An EIC-like Polarized Electron Injector: Source, Beam & Polarimeter(s) Joe Grames Jefferson Lab

Workshop on Precision Electron Beam Polarimetry for the Electron Ion Collider, University of Michigan, Aug 23-24, 2007

An EIC-like Polarized Electron Injector:Source, Beam & Polarimeter(s)

Joe GramesJefferson Lab

August 23, 2007


Challenging e- Source Requirements

High Electron Polarization> 80 %

High Current~ 1 mA for JLab ELIC~ 25 (250) mA for BNL eRHIC~ Surface Charge Limit

Sustainable Photocathode (QE ~1%)2.5 mA => 210 C/day (~ state of the art lifetime)25 mA => 2.1 kC/day (~ represents significant effort)250 mA => 21 kC/day (~ ?)

Laser Heating (6 mA/Watt/%)photocathode cooling becomes necessary I > ~few mA?photocathode lifetime => laser “headroom” necessary

(~10x)


GaAs Photocathodes: last 30 years…

High QE ~ 8%Pol ~ 35%@ 780 nm

Superlattice GaAs: Layers of GaAs on

GaAsPBulk GaAs

“conventional” materialQE ~ 0.2%Pol ~ 75%@ 850 nm

Strained GaAs: GaAs on GaAsP

100

nm

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

100

nm

14 pairs

FOM I P2


P I 2

P I 2sup.

str.

= 1.38

Experiment Figure of Merit

Routinely Achieve Stable Polarization ~ 85%

Pola

riza

tion

85%

No depolarization over time!

QE reduced by 2x, but polarization constant One sample used from Sept ‘05 through Apr ‘07 7 activations, ~ 1000 C extracted, I ~ 200uA Max QE: 0.7 to 0.4%, P ~ constant at 85%


New Fiber Technology Based Drive 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

Diode


Compact, Off-The Shelf, Rack Mountable…

RF locked low-power1560 nm fiber diode

High power 1560 nm fiber amplifier

Non-linear frequency doubling converter for 1560 nm to 780 nm


New CEBAF “load-lock” gun

“suitcase”

Loading chamber

Preparation/activation chamber

HV chamber

Vent/bake gun

x4

Goal: 8 hours swap photocathodePresent: ~12 hours


Outgassing Rates vs. Bakes

0 E+00

1 E-12

2 E-12

3 E-12

4 E-12

5 E-12

6 E-12

0 2 4 6 8 10 12 14

Bake number

Ou

tgas

sin

g R

ate

304SS without (blue) and with (red) electroplishing and vacuum firing

Improvements to Gun Vacuum

304 SS: Electropolished & Vacuum Fired(AVS: 3 hrs @ 900 C @ 3x10-6 T)

NEG coating(Ti/Zr/V)100 hrs @ 70 C200 L/sec


Comparing NEW & OLD load locked guns

OLD

NEW

Bulk GaAs, Green light and DC beam

“Further Measurements of Photocathode Operational Lifetime at Beam Current > 1mA using an Improved 100 kV DC High Voltage GaAs Photogun,” J. Grames, et al., Proceedings Polarized Electron Source Workshop, SPIN06,

Tokyo, Japan

Vacuum gauges indicated same pressure in both guns, Suggesting gauges don’t work below 1.5x10-11Torr


1 mA from a High Polarization Photocathode*

Parameter Value

Laser Rep Rate 499 MHz

Laser Pulselength 30 ps

Wavelength 780 nm

Laser Spot Size 450 mm

Current 1 mA

Duration 8.25 hr

Charge 30.3 C

Lifetime 210 C

Charge Lifetime 160 kC/cm2

High Initial QE

•Vacuum signals•Laser Power•Beam Current

* Electron polarization not measured


An EIC-like Polarized e- Source

100

nm

Superlattice GaAs: Layers of GaAs on

GaAsP

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

chekc14 pairs

Fiber-based Laser

Good gun Good Photocathode Good Laser


What are desirable (necessary) features of electron polarimeters?

