Review of Superconducting RF Guns

In the first topic the superconducting RF (SRF) photo-injector projects are reviewed. The DC SC photo-injector at the Peking University Accelerator Facility, the all-niobium SRF gun of Brookhaven and the high current SRF photo-injector project of AES and Brookhaven are discussed. Special attention is paid to the project of the FZ Rossendorf. The experimental set-up with a 1.3 GHz half-cell cavity has demonstrated the stable operation of a SRF photoelectron gun for the first time. The manufacture and the warm tuning of two 3 1/2 cell niobium cavities for the new photo-injector project are finished. A new cryostat with a special tuning and cathode transfer system has been designed. The status of manufacture for different components is shown. In future it is planed to operate this SRF gun with an energy of 9.5 MeV as a low-emittance photo-injector for the ELBE accelerator. In the second topic of the review some special points are discussed, which are specific for superconducting RF guns. The advantages and disadvantages of different shapes for the first cavity cell (gun cell) are discussed. Different kinds of photo-cathodes (normal conducting, superconducting, all niobium) and their integration into a superconducting cavity are explained. Different possibilities (RF focussing, magnetic RF field (TE - mode), static magnetic field outside the cavity) are discussed for beam focussing and emittance compensation inside the superconducting cavity.