Probing Hadron Structure at CEBAF Using Polarized Electron Scattering

M. Poelker, Jefferson LabAPS Meeting, Dallas, TX, April 2006

Structure Functions, Form Factors, Parity Violation, DVCS, GPD, more?

Outline; CEBAF Overview What Can You Expect at CEBAF? Parity Violation Experiments (becoming routine?) New Developments for New Experiments

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

Continuous Electron Beam Accelerator Facility

Chopper

CEBAF Headaches? … … …

CEBAF Benefits; Recirculating LINACs Superconducting Cavities Three Halls; 3x the physics

CEBAF Overview

What I’m going to

talk about

CEBAF Headaches?

Many shared components link experimental programs at neighboring halls Ambitious schedule with frequent energy changes: demands precise knowledge of magnet field maps All beams originate from the same polarized photogun: more complicated compared to thermionic gun Experiments grow more complicated, Beam specifications grow more demanding. Commissioning

at one hall inconvenient to other halls Beamtime oversubscribed: rush to complete 6GeV program

Everyone Gets Beam from Polarized Electron Gun!

CEBAF’s first polarized e-beam experiment 1997 Now polarized e-beam experiments comprise ~80% of our physics program All beams originate from the same 0.5mm spot on one photocathode inside 100kV GaAs photogun (we removed the thermionic gun in 2000) At the moment, there are three polarized e-beam

experiments on the floor;

Hall A: GEn (10uA)

Hall B: GDH (3nA)

Hall C: G0 Backward Angle (60uA)

Shared Spin Manipulator, Shared LINAC

Wien filter spin manipulator at

injector, used to properly orient

spin at Hall

Spin precession at arcs and transport

linesSpin precession angle:

Shared Spin Manipulator, Shared LINAC

Pure longitudinal pol for one hall at any beam energy Many energy and pass configurations provide

simultaneous longitudinal polarization at two halls Simultaneous longitudinal polarization at three halls limited to ~ 2 and 4 GeV In practice however, many settings provide nearly

longitudinal polarization to all three halls

Hall A

Hall B Hall C

No depolarization

through machine

At 5-pass, precession angle

>10,000 degrees!

Wien AngleJ. Grames, et al. PRST-AB 7, 042802 (2004)

CEBAF Photoinjector

Long photocathode lifetime:• Good vacuum with NEGs• Spare-gun• NEG-coated beampipe• No short focal length elements• Wien filter• Photocathodes with anodized edge• Synchronous photoinjection

1997

1998

NOW

Synchronous Photoinjection

DC Light, Most beam thrown away

Three independent RF-Pulsed lasers

Now add prebuncher

Shared Injector Chopper

A

B

C

Efficient beam extraction prolongs

operating lifetime of photogun.

Lasers with GHz pulse repetition rates have been

hard to come by

Lasers don’t turn completely OFF between pulses:

Leakage (aka crosstalk,

bleedthrough)

CEBAF LasersDiode-seed + diode-amp 1996

2000

Harmonic-modelocked Ti-Sapphire

M. Poelker, Appl. Phys. Lett. 67, 2762 (1995).

C. Hovater and M. Poelker, Nucl. Instrum. Meth. A 418, 280 (1998);

Commercial Ti-Sapphire

• 1st commerical 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

Complicated Laser Table

Many lossy optical components; tune mode generators, IAs, isolatorsTime consuming alignment to ensure coincident, colinear beamsNo “clean-up” polarizer for parity UsersFussy Ti-Sapphire lasers; lose phase lock, require weekly maintenance

New Fiber-Based Drive Laser

CEBAFs last laser! Gain-switching better than modelocking; no phase lock problems Very high power Telecom industry spurs growth Useful only because of superlattice photocathode…

J. Hansknecht and M. Poelker, submitted PRST-AB

Other Benefits of Fiber-Based Laser?

Replace lossy laser-table components with telecom stuff?Tune mode generator (fast phase shifter and injector chopper)IA and laser attenuator: fiber amplitude modulatorFiber optic beam combiners?

Extremly good mode quality, good for parity Users?

Low repetition rate beam for particle ID and background studies, using beat frequncy method.

Polarized beam without Pockels cell?

Green version good for RF-pulsed Compton Polarimeter?

Photocathode Material

High QE ~ 10%Pol ~ 35%

Superlattice GaAs: Layers of GaAs on

GaAsPBulk GaAs

Both are results of successful SBIR Programs

“conventional” materialQE ~ 0.15%Pol ~ 75%@ 850 nm

Strained GaAs: GaAs on GaAsP

100

nm

No strain relaxationQE ~ 0.8%Pol ~ 85%@ 780 nm

100

nm

14 pairs

Superlattice reference; T. Maruyama et al, Appl. Phys. Lett. 85, 2640 (2004)

Beam Polarization at CEBAF

Reasonable to request >80% polarization in PAC proposals

P I 2

P I 2sup.

str.

= 1.38

Superlattice Photocathodes

No depolarization over time Cannot be hydrogen cleaned Arsenic-capped No solvents during preparation!

Oct 13 Nov 9QE dropped by factor of 2

Anodized edge: a critical step

Availability1, 2, and 3 Hall Availability History

50%

60%

70%

80%

90%

100%

FY98 FY99 FY00 FY01 FY02 FY03 FY04 FY05 FY06

1-Hall Ops 2-Hall Ops 3-Hall Ops Linear ( 1-Hall Ops)

Linear ( 2-Hall Ops) Linear ( 3-Hall Ops)

What Can a User Expect at CEBAF?

