The insulating vacuum system of the SNS cryomodules

The 81 superconducting RF cavities of the Spallation Neutron Source (SNS) accelerate the beam from 186 to 1000 MeV [1]. These cavities are fabricated of niobium and installed in 23 cryomodules each containing either 3 or 4 cavities. To achieve the required performance the cavities are operated in a superconducting state at ~2K achieved by submerging the cavities in individual cryostats of supercritical helium. The cryomodules provide a vacuum jacket around these cryostats to minimize heat leak due to gas convection and need to be maintained at a pressure <1e-5 torr. During initial operation helium leaks were experienced in some cryomodules requiring them to be actively pumped to maintain this pressure. This paper provides an overview of the design, installation and operation of the Insulating Vacuum System (IVS) installed for this purpose.