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 CsGaAs 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 lt 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 250C.

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
Photocathode puck cross section heat to 600oC
for up to 3 hours. Neighbouring photocathodes
must not exceed lt 100oC during heatcleaning.
carousel
Keep gap small (lt2mm) 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)