High pressure rf cavities in magnetic fields

The RF breakdown of dense hydrogen gas between metallic electrodes has been studied as part of a program to develop gas-filled RF cavities to be used for muon ionization cooling. A pressurized 805 MHz test cell was used at Fermilab to measure the increase in the breakdown threshold as a function of gas density, known as the Paschen effect. The breakdown behavior of copper, molybdenum, and beryllium electrodes as functions of hydrogen gas density are also reported. First results are reported of RF breakdown using molybdenum electrodes in dense hydrogen in a strong external magnetic field. Unlike evacuated cavities, the pressurized cavity shows no degradation of maximum stable gradient with increasing magnetic field