微波大气击穿阈值的理论研究

Theoretical study on the microwave air breakdown threshold

本文通过对使用有效场强(或均方根场强)得到的微波大气击穿阈值表达式进行讨论,指出其推导中所做的假设及这些假设应用到微波大气击穿过程中存在的问题.然后分别使用解析理论和数值模拟对微波大气击穿过程中的有效电子温度变化过程和击穿阈值进行研究,并将其与直流电场进行比较.分析发现在高气压下,电子能量转移频率高,有效电子温度随电场大幅振荡,由于电离频率随有效电子温度的增长率大于电子能量损失随有效电子温度的增长率,因此在高气压时,微波大气击穿阈值低于使用有效场强的击穿阈值.通过大量分析,给出了理论推导和数值模拟得到的微波大气击穿阈值拟合表达式.

By analyzing the breakdown threshold obtained from effective magnitude or RMS （root mean square） of microwave field, it is pointed out that the assumption of threshold is not suitable for microwave air breakdown. Variations of effective electron temperature and breakdown threshold in microwave fields, which are studied theoretically and numerically by fluid-based plasma equations coupled with the Maxwell equations, are compared with those in static fields. It is found that the effective electron temperature varies greatly with the microwave field at high pressures （electron energy transfer frequency is much larger than microwave frequency） due to its high energy transfer frequency. This causes the microwave air breakdown threshold to be smaller than that obtained from the RMS of field at high pressures because the ionization frequency varies faster than electron energy loss at an effective electron temperature. On the other hand, the effective electron temperature variation with the microwave field is insignificant at low pressures （electron energy transfer frequency is much smaller than microwave frequency） due to the negligible electron energy loss in a microwave period. In this case, the microwave air breakdown threshold is approximately equal to that obtained from the RMS one of the field. The fit formula of microwave air breakdown threshold is obtained by numerical and theoretical analysis.