Plasma Evolution of Gas Breakdown in a Metal-Insulatormetal Gap

This paper is focused on plasma evolution of a metal-insulator-metal gap (MIM), which consists of two metallic electrodes and one alumina ceramic insulator, involving both gas discharge and insulator flashover. The MIM gap is characterized by the ratio $\theta$ of insulator flash-over length and total gap distance. Anode is grounded and cathode is applied with negative $\sim \mathrm{kV}$ pulse. Plasma evolution here is studied both by high speed observation experiments and PIC-MCC (particle-in-cell coupling with Monte Carlo) numerical simulation ${ }^{2}$. In the first part, observation by high speed camera indicates that there exist two modes of plasma evolution in the MIM gap: total gas breakdown (mode A) and gas breakdown with partial surface flash-over (mode B). When $\theta \leq 0.3$, the breakdown is dominated by mode A in continuous 20 shots. When $\theta \geq 0.49$, the breakdown is dominated by mode B. There exists a critical range $(0.3\lt \theta\lt 0.49)$ where two modes occur alternatively, and even meanwhile at some moment. To illustrate more details of plasma evolution by both modes in the MIM gap, in the second part a two-dimensional numerical model is built and solved by PIC-MCC method. Simulation results indicate that electrons are emitted from the cathode surface and the local electric field would get more enhanced near the edge of cathode. When the ratio $\theta$ is small (0.1), plasma evolution propagates along the electric field through collision ionization, just as the common gas breakdown (mode A). If the ratio $\theta$ is relatively large (such as 0.9), even though the electrons emitted from the middle of cathode is less than that from the edge, the strong ability of secondary electron emission on the ceramic surface would cover the margin and the electron avalanche propagates along the ceramic surface rapidly, leading the breakdown of mode B.