论文标题
观察雪崩电离中的故障机制产生的大气等离子体由Picsecond Co $ _2 $激光产生
Observation of breakdown wave mechanism in avalanche ionization produced atmospheric plasma generated by a picosecond CO$_2$ laser
论文作者
论文摘要
了解超短，长波长IR（LWIR）脉冲产生的大气等离子体的形成和长时间的演变是一个重要但已部分理解的问题。特别有趣的是在空气中产生的等离子体，并具有峰值激光强度$ \ sim $ 10 $^{12} $ w/cm $^2 $，这是在LWIR大气指导实验中观察到的所谓夹紧强度，其中隧道和多个photon离子化功能在近乎近乎ir或ShoreTer or Sorterter波长处是Inoperative。我们发现，当使用3 ps 10.6 $ $ m $ m $ m的激光脉冲时，使用了200 gw/cm $^2 $阈值，在气溶胶（灰尘）颗粒表面上的雪崩崩溃可以作用于在200 gw/cm $^2 $阈值上播种。当血浆密度接近临界密度并使前进的传播不可能时，崩溃首先出现在最佳焦点上，但向后传播焦点视频。向后传播分解的速度可以高达10 $^9 $ cm/s，比用NS脉冲产生的分解测得的数量级要大，可以通过所谓的分解波机制来很好地解释。紫外线光电离有助于以相似速度的横向等离子体扩展，并在大约10％的拍摄中观察到是次级纵向崩溃机制。当CM大小，TW电束传播时，可以保证气溶胶颗粒的截距，并且出现了几个（40 cm $^{ - 3} $）崩溃位点，每个位点出现，每个位点最初产生近乎临界的密度等离子体。在10 NS-1 $ $ $ S的时间表上，每个站点的冲击波径向扩展并聚集以产生大型热气通道。扩展的径向速度与为超快大气爆炸开发的爆炸波理论的预测非常吻合。
Understanding the formation and long-timescale evolution of atmospheric plasmas produced by ultrashort, long wavelength IR (LWIR) pulses is an important but partially understood problem. Of particular interest are plasmas produced in air with a peak laser intensity $\sim$10$^{12}$ W/cm$^2$, the so-called clamping intensity observed in LWIR atmospheric guiding experiments where tunneling and multi-photon ionization operative at near-IR or shorter wavelengths are inoperative. We find that avalanche breakdown on the surface of aerosol (dust) particles can act to seed the breakdown of air observed above the 200 GW/cm$^2$ threshold when a train of 3 ps 10.6 $μ$m laser pulses separated by 18 ps are used. The breakdown first appears at the best focus but propagates backwards towards the focusing optic as the plasma density approaches critical density and makes forward propagation impossible. The velocity of the backward propagating breakdown can be as high as 10$^9$ cm/s, an order of magnitude greater than measured with ns pulse-produced breakdown and can be explained rather well by the so-called breakdown wave mechanism. Transverse plasma expansion with a similar velocity is assisted by UV photoionization and is observed as a secondary longitudinal breakdown mechanism in roughly 10 percent of the shots. When a cm size, TW power beam is propagated, interception of aerosol particles is guaranteed and several (40 cm$^{-3}$) breakdown sites appear, each initially producing a near critical density plasma. On a 10 ns-1 $μ$s timescale, shockwaves from each site expand radially and coalesce to produce a large hot gas channel. The radial velocity of the expansion agrees well with the prediction of the blast wave theory developed for ultrafast atmospheric detonations.
