Polarized Electrons Polarized Electrons for Linear Collidersfor Linear Colliders

J. E. Clendenin, A. Brachmann, E. J. E. Clendenin, A. Brachmann, E. L. Garwin, R. E. Kirby, D.-A. Luh, T. L. Garwin, R. E. Kirby, D.-A. Luh, T.

Maruyama, R. Prepost, C. Y. Maruyama, R. Prepost, C. Y. Prescott, J. C. Sheppard, and J. Prescott, J. C. Sheppard, and J.

Turner Turner

OutlineOutline

1.1. ChargeCharge

2.2. PolarizationPolarization

3.3. OtherOther

4.4. ConclusionsConclusions

1.1. Collider Charge Collider Charge RequirementsRequirements

ParameterParameter ILCILC ILCILCat Sourceat Source SCRFSCRF NCRFNCRF

NNe,e,pulsepulse nCnC 6.4*6.4* 2.4*2.4*

zz nsns 22 0.50.5

IIpulse, avgpulse, avg AA 3.23.2 4.84.8

*Twice the IP requirement*Twice the IP requirement

Generating Polarized Electrons Generating Polarized Electrons

from GaAsfrom GaAs Illuminate p-doped GaAs (or itsIlluminate p-doped GaAs (or itsanalogues) crystal with circularlyanalogues) crystal with circularlypolarized monochromatic lightpolarized monochromatic lighttuned to the band-gap edge.tuned to the band-gap edge.Absorbed photons promote eAbsorbed photons promote e--

from filled VB states to CB. CB efrom filled VB states to CB. CB e--

eventually reach surface.eventually reach surface.

with Cs(O) lowers it several additional eV, resulting in the vacuum level beinglower than the CBM in the bulk (NEA surface). If the cathode is biasednegative, CB electrons at the surface are emitted into vacuum.

In p-doped materials,band-bending lowers workfunction by ~1/3 of the 1.4 eVband gap. Treating surface

Space Charge Limit (SCL)Space Charge Limit (SCL)

(Child’s Law)(Child’s Law)

SLC DC gun:SLC DC gun:● ● Cathode bias -120 kV to keep max fieldsCathode bias -120 kV to keep max fields

<8 MV/m<8 MV/m● ● Low fields necessary to minimize the dark Low fields necessary to minimize the dark

current that degrades the QEcurrent that degrades the QE

● ● GaAs crystal 2-cm dia. decreases GaAs crystal 2-cm dia. decreases jjee, , butbutincreases beam emittance at sourceincreases beam emittance at source

23

Vje

For SLC, low-energy beam transport (various apertures in For SLC, low-energy beam transport (various apertures in the 3-4 cm range) designed for 20 nC in 3 ns with the 3-4 cm range) designed for 20 nC in 3 ns with beam interception in first m <0.1%, in first 3-m <1%.beam interception in first m <0.1%, in first 3-m <1%.

Laser 3 ns, thick GaAs cathode dia=1.5 cm, bias -160 kV, 20 nC,

thus Ipulse,avg =6.7 A

[Eppley et al., PAC91, p. 1964]

cath-dia -bias SCL cm kV A

Eppley 1.5 160 10SLC 2.0 160 17SLC 2.0 120 11

ParameterParameter ILCILC ILCILC ILCILC SLCSLCat Sourceat Source SCRFSCRFNCRFNCRF NCRF-Inj/NCRF-Inj/

DesignDesignSCRF-LinacSCRF-Linac (2-cm)(2-cm)

nnee nCnC 6.46.4 2.42.4 6.46.4 2020

zz nsns 22 0.50.5 0.50.5 33

IIpulse, avgpulse, avg AA 3.23.2 4.84.8 12.812.8 6.76.7

IIpulse, peakpulse, peak AA 11 (SCL)11 (SCL)

Conclusion: Space charge limit a problem for ILC source only if tryConclusion: Space charge limit a problem for ILC source only if try

to operate with NCRF injector S-band linacto operate with NCRF injector S-band linac

Surface Photovoltaic (SPV) Surface Photovoltaic (SPV) EffectEffect

The Surface Charge Limit Eff ect

pump probe

Two short pulses

Prob

e sig

nal

Long pulse

[Clendenin et al.,to be publishedin NIM A (2004)]Elsevier B.V.

Higher doping solves the SPV problem: can be restricted to last few nm atsurface (“gradient doping”) to avoid depolarization effects in bulk*

*Creates the practical problem of how to clean the surface at low T priorto Cs(O) activation

Four samples withdifferent doping levels: 51018 cm-3

11019 cm-3

21019 cm-3

51019 cm-3

[Clendenin et al, to be published in NIM A (2004)] Elsevier B.V.

