Continuous Wave (CW) mode is the origin of the Superconducting Radio-Frequency (SRF) accelerator technology. European XFEL project1 was based on the Linear Collider (LC) technology (TESLA) operating in the pulsed RF power mode (10Hz / 650μs 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 ~50% of that energy with 25μA (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. Very encouraging results have been obtained at DESY on Cryo Module Test Bench (CMTB) during CW/LP tests of EXFEL 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.
Results are reported on using evaporation and UHV arc lead deposition to create thin-layer superconducting Pb photocathodes on niobium wall of electron gun. Evaporated photocathodes were prepared and tested for the first time in 2014. A complete XFEL-type photo-injector with an evaporated photocathode underwent successful quality check at DESY - an acceptable working point was reached. On the other hand poor adhesion to niobium proved to be the most serious shortcoming of the evaporated Pb layers. UHV arc deposition seems to be much more promising in this context as it allows energetic coating. Filtered arc coating lead to creation of uniform, 2 μm thick lead layers with casual spherical extrusions which enhance locally electric field and leads to high dark current. Conditioning in electric field is needed to reduce the field emission effects from these layers to acceptably low value. Using non-filtered UHV lead deposition enabled fast coating up to a thickness above 10 μm. Pb films obtained in this way require further post-processing in pulsed plasma ion beams in a rod plasma injector. In order to reach a sufficiently planar film surface the pulsed heat flow through a lead layer on niobium was modeled and computed.