Continuous Wave (CW) mode is the origin of the Superconducting Radio-Frequency (SRF) accelerator technology. European XFEL project<sup>1</sup> 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 CM<sup>4</sup>.
The Polish Free Electron Laser, PolFEL was proposed more than decade ago and at that time was accepted for the Polish Roadmap for Research Infrastructures. The facility was proposed to be built in two stages, at first, with fewer accelerating sections and lower beam energy and the second one, with more accelerating sections, delivering 600 MeV electrons to VUV undulator, generating in the Self Amplified Spontaneous Emission process coherent radiation at wavelength ranged down to 27 nm and 9 nm in the first and third harmonic mode, respectively. Over past decade new experimental methods have been proposed and developed, delivering interesting results obtained with relatively low energy coherent and non-coherent photon beams, for example with IR-UV and THz radiation. In this contribution, subsystems of an updated version of the first stage PolFEL facility will be discussed. The project has recently received funds from the Smart Growth Operational Programme, Measure 4.2: Development of modern research infrastructure of the science sector, and is currently in a preparation phase of the construction, which will begin in 2019.
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.
We report the efforts undertaken at NCBJ and some of its collaborating laboratories dedicated to prepare pure and flat lead film coated onto niobium to operate as superconducting photocathodes. Three approaches to lead cathodic arc deposition have been implemented and tested: active plasma flux filtering, passive filtering and unfiltered flux. None of them allowed us to find a proper balance between thickness and surface roughness of a cathode. At that point efforts were taken to establish post-deposition heat treatment of lead film.