SLAC Experimental Results Using High-Polarization SLAC Experimental Results Using High-Polarization Gradient-Doped Cathodes and Long Pulse LaserGradient-Doped Cathodes and Long Pulse Laser

Cathode Laser Pulse Charge QE Laser e- Pk Dia. Current

Energy Length per pulse Pk

power Current Density

J nm ns nC % kW A mm Acm-2 2002 [a] BW-Semi 200 805 100 368 0.31 2 3.7 20 1.1 strained-layer GaAsP/GaAs 150 805 100 224 0.25 1.5 2.2 14 1.5 2003 [b] SVT strained 247 780 75 416 0.44 3.3 5.5 14 3.6 GaAsP/GaAs superlattice 225 780 75 320 0.42 3 4.3 10 5.5

[a] Maruyama et al., NIM A 492 (2002), 199, Fig. 18[b] Clendenin et al., to be published in NIM A (2004)

Vary laser spot diameter

25

20

15

10

5

0

Elec

tron/

Puls

e (1

011

)

200150100500

Laser Energy/ Pulse (uJ)

SVT-4353780nm, 75ns

18mmØ 14mmØ 10mmØ 7mmØ 5mmØ

0.14

0.12

0.10

0.08

0.06

0.04

0.02

0.00

Cur

rent

Den

sity

(A/m

m2 )

140120100806040200

Laser Power Density (W/mm2)

SVT-4353, 780nm

Field gradient @ cathode surface ~1.8MV/m

space charge limit ~0.0477A/mm2

Very high current densities achieved by reducing thelaser spot diameter at the cathode

2.2. PolarizationPolarization

Highest polarization from thin (~100 Highest polarization from thin (~100 nm) epilayer having a biaxial nm) epilayer having a biaxial compressive strain. Strain produced by compressive strain. Strain produced by lattice mismatch with substrate and/or lattice mismatch with substrate and/or by quantum confinement associated by quantum confinement associated with short-period superlattice structures.with short-period superlattice structures.

Strain breaks the degeneracy of hh and Strain breaks the degeneracy of hh and lh energy bands at the VBM. A lh energy bands at the VBM. A separation of 50-80 meV now readily separation of 50-80 meV now readily achieved.achieved.

On absorption of photon, VB electron promoted to CB. The hh-lh On absorption of photon, VB electron promoted to CB. The hh-lh splitting sufficient to select electrons from hh band only, splitting sufficient to select electrons from hh band only, resulting in CB electrons of 1 spin state only.resulting in CB electrons of 1 spin state only.

[Alley et al., NIM A 365 (1995) 1] Elsevier B.V.

Accuracy of SLC Accuracy of SLC PolarimetersPolarimeters

The The CTSCTS (Cathode Test System) (Cathode Test System) MottMott at SLAC is a at SLAC is a compact low-energy (20 kV) retarding-field polarimeter located in compact low-energy (20 kV) retarding-field polarimeter located in the Cathode Test Labthe Cathode Test Lab

The The GTLGTL (Gun Test Lab) (Gun Test Lab) MottMott at SLAC is a medium-at SLAC is a medium-energy (120 kV) multiple-foil polarimeter located in the GTLenergy (120 kV) multiple-foil polarimeter located in the GTL

SLCSLC ComptonCompton polarimeterpolarimeter was located at the IP (50 GeV)was located at the IP (50 GeV)

Same Mott polarimeters in operation at SLAC today

Run CTS-Mott GTL-Mott Compton

’97-9877% 72.92%±0.38%

’96 78 79 76.16%±0.40%

For ’96 and ‘97/’98, error of SLC Compton polarimeter measurements ~0.5%, dominated by systematic uncertainties. [Abe et al., PRL 84 (2001) 5945]

Known depolarization in NDR and NARC: ~2% (NDR 0.8%; NARC 0.7% energy spread, 0.3% synchrotron radiation, 0.4% beam emittance).

Thus, during SLC, the Mott measurements (in lab) were consistently ~2% higher than Compton corrected for known depolarization effects. Some of the difference may be spin de-tuning in NARC.

E-158 ResultsE-158 Results E-158 an experiment (2001-2003) to E-158 an experiment (2001-2003) to

measure parity violation at 50 GeV in measure parity violation at 50 GeV in electron-electron scattering at SLACelectron-electron scattering at SLAC

Moller polarimeter at 50 GeV, similar to Moller polarimeter at 50 GeV, similar to JLab’s. Depolarization in A-line ~1%.JLab’s. Depolarization in A-line ~1%.

Runs 1,2 used GaAsP/GaAs strained-Runs 1,2 used GaAsP/GaAs strained-layer cathodelayer cathode

Run 3 used GaAsP/GaAs superlattice (SL)Run 3 used GaAsP/GaAs superlattice (SL)

GaAsP/GaAs Superlattice GaAsP/GaAs Superlattice (SL)(SL)

Data showing high polarization from MOCVD-Data showing high polarization from MOCVD-grown version first presented by Nishitani et grown version first presented by Nishitani et al. at the PESP 2000 Workshop in Nagoya.al. at the PESP 2000 Workshop in Nagoya.

SVT Associates and SLAC collaborated to SVT Associates and SLAC collaborated to explore parameter space for MBE-grown explore parameter space for MBE-grown version.version.– Results show an amazingly stable high Results show an amazingly stable high

polarization over a wide range of parameter space polarization over a wide range of parameter space [Maruyama et al., Appl. Phys. Lett. 85 (2004) [Maruyama et al., Appl. Phys. Lett. 85 (2004) 2640] while maintaining a high QE.2640] while maintaining a high QE.

