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

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

3. 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

4. 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 ….

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

6. 2.1 SRF guns running at ELBE
e--beam
SRF Gun I
SRF Gun II
SRF Gun-II in ELBE hall

7. 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

8. 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 MV/m
No cathode in SRF cavity: No degradation with cathode

9. 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

10. 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): dB
R/Q für 80% Feldverteilung bei 40MV/m: r=168 Ohm
Epk/Eacc=2.556, Ekin=7.82MeV

11. 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

12. 3. Performance of SC cavity at ELBE
Contaminated by cesium compound
- disappeared Cs2Te cathodes # , # 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
in gun
Cs2Te layer disappeared
in gun

13. 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.

14. ? 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

15. 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)

16. 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.

17. Thank you !