First operation of a superconducting rf photoelectron gun

RF photo cathode electron guns are the source of choice for most high-performance accelerator systems. Their advantages are the ability to produce very bright beams of electrons with high bunch charges and small transverse and longitudinal emittance. At present, their drawback is the limited average current since the high RF fields in the cavities require a pulsed operation. On the other hand, advanced light sources uses linear accelerators with superconducting RF cavities which can operate in the cw mode and produce high average current electron beams. A preferred injector for these accelerators would be a photo electron gun with superconducting RF cavities.
In a research project [1], a superconducting RF photo electron gun (SRF gun) has been developed during the last years at the Forschungszentrum Rossendorf. The SRF gun was put successfully into operation and the efficiency of the concept could be demonstrated. A helium bath cryostat was used to cool down the cavity to 4.2 K. In a daily cycle the cryostat was filled up with liquid helium in the morning followed by a measuring time in the afternoon. The resonator is a niobium half cell with TESLA geometry operating at 1.3 GHz. The photo cathode had a Cs2Te layer deposited in a preparation chamber connected with the gun. In contrast to the cavity the cathode is not superconducting. It is thermally isolated from the cavity and its heat load goes in a special nitrogen cooling system. The cathode was illuminated by an UV laser of 263 nm with a micropulse frequency of 26 MHz and a maximum energy per pulse of 40 nJ. For evaluation of the produced electron beam a beam line is installed with two solenoids, insertable mask and view screens, spectrometer magnet, kicker cavity and Faraday-cup. Measurements of beam parameters and operation experience are reported.

1] E. Barthels et al., Nucl. Instr. and Meth. A 445 (2000) 408