介质表面附近微波大气击穿的理论研究

将麦克斯韦方程组和简化等离子体方程耦合求解, 对介质表面附近大气击穿形成等离子体的过程进行了理论研究. 分别使用一维、二维模型对等离子体的形成过程及等离子体对电磁波的反射、吸收过程进行了模拟研究. 一维计算结果发现在n_e = 0, j = 0两种边界条件下, 虽然形成的等离子体密度分布相差较大, 但二者得到的微波反射、吸收、透射波形彼此相差不大. 初始电子数密度厚度为20 mm的条件下, 得到界面附近的等离子体密度大于5 mm厚度的情况. 二维计算结果发现, 由于TE10模在波导中心位置处的微波电场最强, 电子碰撞电离首先在中心位置处形成等离子体, 当等离子体密度达到一定值(临界密度附近)时, 波导中心介质表面处微波场强减小, 等离子体区域沿着介质表面向两侧移动. TE10模在波导边缘处微波电场强度小于击穿阈值, 因此等离子体区域不可能移动到波导边缘附近.

Theoretical study of the microwave air breakdown at dielectric surface

Microwave air breakdown at dielectric surface is investigated by numerically solving the fluid-based plasma equations coupled with the Maxwell equations. The plasma formation and microwave scattering and absorption by plasma are investigated by one-dimensional (1D) and two-dimensional (2D) models. In the 1D model, it is found that at the initial stage of microwave breakdown, the plasma develops in the whole plasma region. As time increases, the plasma in the upstream grows much faster than in the downstream. Although the electron density distributions for n_e = 0 and j = 0 are different, the microwave reflection, absorption and transmission are almost the same. It is found that the electron number density in the upstream region for 20 mm is larger than for 5 mm. In the 2D model, it is found for TE10 mode that the plasmoid first grows in the middle of waveguide until its density becomes large enough to diffract the incident field, then the plasma region moves along the surface to both sides. The plasma region cannot reach the wall of waveguide, where the electric field is smaller than the breakdown threshold. After comparison between the computational and experimental results, it is found that the simulated absorbed power is larger than the measured one, and the transmitted power is smaller than than measured one. The reason is that the initial electron densities in 1D and 2D simulation are both assumed to cover the whole dielectric surface, but the plasma in experiment develops in a very small region.