High gradient experiments with X-band cryogenic copper accelerating cavities

Vacuum radio-frequency (rf) breakdown is one of the major factors that limit operating accelerating gradients in rf particle accelerators. The occurrence of rf breakdowns was shown to be probabilistic, and can be characterized by a breakdown rate. Experiments with hard copper cavities showed that harder materials can reach larger accelerating gradients for the same breakdown rate. We study the effect of cavity material on rf breakdowns with short X-band standing wave accelerating structures. Here we report results from tests of a structure at cryogenic temperatures. At gradients greater than 150 MV/m we observed a degradation in the intrinsic cavity quality factor, Q(0). This decrease in Q(0) is consistent with rf power being absorbed by field emission currents, and is accounted for in the determination of accelerating gradients. The structure was conditioned up to an accelerating gradient of 250 MV/m at 45 K with 10(8) rf pulses and a breakdown rate of 2 x 10(-4)/pulse/m. For this breakdown rate, the cryogenic structure has the largest reported accelerating gradient. This improved performance over room temperatures structures supports the hypothesis that breakdown rate can be reduced by immobilizing crystal defects and decreasing thermally induced stresses.