高功率微波窗内外表面闪络击穿流体模拟研究

Numerical investigation on high power microwave flashover and breakdown on inner and outer surface of output-window by EM-fluid simulation

建立理论模型,将电磁场时域有限差分方法与等离子体流体模型结合,编制一维电磁场与等离子流体耦合程序,数值研究了3 GHz高功率微波窗内外表面闪络击穿的不同物理过程.研究结果表明:外表面闪络击穿中,输出微波脉宽缩短(未完全截止),窗体前均方根场强呈驻波分布,波节与波腹位置不变,窗体外表面形成有一层高密(约10^(21)·m^(-3)量级)极薄(约mm量级)等离子体(扩散缓慢),入射波可部分透过该薄层等离子体,脉宽缩短主要源于等离子体吸收效应;降低初始等离子体密度、厚度、入射波场强及缩短入射波脉宽等方式,可不同程度地改善输出脉宽缩短效应.内表面闪络击穿中,窗体前均方根场强亦出现驻波分布f但波节与波腹位置随时间变化),等离子体向波源方向运动;强释气下,输出脉宽缩短(未完全截止),形成多丝状高密(约10^(21)·m^(-3)量级)极薄(约mm量级)等离子体区域(扩散缓慢),间距1/4微波波长,脉宽缩短主要源于等离子体吸收效应;弱释气、低场强下,脉宽缩短有所改善(但最终截止),形成多带状致密(约10^(18)·m^(-3)量级)略厚(mm-cm量级)等离子体区域(扩散较快),间距1/4波长,脉宽缩短主要源于等离子体吸收效应;弱释气、高场强下,脉宽缩短严重(很快截止),形成块状高密(约10^(21)·m^(-3)量级)较厚(约cm量级)等离子体区域(扩散迅速),脉宽缩短主要源于等离子体反射效应.

In this paper, an electromagnetic-field FDTD method coupled with plasma fluid model is put forward to investigate the different physical phenomena of high power microwave （HPM） flashover and breakdown on inner and outer surface of output-window. Based on the above theoretical models, a one-dimensional （1D） electromagnetic field and plasma interaction code is programmed by authors. By using the code, the HPM flashover and breakdown on inner and outer surface of output-window are simulated. The numerical results could be concluded as follows. For flashover and breakdown on outer surface, output microwave pulse is shortened without cut-off; there is a standing-wave distribution of electric field RMS （Root-Mean-Square） value before the window with fixed-positions of wave nodes and antinodes;there is a ultra-high-density （～ 1021 m-3） and ultra-thin （～mm） plasma shell with slow diffusion, microwave could penetrate the plasma-shell partly; the shortening of output microwave is caused by plasma absorption mostly. The output pulse of microwave could be lengthened by reducing the initial density or depth of plasmas; the other way is to shorten incident microwave pulse or reduce the value of incident microwave power. For flashover and breakdown on inner surface, there is also a standing-wave distribution of electric field RMS value before the window but the positions of wave nodes and antinodes vary with time; the plasma region moves toward the microwave source; with strong-outgassing, output microwave pulse is shortened without cut-off, there are“thread-like”ultra-high-density （～10^21 m^-3） and ultra-thin （～mm） plasma regions with slow diffusion, the distance between two “thread-like” regions is about a quarter of microwave wavelength, the shortening of output microwave is caused by plasma absorption mostly;with weak-outgassing and low electric field value, the output pulse of microwave is lengthened but cut-off finally, there are “belt-like” high-density （～10^18 m^-3） and thin （mm-cm） plasma regions with fast diffusion, the distance between two“belt-like”region is about a quarter of microwave wavelength, the shortening of output microwave is caused by plasma absorption mostly;with weak-outgassing and high electric field value, output pulse of microwave is cut-off quickly,“block-like”diffuse ultra-high-density （～ 10^21 m^-3） and deep （～cm） plasma regions are formed with very fast diffusion, and the shortening of output microwave is caused by plasma reflection mostly.