Pulsed laser ablation of pyrolytic graphite with a 4-J / cm2 KrF laser in backgrounds of air, argon, nitrogen, and helium at pressures up to 10 Torr was performed to study the plume dynamics. Optical emission imaging with a 2-ns gated intensified charged-coupled device camera was used to determine shock front positions and plume trajectories for characterization by free expansion, Sedov–Taylor (ST) blast, and drag models. The plume expands with initial Mach numbers of M = 55, decreasing to M ∼ 20 as the emission becomes too weak to detect. The plumes begin with a planar shock front and thickness of a few mean free paths, but evolve to higher dimensionality, n, depending on pressure and mass of the background gas. The ST energy released in the sudden ablation is typically 33% the laser pulse energy. Blast energy and plume dimensionality are correlated with stopping distances, which are typically greater than 103 mean free paths. Upper bounds for the mass ablated (1 to 2 μg / pulse) and hole depth (∼88 to 183 nm) are inferred from the shock kinetic energy relative to laser pulse energy. The inferred hole depth ranges from 0.32 to 0.67 percent of the thermal diffusion length as the pressure increases from 1 to 10 Torr for these conditions where the fluence is just above twice the ablation threshold.

中文翻译：

---

脉冲激光烧蚀石墨过程中的冲击锋面行为

在空气，氩气，氮气和氦气的背景下，在高达10 Torr的压力下，用4-J / cm2 KrF激光对热解石墨进行脉冲激光烧蚀，以研究羽流动力学。使用2纳秒选通的增强型电荷耦合器件照相机进行的光发射成像可用于确定冲击波的前部位置和羽流轨迹，以通过自由扩展，Sedov-Taylor（ST）爆炸和阻力模型进行表征。羽流以初始Mach数M = 55膨胀，随着发射变得太弱而无法检测，减小到M〜20。羽流以平面冲击波前开始，并具有一些平均自由程的厚度，但根据背景气体的压力和质量，逐渐发展为更高的维数n。突然消融中释放的ST能量通常为激光脉冲能量的33％。爆炸能量和羽流尺寸与停止距离相关，该距离通常大于103个平均自由程。从相对于激光脉冲能量的冲击动能推断出烧蚀质量（1至2μg/脉冲）和孔深度（约88至183 nm）的上限。在这些通量刚好大于消融阈值两倍的情况下，当压力从1Torr增加到10Torr时，推断出的孔深度为热扩散长度的0.32％至0.67％。