High-Power RF Window for Pulsed RF Accelerating Cavities

Modern accelerator facilities for Basic Energy Sciences (BES) such as the Spallation Neutron Source (SNS) require reliable high-power RF components. The RF vacuum window is a critical part of the waveguide couplers to the accelerating cavities. It is the point where the RF feed crosses the vacuum boundary and thus forms part of the confinement barrier. RF windows must be designed to have low power dissipation inside the ceramic, be resistant to mechanical stresses and free of discharges. In response to this problem, this project is developing a novel design and manufacturing process of high-power RF windows that will be used in high average power accelerators. The design will be based on two innovations: the use of waveguide to coaxial transition that allows a significant reduction in window dimensions, allows lower losses, better cooling and easier multipactor suppression, and the use of modern coated low-loss ceramics with low loss tangent and novel secondary electron yield coating. In Phase II of this project, we have optimized the electromagnetic design of coaxial RF window, so its characteristics exceed those of conventional windows, including an order of magnitude lower reflections, twice larger bandwidth and power handling capability. We have completed the engineering design, including robust thermo-mechanical analysis. Importantly, we have received 8ĺĺ coaxial disks, made of novel high-purty AO479U alumina from Kyocera, characterized it and performed brazing studies. Currently, we are in the fabrication process of the waveguide transition and other sub-components as planned in the initial schedule. In the proposed Phase IIA of this project, we plan to rescale the design for 805 MHz and higher power (5 5MW peak, 450kW average) to be used with SNS CCL cavities. We will advance the design from the prototype to a commercial product, scalable for a wide range of products. In particular, we will re-define the ceramic fabrication process to avoid contamination introduced by the metallization process, which caused lengthy delays. Lastly, we will add two additional ports to be used for a thermal camera to provide near real-time thermal imaging of the windows in operation on both the air and vacuum side. The results of this work will be of immediate benefit to SNS DTL and CCL cavities and the window design can be scaled for other frequencies to operate at LANSCE, SSRL, ALS, NSLS-II, APS etc. Also, RF windows are used in most accelerators, and the proposed design can be adapted for other frequency bands.