operated by jefferson science associates, llc for the u.s. department of energy thomas jefferson...

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy

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)


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



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



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)



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



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



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)



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



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



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



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



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



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.



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.,



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



Origins of Helicity Correlated Beam Asymmetries

Pockels Cell = active lens. Laser beam needs to pass through center of cell.


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 -



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



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.



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.



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



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



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%



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



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.




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





Gradient in phase shift leads to gradient in charge

asymmetry which leads to beam profiles whose

centroids shift position with helicity.


Non-uniform polarization across laser beam + QE anisotropy…

Pockels cell aperture

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