ELBE SRF Gun II - New High Gradient SRF Gun, 2009 – Today
After the proof-of-principle demonstration of the world’s first SRF gun operation for a superconducting accelerator i.e. the ELBE LINAC,A second project started with financial support of the German Federal Ministry of Education and Research. The main goal is to achieve high average current (1 mA) and low emittance (1 mm mrad @ 77 pC) for the ELBE LINAC as well as to test new semiconductor cathodes within the BMBF-PCHB project
The main task is to reach the designed specifications with respect to the intrinsic quality factor and the maximum acceleration gradient of the niobium cavity. To meet this goal, an improved cleaning has been applied, but also other modifications, based on the experiences obtained by the previously installed ACC300 cavity, were taken into account [1, 2]. Another significant change is a superconducting solenoid which is integrated into the cryomodule of the gun. Its cooling is realized by a the same helium supply line cooling the cavity while its fine positioning is done by thermally isolated steppors. The whole module is currently under assembly with the commissioning expected to be in summer 2014 .
The JLabFG cavity
The heart of the SRF gun II photo injector is the new JLabFG cavity which is again a 3.5-cell cavity made from so called fine grain niobium. It was built and tested in collaboration with Thomas Jefferson Laboratory  until late 2013. In order to reduce the sensitivity to helium pressure fluctuations and microphonics, the half-cell of the cavity was equipped with additional stiffeners (green parts of the cavity in the picture below).
Jun. 2010: JLabFG cavity manufacturing finished
Dec. 2010: field- and frequency tuning
Aug.2011: first vertical tests (Epeak = 39 MV/m)
Feb. 2012: helium vessel welding and vertical test (Epeak = 41 MV/m)
Nov. 2013: final vertical test (Epeak = 41 MV/m) and string assembly
Apr. 2014: module assembly (see picture below) and commissioning
Aug. 2014: cool down and RF commissioning (Epeak = 25 MV/m )
Feb. 2015: 4.5 MeV CW electron beam from a copper cathode 
Mar. 2015: Cs2Te cathode operation in the gun (Epeak = 20 MV/m , )
Mar. 2016: acceleration of 200 pC electron beam up to 30 MeV by the ELBE LINAC
Feb. 2017: 1st user beam time (several days) for neutron and THz generation 
- until today: stable user operation for THz generation 
(note: Ecath=1.53xEacc and Epeak=2.56xEacc)
 P. Murcek et al., Modified 3½-Cell SC Cavity Made of Large Grain Niobium for the FZD SRF Photoinjector, Proc. SRF 2009 Conference, Sept. 20-25, 2009, Berlin & Dresden, Germany.
 P. Murcek et al., Modified SRF Photoinjector for the ELBE at HZDR, 15th International Conference on RF Superconductivity, 25.-29.07.2011, Chicago, USA.
 H. Vennekate et al., Emittance Compensation for an SRF Photo Injector, SRF 2013 – 16th Int. Conf. on RF Superconductivity, September 23-27, 2013, Paris, France.
 P. Murcek et al., The SRF Photoinjector at ELBE – Design and Status 2013, SRF 2013 – 16th Int. Conf. on RF Superconductivity, September 23-27, 2013, Paris, France.
 A. Arnold et al., Fabrication, Tuning, Treatment and Testing of Two 3.5 Cell Photo-Injektor Cavities for the ELBE Linac, 15th International Conference on RF Superconductivity, 25.-29.07.2011, Chicago, USA.
 A. Arnold, et al., Commissioning Results of the 2nd 3.5 Cell SRF Gun for ELBE, LINAC 2014 - 27th Linear Accelerator Conference, 31st August to 5th September 2014, Geneva, Switzerland
 J. Teichert, et al., First Beam Characterization of SRF Gun II with a Copper Photocathode, 56th ICFA Advanced Beam Dynamics Workshop on Energy Recovery Linacs, Stony Brook, USA, 2015
 A. Arnold, et al., RF Performance Results of the 2nd ELBE SRF Gun, 17th International Conference on RF Superconductivity, September 13-18, 2015, Whistler, British Columbia, Canada
 B. Green, et al., High-Field High-Repetition-Rate Sources for the Coherent THz Control of Matter, Scientific Reports 6, 22256 (2016)
 Hassan A. Hafez, et al., Extremely efficient terahertz high-harmonic generation in graphene by hot Dirac fermions, Nature Vol. 561, 507–511 (2018)