Status Report of ELBE and GaN

The SRF Gun has been running stabile using a magnesium cathode in the last year. Over 200 hours beam time have been provided in CW operation in 2019.
The magnesium bulk cathodes work routinely in ELBE and are polished and chemical cleaned before inserting them into the SRF Gun II, where they are again cleaned with an UV drive laser. Magnesium cathodes derives usually quantum effeciencies (QE) between 0.3 to 0.5% in SRF Gun II and offer a low risk of contaminations and an extreme long lifetime. The UV drive laser cleaning can be repeated several times to guarantee an high quality working cathode.
However, the particle accelerator community is always looking for new materials which can promise high quantum efficiency, a long lifetime and good vacuum stability, fast response time and low thermal emittance. Semiconductors such as GaN as novel materials for photocathodes are showing an enormous potential.
GaN is a semi-conductive material and well known for its high QE when illuminated with UV light. For the activation only caesium is required.
At the beginning of 2020 the first activation of GaN on sapphire substrate was successfully done. At first the GaN is heat treated at 610°C for 15 min and then activated with caesium to form a negative electron affinity surface. With 0.5 % quantum efficiency the first activation is all in all a successfully step for further promising GaN photocathodes.