Abstract The two-dimensional kinetic simulations of the plume expansion process induced by irradiation of a copper target in argon background gas with nanosecond laser pulses in the doughnut beam mode are performed. The target is irradiated by single and double laser pulses, as well as by bursts consisting of up to 35 individual pulses. The simulations are based on a hybrid computational model that includes the direct simulation Monte Carlo method, where the ionization and absorption of laser radiation are considered. The numerical results are systematically compared with the results obtained for a Gaussian beam at the same pulse energy, peak fluence, and peak intensity. The structure of the plumes induced by a doughnut beam is found to drastically differ from the structure of the plumes induced by a Gaussian beam. The doughnut beam initially induces a toroidal shock wave. The self-interaction of this shock at the axis of symmetry results in the formation of multiple internal shock waves that strongly increase the plume temperature and density. The interaction between plumes induced by a double pulse in the doughnut beam mode occurs in two distinct regimes depending on the beam size and inter-pulse separation time. At a relatively small peak laser fluence, the effect of plasma shielding in the plumes induced by a burst of pulses in the doughnut beam mode is found to be smaller compared to the case of a Gaussian beam. At a relatively large fluence, the simulations reveal the opposite trend, but the amount of vaporized material is found to be always larger for a doughnut beam than for a Gaussian one. The obtained results suggest that the absorption of laser radiation in plasma plumes induced by nanosecond laser pulses can be tuned by changing the radial distribution of energy in the incident laser beam.

中文翻译：

---

环形光束模式下单个和多个纳秒激光脉冲照射铜靶引起的羽流中的膨胀动力学和辐射吸收

摘要 对在氩气背景气体中用纳秒激光脉冲在环形光束模式下辐照铜靶引起的羽流膨胀过程进行二维动力学模拟。目标由单激光脉冲和双激光脉冲以及由多达 35 个单独脉冲组成的脉冲串照射。模拟基于混合计算模型，包括直接模拟蒙特卡罗方法，其中考虑了激光辐射的电离和吸收。将数值结果与在相同脉冲能量、峰值注量和峰值强度下获得的高斯光束结果进行系统比较。发现由环形光束引起的羽流结构与由高斯光束引起的羽流结构截然不同。环形光束最初会引起环形冲击波。这种冲击在对称轴上的自相互作用导致形成多个内部冲击波，强烈增加羽流温度和密度。根据光束大小和脉冲间分离时间，在环形光束模式中由双脉冲引起的羽流之间的相互作用以两种不同的方式发生。在相对较小的峰值激光能量密度下，与高斯光束的情况相比，圆环光束模式中的脉冲突发引起的羽流中的等离子体屏蔽效应更小。在相对较大的注量下，模拟显示出相反的趋势，但发现圆环光束的汽化材料量总是大于高斯光束。