SBIR-STTR Award

High gradient experiments with microwave accelerating structures using moderately long radio frequency (RF) pulse widths in the range of 150 to 400 nanoseconds, at both 11.4 and 17.1 gigahertz, have shown that there is little advantage in selecting a shorter operating wavelength to increase the maximum operating accelerating gradient. Extensive high gradient tests at these pulse widths have now established that conventional design structures are unable to operate on a reliable long term basis with an accelerating gradient of greater than 70 to 75 megavolts per meter. Attempts to operate at higher gradients, in order to more closely approach the desired goal of 100 megavolts per meter, have resulted in a serious problem of accelerator foreshortened lifetime, due to surface-errosion-related, irreversible phase changes in the structure. In order to determine the validity of various theories associated with the initiating, nurturing, and quenching mechanisms of RF breakdown in high gradient accelerator structures, this project will design a gradient-hardened, brazed, traveling wave test structure, configured for integration into an existing 17 gigahertz dual recirculating amplifier ring, having a long pulse peak power capability sufficient to develop an accelerating gradient of 100 megavolts per meter when using a RF peak power source of approximately 20 megawatts. Phase I will determine a configuration for a candidate traveling wave structure that would: (1) alleviate the problem of foreshortened lifetime, (2) elevate the maximum accelerating gradient, and (3) reduce processing time. In particular, the brazed metal insertion techniques, adopted to solve cavity surface erosion problems in the output structures of high power 17 gigahertz traveling wave relativistic klystrons, shall be extended to satisfy the operational requirements of a resonant ring accelerator.

Commercial Applications and Other Benefits as described by the awardee:
The technology should eliminate surface-erosion-induced structure phase changes, and lead to a better understanding of the field gradient limitations of traveling wave linear accelerator 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