Joule-heating-supported plasma filamentation and branching during subcritical microwave irradiation

Breakdown physics induced by a subcritical microwave was numerically reproduced by using a two-dimensional effective diffusion model for plasma transport and combining it with Maxwell’s equations and a neutral gas dynamics equation. A discrete plasma structure was obtained when E0,rms/Ec≧0.69, where E0,rms is the root-mean-square of the incident electric field and Ec is the breakdown threshold, because an overcritical field that exceeded the breakdown threshold was formed in a region away from the bulk plasma by the wave reflection when the plasma reflectivity was increased by joule heating. However, a continuous plasma structure with a branching pattern was formed when E0,rms/Ec<0.69 because the enhanced electric-field region away from the bulk plasma never exceeded the breakdown threshold even when the plasma reflectivity increased. The propagation speed of the plasma front drastically decreased when E0,rms/Ec<0.69 because the plasma propagation was sustained by strong gas expansion, which required more time than wave-reflection and ionization processes.