Development of a prototype superconducting radio-frequency cavity for conduction-cooled accelerators
Gianluigi Ciovati, James P. Anderson, S. Balachandran, Gary Cheng, Bruno Coriton, E. Daly, Pashupati Dhakal, A. Gurevich, Fay Hannon, Keith Harding, L. Holland, Frank Marhauser, K. McLaughlin, Drew A. Packard, T. Powers, Uttar Pudasaini, J. Rathke, Robert Rimmer, T. Schultheiss, Hannes Vennekate, Devin Vollmer
Abstract
The higher efficiency of superconducting radio-frequency (SRF) cavities compared to normal-conducting ones enables the development of high-energy continuous-wave linear accelerators (linacs). Recent progress in the development of high-quality ${\mathrm{Nb}}_{3}\mathrm{Sn}$ film coatings along with the availability of cryocoolers with high cooling capacity at 4 K makes it feasible to operate SRF cavities cooled by thermal conduction at relevant accelerating gradients for use in accelerators. A possible use of conduction-cooled SRF linacs is for environmental applications, requiring electron beams with energy of 1--10 MeV and 1 MW of power. We have designed a 915 MHz SRF linac for such an application and developed a prototype single-cell cavity to prove the proposed design by operating it with cryocoolers at the accelerating gradient required for 1 MeV energy gain. The cavity has a $\ensuremath{\sim}3\text{ }\text{ }\mathrm{\ensuremath{\mu}}\mathrm{m}$ thick ${\mathrm{Nb}}_{3}\mathrm{Sn}$ film on the inner surface, deposited on a $\ensuremath{\sim}4\text{ }\text{ }\mathrm{mm}$ thick bulk Nb substrate and a bulk $\ensuremath{\sim}7\text{ }\text{ }\mathrm{mm}$ thick Cu outer shell with three Cu attachment tabs. The cavity was tested up to a peak surface magnetic field of 53 mT in liquid He at 4.3 K. A horizontal test cryostat was designed and built to test the cavity cooled with three Gifford-McMahon cryocoolers. The rf tests of the conduction-cooled cavity, performed at General Atomics, achieved a peak surface magnetic field of 50 mT and stable operation was possible with up to 18.5 W of rf heat load. The peak frequency shift due to microphonics was 23 Hz. These results represent the highest peak surface magnetic field achieved in a conduction-cooled SRF cavity to date and meet the requirements for a 1 MeV energy gain.