Aluminum Nitride RF Windows

72560-New window materials that can handle multi-megawatt radio frequency (RF) powers are needed to increase the reliability of RF windows for linear particle accelerators. Aluminum nitride would be an excellent window material due to its high thermal conductivity, high dielectric strength, low secondary electron emission, and high mechanical strength, if its dielectric loss (loss tangent) could be controlled and/or reduced. This project will develop technology for optimizing aluminum nitride for the RF window application. In particular, the effects of starting powder type and purity, porosity, grain size, and yttria additive concentration on the thermal conductivity and microwave dielectric properties (dielectric constant and loss tangent) of aluminum nitride will be investigated. Phase I identified the variables that control dielectric loss and thermal conductivity of aluminum nitride. The presence of certain impurities in the starting powders, and the type and concentration of the second phase (added as a sintering aid), were found to be responsible for controlling the dielectric loss and thermal conductivity. Phase II will determine the sintering aid concentration and associated processing variables that minimize dielectric loss without significantly reducing thermal conductivity. The material's thermal and dielectric properties will be fully characterized to help the RF window design. Thermetically sealed prototype windows will be prepared by newly developed metallization techniques. Cold and hot tests will be performed to demonstrate the windows¿ power handling capabilities. Commercial Applications and Other Benefits as described by awardee: Almost every medium-to-high vacuum electron device would benefit from the low loss, high thermal conductivity aluminum nitride ceramics. Applications include Klystrons and gyrotrons that supply microwave energy to improve the performance of chemical processes and materials processing; Klystrons for direct broadcast satellites; gyrotrons for magnetic fusion based on electron cyclotron heating; and microwave communications.