Photocathode Preparation System for the ALICE Photoinjector

PESP 2008 JLAB 1st-3rd Oct

Photocathode Preparation System for the ALICE

Photoinjector

Keith MiddlemanVacuum Science Group

ASTeC, DaresburyUK


Outline

ALICE Gun

Problems encountered

New 3 stage loadlock system

Installation on the ALICE gun

Summary


ALICE Gun and Power Supply

Gun Power SupplyCathode Ball

Anode PlateCeramic

Based on the JLabIR-FEL design


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


The Insulating Ceramic & Cathode Ball


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


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

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


%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



Q.E. ~ 1.5%



Test Set-up in Lab

Gun Set-up

Charge collectorCs channels


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


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


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 ……


HV Breakdown

Discharge point on the Corona ring.


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

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


3 Stage Loadlock


Loading Chamber


Loading Chamber Cathode Holder

Magazineholder

ISO Sealing flange


Cathode Mounting and Holder

Kovar cathode holder

Springclip

Mo cathode mount

Cu or Sapphire


Transfer Arm

Coarse screwthread


Z-stage for driving cathode into position for H2 cleaning

Z-stage


Hydrogen Cleaning Chamber


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


Preparation Chamber


Plan View Preparation Chamber


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


Plan View of ALICE Gun + Loadlock


Side loading of cathode into cathode ball


Side loading mechanism of cathode ball


Drive mechanism inside cathode ball


Vertical Ceramic Gun Design

Side view

Rear view

Mechanical advantagesRear loading of the cathodePossible back illumination


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

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)

Photocathode Preparation System for the ALICE Photoinjector

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