A high-brightness SRF photo injector for FEL light sources

Most of the proposed electron accelerator projects for future FELs, ERLs, or 4th generation light sources require electron beams with an unprecedented combination of high-brightness, low emittance and high average current. In all projects photo guns will be applied: DC-photoguns, normal conducting RF photoguns (NC-guns), and superconducting RF photoguns (SRF-guns). While the concepts of DC- and NC-guns are well proofed, the SRF gun development still possesses a high risk. Challenges are the design of the superconducting cavity, the choice of the right photocathode type, its life time and possible cavity contamination, the difficulty of coupling high-average power into the gun, and beam excitation of higher order cavity modes. But in combination with SRF linacs, the SRF guns will be the best solution for high average currents. Continuous wave operation is simple to achieve. Thus, several R&D projects of SRF-gun have been launched.
The talk will give an overview of the progress of the SRF photoinjector development. In detail, the technical concept, performance, and status of the Rossendorf superconducting rf gun project, a collaboration of BESSY, DESY, MBI and FZD, will be presented. Main design parameters of this SRF gun are a final electron energy of 9.5 MeV, an average current of 1 mA, transverse normalized emittances (rms) of 1 mm mrad at 77 pC and 2.5 mm mrad at 1 nC bunch charge. The 1.3 GHz cavity consists of three full cells with TESLA geometry, a specially designed half-cell in which the photo cathode is placed, and a choke filter in order to prevent rf losses at the cathode side of the cavity. The photocathode with a Cs2Te photoemission layer is normal-conducting and are cooled by liquid nitrogen. The SRF gun cryostat consists of a stainless steel vacuum vessel, a warm magnetic shield, a liquid nitrogen cooled thermal shield, a titanium He tank with two-phase supply tube. A heater pot in the He input port will be used for He level control. The 10-kW fundamental power coupler is adopted from the ELBE cryomodule. The gun will be installed at the ELBE superconducting linear accelerator. In a first commissioning and test period the gun will be operated in parallel to the accelerator. A diagnostic beamline will allow beam parameter measurement and further optimization of the SRF gun.