Progress towards Continuous Wave Operation of the SRF Linac at DESY

Continuous Wave (CW) mode is the origin of the Superconducting Radio-Frequency (SRF) accelerator technology. European XFEL project(1) was based on the Linear Collider (LC) technology (TESLA) operating in the pulsed RF power mode (10Hz / 650 mu s beam pulse). Many FEL user experiments will get an advantage (or become possible) with CW mode operation. European XFEL (E-XFEL) SRF accelerator recently reached its project goal of 17.5 GeV electron beam energy. Possible CW mode linac operation scenario with 17 modified injector section cryo-modules (CM) may reach similar to 50% of that energy with 25 mu A (100pC and 250kHz) CW beam in E-XFEL. A Long Pulse (LP) mode (duty factor < 100%) may provide even higher beam energies and still long enough FEL radiation pulses(2,3). Very encouraging results have been obtained at DESY on Cryo Module Test Bench (CMTB) during CW/LP tests of E-XFEL prototype CMs. The possibility to run an E-XFEL accelerating module in CW/LP mode was clearly shown together with reaching higher unloaded Q-factor of the cavities in the CM4.