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Photocathodes for Rossendorf SRF gun

1 st Topical Workshop on laser based particle sources 20-22, February, 2013 CERN. Photocathodes for Rossendorf SRF gun. Rong Xiang for HZDR SRF- gun team. ELBE radiation source and SRF gun. Since 2007 in operation. OUTLINE Rossendorf SRF gun and cathode system

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Photocathodes for Rossendorf SRF gun

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  1. 1st Topical Workshop on laser based particle sources 20-22, February, 2013 CERN Photocathodes for Rossendorf SRF gun Rong Xiang for HZDR SRF-gunteam

  2. ELBE radiation source and SRF gun Since 2007 in operation

  3. OUTLINE Rossendorf SRF gunandcathodesystem Running experience on Cs2Te in SRF gun Tests on new photocathode candidates GaAs photocathodes for the SRF gun

  4. Choke-Filter HOM Coupler TESLA-Cells Electron Beam 9.4 MeV, 1 mA, 9.4 kW 77 pC - 1 nC Cathode Half-Cell Main Coupler Rossendorf SRF gun and cathode system JochenTeichert, Talk on Friday Laser 1 W, 262 nm • 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

  5. Rossendorf SRF gun and cathode system Cathode and cooling system SRF Gun Transport chamber Load-lock GaAs Mo Mo Pb/Nb Cs2Te Cs2Te

  6. Rossendorf SRF gun and cathode system Transport chamber Requirements for cathode transport: • Load lock system with < 10-9 mbar • Exchange without cavity warmingup & in short time and low particle generation

  7. 2. Cs2Te photocathodes for SRF gun Preparation Lab Monitor1 • UHV (10-10 - 10-9 mbar ) • Halogen-light heating • Ar+ beam cleaning • Evaporation • Qualification Te Cs Monitor2 R. Xiang, PhysRevSpecialTopics_13_043501

  8. 2. Cs2Te photocathodes for SRF gun Typical preparation process: Plug mechanical polish ( Ra=8-10nm) Clean Assemble in clean room Pump and bake Deposited in Prep. Chamber Stored in transport chamber Transport to SRF gun Standard Te+Cs

  9. 2. Cs2Te photocathodes for SRF gun Cathode #250310Mo in gun from 2010.5.25 to2011.7.26 Q.E. 1% total beam time 1013 h extracted charge 35 C SEM / EDX for Cs2Te on Mo Cs/Te=2.3

  10. 2. Cs2Te photocathodes for SRF gun • Open questions: • QE decrease during transport • Strong multipacting around cathode plug • Dark current • layer destroyed during operation • 8 Cs2Te cathodes tested in gun • QE ~1 % in months • I dark~ 120 nA@ 15 MV/m • Cs2Te contributed 30% of dark current • ɛthermal~ 0.7 mm·mrad/r(mm) 120nA R. Xiang, et al., FEL2010, Sweden

  11. 3. Tests new photocathode candidates : Cs3Sb Sb-source in boat • Use Cs2Te chamber, vacuum ~10-9 mbar • On Mo plug, @ 160oC, two-cycle “Yo-yo” process • fresh QE reached 1.5 % • SEM surface morphology Cs3Sb out from chamber Optical-polished Mo substratum On Mo Substrate, Cs/Sb rate 2.7 R.Xiang, et al., IPAC2012

  12. 3. Tests new photocathode candidates: GaN (Cs) • wide band gap (3.4 eV) • negative electron affinity (NEA) • Robust: 10-9 mbar • GaN on sapphire with Au/Ni electrode • Onlycesium • 262 nmlaser • ~ 10-9mbar • R.Xiang, et al., ERL 2011

  13. 4. GaAsphotocathodesforthe 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. Plug on transport plate 4. GaAsphotocathodesforthe SRF gun Plug handle • Transport only small plugs (up to 6 plugs) • HZB-carrier for storage and transport • Cathode 106 mm long • plug ɸ10mm x 6mm (modified from HZB-plug) • Easier for transport • Easier for heat-cleaning GaAs

  15. 4. GaAsphotocathodesforthe SRF gun wafer treatment: 1. Wash in 4:1:1 acid solution (H2SO4, H2O2 and H2O) 2. rinse with pure water and methanol 3. Blow with dry N2 Result: reduce the Oxygen content on the surface p-GaAs(100) crystal 400μm, 5 x1018- 5 x1019 /cm3 Zn-doping Resistivity of 0.0053 Ohm cm Mobility of 209 cm2/V s SEM / EDX Etched GaAs sample

  16. 4. GaAsphotocathodesforthe SRF gun time (hh:mm) GunwithGaAscrystal (0.35mm) and holder Without RF, PN2=34.52W WithRFmax=5.5MV/m on surface, PN2=35 W No detectable dark current from GaAs-crystal. But after activation? R. Xiang, Task 10.7, EuCARD-SRF Annual Review 2012

  17. 4. GaAsphotocathodesforthe 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 • Designed process: • Heat @ 640oC for 5min with focused Halogen light, cool down to 100oC • Activate by using SAES cesium dispenser and computer controlled O2 leak valve • Green laser for photocurrent detect Preparation chamber with load lock system

  18. 4. GaAsphotocathodesforthe SRF gun New photocathode Lab. in accelerator building is in building. Cs2Te preparation system and the new GaAs chamber will be moved together.

  19. Thank you! 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.

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