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Photocathode Preparation System for the ALICE Photoinjector. Keith Middleman Vacuum Science Group ASTeC, Daresbury UK. Outline. ALICE Gun Problems encountered New 3 stage loadlock system Installation on the ALICE gun Summary. ALICE Gun and Power Supply. Gun Power Supply Cathode Ball

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photocathode preparation system for the alice photoinjector

Photocathode Preparation System for the ALICE Photoinjector

Keith Middleman

Vacuum Science Group

ASTeC, Daresbury

UK

outline
Outline
  • ALICE Gun
  • Problems encountered
  • New 3 stage loadlock system
  • Installation on the ALICE gun
  • Summary
alice gun and power supply
ALICE Gun and Power Supply

Gun Power Supply

Cathode Ball

Anode Plate

Ceramic

Based on the JLab

IR-FEL design

slide4

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

original ceramic design
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
new tapered design
New Tapered Design

Ceramic is tapered and it is thought this will aid the braze of the Kovar ring to the ceramic

cathode lifetime problems
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

slide9

Problems During Cathode Activation

Peak current: 770 nA

Dark current: 90 nA

Photo current: 680 nA

Laser power: 45 mW

Laser wavelength: 532 nm

slide11

Problems During Cathode Activation

Test Set-up in Lab

Gun Set-up

Charge collector

Cs channels

cathode lifetime plot 1 st cathode
Cathode Lifetime Plot (1st Cathode)

1/e lifetime = 20.8 hours

improvement in cathode lifetime
Improvement in Cathode Lifetime

1/e lifetime = 102 hours

hv problems
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 ……

hv breakdown
HV Breakdown

Discharge point on the Corona ring.

current alice issues
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
loadlock design
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
loading chamber cathode holder
Loading Chamber Cathode Holder

Magazine

holder

ISO Sealing flange

cathode mounting and holder
Cathode Mounting and Holder

Kovar cathode holder

Spring

clip

Cu or Sapphire

Mo cathode mount

transfer arm
Transfer Arm

Coarse screw

thread

cathode heat cycling
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

cathode carousel
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

vertical ceramic gun design
Vertical Ceramic Gun Design

Side view

Rear view

Mechanical advantages

Rear loading of the cathode

Possible back illumination

summary
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
acknowledgements
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)