• Large total analyzing power

• High luminosity to rapidly achieve small statistical uncertainty

• Non-invasive continuous measurement does not disrupt experiment

• Designs with reduced sensitivity to major systematics

An EIC-like Injector Polarimeter

= Pbeam · Ptarget · Atotal analyzing power

N= ·(Pb·Pt ·A) 2Pb

Pb

2-1


Mott Scattering

Spin-orbit coupling of the beamelectron and the target nucleus.

= 1+ S() Pb·k k´

| k k´|

Sherman Phys.Rev. 103(6) 1956, p1601-7

(-)

Ato

t Target thickness effects• Dilution by multiple/plural scattering• Sherman function sets scale• Target thickness extrapolation necessary• MeV double-scattering is important


High cross-section of low energy (<1 MeV) Mott polarimeters is problematic:

• Significant plural and multiple scattering => reduces effective analyzing power• Beam current limited to nanoamps

High energy Mott scattering (MAMI, 1994)• J. Sromicki demonstrated Mott scattering experiments from lead at 14 MeV

J. Sromicki, Phys.Rev.Let. 81(1), 1999, p.57-60• Reduced cross-section => A currents are tolerated and dilution of the analyzing power is suppressed => sensitivity to target thickness is similarly reducedUncertainty of Sherman function

• Coulomb screening at lower energy Ross et.al Phys.Rev A 38(12) 1988, p6055-8 • Finite nuclear size at higher energy

Ugincius et. al Nucl.Phys. A158 1970, p418-32

20% effect at 14 MeV1.5% effect at 5 MeV

Mott at Higher (MeV) Energy


Jlab 5 MeV Polarimeter• Jlab built a 5 MeV Mott polarimeter (typ. 1m Au foil and 2 A beam current)

J.S. Price et al., Pol. Gas Targets and Pol. Beams 7th Int’l. Workshop, Urbana, IL 1997• Inelastic background discrimination was the largest problem • HAPPEX used injector Mott results with ~5% uncertainity

JLab 5 MeV Mott Polarimeter

Pb ~ 70%Atot ~ 40%Target: 1m Au

= Atot · Pb =N+ + N-

N+ - N-


Late 90’s M. Steigerwald joined the source group from MAMI

• Dramatic improvement eliminating background signal by means of collimation, shielding, time of flight, and coincidence methods

• Mott studied over range of 2-8 MeV with Au, Ag, Cu foils.

• Results presented at Spin 2000; M. Steigerwald, 14th International Spin Physics Symposium

500 ÅGold

Reducing Mott Systematics


Effective Analyzing Power

Collaboration with Horowitz at Indiana Univ. for Sherman function calculations (dominant contribution of total uncertainty about 1%)

Applied double-scattering model to describe dilution of AP in targets of finite thickness.

PRL describing analysis, model of double scattering, and results of 1.1% total measurement was drafted, but not published.


Mott @ 20 keV, 100 keV, 500 keV & 5 MeV

20 kV “mini”-Mott will mate to new Load Lock Gun

5 MeV100 keV

or500 keV

New,July 2007


GaAs Photoguns @ Higher Average Current

Year

Ave

. Bea

m C

urre

nt (

mA

)

First polarized beam from GaAs photogun

eRHIC

First low polarization, then

high polarization at CEBAF

JLab FEL program with unpolarized

beam

Cornell/JLab(construction)

Qweak

(2009)

ELIC(LL demo)

JLab Load Lock


Conclusions

EIC-like Electron SourceGreat progress, but work still to be done…

• 10-100x increase in superlattice lifetime• laser cooling for 1 … 10 …100 mA sources• gun technology for Vacuum or HV (e.g., 200 kV gun)• surface charge limit (working with vendor)

EIC-like Injector Polarimeter~MeV Mott can be a “1%”-ish polarimeter

• limiting uncertainty is Sherman function• invasive, yet rapid• useful for the e- source scientist• useful for polarimeter cross-comparisons

An EIC-like Polarized Electron Injector: Source, Beam & Polarimeter(s) Joe Grames Jefferson Lab

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