The Rossendorf RF Gun with a Superconducting Niobium Half-cell Cavity - Worldwide First Electron Beam of a Superconducting RF Gun
In common research project together with the Budker Institute of Nuclear Physics, Novosibirsk and collaboration with DESY and the Max Born Institute, the first successfully operating superconducting radio-frequency photoelectron source had been developed . The RF resonator was an elliptical shaped, half-cell niobium cavity with a resonant frequency of 1.3 GHz. The back wall of the half-cell has an opening with an attached tube of 12 mm diameter for the insertion of the photo cathode. The tube ended in a superconducting choke flange filter preventing RF power leakage at the cathode side. The cavity was mounted in a cryostat and cooled with liquid helium to a temperature of 4.2 K. In the cavity the electron bunches were produced with a pulsed laser beam hitting a photo cathode. This photo cathode was hold and cooled with liquid nitrogen by a special support and cooling system. Thereby the cathode was thermally and electrically isolated from the cavity . This design allowed the use of normal-conducting photo cathodes like Cs2Te in the superconducting cavity. Cesium telluride has a high quantum efficiency of more than 10 % but requires an UV laser.
The gun had been in operation together with a dedicated diagnostic beamline for electron beam parameter measurements in 2002. For the first time it was shown that a RF electron source with a photo cathode inside a superconducting cavity can work stably . At a temperature of 4.2 K an intrinsic quality factor of 2.5 x 108 was obtained without any degradation in the operation period of seven weeks. The cavity reached a maximum peak field of 22 MV/m and produced an electron beam with 900 keV kinetic energy.
 Peter vom Stein, “Hochfrequenz Elektroneninjektoren für cw-Beschleuniger”, PhD thesis, Report FZR-227, Juli 1998.
 D. Janssen et al., “First operation of a superconducting RF gun”, Nucl. Instr. Meth. A 507 (2003) 314 – 317.