微波瑞利散射法测定空气电火花激波等离子体射流的时变电子密度

大气压空气电火花激波等离子体射流的电子密度在亚微秒时间尺度上瞬变,其电子密度的测定很难.基于微波瑞利散射原理,本文测量了空气电火花冲击波流注放电等离子体射流的时变电子密度.实验结果表明:测量系统的标定参数A为1.04 × 105 V·Ω·m–2;空气流注放电等离子体射流的电子密度与等离子体射流的半径和长度有关,结合高速放电影像展示的等离子体射流的等效半径和等效长度,测定的电子密度在1020 m–3的量级,且随时间先快速增长至峰值再成指数衰减.此外,本文还探讨了等离子体射流的不同等效尺度对测定结果的影响;分析结果表明,采用时变等效半径和时变等效长度的计算结果最有效,且第1个快速波峰是由光电离的电离波导致的.

Measurement of time-varying electron density of air spark shock wave plasma jet by the method of microwave Rayleigh scattering

It is difficult in measuring the electron density of an atmospheric air spark shock wave plasma jet,due to its variation on the time scale of sub-microseconds.In this paper,the time-varying electron density of air spark shock wave plasma jet is measured,based on the principle of microwave Rayleigh scattering.The system constant A is determined by using calibration of materials with known properties;the results show that the system constant is obtained as A=1.04×105 V·Ω·m-2.According to the principle of microwave Rayleigh scattering,the electron density of the plasma jet is related to its radius and length of the plasma jet plume.Combined with the discharge image captured by ICCD camera,it is observed that the plasma jet plumes are with irregular patterns.In order to facilitate the calculation,the plasma jet plumes are replaced by cylinders with the same volume as the original shapes.Thus,the equivalent radius and length of the plasma jet plume are obtained.According to the known data,the electron density is determined to be in the order of 1020 m-3;its value increases rapidly to the peak value,and after then exponential attenuates along with time.In addition,the effect of different equivalent dimensions of the plasma jet plume on the measurement results is also discussed.It is shown that the calculation result with the time-varying equivalent radius and the time-varying equivalent length is the most effective one.In addition,the first fast peak is caused by the ionization wave of the photo ionization.The actual ionization process is that the air discharge in the cathode cavity releases a large number of high energy photons,which pass through the cathode nozzle and project into the region outside the nozzle;and then the 02 molecule in the ambient air are ionized by those high energy photons to form the plasma jet plume at the time of 1μs.