KINETIC MODELING OF RF BREAKDOWN IN HIGH-PRESSURE GAS-FILLED CAVITIES ∗

Recent studies have shown that high field gradients can be achieved quickly in high-pressure gas-filled cavities without the need for long conditioning times, because the dense gas can dramatically reduce dark currents and multipacting. In this project we use this high pressure technique to suppress effects of residual vacuum and geometry found in evacuated cavities to isolate and study the role of the metallic surfaces in RF cavity breakdown as a function of operating frequency and surface preparation. A series of experiments at 805 MHz using hydrogen fill pressures up to 0.01 g/cm 3 of H2 have demonstrated high electric field gradients and scaling with the DC Paschen law limit, up to ∼30 MV/m, dependingon the choice of electrode material. At higher pressures, the breakdown characteristics deviate from the Paschen law scaling. Fully-kinetic 0D collisional particle-in-cell (PIC) simulations give breakdown characteristics inH2 andH2/SF6 mixturesin goodagreementwith the 805 MHz experimental results below this field stress threshold. At higher pressures the formation of streamers at operating parameters below the Paschen limit are examined using 2D simulations.