Suppression of Voltage Breakdown in High-Gradient RF Structures

Experimental results of a promising concept for raising the breakdown limit in accelerating structures by the use of semiconducting or insulating cavity coatings are presented. Extensive experimental measurements of various coatings on OFHC Cu electrodes in the dc regime show that electrical breakdown can be increased from a value of 40 MV/m for bare Copper to 115 MV/m for a specially-coated Copper electrode. TiN-coated electrodes at use in the Stanford Linear Accelerator Center (SLAC) were measured to undergo breakdown at 50 MV/m. Dark current levels from our special coatings are over six orders of magnitude less than TiN-coated Copper even after arcing. These coatings can decrease the secondary emission levels, are mechanically stable, are not sensitive to radiation, do not affect the cavity Q, and will not poison the cathode. Hot-tests of coated X-band cavities will be performed in collaboration with SLAC.