Overview of the Cornell ERL injector cryomodule

The Laboratory for Elementary-Particle Physics, Cornell University, in collaboration with Jefferson Lab is exploring the potential of a Synchrotron Radiation User Facility based on a multi-GeV, low emittance, Energy-Recovery Linac (ERL) with a 100 mA CW beam. The ERL injector will accelerate bunches from the electron source from 0.5 MeV to 5 MeV with minimal emittance growth. The injector and main linac of the ERL will be based on superconducting RF technology to provide CW operation. There will be one cryomodule with five 1300 MHz 2-cell cavities, each providing one MV of acceleration, corresponding to an accelerating field of about 4.3 MV/m in CW operation. Besides standard features such as an integrated helium vessel and mechanical tuner, each cavity has two input couplers, symmetrically placed on the beam pipe to cancel kicks due to coupler fields. For a 100 mA maximum injected beam current, each coupler must deliver 50 kW of beam power leading to a Qext of 4.6 × 10⁴ for matched beam loading conditions. Antenna- and loop-based HOM couplers can disturb beam emittance through kicks. We plan to avoid the use of such couplers. Following the strategy for B-factory SRF cavities, the beam pipe aperture has been enlarged on one side to propagate all higher order modes out to symmetric ferrite beam pipe loads. These are positioned outside the helium vessel and cooled to liquid nitrogen temperature. Ferrite properties at 77 K have been measured and the corresponding damping evaluated. To explore the full capabilities of the injector, energy gains up to 3 MV per cavity will be considered at lower beam currents. For this flexibility, the input coupling needs to be adjustable by a factor of 9.