Operating SRF in a "dirty" machine 14 – 15. Sep. 2017 , HZB

Compatibility of SC cavity and "foreigner" photocathodes in SRF gun Rong Xiang on behalf of the SRF Gun Group at ELBE

Outline Concept of SC RF photo-injector (SRF gun) SRF guns running at ELBE, BNL, PKU Performance of SC cavity at ELBE Quality control of photocathode Summary

Electron emitter Accelerating field 1. Concept of SC RF photo-injector (SRF gun) Electron emitter Photocathodes Accelerating field SC RF cavity Ultra short, high brightness source Higher gradient than DC gaps MeV energy, simple injector Better vacuum than NC RF gun CW operation possible … NC & SC Complicated SRF system Sensitive photocathode Cathode life time Contamination to SRF cavity ….

1. Concept of SC RF photo-injector (SRF gun) Ref: A. Arnold, J. Teichert, Phys. Rev. ST AB 2 14 (2011), S. 024801.

2.1 SRF guns running at ELBE 2007-2014 e--beam SRF Gun I SRF Gun II SRF Gun-II in ELBE hall

Future – ELBE User operation 2.2 SRF guns running at BNL Cathode contamination to the gun due to severe cathode stalk RF conditioning. Copper ring around the choke joint 1.3 GHz SC cavity + GaAs & Diamond 704 MHz gun + K2CsSb 2017 112 MHz gun + K2CsSb

No cathode in SRF cavity: No degradation with cathode 2.3 SRF guns running at PKU 1.3GHz SC cavity + DC pierce structure + Cs2Te E max on the wall 49.8 MV/m No cathode in SRF cavity: No degradation with cathode

Outline Concept of SC RF photo-injector (SRF gun) SRF guns running at ELBE, BNL, PKU Performance of SC cavity at ELBE Quality control of photocathode Summary

3. Performance of SC cavity at ELBE SRF gun II status 8.0 MV/m acc. gradient 20 MV/m cavity peak field 12.3 MV/m field at cathode 4.0 MeV kinetic energy Bandbreite ohne 3-Stubtuner: 140 Hz Bandbreite während Messung: 230 Hz Gesamtkabeldämpfung der Pickupleitung bis zum Powermeter (am Richtkoppler): -14.62dB R/Q für 80% Feldverteilung bei 40MV/m: r=168 Ohm Epk/Eacc=2.556, Ekin=7.82MeV

3. Performance of SC cavity at ELBE Contaminated by metallic particles Strong field emission Permanent effect Scratches on cathode Defect on cathode after field emission

3. Performance of SC cavity at ELBE Contaminated by cesium compound - disappeared Cs2Te cathodes #2016.03, #2017.03 in SRF gun - Can be recovered by warming up to room temp. and cooling down again. Before Accident HPP Warming up cycle ɸ 4 mm layer ?? Cs2Te cathode in gun 2017-2-6 in gun Cs2Te layer disappeared 2017-2-24 in gun

3. Performance of SC cavity at ELBE SRF Gun II histroy contamination of particles and cesium compound is the main problem (field emission) . by warming up to room temperature, Q0 can be partly recovered. H2, H2O, N2 are the most rest gas detected during the warming up.

? 4. Quality control of photocathode before deposition polishing (roughness ~ 10 nm) clean room cleaning microscope view dry ice cleaning baking in vacuum particle searching Cs2Te deposition transport to gun before deposition after deposition

5. Summary Photocathodes bring contamination to the SRF guns at ELBE, BNL - particles - alkali-compounds Photocathode exchange and operation are also contamination sources - careful quality check of cathodes - improved mechanics to avoid particle production Metallic photocathodes can easily be used in SC cavity - less multipacting and lower dark current ( <10 nA)

Acknowledgement Thank for the reports from: Dr. Jiankui Hao, Prof. Kexin Liu (PKU) Dr. Erdong Wang (BNL) Thank the whole ELBE team for their help and assistance.

Thank you !