– One of these SL wafers used for E-158-III.One of these SL wafers used for E-158-III.

Comparison of 3 photocathodes representing 2 Comparison of 3 photocathodes representing 2 structuresstructures

Cathode Structure

Growth Method

Pemax

0

(nm) QEmax(o) Polarimeter Ref

1a GaAsP/GaAs strained SL

MOCVD

0.92 775 warm

0.005 Mott Nagoya

a

1b GaAsP/GaAs strained SL

MBE

0.86 783 warm

0.012 CTS Mott SLAC

b

0.90 780 cold

0.008 Møller E158-III SLAC

c

2 GaAsP/GaAs strained-layer

MOCVD

0.82 805 warm

0.001 CTS Mott SLAC

d

0.85 800 cold

0.004 Møller E158-I SLAC

e

a T. Nishitani et al., in SPIN 2000, AIP Conf. Proc. 570 (2001),p. 1021

b T. Maruyama et al., Appl. Phys. Lett. 85 (2004) 2640c On line, preliminary value of Pemax.d T. Maruyama et al., Nucl. Instrum. and Meth. A 492 (2002)

199, Fig. 13e P.L. Anthony et al., Phys. Rev. Lett. 92 (2004) 181602

““Spin Dance” at Jefferson LabSpin Dance” at Jefferson Lab

Relative analyzing power for 5 JLab polarimeters operated simultaneously tomeasure polarization of common beam on pulse-to-pulse basis. Error barsrepresent fits to the data only, statistical (much larger) and systematic errorsnot included. The Moller A value reduced to 1.04 if data set limited to within25% of max measured polarization (but error bars increase).

[Grames et al.,PRST-AB 7(2004) 042802]American PhysicalSociety

Maximum Polarization of SVT Maximum Polarization of SVT SLSL

CTS MottCTS Mott (86±5)%(86±5)% E-158-III MollerE-158-III Moller (91±5)%(91±5)%

(corrected for source)(corrected for source) AverageAverage (88±4)%(88±4)%

3. Other Issues3. Other Issues Cathode QE, QE uniformity, anisotropy, lifetimeCathode QE, QE uniformity, anisotropy, lifetime

– QE determines required laser energy—the higher the QE QE determines required laser energy—the higher the QE the more reliable the laser system can bethe more reliable the laser system can be

– QE non-uniformity affects low-energy beam optics, thus QE non-uniformity affects low-energy beam optics, thus needs to be stableneeds to be stable

– QE anisotropy very low for SLQE anisotropy very low for SL– QE lifetime must be >100 h to ensure stable operating QE lifetime must be >100 h to ensure stable operating

conditionsconditions Cannot always compensate for low QE with more laser Cannot always compensate for low QE with more laser

energy because of SPV effectenergy because of SPV effect Can restore QE by re-cesiating, takes ~15 min.Can restore QE by re-cesiating, takes ~15 min. SLC lifetimes typically >400 h [J.E. Clendenin et al, in AIP SLC lifetimes typically >400 h [J.E. Clendenin et al, in AIP

CP-421 (1998), p. 250]CP-421 (1998), p. 250]

Source VacuumSource Vacuum– Critical for high QE and long lifetimeCritical for high QE and long lifetime– Affects ion back bombardmentAffects ion back bombardment

JLab 10,000 C/cmJLab 10,000 C/cm22 equivalent to 1/e lifetime [C. Sinclair, equivalent to 1/e lifetime [C. Sinclair, PAC99, p. 65]PAC99, p. 65]

ILC maximum 1000 C/cmILC maximum 1000 C/cm22 per year (SLC type source) per year (SLC type source) High voltage cathode biasHigh voltage cathode bias

– Beam loading effects: each Beam loading effects: each pulse ~1 mJ, 1-ms pulse pulse ~1 mJ, 1-ms pulse train ~3 Jtrain ~3 J

– Pulsed HV can be shapedPulsed HV can be shaped Laser systemLaser system

– Laser to be modulated at Laser to be modulated at pulse frequency, i.e., at ~3 pulse frequency, i.e., at ~3 MHzMHz

– The pulse train envelope can be shapedThe pulse train envelope can be shaped

Next Generation Polarized Electron SourcesNext Generation Polarized Electron Sources

Higher voltageHigher voltage RG gunsRG guns

4. Conclusions4. Conclusions High probability that required charge for High probability that required charge for

ILC can be produced using SLC type PES. ILC can be produced using SLC type PES. Numerous problems introduced if Numerous problems introduced if bunch bunch spacing is reduced to significantly <300 spacing is reduced to significantly <300 ns.ns.

PPe e 85% is assured using well-tested 85% is assured using well-tested GaAs/GaAsP SL structure. GaAs/GaAsP SL structure.

Various relatively minor issues remain.Various relatively minor issues remain.