Development of low-loss cryo-accelerating structure with high-purity copper

In this paper, the development of an accelerating structure with high-purity copper operated at approximately 20  K is described. The aim is to obtain a dramatic increase in the quality factor (Q factor), which is an important parameter as regards enhancement of the acceleration gradient in a normal-conducting accelerating structure that has been operated at room temperature in the past. First, the Q factors of pillbox cavities comprised of two types of copper with different purities (99.99% and 99.99998%) were measured, at room temperature (300 K) and at a relatively low temperature (20 K); the resultant ratios, Q0(20 K)/Q0(300 K), were then compared with calculated values. Hence, it was found that the experimental and calculated values agree to within several percent, and converge to a constant value (approximately 5.3) when the residual resistance ratio is above 500. Next, a C-band accelerating structure was fabricated using a copper material having a purity of 99.99998%, and high-power testing of the structure was conducted at 20  K. Hence, it was found that the accelerating gradient Eacc reaches 30.9  MV/m with a 1.0-μs pulse width and 50-Hz repetition rate. In the high-power test, it was revealed that the radiation dose rate for C-band structure (20  K) was about two orders of magnitude lower than that for S-band structure (303  K) under the same conditions. Further, the magnitude of the dark current generated in this structure was estimated to be several femtoamps, which was below the detection limit. This finding was based on the assumption that the dark current magnitude is proportional to the radiation dose; however, no dark current was observed.