利用激光离焦的方法优化超强激光驱动的质子加速

Enhancement of ion generation in low-contrast laser-foil interactions by defocusing

在中国科学院物理研究所"极光Ⅱ号"飞秒激光装置上,对激光与薄膜靶相互作用产生的靶后质子束特性进行了实验研究.结果发现,在主脉冲前存在较强的飞秒预脉冲的情况下,通过适当地对激光束散焦,可以使质子束的转换效率提高3个数量级,并同时改善质子束的准直性.分析表明,激光散焦的方法可以有效地抑制预脉冲的流体力学效应对质子加速的负面影响,从而提高质子的转换效率.此外,粒子模拟还发现,散焦量增大时可以产生更多的中低能超热电子,这也有利于建立高质量的质子加速电场.

The characteristics of proton beams generated from ultra-intense laser-foil interactions under varied focal offsets are experimentally studied on XL-Ⅱ laser facility.The experimental results show that when strong femtosecond prepulses are presented before the main pulse,the conversion efficiency from the laser energy to the protons is enhanced by 3 orders of magnitude with effective improvement of the beam collimation by properly defocusing the laser pulses.The depression of the prepulse effect on the proton acceleration is believed to be one of the main reasons for the significant optimization.Besides,the particle-in-cell（PIC） simulation results indicate that the population of moderate energy electrons increases under large focal offsets,which in turn effectively improves the acceleration electric field and the beam quality.