Commercially available cryocoolers typically provide about 2 Watts of cooling at 4.5 K. The ANL Physics Division accelerator group has been testing a new two stage Gifford-McMahon/Joule-Thomson (GM-JT) cryocooler from Sumitomo Heavy Industries. In tests at both RadiaBeam and Argonne the measured cooling capacity is approximately 9.5 Watts at 4.5 K. The cryocooler has also been used to condense helium at approximately 4.3 K at a rate of about 0.4 L/hr into a small helium container. We report here on the experimental hardware used to perform these measurements and discuss the data. The device appears to be an excellent complement to new low-loss Nb3Sn cavities under development at ANL and elsewhere. We comment on this application and the outlook for future uses of the device in ATLAS and elsewhere.

Niobium-tin has been identified as the most promising next-generation superconducting material for accelerator cavities. This is due to the higher critical temperature (Tc = 18 K) of Nb3Sn compared to niobium (TC = 9.2 K), which leads to greatly reduced RF losses in the cavity during 4.5 K operation. This allows two important changes during cavity and cryomodule design. First, the higher Tc leads to negligible BCS losses when operated at 4.5 K, which allows for a higher frequency to be used, translating to significantly smaller cavities and cryomodules. Second, the reduced dissipated power lowers the required cryogenic cooling capacity, meaning that cavities can feasibly be operated on 5-10 W cryocoolers instead of a centralized helium refrigeration plant. These plants and distribution systems are costly and complex, requiring skilled technicians for operation and maintenance. These fundamental changes present an opportunity for a paradigm shift in how low-beta linacs are designed and operated. Fabrication and testing results of first prototypes are discussed.

Niobium-Tin (Nb3Sn) is a promising alternative to pure niobium for low-beta ion accelerators due to low RF losses even at high frequencies. At 4.5K, the expected Bardeen-Cooper-Schrieffer resistance of Nb3Sn is a few nano-Ohms at 1GHz, with two orders of magnitude improvement compared to pure niobium. The low Nb3Sn RF losses allow the use of higher frequency, much smaller cavities while maintaining 4.5K operation that is compatible with cryocoolers. This work aims to demonstrate the feasibility of coating a high-frequency 1GHz, compact quarter-wave cavity. The development of this cavity has the potential to transform low-beta ion accelerators through size reductions and by enabling the replacement of large helium cryoplants with small plug-in cryocoolers. We have designed, built, and tested a 1GHz Nb3Sn coated quarter-wave cavity. Nb3Sn coating has been performed once by vapor diffusion at Fermilab. Cryogenic cold tests show a quality factor of ~1E9 at low accelerating electric fields or ~7 times higher than the theoretical limit for pure niobium at 1GHz. This first coating does not yet meet our quality factor or gradient goals; however, we plan to continue to develop this cavity.