Design of the SNS cryomodule

The Spallation Neutron Source (SNS) has changed the high-energy end of its linac to a superconducting RF (SRF) system. The design uses medium and high beta cavities installed into cryomodules to span the energy region of 185 MeV up to 840-1300 MeV. This paper describes the design of these cryomodules that provide a significant portion of the total acceleration to the negative Hydrogen ion beam. The design minimizes the effect of losses on cavity performance by preserving cavity cleanliness, maintaining high vacuum and shielding the cavity from the earth's magnetic field. A modal analysis was performed to maintain the cavities on resonance by minimizing cavity microphonics and providing cavity tuning. The cryomodule design optimizes the cryostat heat load to the refrigerator on the primary (2 K), secondary (5 K) and shield (50 K) circuits and facilitates production of multiple modules (32 maximum) at a reasonable cost. The paper describes the two types of cavities, the helium vessel, the tuner, the magnetic shields, the thermal shield, the fundamental power couplers, the space frame, the vacuum tank and the end cans that make up the medium and high beta cryomodules.