High Gradient Research on 17 Gigahertz Accelerating Structures

78756S 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.