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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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Vacuum Science Group
Gun Power Supply
Based on the JLab
Moved to a Cu braze design
Using Kovar Rings
Ceramic is tapered and it is thought this will aid the braze of the Kovar ring to the ceramic
Base Pressure = 8 x 10-11 mbar
Base Pressure = 1.6 x 10-11 mbar
Peak current: 770 nA
Dark current: 90 nA
Photo current: 680 nA
Laser power: 45 mW
Laser wavelength: 532 nm
Q.E. ~ 1.5%
Test Set-up in Lab
1/e lifetime = 20.8 hours
1/e lifetime = 102 hours
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 ……
Discharge point on the Corona ring.
ISO Sealing flange
Kovar cathode holder
Cu or Sapphire
Mo cathode mount
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
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
Rear loading of the cathode
Possible back illumination
Julian McKenzie – 3D Modelling of the ALICE Photoinjector Upgrade (Friday 3rd October)