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Thomas Jefferson National Accelerator Facility
Polarized Electron Beam at CEBAF
Matt Poelker
13 June, 2006
Science and Technology ReviewJefferson Lab
June 12-13, 2006
Polarized Source Group: M. Poelker, P. Adderley, J. Brittian, J. Clark, J. Grames,J. Hansknecht, James McCarter, M. Stutzman, K. Surles-Law
(3 scientists, 4 technical staff, 2 graduate students)
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Thomas Jefferson National Accelerator Facility 2
Highlights Since the Last S&T Review
Beam Polarization 85% typical, 80% guaranteed
New Fiber-Based Drive Laser: high power, reliable
Parity Violation Experiments: becoming more routine
Load-Locked Gun developments for high current future experiments
Low Voltage Mott polarimeter for photocathode studies
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Thomas Jefferson National Accelerator Facility 3
Continuous Electron Beam Accelerator Facility
AB
CA
B
C
A B C
Pockels cell
Gun
0.6 GeV linac(20 cryomodules)
1497 MHz67 MeV injector
(2 1/4 cryomodules)1497 MHz
RF separators499 MHz
Double sidedseptum
499 MHz, = 120
RF-pulsed drive lasers
Wien filter
Chopper
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Everyone Gets Beam from Pol. Electron Gun!
• CEBAF’s first polarized e-beam experiment 1997
• Now polarized beam experiments comprise ~ 80% of our physics program
• All beam originates from the same 0.5mm spot on one photocathode inside 100kV GaAs photogun (the thermionic gun was removed in 2000)
• For example, during April 2006 there were three high profile polarized beam experiments on the floor simultaneously;
– Hall A: Gen (10uA)
– Hall B: GDH (3nA)
– Hall C: G0 Backward Angle (60uA)
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Photocathode Material
High QE ~ 10%Pol ~ 35%
Bulk GaAs
“conventional” materialQE ~ 0.15%Pol ~ 75%@ 850 nm
Strained GaAs: GaAs on GaAsP
100
nm
Superlattice GaAs: Layers of GaAs on GaAsP
No strain relaxationQE ~ 0.8%Pol ~ 85%@ 780 nm
100
nm
14 pairs
Both are results of successful SBIR Programs
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Beam Polarization at CEBAF
P I 2
P I 2sup.
str.
= 1.38
Experiment Figure of
Merit
Reasonable to request >80% polarization in PAC proposals
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Superlattice Photocathodes• Success required ~ 1 year of effort• Cannot be hydrogen cleaned (M. Baylac)• Arsenic capped (worked with vendor SVT)• No solvents during preparation! (M. Stutzman)
Anodized edge: a critical step
No depolarization over time!
Oct 13 Nov 9QE dropped by factor of 2
Pol
ariz
atio
n
M. Baylac et al., “Effects of atomic hydrogen and deuterium exposure on high polarization GaAs photocathodes” PRST-AB 8, 123501 (2005)
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Synchronous Photoinjection
Only electrons within 110 ps window can be accelerated. Electrons outside window are dumped in the chopper.
Efficient beam extraction prolongs operating lifetime of
photogun
Lasers with GHz pulse repetition rates have been hard to come by
Three independent RF-Pulsed lasers
DC drive laser, Most beam thrown away
Chopper viewer
A
B
C
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Commercial Ti-Sapphire Laser
• 1st commercial laser w/ 499 MHz rep rate• Higher power compared to diode lasers• Wavelength tunable for highest
polarization• Feedback electronics to lock optical pulse
train to accelerator RF
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Thomas Jefferson National Accelerator Facility 10
TJNAF 12 Month Time Accounting by System
0
24
48
72
96
120
144
168
192
216
240
264
288
312
336
360
384
408
June'05 55.4%358/648hrs. July'05 65.2%227/570hrsAug'05 82.0%155/742hrsSept'05 84.6%146/560hrsOct'05 82.7%123/473hrsNov'05 75.2% 130/552hrsDec'05 85.2%80/535hrsFeb'06 90.0%117/571hrsMarch'06 79.9%268/699hrsApril'06 81.2%125/705hrsMay'06 84.1%113/413hrs
1% line
Hours Lost
System Availability FY05Q4 – FY06Q3
Realign Ti-Sapphire lasers
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New Fiber-Based Drive Laser
CEBAF’s last laser! Gain-switching better than modelocking; no phase lock problems Very high power Telecom industry spurs growth, ensures availability Useful because of superlattice photocathode (requires 780nm)
J. Hansknecht and M. Poelker, Phys. Rev. ST Accel. Beams 9, 063501 (2006)
Ti-Sap power
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Other Benefits of Fiber Drive Laser• Maybe replace some lossy optics components with telecom stuff?
