1. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Photocathodes for Rossendorf SRF gun Rong Xiang for HZDR SRF-gun team 1 st Topical Workshop on laser based particle sources 20-22, February, 2013 CERN

2. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 2 ELBE radiation source and SRF gun Since 2007 in operation

3. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 3 OUTLINE 1.Rossendorf SRF gun and cathode system 2.Running experience on Cs 2 Te in SRF gun 3.Tests on new photocathode candidates 4.GaAs photocathodes for the SRF gun

4. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 4 1.Rossendorf SRF gun and cathode system HOM Coupler TESLA-Cells Choke-Filter Cathode Electron Beam 9.4 MeV, 1 mA, 9.4 kW 77 pC - 1 nC Laser 1 W, 262 nm Half-Cell Main Coupler Laser and photo cathode short and highly charged bunches RF field low emittance and high peak brightness Superconducting cavity CW operation and high average current More info: A.Arnold, et al., NIM A 593(2008) J. Teichert, et al., J. Phys.: Conf. Ser. 298 012008 Jochen Teichert, Talk on Friday

5. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 5 1.Rossendorf SRF gun and cathode system Transport chamber Load-lock Cs 2 Te Pb/Nb Mo Cs 2 Te SRF Gun GaAs Cathode and cooling system

6. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 6 Requirements for cathode transport: Load lock system with < 10 -9 mbar Exchange without cavity warming up & in short time and low particle generation Transport chamber 1.Rossendorf SRF gun and cathode system

7. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 7 Preparation Lab 2. Cs 2 Te photocathodes for SRF gun R. Xiang, PhysRevSpecialTopics_13_043501 Te Cs Monitor1 Monitor2 UHV (10 -10 - 10 -9 mbar ) Halogen-light heating Ar+ beam cleaning Evaporation Qualification

8. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 8 2. Cs 2 Te photocathodes for SRF gun Typical preparation process: 1.Plug mechanical polish ( Ra=8-10nm) 2.Clean 3.Assemble in clean room 4.Pump and bake 5.Deposited in Prep. Chamber 6.Stored in transport chamber 7.Transport to SRF gun Standard Te+Cs

9. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 9 Cathode #250310Mo in gun from 2010.5.25 to 2011.7.26 Q.E. 1% total beam time 1013 h extracted charge 35 C Cs/Te=2.3 SEM / EDX for Cs 2 Te on Mo 2. Cs 2 Te photocathodes for SRF gun

10. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 10 2. Cs 2 Te photocathodes for SRF gun Open questions: QE decrease during transport Strong multipacting around cathode plug Dark current layer destroyed during operation 120nA 8 Cs 2 Te cathodes tested in gun QE ~1 % in months I dark ~ 120 nA @ 15 MV/m Cs 2 Te contributed 30% of dark current ɛ thermal ~ 0.7 mm·mrad/r(mm) R. Xiang, et al., FEL2010, Sweden

11. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 11 3. Tests new photocathode candidates : Use Cs 2 Te chamber, vacuum ~10 -9 mbar On Mo plug, @ 160 o C, two-cycle “Yo-yo” process fresh QE reached 1.5 % SEM surface morphology On Mo Substrate, Cs/Sb rate 2.7 Optical-polished Mo substratum Cs 3 Sb R.Xiang, et al., IPAC2012 Sb-source in boat Cs 3 Sb out from chamber

12. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 12 GaN (Cs) GaN on sapphire with Au/Ni electrode Only cesium 262 nm laser ~ 10 -9 mbar wide band gap (3.4 eV) negative electron affinity (NEA) Robust: 10 -9 mbar R.Xiang, et al., ERL 2011 3. Tests new photocathode candidates:

13. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 13 4. GaAs photocathodes for the SRF gun preparation and transfer system for GaAs (Cs,O) Close to SRF gun XHV chamber 10 -11 mbar Green light First cathode planned in 2013 Interesting points: QE / lifetime in SRF gun dielectric rf loss on crystal temporal response thermal emittance potential field emission dark current polarized beam

14. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 14 Cathode 106 mm long plug ɸ10mm x 6mm (modified from HZB-plug) Easier for transport Easier for heat-cleaning 4. GaAs photocathodes for the SRF gun GaAs Transport only small plugs (up to 6 plugs) HZB-carrier for storage and transport Plug on transport plate Plug handle

15. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 15 original Etched sample O content2.5%1.2% Ga/As1.191.15 Etched GaAs sample 4. GaAs photocathodes for the SRF gun wafer treatment: 1. Wash in 4:1:1 acid solution (H 2 SO 4, H 2 O 2 and H 2 O) 2. rinse with pure water and methanol 3. Blow with dry N2 Result: reduce the Oxygen content on the surface SEM / EDX p-GaAs(100) crystal 400μm, 5 x10 18 - 5 x10 19 /cm 3 Zn-doping Resistivity of 0.0053 Ohm cm Mobility of 209 cm 2 /V s

16. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 16 No detectable dark current from GaAs-crystal. But after activation? time (hh:mm) Gun with GaAs crystal (0.35mm) and holder Without RF, P N 2 =34.52W With RF max =5.5MV/m on surface, P N 2 =35 W 4. GaAs photocathodes for the SRF gun R. Xiang, Task 10.7, EuCARD-SRF Annual Review 2012

17. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 17 4. GaAs photocathodes for the SRF gun Preparation chamber for activation test: Ø300 SS chamber, with NEG pumps and ion getter pump. First activation will be performed in the following months samples through load lock system directly to preparation chamber Preparation chamber with load lock system Designed process: 1.Heat @ 640 o C for 5min with focused Halogen light, cool down to 100 o C 2.Activate by using SAES cesium dispenser and computer controlled O 2 leak valve 3.Green laser for photocurrent detect

18. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 18 4. GaAs photocathodes for the SRF gun New photocathode Lab. in accelerator building is in building. Cs 2 Te preparation system and the new GaAs chamber will be moved together.

19. Rong Xiang I r.xiang@hzdr.de I www.hzdr.de Member of the Helmholtz Association Page 19 Acknowledgement We acknowledge the support of the European Community-Research Infrastructure Activity under the FP7 programme since 2009 (EuCARD, contract number 227579) as well as the support of the German Federal Ministry of Education and Research grant 05 ES4BR1/8. Thank you!