High Performance Lossy Dielectric HOM Absorbers for SRF Cavities

Lossy dielectrics are used as high order mode (HOM) absorbers in superconducting radio frequency (SRF) cavities in linear accelerators and microwave tubes. Recently, lossy dielectrics based on high thermal conductivity aluminum nitride (AlN) have been developed, which can replace the lossy dielectrics based on toxic beryllia (BeO). These AlN-based lossy dielectrics must be joined to metallic copper members in these applications. However, the lack of suitable metallization and brazing technologies hampers the insertion of AlN based lossy dielectrics into SRF cavities and other vacuum electron devices. This project will develop a family of vacuum-compatible, robust joining technologies to attach AlN based lossy dielectrics to copper members for SRF cavities and power microwave tubes. Phase I will produce AlN-based lossy dielectrics with tailored loss properties and develop two processes (vacuum-compatible diffusion bonding process and active metal-brazing) to attach them to copper members. Adhesion between the copper member and AlN-based lossy dielectrics will be measured, and the vacuum performance and thermal cycling behavior of the brazed joints will be evaluated. Commercial Applications and other Benefits as described by the awardee: SRF cavities in linear accelerators and medium-to-high vacuum electron devices (microwave tubes) would benefit from AlN-based lossy dielectric HOM absorbers that are robustly joined to Cu components. The commercial applications include linear particle accelerators for scientific research; x-ray sources for medical diagnostic and treatment devices; klystrons and gyrotrons to 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.