pesp 2008 jlab 1st-3rd oct photocathode preparation system for the alice photoinjector keith...
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PESP 2008 JLAB 1st-3rd Oct
Photocathode Preparation System for the ALICE
Photoinjector
Keith MiddlemanVacuum Science Group
ASTeC, DaresburyUK
PESP 2008 JLAB 1st-3rd Oct
Outline
ALICE Gun
Problems encountered
New 3 stage loadlock system
Installation on the ALICE gun
Summary
PESP 2008 JLAB 1st-3rd Oct
ALICE Gun and Power Supply
Gun Power SupplyCathode Ball
Anode PlateCeramic
Based on the JLabIR-FEL design
PESP 2008 JLAB 1st-3rd Oct
Electron
s
XHV
CeramicCathode
SF6Vessel removed
Cathode ball
Stem
Laser
Anode Plate
• JLab design Cs:GaAs cathode
•500 kV DC supply
•Single Piece Ceramic
•WESGO Proprietary Ceramic
Gun Assembly
PESP 2008 JLAB 1st-3rd Oct
The Insulating Ceramic & Cathode Ball
PESP 2008 JLAB 1st-3rd Oct
Original Ceramic Design
Using Kovar Rings
Moved to a Cu braze design
• 9 leaks, 1 major contamination• All four of the DL ceramics have failed at the Cu brazed joints during the bake cycle
PESP 2008 JLAB 1st-3rd Oct
New Tapered Design
Ceramic is tapered and it is thought this will aid the braze of the Kovar ring to the ceramic
PESP 2008 JLAB 1st-3rd Oct
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
PESP 2008 JLAB 1st-3rd Oct
%5.3
124..
laserP
IEQ
Problems During Cathode Activation
Peak current: 770 nA
Dark current: 90 nA
Photo current: 680 nA
Laser power: 45 W
Laser wavelength: 532 nm
PESP 2008 JLAB 1st-3rd Oct
Problems During Cathode Activation
Q.E. ~ 1.5%
PESP 2008 JLAB 1st-3rd Oct
Problems During Cathode Activation
Test Set-up in Lab
Gun Set-up
Charge collectorCs channels
PESP 2008 JLAB 1st-3rd Oct
Cathode Lifetime Plot (1st Cathode)
y = 0.4221e-0.048x
0
0.5
1
1.5
2
2.5
3
3.5
4
0 5 10 15 20 25 30 35 40 45 50
Time (hours)
QE
(%
)
1/e lifetime = 20.8 hours
PESP 2008 JLAB 1st-3rd Oct
Improvement in Cathode Lifetime
y = 2.6105e-0.0098x
0
0.5
1
1.5
2
2.5
3
3.5
4
0 50 100 150 200 250
Time (hours)
QE
(%
)
1/e lifetime = 102 hours
PESP 2008 JLAB 1st-3rd Oct
HV Problems
-0.1
0
0.1
0.2
0.3
0.4
0 20 40 60 80 100 120
DC Gun HV testsData: 20/09/2006 #166,179,184,190
Ic(curr lim)Icond(#179)Irun(#179)I(#184)I(#166)I(#190)
Cur
rent
, m
A
Voltage, kV
Runresistor
Conditioningresistor
before bake-out
Rods, NO Dome, enclosure present(+100kV) (#184)
NO Rods, NO Dome,negative HV, condit. resistor (#190)
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
0
20
40
60
80
100
120
140
0 20 40 60 80 100 120
Data #194 (30/09/2006)
I(cond)
I(run)
Vac(cond)
Vac(run)
Cur
ren
t, m
A
Va
cuu
m,
x10
E-1
1 m
bar
U
Runresistor
Cond.resistor
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 ……
PESP 2008 JLAB 1st-3rd Oct
HV Breakdown
Discharge point on the Corona ring.
PESP 2008 JLAB 1st-3rd Oct
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
PESP 2008 JLAB 1st-3rd Oct
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
PESP 2008 JLAB 1st-3rd Oct
3 Stage Loadlock
PESP 2008 JLAB 1st-3rd Oct
Loading Chamber
PESP 2008 JLAB 1st-3rd Oct
Loading Chamber Cathode Holder
Magazineholder
ISO Sealing flange
PESP 2008 JLAB 1st-3rd Oct
Cathode Mounting and Holder
Kovar cathode holder
Springclip
Mo cathode mount
Cu or Sapphire
PESP 2008 JLAB 1st-3rd Oct
Transfer Arm
Coarse screwthread
PESP 2008 JLAB 1st-3rd Oct
Z-stage for driving cathode into position for H2 cleaning
Z-stage
PESP 2008 JLAB 1st-3rd Oct
Hydrogen Cleaning Chamber
PESP 2008 JLAB 1st-3rd Oct
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
carousel
Cathode Heat Cycling
PESP 2008 JLAB 1st-3rd Oct
Preparation Chamber
PESP 2008 JLAB 1st-3rd Oct
Plan View Preparation Chamber
PESP 2008 JLAB 1st-3rd Oct
Cathode Carousel
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) Spring clip (red) Inconel
Mounting slot (yellow) Kovar
Carousel plate 2mm Ti (or 316 St.Steel)
Photocathode puck holder
PESP 2008 JLAB 1st-3rd Oct
Plan View of ALICE Gun + Loadlock
PESP 2008 JLAB 1st-3rd Oct
Side loading of cathode into cathode ball
PESP 2008 JLAB 1st-3rd Oct
Side loading mechanism of cathode ball
PESP 2008 JLAB 1st-3rd Oct
Drive mechanism inside cathode ball
PESP 2008 JLAB 1st-3rd Oct
Vertical Ceramic Gun Design
Side view
Rear view
Mechanical advantagesRear loading of the cathodePossible back illumination
PESP 2008 JLAB 1st-3rd Oct
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
PESP 2008 JLAB 1st-3rd Oct
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)