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 I we identified the ceramic candidates with low loss tangents, low SEY and large thermal expansion coefficients that best match the surrounding metallic wall structures. Then we purchased, inspected, coated and measured them in order to select the optimal one. We performed RF design to achieve the parameters required for SNS DTL couplers: 402.5 MHz frequency, 3 MW peak power at 10% duty factor and >4 MHz bandwidth. We performed conceptual engineering design of the window and prepared for prototype fabrication. In the Phase II project, we will finalize the electromagnetic and engineering design of the window. The device will be fabricated and assembled, and then delivered to SNS, where the commissioning program will include high-power tests after Phase II. The results of this work will be of immediate benefit to SNS DTL cavities and the window design can be scaled for other frequencies to operate at 805 MHz with SNS CCL cavities, SSRL at 476 MHz, ALS and NSLS-II at 500, APS at 352, etc. Also, RF windows are used in most accelerators, and the proposed design can be adapted for other frequency bands.