Advances in modeling microwave window breakdown

Summary form only given. Dielectric window breakdown remains a significant issue for high power microwave systems. Using a particle-in-cell (PIC) model with Monte Carlo collisions, we investigate the transition of dielectric window breakdown from a vacuum multipactor discharge to a collisional microwave discharge in a number of noble gases. At low pressure, the dominant mechanism of electron creation is the single-surface multipactor, and the mean energy of the electron population is hundreds of eV. As pressure increases to 10-50 torr, an intermediate regime is obtained in which electrons generated in volumetric ionization compete with the multipactor electrons generated at the dielectric window surface. In this regime, the mean energy declines significantly, and we observe two distinct electron populations: a surface population participating in the multipactor process, and a detached population shielded from the surface fields by ions. Approaching atmospheric pressure, volumetric ionization dominates, and the multipactor process is extinguished as the electron mean energy drops to a few eV. In this collisional regime, the nearly neutral discharge detaches from the dielectric window surface, and the surface charge and field that drives electrons into the window is also eliminated. The electron energy probability function changes from a bi- Maxwellian at low pressure to a higher order kinetic form at high pressure. The time to achieve breakdown is described across a broad range of pressures for Ne, Ar, and Xe, and a general analytic scaling law was deduced. Using the electron generation rates from the kinetic PIC model, we developed a global model with an enhanced electron energy distribution function form along with an energy equation which matches the kinetic results well across a broad range of pressure. We are now extending that model to air, a broader range of frequency and microwave power levels.