Kline J L, Montgomery D S, Flippo K A, Johnson R P, Rose H A, Shimada T, Williams E A
Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA.
Rev Sci Instrum. 2008 Oct;79(10):10F551. doi: 10.1063/1.2955927.
A short pulse (few picoseconds) laser probe provides high temporal resolution measurements to elucidate details of fast dynamic phenomena not observable with typical longer laser pulse probes and gated diagnostics. Such a short pulse laser probe (SPLP) has been used to measure filamentation of a random phase plate (RPP) smoothed laser beam in a gas-jet plasma. The plasma index of refraction due to driven density and temperature fluctuations by the RPP beam perturbs the phase front of a SPLP propagating at a 90 degree angle with respect to the RPP interaction beam. The density and temperature fluctuations are quasistatic on the time scale of the SPLP (approximately 2 ps). The transmitted near-field intensity distribution from the SPLP provides a measure of the phase front perturbation. At low plasma densities, the transmitted intensity pattern is asymmetric with striations across the entire probe beam in the direction of the RPP smoothed beam. As the plasma density increases, the striations break up into smaller sizes along the direction of the RPP beam propagation. The breakup of the intensity pattern is consistent with self-focusing of the RPP smoothed interaction beam. Simulations of the experiment using the wave propagation code, PF3D, are in qualitative agreement demonstrating that the asymmetric striations can be attributed to the RPP driven density fluctuations. Quantification of the beam breakup measured by the transmitted SPLP could lead to a new method for measuring self-focusing of lasers in underdense plasmas.
短脉冲（几皮秒）激光探针可提供高时间分辨率测量，以阐明快速动态现象的细节，而这些细节是典型的较长激光脉冲探针和选通诊断无法观测到的。这样的短脉冲激光探针（SPLP）已被用于测量气体喷射等离子体中随机相位板（RPP）平滑激光束的丝状化。由RPP光束驱动的密度和温度波动引起的等离子体折射率会扰动相对于RPP相互作用光束以90度角传播的SPLP的波前。密度和温度波动在SPLP的时间尺度（约2皮秒）上是准静态的。SPLP的透射近场强度分布提供了波前扰动的一种度量。在低等离子体密度下，透射强度图案是不对称的，在RPP平滑光束方向上的整个探测光束上有条纹。随着等离子体密度增加，条纹沿RPP光束传播方向分解为更小的尺寸。强度图案的分解与RPP平滑相互作用光束的自聚焦一致。使用波传播代码PF3D对该实验进行的模拟在定性上是一致的，表明不对称条纹可归因于RPP驱动的密度波动。通过透射SPLP测量光束分解的量化可能会导致一种测量低密度等离子体中激光自聚焦的新方法。
