K. Floettmann WSHQE, OCT. 5, 2006 WSHQE Oct. 5, 2006 Klaus Floettmann Photo cathode requirements for the European XFEL

K. Floettmann WSHQE, OCT. 5, 2006 QE and Lifetime requirements A lifetime of some month with a QE > 0.5% is required. This is achieved with Cs 2 Te Cathodes e.g. at Flash. Can we go home now?

K. Floettmann WSHQE, OCT. 5, 2006 Problem I: roughly half of the predicted emittance of the XFEL injector is due to the thermal emittance of the cathode

K. Floettmann WSHQE, OCT. 5, 2006 Development of the transverse Emittance – 1nC charge – uniform transverse distribution – longitudinal flat-top with 2 ps rise time – incl. thermal emittance

K. Floettmann WSHQE, OCT. 5, 2006 Slice Emittance at the Injector exit emittance in longitudinal slices of the bunch

K. Floettmann WSHQE, OCT. 5, 2006 Slice Emittance at the Injector exit w.o. thermal emittance

K. Floettmann WSHQE, OCT. 5, 2006 The measured thermal emittance is even higher than expected from simple theoretical predictions (Schottky effect, cathode roughness) How can we reduce the thermal emittance? reduce cathode roughness control the work function …?

K. Floettmann WSHQE, OCT. 5, 2006 Problem II: dark current presents a sever operational problem for the XFEL injector

K. Floettmann WSHQE, OCT. 5, 2006 Dark Current extrapolation to 60 MV/m no conditioning effect taken into account

K. Floettmann WSHQE, OCT. 5, 2006 A large fraction of the dark current is generated at or near the cathode. Collimation at low energies is very difficult because the dark current has similar properties as the beam How can we reduce the dark current generation reduce cathode roughness reduce number of emitters by better cleaning techniques …?

K. Floettmann WSHQE, OCT. 5, 2006 Dry Ice Cleaning Principle CO 2 cold Nitrogen

K. Floettmann WSHQE, OCT. 5, 2006 CO2 Nozzle

K. Floettmann WSHQE, OCT. 5, 2006 No field emission was observed up to 90 MV/m on dry ice cleaned Cu samples in field emission scans. Dry ice cleaning of the gun cavity is in preparation. Dry Ice Cleaning of the cathode plugs should be tested next. To monitor the deposition of field emitters during the cathode preparation process a field emission scanning apparatus for the cathodes will be designed and build in collaboration with the University of Wuppertal.