Commissioning of the CEBAF cryomodules

When complete, the Continuous Electron Beam Accelerator Facility will house a 4 GeV recirculating linear accelerator containing 42 1/4 cryomodules arrayed in two antiparallel linacs and an injector. Currently, over half of the cryomodules have been installed. Each cryomodule contains eight superconducting niobium 5-cell RF cavities that operate at 1.497 GHz. A cryomodule must provide an energy gain of 20 MeV to the 200 μA beam. The resultant dynamic heat load must be less than 45 W. The cavity parameters that are measured during the commissioning process include the external Q´s of the cavity ports, the unloaded Q (Q₀) of the cavity as a function of accelerating gradient, and the maximum operating gradient of the cavity. The sensitivity of the resonant frequency to changes in pressure and gradient is also measured. Finally, the mechanical tuners are cycled and characterized. In all cases, the performance of CEBAF cryomodules has exceeded the design requirements. A portable test stand allows local control of the RF system and provides automated data acquisition. This paper describes the cryomodule commissioning hardware, software, and measurements