Improving the Performance of High Gradient RF Structures by Surface Treatment with Gas Cluster Ion Beams

75857S Mitigation of high voltage breakdowns is a major concern in the development of higher-field radio frequency (RF) cavities for next generation accelerators. It is widely believed that the local surface condition of high-voltage electrodes, at atomic-scales, is the critical factor that determines the maximum electrical field strength that can be maintained without breakdown. However, all existing techniques for electrode surface preparation and conditioning fail to provide adequate correction and passivation of atomic scale defects and asperities. As a result, RF cavities invariably must be operated at much lower potentials than would otherwise be possible. This project will apply an established surface processing technique, Gas Cluster Ion Beam (GCIB) smoothing, to produce surface smoothing to atomic levels on RF cavity surfaces. It is anticipated that use of GCIB smoothing will dramatically increase breakdown strengths and result in a major improvement of operating stabilities. In Phase I, samples of electrode materials with typical surface finish were treated with various GCIB processes. It was found that large asperities could be removed, and that smoothing effects could be extended up to 2¿m. A new theory of the role of nanoscale roughness in RF breakdown was developed, and a useful model of cluster smoothing of large asperities was demonstrated. In Phase II, the GCIB chemical removal of the grain structure from Nb electrode material will be further investigated. Studies of the RF breakdown of GCIB-treated electrodes will be conducted, and an apparatus for in situ GCIB treatment of RF cavities will be designed and built. Commercial Applications and Other Benefits as described by the awardee: Gas Cluster Ion Beam processing of RF cavities would significantly reduce the size and cost of high-energy particle accelerators by allowing reliable operation at higher acceleration gradients. The processing also should increase the service lifetime of RF components by reducing damage caused by RF breakdown, and it may decrease the time and expense of conditioning RF cavities by eliminating steps used to prepare them for high-vacuum, high-voltage operation. In addition, electrode smoothing could benefit any application where achieving the maximum surface field strength is critical.