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

75857-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 must invariably 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 actual RF electrode materials will be obtained, and various candidate GCIB processes will be tested for effectiveness. Samples with and without GCIB processing will be evaluated using Fowler-Nordheim determinations of DC field emission currents, high voltage RF stress testing, Atomic Force Microscopy, and Scanning Electron Microscopy. Experimental observations will be compared with theoretical predictions. Commercial Applications and Other Benefits as described by the awardee: Gas Cluster Ion Beam processing of RF cavities should 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 breakdowns, 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 maximum surface field strength is critical.