High Gradient Research on 17 Gigahertz Accelerating Structures

High power experiments with full length linear accelerator structures, using radio-frequency pulse widths in the range of 150 to 400 nanoseconds at both 11.4 and 17.1 gigahertz, have indicated that the maximum practical accelerating gradient is essentially independent of the frequency. Moreover, an extensive 11.4 gigahertz test program, using copper structures, has established that, for reliable long term operation, the upper limit of the accelerating gradient is 70 to 75 megavolts/meter. Attempts to operate accelerator structures at higher gradients, in order to satisfy the 100 to 200 megavolt/meter requirement of future linear colliders, have resulted in microwave breakdown of the cavities and the coupling irises; in several instances, permanent phase changes have occurred due to erosion of the copper cavity surfaces. This project will design, fabricate, and test a gradient-hardened accelerating structure that features high-temperature brazed and machined stainless steel inserts in the high stress regions of the cavity and coupler irises. Phase I investigated the microwave characteristics of accelerator cavities and dual-feed racetrack-shaped couplers, modified with stainless steel (and molybdenum) inserts. Brazed cavity test data was obtained, the microwave design parameters of a gradient-hardened 17 gigahertz accelerator structure were established, and overall system and component layout drawings were developed. In Phase II, a prototype accelerating structure will be engineered, fabricated, and tested at high gradients. Commercial Applications and other Benefits as described by the awardee: The new accelerating structures should avoid radio-frequency breakdown in traveling-wave-linear-accelerator mixed-metal structures, as well as the high-gradient structure, reducing the lifetime foreshortening caused by surface erosion detuning