Multipactor coatings for sapphire windows using remote plasma assisted deposition

Single crystal sapphire provides several properties advantageous for high power RF windows, including high strength, low loss, and clear visual detection of impurities and material defects. These should allow applications for both high peak and average power operation. In fact, sapphire windows are commonly used in gyrotrons producing more than 500 kW of average power. Their use for peak power applications, however, has been prevented by poor adhesion of sputtered multipactor coatings. Previous efforts to develop high peak power sapphire windows failed when minor processing arcs removed the coating. This presentation describes adaptation of plasma processes to apply TiN to the crystal sapphire surface. This creates a molecular bond that can survive high temperature furnace operations as well as arcing during high power processing. In addition to cleaning, the process uses three plasma operations. The first creates an AI-oxide coating approximately 0.5 run thick on the sapphire surface. This activates the surface for a subsequent plasma operation that bonds TiN to the surface. A subsequent nitridation process replaces any remaining TiO bonds with with TiN. The fmal result is a uniform coating of TiN molecularly bonded to the sapphire substrate.substrate. Secondary electron yield measurements at the University of New Mexico confirmed the effectiveness of the coating. Calabazas Creek Research, Inc. built a high power, RF window at 11.424 GHz that was tested at SLAC National Accelerator Laboratory. The coating survived a high temperature braze and numerous processing arcs due to less than optimum vacuum in the transmission line. The window operated for several days, transmitting power levels up to 45 MW, the maximum power available. There was no indication of multipactor emission, and the coating appeared visually unaffected.