• Green version for RF-pulsed Compton Polarimetry, FEL Drive Laser
• “Beat Frequency Technique” to create Low Rep Rate Beam for Particle Identification at Halls:
Beat Frequency Technique;One laser at 467.8125 MHz
Normal Ops; Three beams at 499 MHz
A
B
C
Every 15th pulse delivered to hall:
31 MHz beam
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What is “Parity Quality”?Helicity-correlated asymmetry specifications
ExperimentPhysics
Asymmetry
Max run-average helicity correlated
Position Asymmetry
Max run-average helicity correlated Current Asymmetry
Spec Achieved Spec Achieved
HAPPEx-I 13 ppm 10 nm 10 nm 1 ppm 0.4 ppm
G0 Forward 2 to 50 ppm 20 nm (4 ± 4) nm 1 ppm (0.14 ± 0.3) ppm
HAPPEx-He [2004]HAPPEx-He [2005]
8 ppm 3 nm3 nm20* nm
0.6 ppm0.08 ppm0.1 ppm
HAPPEx-II-H [2004]HAPPEx-II-H [2005]
1.3 ppm 2 nm8** nm1 nm
0.6 ppm2.6** ppm0.1 ppm
Lead 0.5 ppm 1 nm - 0.1 ppm -
Qweak 0.3 ppm 20 nm - 0.1 ppm -
1999
2008 * Results affected by electronic crosstalk at injector. ** Results at Hall A affected by Hall C operation. Spec was met in 2005 run.
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Routine Parity Violation Experiments?
We need: Long lifetime photogun (i.e., slow QE decay) Stable injector (especially RF phases) Properly aligned laser table, pockels cell (HAPPEx method) Proper beam-envelope matching throughout machine for
optimum adiabatic damping Set the phase advance of the machine to minimize position
asymmetry at target Eliminate electronic ground loops: isolate electronics Feedback loops; charge and position asymmetry Specific requirements for each experiment; e.g., 31 MHz
pulse repetition rate, 300 Hz helicity flipping, beam halo < , etc.,
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What is HAPPEx Method?
• Developed jointly with Source Group• Identify Pockels cells with desirable properties:
– Minimal birefringence gradients– Minimal steering– Must be verified through testing!
• Install Pockels cell using good diagnostics:– Center to minimize steering– Rotationally align to minimize unwanted birefringence
• Adjust axes to get small (but not too small) analyzing power.• Adjust voltage to get maximum circular polarization!• Use feedback to reduce charge asymmetry.
– Pockels cell voltage feedback maximizes circular polarization.– “Intensity Asymmetry” Pockels cell provides most rapid feedback.– During SLAC E158, both were used.
• If necessary, use position feedback, keeping in mind you may just be pushing your problem to the next highest order.
From G. Cates presentation, PAVI04 June 11, 2004
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Origins of Helicity Correlated Beam Asymmetries
Pockels Cell = active lens. Laser beam needs to pass through center of cell.
From G. Cates presentation, PAVI04 June 11, 2004
Translation (inches)
X p
osit
ion
dif
f. (
um
)Y
pos
itio
n d
iff.
(u
m)
Red, IHWP Out
Blue, IHWP INNo HV
HV +
HV -
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New Developments
High Current at High Polarization;Qweak to test standard model, 2008180 uA at 85% polarization
Higher Current, High Polarization; ~ > 1 mAProposed new facilities ELIC, eRHIC
High Current, No Polarization: ~ 100mAJLab FEL, electron cooling
Solution: Fiber-based laser + Load locked gun
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Load Locked Gun for Qweak
Bulk GaAs
100 kV load locked gun
Faraday CupBaked to 450C
NEG-coated large aperture beam pipe
DifferentialPumps w/ NEG’s
1W green laser, DC, 532 nm
Focusing lens on x/y stage
Spot size diagnostic
Insertable mirror
Load locked gun: replace photocathodes quickly without bakeout. 8 hours versus 4 days.
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Lifetime versus Laser Spot Size
• Exceptionally high charge lifetime, >1000C at beam current to 10mA!
• Lifetime scales with laser spot size but simple scaling not valid. Factor 10 instead of factor 20.
