Polarized Electrons for Linear Colliders

1
Polarized Electrons for Linear Colliders

• J. E. Clendenin, A. Brachmann, E. L. Garwin, R.
  E. Kirby, D.-A. Luh, T. Maruyama, R. Prepost, C.
  Y. Prescott, J. C. Sheppard, and J. Turner

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Outline

• Charge
• Polarization
• Other
• Conclusions

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1. Collider Charge Requirements

• Parameter ILC ILC
• at Source SCRF NCRF
• Ne,mpulse nC 6.4 2.4
• Dz ns 2 0.5
• Impulse, avg A 3.2 4.8
• Twice the IP requirement

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Generating Polarized Electrons from GaAs

• Illuminate p-doped GaAs (or its
• analogues) crystal with circularly
• polarized monochromatic light
• tuned to the band-gap edge.
• Absorbed photons promote e-
• from filled VB states to CB. CB e-
• eventually reach surface.

In p-doped materials, band-bending lowers
work function by 1/3 of the 1.4 eV band gap.
Treating surface
with Cs(O) lowers it several additional eV,
resulting in the vacuum level being lower than
the CBM in the bulk (NEA surface). If the cathode
is biased negative, CB electrons at the surface
are emitted into vacuum.
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Space Charge Limit (SCL)

• (Childs Law)
• SLC DC gun
• ? Cathode bias -120 kV to keep max fields
• lt8 MV/m
• ? Low fields necessary to minimize the dark
  current that degrades the QE
• ? GaAs crystal 2-cm dia. decreases je, but
• increases beam emittance at source

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• For SLC, low-energy beam transport (various
  apertures in the 3-4 cm range) designed for 20 nC
  in 3 ns with beam interception in first m lt0.1,
  in first 3-m lt1.

Eppley et al., PAC91, p. 1964
cath-dia -bias SCL cm kV A Eppley 1.5 16
0 10 SLC 2.0 160 17 SLC 2.0 120 11
Laser 3 ns, thick GaAs cathode dia1.5 cm, bias
-160 kV, 20 nC, thus Impulse,avg 6.7 A
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• Parameter ILC ILC ILC SLC
• at Source SCRF NCRF NCRF-Inj/ Design
• SCRF-Linac (2-cm)
• ne nC 6.4 2.4 6.4 20
• Dz ns 2 0.5 0.5 3
• Impulse, avg A 3.2 4.8 12.8 6.7
• Impulse, peak A 11 (SCL)
• Conclusion Space charge limit a problem for ILC
  source only if try
• to operate with NCRF injector S-band linac

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Surface Photovoltaic (SPV) Effect
Clendenin et al., to be published in NIM A
(2004) ?Elsevier B.V.
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Higher doping solves the SPV problem can be
restricted to last few nm at surface (gradient
doping) to avoid depolarization effects in bulk
Clendenin et al, to be published in NIM A
(2004) ?Elsevier B.V.
Four samples with different doping levels
5?1018 cm-3 1?1019 cm-3 2?1019 cm-3
5?1019 cm-3
Creates the practical problem of how to clean
the surface at low T prior to Cs(O) activation
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SLAC Experimental Results Using High-Polarization
Gradient-Doped Cathodes and Long Pulse Laser
a Maruyama et al., NIM A 492 (2002), 199, Fig.
18 b Clendenin et al., to be published in NIM A
(2004)
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Very high current densities achieved by reducing
the laser spot diameter at the cathode
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2. Polarization

• Highest polarization from thin (100 nm) epilayer
  having a biaxial compressive strain. Strain
  produced by lattice mismatch with substrate
  and/or by quantum confinement associated with
  short-period superlattice structures.
• Strain breaks the degeneracy of hh and lh energy
  bands at the VBM. A separation of 50-80 meV now
  readily achieved.

