RF and thermal studies on conduction cooled Nb₃Sn SRF cavity

Advancements in the development of Nb3Sn coatings for superconducting radio-frequency (SRF) cavities have enabled efficient RF operation at 4.2 K. This has made the use of new cooling methods possible, namely those based on conduction cooling from commercial cryocoolers. Using cryocoolers in place of liquid helium as a cooling source eliminates the need for expensive and complex cryogenic infrastructure, making SRF technology accessible to small-scale applications in fields such as medicine, industry, environmental sustainability and more. At Cornell University, we have developed a new cavity testing assembly which uses a Cryomech PT420-RM cryocooler to cool a 2.6 GHz Nb3Sn cavity. We have performed several rounds of RF and diagnostic testing using this new assembly. Our best results demonstrated stable CW operation at 10 MV/m, with the cavity remaining at 4.2 K or lower. This represents breakthrough performance for a conduction cooled cavity, in which accelerating gradients relevant to some industrial applications were achieved. Our analysis highlights the importance of reducing thermal gradients across the cavity during cooldown; different methods for achieving this were successfully developed and demonstrated. We also found close agreement regarding thermal behavior between experimental measurements and numerical simulations, validating our chosen conduction cooling methods and providing guidance for future improvements. These findings will serve as a foundation for designing a new cryocooler-based cryomodule which will provide beam energy gains on the order of 1 MeV for beam currents up to 100 mA.