Electron density and gas density measurements in a millimeter-wave discharge

Electron density and neutral gas density have been measured in a non-equilibrium air breakdown plasma using optical emission spectroscopy and two-dimensional laser interferometry, respectively. A plasma was created with a focused high frequency microwave beam in air. Experiments were run with 110 GHz and 124.5 GHz microwaves at powers up to 1.2 MW. Microwave pulses were 3 microsec. long at 110 GHz and 2.2 microsec. long at 124.5 GHz. Electron density was measured over a pressure range of 25 to 700 Torr as the input microwave power was varied. Electron density was found to be close to the critical density over the pressure range studied and to vary weakly with input power. Neutral gas density was measured over a pressure range from 150 to 750 Torr at power levels high above the threshold for initiating breakdown. The two-dimensional structure of the neutral gas density was resolved. Intense, localized heating was found to occur hundreds of nanoseconds after visible plasma formed. This heating led to neutral gas density reductions of greater than 80% where peak plasma densities occurred. Spatial and temporal structure of gas heating at atmospheric pressure were found to agree well with numerical simulations.

本研究分别采用光学发射光谱法与二维激光干涉法，对非平衡空气击穿等离子体中的电子密度与中性气体密度进行了测量。实验中通过聚焦高频微波束在空气中激发产生等离子体，所用微波的频率分别为110 GHz与124.5 GHz，最大入射功率达1.2 MW；其中110 GHz微波脉冲时长为3微秒，124.5 GHz微波脉冲时长为2.2微秒。在入射微波功率可变的条件下，我们于25至700 Torr的压力范围内测量了电子密度，发现在所研究的压力区间内，电子密度接近临界密度，且随入射功率的变化幅度极小。在远高于击穿阈值的功率条件下，我们于150至750 Torr的压力范围内测量了中性气体密度，并解析得到了中性气体密度的二维分布结构。研究发现，在可见等离子体形成数百纳秒后，会发生强烈的局域加热现象；该加热过程会在等离子体峰值密度所在区域造成中性气体密度降低80%以上。在大气压条件下，气体加热的时空分布结构与数值模拟结果吻合良好。