RF Breakdown Studies using Pressurized Cavities

Many present and future particle accelerators are limited by the maximum electric gradient and peak surface fields that can be realized in RF cavities. Despite considerable effort, a comprehensive theory of RF breakdown has not been achieved and mitigation techniques to improve practical maximum accelerating gradients have had only limited success. Part of the problem is that RF breakdown in an evacuated cavity involves a complex mixture of effects, which include the geometry, metallurgy, and surface preparation of the accelerating structures and the make-up and pressure of the residual gas in which plasmas form. Recent studies have shown that high gradients can be achieved quickly in 805 MHz RF cavities pressurized with dense hydrogen gas, without the need for long conditioning times. In this project, high pressure techniques will be used to suppress the effects of residual vacuum and geometry found in evacuated cavities, so that the role of the metallic surfaces in RF cavity breakdown can be isolated and studied as a function of magnetic field, frequency, and surface preparation. In Phase I, an RF test cell and pressure barrier, capable of operating both at high pressure and in vacuum, will be designed, built, and tested. The test cell will have replaceable electrodes (e.g., Mo, Cu, Be, W, and Nb) that are compatible with analysis devices such as scanning tunneling microscopes and atomic force microscopes Commercial Applications and other Benefits as described by the awardee: The understanding of the role of the metals in RF breakdown will lead to better designs of RF cavities for many applications, including the International Linear Collider. An increase in the operating accelerating gradient, improved reliability, and shorter conditioning times should provide very significant cost savings