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On absorption of photon, VB electron promoted to
CB. The hh-lh splitting sufficient to select
electrons from hh band only, resulting in CB
electrons of 1 spin state only.
Alley et al., NIM A 365 (1995) 1 ?Elsevier B.V.
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Accuracy of SLC Polarimeters

• The CTS (Cathode Test System) Mott at SLAC is a
  compact low-energy (20 kV) retarding-field
  polarimeter located in the Cathode Test Lab
• The GTL (Gun Test Lab) Mott at SLAC is a
  medium-energy (120 kV) multiple-foil polarimeter
  located in the GTL
• SLC Compton polarimeter was located at the IP (50
  GeV)

Same Mott polarimeters in operation at SLAC today
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For 96 and 97/98, error of SLC Compton
polarimeter measurements 0.5, dominated by
systematic uncertainties. Abe et al., PRL 84
(2001) 5945
Run CTS-Mott GTL-Mott Compton
97-98 77 72.920.38 96 78
79 76.160.40
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.
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E-158 Results

• E-158 an experiment (2001-2003) to measure parity
  violation at 50 GeV in electron-electron
  scattering at SLAC
• Moller polarimeter at 50 GeV, similar to JLabs.
  Depolarization in A-line 1.
• Runs 1,2 used GaAsP/GaAs strained-layer cathode
• Run 3 used GaAsP/GaAs superlattice (SL)

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GaAsP/GaAs Superlattice (SL)

• Data showing high polarization from MOCVD-grown
  version first presented by Nishitani et al. at
  the PESP 2000 Workshop in Nagoya.
• SVT Associates and SLAC collaborated to explore
  parameter space for MBE-grown version.
• Results show an amazingly stable high
  polarization over a wide range of parameter space
  Maruyama et al., Appl. Phys. Lett. 85 (2004)
  2640 while maintaining a high QE.
• One of these SL wafers used for E-158-III.

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Comparison of 3 photocathodes representing 2
structures
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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) 2640 c On line,
preliminary value of Pemax. d T. Maruyama et al.,
Nucl. Instrum. and Meth. A 492 (2002) 199, Fig.
13 e P.L. Anthony et al., Phys. Rev. Lett. 92
(2004) 181602
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Spin Dance at Jefferson Lab
Grames et al., PRST-AB 7 (2004)
042802 ?American Physical Society
Relative analyzing power for 5 JLab polarimeters
operated simultaneously to measure polarization
of common beam on pulse-to-pulse basis. Error
bars represent fits to the data only, statistical
(much larger) and systematic errors not included.
The Moller A value reduced to 1.04 if data set
limited to within 25 of max measured
polarization (but error bars increase).
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Maximum Polarization of SVT SL

• CTS Mott (865)
• E-158-III Moller (915)
• (corrected for source)
• Average (884)

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3. Other Issues

• Cathode QE, QE uniformity, anisotropy, lifetime
• QE determines required laser energythe higher
  the QE the more reliable the laser system can be
• QE non-uniformity affects low-energy beam optics,
  thus needs to be stable
• QE anisotropy very low for SL
• QE lifetime must be gt100 h to ensure stable
  operating conditions
• Cannot always compensate for low QE with more
  laser energy because of SPV effect
• Can restore QE by re-cesiating, takes 15 min.
• SLC lifetimes typically gt400 h J.E. Clendenin et
  al, in AIP CP-421 (1998), p. 250

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• Source Vacuum
• Critical for high QE and long lifetime
• Affects ion back bombardment
• JLab 10,000 C/cm2 equivalent to 1/e lifetime C.
  Sinclair, PAC99, p. 65
• ILC maximum 1000 C/cm2 per year (SLC type source)
• High voltage cathode bias
• Beam loading effects each mpulse 1 mJ, 1-ms
  pulse train 3 J
• Pulsed HV can be shaped
• Laser system
• Laser to be modulated at mpulse frequency, i.e.,
  at 3 MHz
• The pulse train envelope can be shaped

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Next Generation Polarized Electron Sources

• Higher voltage
• RG guns

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4. Conclusions

• High probability that required charge for ILC can
  be produced using SLC type PES. Numerous problems
  introduced if mbunch spacing is reduced to
  significantly lt300 ns.
• Pe ? 85 is assured using well-tested GaAs/GaAsP
  SL structure.
• Various relatively minor issues remain.