Performance of the FEL cryomodules

The Thomas Jefferson National Accelerator Facility (Jefferson Lab, formerly known as CEBAF) is building a highly efficient, kilowatt-level infrared free-electron laser, the IR Demo FEL. The IR FEL uses superconducting radiofrequency (SRF) cavities to accelerate the electron beam that provides energy for the laser. These cavities provide the high-gradient acceleration for the high average currents necessary for a compact FEL design. Currently, a quarter cryomodule injector and a full eight-cavity cryomodule have been installed in the FEL linac. These units were tested as part of the IR FEL commissioning process. The main focus of these tests was to determine the maximum stable operating gradient. The average maximum gradient reached by these ten cavities was 11 MV/m. Other tests included measurement of cavity parameters such as the unloaded Q (Qo) vs. gradient, the input coupling, calibration of field probes and behavior of the tuner mechanisms. This paper presents the results of those tests.