Photocathode Preparation System for the ALICE Photoinjector

1. Photocathode Preparation System for the ALICE Photoinjector Keith Middleman Vacuum Science Group ASTeC, Daresbury UK

2. Outline • ALICE Gun • Problems encountered • New 3 stage loadlock system • Installation on the ALICE gun • Summary

3. ALICE Gun and Power Supply Gun Power Supply Cathode Ball Anode Plate Ceramic Based on the JLab IR-FEL design

4. Gun Assembly Cathode ball • JLab design Cs:GaAs cathode • 500 kV DC supply • Single Piece Ceramic • WESGO Proprietary Ceramic Ceramic Cathode SF6 Vessel removed Electrons Laser XHV Stem Anode Plate

5. The Insulating Ceramic & Cathode Ball

6. Original Ceramic Design Moved to a Cu braze design Using Kovar Rings • 9 leaks, 1 major contamination • All four of the DL ceramics have failed at the Cu brazed joints during the bake cycle

7. New Tapered Design Ceramic is tapered and it is thought this will aid the braze of the Kovar ring to the ceramic

8. Cathode Lifetime Problems • During the early bakeouts on the photoinjector, the criteria was to achieve a < 10% pressure drop over 24 hours • This specification left the photoinjector with a residual gas spectrum shown in Scan 1. This led to poor lifetimes due to contaminant species. • The specification was changed as detailed above and Scan 2 shows the residual gas spectrum whilst the photoinjector was at 250°C. Base Pressure = 8 x 10-11 mbar Base Pressure = 1.6 x 10-11 mbar

9. Problems During Cathode Activation Peak current: 770 nA Dark current: 90 nA Photo current: 680 nA Laser power: 45 mW Laser wavelength: 532 nm

10. Problems During Cathode Activation Q.E. ~ 1.5%

11. Problems During Cathode Activation Test Set-up in Lab Gun Set-up Charge collector Cs channels

12. Cathode Lifetime Plot (1st Cathode) 1/e lifetime = 20.8 hours

13. Improvement in Cathode Lifetime 1/e lifetime = 102 hours

14. HV Problems After cathode re-activation on August 30th 2006, the gun exhibited huge out-gassing during HV conditioning. The ensuing vacuum spikes resulted in frequent HV PSU trips, and progress was slow. When we eventually reached 320 – 340 kV, it became clear that the HV PSU current was erratic and then ……

15. HV Breakdown Discharge point on the Corona ring.

16. Current ALICE Issues • Single chamber design • No cathode change possible • In-situ cathode activation • Cathode exposed to bakeout process • Vacuum performance affected by cathode heating • No Hydrogen cleaning possible • Operational downtime due to cathode activation • Mechanical stresses on key components • Repeated problems mean repeated bake processes • Pumping arrangement not optimised for bakeout

17. Loadlock Design • Vacuum Requirements • Loading chamber – 10-9 mbar • Hydrogen cleaning chamber – 10-10 mbar • Preparation chamber – 10-12 mbar • Primarily NEG and SIP pumping • Vacuum firing of all vacuum components (where possible) • Chamber constructed from 316L Stainless Steel • All flanges are 316LN Stainless Steel • Decided not to electropolish the inner surfaces

18. 3 Stage Loadlock

19. Loading Chamber

20. Loading Chamber Cathode Holder Magazine holder ISO Sealing flange

21. Cathode Mounting and Holder Kovar cathode holder Spring clip Cu or Sapphire Mo cathode mount

22. Transfer Arm Coarse screw thread

23. Z-stage for driving cathode into position for H2 cleaning Z-stage

24. Hydrogen Cleaning Chamber

25. Cathode Heat Cycling carousel Photocathode ‘puck’ cross section – heat to 600oC for up to 3 hours. Neighbouring photocathodes must not exceed < 100oC during heatcleaning. Keep gap small (<2mm) to minimise radiant heat leak Double wall radiation ‘baffle’ reflector . Inner surfaces highly polished. Forms closed radiant heat ‘autoclave’ 250W halogen lamp – applied power up to 25W(max). Long term experience good at Novisibirsk and Heidelberg. Alternative is open Nichrome or Tungsten filament

26. Preparation Chamber

27. Plan View Preparation Chamber

28. Cathode Carousel Photocathode puck holder Carousel plate 2mm Ti (or 316 St.Steel) Radial slots & 2mm thin carousel plate limit in-plane heat conduction to neighbouring photocathodes during heating. Calculations indicate that nearest neighbour should not exceed 100oC whilst the heated photocathode can reach to 600oC (at equilbrium steady state condition) Mounting slot (yellow) Kovar Spring clip (red) Inconel

29. Plan View of ALICE Gun + Loadlock

30. Side loading of cathode into cathode ball

31. Side loading mechanism of cathode ball

32. Drive mechanism inside cathode ball

33. Vertical Ceramic Gun Design Side view Rear view Mechanical advantages Rear loading of the cathode Possible back illumination

34. Summary • Ceramic vacuum failures have limited the operation of the ALICE gun • Improved vacuum and activation procedures have led to an improvement in cathode performance. • Repeated failures have led to a new 3 stage loadlock system being designed • Introduced the 3 stage loadlock design • Carousel limited to holding 6 cathodes to ensure minimal heat transfer to other cathodes • Side loading of cathode into cathode ball • Future upgrade may include vertical gun design

35. Acknowledgements • I would like to thank the following people for their contributions to this work: • Lee Jones • Boris Militsyn • Ian Burrows • Barry Fell • Ryan Cash • Julian McKenzie • Alex Terekhov Related presentation: Julian McKenzie – 3D Modelling of the ALICE Photoinjector Upgrade (Friday 3rd October)