Transcript DC photogun vacuum characterization through photocathode

DC photogun vacuum characterization
through photocathode lifetime studies
Marcy L. Stutzman, Philip Adderley,
Joseph Grames, Matthew Poelker, Ken Surles-Law
Thomas Jefferson National Accelerator Facility
Newport News, Virginia, USA
Jefferson Lab
A
B
C
• CEBAF: Nuclear physics
electron accelerator
laboratory
• 100 kV DC polarized
electron gun
• Recirculating linear
accelerators
• Three experimental halls
running simultaneously
• User community of 2000+
physicists
• Operates 310 days/year,
24 hours/day
• FEL program at JLab
Marcy Stutzman IVC-17
Existing CEBAF photoguns
•Gun chambers vented and baked
each time new photocathode installed
•Activation chemicals in HV chamber
•Full NEG activation may harm
photocathode
• GaAs Photoelectron
gun
– 100 kV
– 200 μA
– 85% polarization
– delivers beam
simultaneously to
three experimental
halls
• CEBAF pressures
~1.2x10-11 Torr
• Guns pumped with
combination of NEG
and ion pumps
Marcy Stutzman IVC-17
Vacuum Characterizations
Ultimate Pressure
measured
1E-11
predicted
1E-12
0
0.2
0.4
0.6
0.8
1
1.2
2
Outgassing Rate (Torr•L/s•cm )
Getter Surface Area (m )
2
Pressure (Torr)
1E-10
Outgassing Rate
5E-12
New 304: 250°C bake
EP: 150°C bake
EP: 250°C bake
4E-12
3E-12
2E-12
1E-12
0
0
5
10
• Ultimate Pressure discrepancy
• Outgassing rates
– Accumulation with
spinning rotor gauge
– throughputmethod
– good agreement
• Untreated 304 SS as expected
• EP/vacuum fire (900ºC, 4 hrs)
– slightly better
– fewer bakes
– fits to guide the eye
• Outgassing rate does not
explain ultimate pressure
discrepancy
15
Bake number
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Pump speed measurement
ST707 Getter
1400
•
Pump Speed (L/s)
1200
1000
•
•
•
800
600
400
200
•
0
0
5.0E-11
1.0E-10
1.5E-10
2.0E-10
Pressure (Torr)
Ti/Zr/V coating
250
Pump Speed (L/s)
200
•
150
100
50
•
0
0
1E-10
2E-10
3E-10
4E-10
5E-10
Hydrogen Pump speed
measurements
– SAES getter ST707
– WP950 modules
Throughput method
High speed >5x10-11 Torr
Activation protocol increases pump
speed
Decrease at low pressures
– Pressure measurement?
– Artifact of measurement?
 P 
S  S max 1  o 
P

Homemade NEG coating
– Higher pressures
– Same behavior
Still no explanation for ultimate
pressure discrepancy
Pressure (Torr)
Marcy Stutzman IVC-17
Photocathode lifetime to diagnose vacuum
laser light IN
electron beam OUT
anode
residual gas
cathode
• GaAs photocathode
activated with cesium and
NF3 or O2
• Quantum efficiency (QE)
falls during operation
– Electrons out per unit
photons in
• Mechanism for QE decay:
– Surface chemistry
corruption
– Crystal damage
• Both mechanisms
dependent on vacuum
Marcy Stutzman IVC-17
Load-locked future of JLab gun
• Successfully run program so far with “vent and bake” gun
• Upcoming experiments with very high current (300 μA)
will require frequent cathode maintenance
• Load-locked designs allow
– Faster cathode change
– Separation of surface chemistry from
• High voltage electrodes
• High voltage (HV) chamber vacuum
– Allow full activation of NEG cartridges in HV chamber
Marcy Stutzman IVC-17
Prototype CEBAF LL gun
• 304 Stainless Steel
• Electropolished, not vacuum
fired
• Cartridge getter pumps on 8
inch ports
– Conductance limitations
• 220 L/s ion pump
– Does fast pump speed
imply good ultimate
pressure?
• Photocathode heating in
relatively poor vacuum of load
chamber
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New CEBAF load-locked gun
Heat/activation chamber
Suitcase
HV chamber
Small bake Load region
• To be installed July,
2007
• High voltage chamber
– 304 SS
– Electropolished
– vacuum fired
– NEG coated
• Smaller surface area
• NEG modules
surround
cathode/anode gap
• Full NEG activation
possible
Additional Benefits
• Photocathode activation through mask
• Four photocathodes can be exchanged in 12 hours
Marcy Stutzman IVC-17
Vacuum measurements
1E-10
New LL
Tunnel
Pressure (Torr)
measured
prototype LL
1E-11
predicted
1E-12
0
0.2
0.4
0.6
0.8
1
1.2
Getter Surface Area (m2)
• Significant design improvement
• Extractor gauge readings no better
– Gauge characterization (x-ray limit)
– Gauge utilization (electrometer, cables)
Marcy Stutzman IVC-17
Photocathode lifetime improvement
Bulk GaAs, green light
• Lifetime improves
with better vacuum
design
– factor >2
• Damage analysis:
TEM, SIMS
• Improve vacuum
design regarding
most damaging
species
Ref: J. Grames et. al SPIN 2006 Kyoto
Marcy Stutzman IVC-17
High Polarization Superlattice Lifetime
Lifetime (Coulombs)
250
200
150
new load lock
100
operational gun
50
0
0
200
400
600
800
1000
Photocurrent (m icroA)
• Tunnel: Lifetimes at high polarization typically
~100 Coulombs running ~100 μA
• New LL gun: Lifetimes over 200 Coulombs
at 250 μA or 1 mA on test beamline
• We anticipate (hope for!) improved lifetime during
operation with new load lock
Reference:
J. Grames et. al
PAC’07 proceedings
Marcy Stutzman IVC-17
Conclusions
• Vacuum improvements incorporated during construction of
New LL gun
– Little improvement seen in ultimate pressure
measurement
– Significant improvement in photocathode lifetime
• Future work
– Better materials
• 316 LN electropolished, then baked at 400ºC for
~1 week - JLab FEL
– Ion pump limitations?
• Anticipate positive operational experience with new LL
gun in CEBAF machine
• Future machines: ILC, EIC, UV FELs…
Marcy Stutzman IVC-17