Beam current from 100pA to 120 uA Polarization > 80% Photogun Lifetime ~ 100C (weeks of uninterrupted operation of gun) Availability ~ 70% Leakage from neighboring beams, < 3% Energy Spread 1E-4 (can be made smaller) Charge asymmetry 500ppm routine Parity-Quality…

What is Parity Quality?

ExperimentPhysics

Asymmetry

Max run-average helicity correlated

Position Asymmetry

Max run-average helicity correlated

Current Asymmetry

HAPPEX-I 13 ppm 10 (10) nm 1 (0.4) ppm

G0 Forward 2 to 50 ppm 20 (4 ± 4) nm 1 (0.14 ± 0.3) ppm

HAPPEX-He* 8 ppm 3 nm (3) nm 0.6 (0.08) ppm

HAPPEX-II* 1.3 ppm 2 nm (8)$ nm 0.6 (2.6)$ ppm

Lead 0.5 ppm 1 nm 0.1 ppm

Qweak 0.3 ppm 40 nm 0.1 ppm

Helicity-correlated asymmetry specifications (achieved)

1999

2007

HAPPEx notes: * Part 1 completed 2004, Part 2 during 2005, awaiting final numbers$ Results at Hall A affected by Hall C operation. Expect specs were met in part2

Routine Parity Violation Experiments?

We need: Long lifetime photogun (i.e., slow QE decay) Stable injector Properly aligned laser table (HAPPEx method) Eliminate electronic ground loops Proper beam-envelope matching throughout machine for optimum adiabatic damping: need to develop tools Set the phase advance of the machine to minimize

position asymmetry at target Feedback loops; charge and position asymmetry Specific requirements for each experiment; e.g., 31

MHz pulse repeitition rate, 300 Hz helicity flipping, beam halo < , etc.,

What is HAPPEx Method?• Identify Pockels cells with desirable properites:

– 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 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 HC 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

Origins of HC beam asymmetries cont.

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

Origins of HC Asymmetries cont.

Pockels Cell acts as active lens

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 IN

Use quad photodiode to

minimize position differences

New Developments

Higher Current and High Polarization; > 1 mAProposed new facilities ELIC, eRHIC

Solution: Fiber-based laser + Load locked gun

High Current at High Polarization;Qweak to test standard model180uA at 85% polarization

CEBAF and ELIC

Test Cave LL-Gun and 100 kV Beamline

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

Side-view

Ion Backbombardment Limits Photocathode Lifetime(Best Solution – Improve Vacuum, but this is not easy)

electron beam OUT

residual gas

cathodeionized residual gas hits photocathode

anode

laser light IN

Can increasing the laser spot size improve charge lifetime?

Bigger laser spot –same # electrons,

same # ions

But QE at (x ,y ) degrades more slowly because ion damage distributed over larger area (?)

i i

Reality more complicated, Ions focused to electrostatic center

High Current Lifetime Experiments

342 um and 1538 um laser spots Exceptionally high charge lifetime, >1000C at beam current to 10mA! Lifetime scales with laser spot size but simple scaling not valid

Repeat measurements with high polarization photocathode material

Load Locked Gun Development

No more gun bakeouts! Photocathode replaced in 8 hours versus 4

days.

Installation at CEBAF September, 2006

Plus: • Multiple samples,• No more anodizing,• Better gun vacuum

•Less surface area•No more venting

Longer photocathode lifetime?

Beat Frequency TechniqueNormal Ops;

Three beams at 499 MHzBeat Frequency Technique;One laser at 467.8125 MHz

Halls receives Low Rep Rate Beam at Beat Frequency between Laser and Chopper RF, in this case, 31.1875 MHzWhy? Particle identification, background studies

A

B

C

Polarized beam without PC

Fast RF phase shifter

/2

/2

/4

atten

attensteering mirror

Fiber-based laser

p-polarized

s-polarized

s and ppolarized

60 degree optical delay line

Fast phase shifter moves beam IN/OUT of slit;Downside: extract 2x required beam current

CEBAF Headaches not so bad

Healthy polarized beam program at CEBAF with (mostly) happy Users. Easy to satisfy ~60uA experiments. 100uA beam experiments at high polarization still keep us on our toes (i.e., we have to provide photocathode maintenence 1/mo.). Ongoing gun and laser development to support high current Ops. Parity violation experiments are not yet “routine” but we are getting there. Experience helps, new tools are being developed, better hardwareFiber laser and load locked gun will help a great dealWe’ve enjoyed a great relationship with our Users, hopefully Users feel simialrly about CEBAF accelerator staff.

Routine Parity Violation Experiments

• HC position differences are generated at the source.• “Matching” the beam emittance to the accelerator acceptance realizes damping,

• Well matched beam => position differences reduced.• Poorly matched beam => reduced damping (or even growth).

• Accelerator matching (linacs & arcs) routinely demonstrated.• Injector matching has been arduous, long (~2 year) process.

X-PZT(Source)

Y-PZT(Source)

X-BPM (mm) Y-BPM (mm)

1C-Line

X-BPM (mm) Y-BPM (mm)

1C-Line

1C-Line

1C-Line

without

with