• Repeat measurements with high polarization photocathode material
Imperfect vacuum limits photocathode lifetime - damage from ion backbombardment
Can we increase operating lifetime by merely increasing the laser spot size? Same number electrons, same number ions, but distributed over larger area.
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New Load-Locked Gun
• Better than first load locked gun design
• No more edge-anodizing
• Multiple samples
• Better vacuum in high voltage chamber
– No more venting
– Less surface area
– NEG coated
• Longer photocathode lifetime?
Commissioning now, Ready for installation Fall 2006
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R&D program to obtain polarization > 90%
• “Spintronics”, with graduate student James McCarter and Dr. Stuart Wolf of University of Virginia. New photocathode material.
• Collaborating with Dr. Tim Gay of University of Nebraska, polarimeter expert. (We have borrowed his polarimeter)
Low voltage gun and mini-Mott polarimeter
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Thomas Jefferson National Accelerator Facility 22
Conclusions and Future Plans
The Polarized Source Group will:
Continue to deliver high polarization beam from long lifetime photoguns, using superlattice photocathodes and reliable fiber-based lasers
Install our new load-locked gun, to improve operating lifetime and support Qweak and other high current experiments
Support parity violation experiments that have tighter and tighter beam specifications
Continue working on exciting R&D projects:
• Lifetime studies at current > 1 mA using load locked gun and high polarization photocathode material
• Mini-Mott commissioning + photocathode studies to provide polarization >90%
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Based on Optimistic 07 Budget
We will:
Purchase more superlattice photocathode material, to keep us happy for many years, just in case vendor loses interest.
Purchase two more fiber-based laser systems (that would give us one for each hall plus green version for unpolarized beam experiments)
One more staff scientist to manage R&D program at Test Cave, to replace Maud Baylac who returned to France
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Source Group Recent PublicationsPapers:
“A High Average Current Polarized Electron Source with Long Cathode Operational Lifetime,” C. K. Sinclair, M. Poelker, P. A. Adderley, B. M. Dunham, J. C. Hansknecht, P. Hartmann, J. S. Price, P. M. Rutt, W. J. Schneider, and M. Steigerwald, in press.
M. Poelker, J. Grames, J. Hansknecht, R. Kazimi, J. Musson, “Generation of Electron Microbunches at Low Repetition Rates Using Beat Frequency Technique”, in press.
“Synchronous Photoinjection Using a Frequency-Doubled Gain-Switched Fiber-Coupled Seed Laser and ErYb-Doped Fiber
Amplifier,” J. Hansknecht and M. Poelker, Phys. Rev. ST Accel. Beams 9, 063501 (2006) “The Effects of Atomic Hydrogen and Deuterium Exposure on High Polarization GaAs Photocathodes,” M. Baylac, P.
Adderley, J. Brittian, J. Clark, T. Day, J. Grames, J. Hansknecht, M. Poelker, M. Stutzman, A.S. Terekhov and A.T. Wu, Phys. Rev. ST Accel. Beams 8, 123501 (2005).
Conferences:
“Probing Hadron Structure at CEBAF Using Polarized Electron Scattering,” M. Poelker, presented at the annual meeting of the American Physical Society, Dallas, TX, April 2006.
“Operation of CEBAF photoguns at average beam current > 1 mA,” M. Poelker, J. Grames, P. Adderley, J. Brittian, J. Clark, J.
Hansknecht, M. Stutzman, Polarized Sources and Targets Workshop, Nov. 14-17, 2005, Tokyo, JAPAN. “Polarized Photoguns and Prospects for Higher Current,” M. Poelker, Workshop on Energy Recovered Linacs,
Jefferson Lab, March 19-22, 2005.
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Thomas Jefferson National Accelerator Facility 25
Origins of Helicity Correlated Beam Asymmetries
maximumanalyzingpower
minimumanalyzingpower
Bea
m C
harg
e A
sym
met
ry
Rotating Halfwaveplate Angle
Photocathode QE Anisotropy, aka Analyzing Power
Different QE for different orientation of linear polarization
GaAs photocathode
From G. Cates presentation, PAVI04 June 11, 2004
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Thomas Jefferson National Accelerator Facility 26
Origins of Helicity Correlated Beam Asymmetries
Gradient in phase shift leads to gradient in charge
asymmetry which leads to beam profiles whose
centroids shift position with helicity.
From G. Cates presentation, PAVI04 June 11, 2004
Non-uniform polarization across laser beam + QE anisotropy…
Pockels cell aperture