Hz 200 TW飞秒激光加速器实验装置上利用68 TW(1.7 J,25 fs)的激光与混合气体(99%He掺杂1%N_2)进行激光电子加速的初步实验结果与理论分析.在实验中观测到了最大截止能量为290 MeV的连续电子能谱,并且最大输出能量在一定的聚焦范围内基本不变.二维particle-in-cell模拟表明:电离注入导致电子不断注入,使得纵向相空间在激光传播几个毫米后基本被电子填满;之后相空间中电子分布基本保持稳定,随着激光传播距离的增加,输出电子最大能量几乎不变,这与实验观察到的最大输出能量随激光聚焦位置在一定范围内不变的现象一致.实验与模拟结果揭示了在当前实验条件下连续电离注入对电子束品质的影响,为今后进一步优化电离注入电子品质提供了依据.

Electrons can be accelerated to a GeV level in centimeters by plasma wakefield driven by laser. With the develop- ment of chirped pulse amplification technique, the accelerating field can reach 100 GV/m. The laser driven wakefield acceleration experiments with ionization injection are carried out using 68 TW （1.7 J, 25 fs） laser and a mixture gas of 99~ He and 1% N2. In experiment, the output electron beam has broadband spectrum with a maximum cut-off energy of about 290 MeV and a maximum output energy is quite stable in a certain range of laser focal positions. Two-dimensional particle-in-cell simulation is carried out. It is found that the longitudinal phase space is occupied by the continuously injected electrons and the phase space distribution is quite stable after the laser has propagated several millimeters inside plasma. This acceleration process can lead to quite stable maximum output energy of electron beam. These experiments reveal the physical nature of continuous ionization injection, which is very important for improving the performance of ionization injection.