释气对介质沿面闪络击穿影响的粒子模拟

Particle-in-cell simulation on effect of outgassing on flashover and breakdown on dielectric surface in high-power microwave environment

为研究释气下的高功率微波介质沿面闪络击穿物理机制,首先建立了理论模型,包括:动力学方程、粒子模拟算法、次级电子发射、蒙特卡罗碰撞模型以及碰撞退吸附气体分子模型;其次,基于理论模型,编制了1D3V PIC-MCC程序,分别研究了弱退吸附、强退吸附以及释气分子运动速率对沿面闪络击穿的影响.研究结果表明:介质沿面闪络击穿本质是沉积功率的持续增加.弱退吸附下,次级电子倍增占优,随着退吸附系数的增加,碰撞电离效应对次级电子倍增有促进作用,主要表现为介质窗表面静电场、表面碰撞电子平均能量以及表面碰撞电子数目的增加,此处的表面碰撞电子主要是次级电子倍增形成的;释气分子运动速率高导致介质面附近气压下降,不利于次级电子与气体分子间碰撞电离过程形成.强退吸附下,气体碰撞电离效应占优,随着退吸附系数的增加,离子数增加速度表现为电离频率增加的指数增长形式,碰撞电离效应对次级电子倍增有抑制作用,主要表现为介质窗表面静电场为负、表面碰撞电子平均能量的降低,但是表面碰撞电子数目却得以增加,此处的表面碰撞电子主要是贴近介质面的气体碰撞电离形成的;释气分子运动速率高导致气体厚度增加,扩大了气体碰撞电离作用区域,有利于气体碰撞电离.

For investigating the mechanism of high power microwave flashover and breakdown on dielectric surface with outgassing, firstly, the theoretical modeling is put forward, including dynamic equations, particle-in-cell （PIC） method, secondary emission, Monte-Carlo collision （MCC） method and outgassing model. Secondly, based on the theoretical modeling, the 1D3V PIC-MCC code is programmed by authors. By using this code, the flashover and breakdown on dielectric surface with weak and strong outgassing course under different gas moving velocities are studied numerically. The numerical results are concluded in the following. The flashover and breakdown on dielectric surface are caused by continuous increase of deposited power. For weak outgassing, multipacting is dominant. As outgassing coefficient increases, multipacting is promoted by ionization collision. The typical phenomena are the increases of space-charge field, average energy of surface-collision electrons and the number of surface-collision electrons. Here, the surface-collision electrons are caused by multipacting mostly. With the increase of gas molecule velocity, ionization course is suppressed by gas pressure decreasing near to the dielectric surface. For strong outgassing, ionization collision is dominant. As outgassing coefficient increases, the number of ions increases exponentially with ionization frequency increasing, multipacting is suppressed by ionization collision. The typical phenomena are the negative value of space-charge field on dielectric surface, the decrease of average energy of surface-collision electrons, and the exponential increase of surface-collision electrons caused by ionization collision near to dielectric surface. Here, the surface-collision electrons are caused by ionization mostly. With the increase of gas molecule velocity, the depth of gas is enlarged, thereby promoting the ionization collision.