Recent Polarized Cathodes R

1
Recent Polarized Cathodes RD at SLAC and
Future Plans
Feng Zhou Axel Brachmann, Jym
Clendenin (ret.),
Takashi Maruyama, and John Sheppard SLAC Workshop
on polarized e- sources, JLAB, Oct. 1 Oct. 3,
2008
2
Outline

• Critical RD goals demonstrate full charge
  productions (highly polarized) for future linear
  colliders
• Recent measurements on InAlGaAs/AlGaAs
• Measurements at Cathode Test System (CTS)
• Measurements at Gun Test Lab (GTL)
• Summary for the measurements
• Future plans on polarized cathode developments

F. Zhou/SLAC
PESP workshop at JLAB, Oct. 1-3, 2008
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Major parameters of ILC and CLIC e- sources
Parameters ILC CLIC
Electrons/microbunch 3E10 6E9
Number of microbunches 2625 312
Width of Microbunch 1 ns 100 ps
Time between microbunches 360 ns 500.2 ps
Width of Macropulse 1 ms 156 ns
Macropulse repetition rate 5 Hz 50 Hz
Charge per macropulse 12600 nC 300 nC
Average current from gun 63 ?A 15 ?A
Peak current of microbunch 4.8 A 9.6 A
Current density (1 cm radius) 1.5 A/cm 3.0 A/cm
Polarization gt80 gt80
2
2
PESP workshop at JLAB, Oct. 1-3, 2008
F. Zhou/SLAC
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Challenges

• Full charge production limited by space charge
  and surface charge
• Lasers to demonstrate production of electron beam
  with ILC and CLIC time structures
• Good polarized cathodes to overcome surface
  charges with high QE and polarization
• H.V. gun to overcome space charge, and high
  vacuum to overcome contamination.
• Cathode candidates for linear colliders
• Less charge limit (surface charge and space
  charge)
• High polarization (gt85)
• High QE and QE lifetime

PESP workshop at JLAB, Oct. 1-3, 2008
F. Zhou/SLAC
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InAlGaAs/AlGaAs cathode

• Strained-well InAlGaAs/AlGaAs structures designed
  and grown by St. Petersburg in Russia
• Large valence band splitting (60 meV) due to
  combination of deformation and quantum
  confinement effects in quantum well.
• Good BBR engineering
• Thick working layer without strain relaxation
• 88-93 of polarization obtained at Russia

F. Zhou/SLAC
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Capabilities at Cathode Test System

• Cathode preparation and cleaning processes
• QE and polarization measured at 20 KV
• QE and polarization of Cathode can be quickly
  characterized in few days.
• More fast and convenient compared with GTL
• Drawback is unable to characterize surface
  charge limit (time evolution of bunch).

PESP workshop at JLAB, Oct. 1-3, 2008
F. Zhou/SLAC
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InAlGaAs/AlGaAs _at_ CTS

• Polarized cathode measurements
• Cathode preparation
• Chemical cleaning
• Load lock system to change cathode in UHV
• Heat cleaning
• Atomic hydrogen cleaning
• 4 samples measured at CTS 0.1-0.3 QE and
  82-87 of polarization.

PESP workshop at JLAB, Oct. 1-3, 2008
F. Zhou/SLAC
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SBIR graded AlxGa(1-x)As/GaAs (Grown by SVT)

• The graded bandgap active region provides an
  internal accelerating field for the
  photo-generated electrons in the conduction band.
  QE is increased by the field.
• But, the polarization is decreased need to tune
  the structure parameters in SBIR phase II.

F. Zhou/SLAC
PESP workshop at JLAB, Oct. 1-3, 2008
9
Capabilities at Gun Test Lab

• Re-established all measurements at GTL after
  three-year down time
• Charge limit (time evolution of bunch)
• QE and QE lifetime
• Polarization
• Take full measurements at GTL
• 1st sample InAlGaAs/AlGaAs (measurements done)
• 2nd sample InAlGaAs/AlGaAs installed
• Internal graded sample AlxGa(1-x)As/GaAs
• To demonstrate full charge production once it is
  mated to ILC and CLIC lasers
• Planned programs on cathode developments
• System is also available for other RD projects,
  such as test different electrodes and guns.

PESP workshop at JLAB, Oct. 1-3, 2008
F. Zhou/SLAC
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GTL layout
60-300 ns Flash TiSaphhire _at_60 Hz
120 kV DC-gun
Nanoammeter to measure QE
Fast Faraday cup to measure time evolution of
electron bunch
Mott polarimeter
Spin rotator
PESP workshop at JLAB, Oct. 1-3, 2008
F. Zhou/SLAC
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InAlGaAs/AlGaAs QE uniformity
0.75
0.75
0.75
0.86
0.82
0.72
20 mm
1
1
0.75
0.84
0.84
0.81
0.86
0.79
0.83
0.78
PESP workshop at JLAB, Oct. 1-3, 2008
F. Zhou/SLAC
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InAlGaAs/AlGaAs QE lifetime
PESP workshop at JLAB, Oct. 1-3, 2008
F. Zhou/SLAC
13
QE vs wavelength
_at_ certain laser energy ?10 mm
bandgap
PESP workshop at JLAB, Oct. 1-3, 2008
F. Zhou/SLAC
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InAlGaAs/AlGaAs polarization measurements
PESP workshop at JLAB, Oct. 1-3, 2008
F. Zhou/SLAC
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InAlGaAs/AlGaAs polarization vs surface charge
limit

• The cathode is driven into saturation, electrons
  photoexcited into conduction band can still
  escape if they diffuse to a non-saturated region.
  
• But, these electrons spend long time inside
  structure so it is likely that they suffer spin
  relaxation.

PESP workshop at JLAB, Oct. 1-3, 2008
F. Zhou/SLAC
16
Surface photovoltaic effect surface charge
limit

• Photon absorption excites electrons to conduction
  band
• Electrons can be trapped near the surface
• Electrostatic potential from trapped electrons
  raised affinity.
• Increased affinity decreases emission
  probability.

PESP workshop at JLAB, Oct. 1-3, 2008
F. Zhou/SLAC
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Surface charge vs laser energy
10
10
2.5x10 e- _at_ 8x laser energy 10 mm full size
0.73x10 e- _at_ 1x laser energy 10 mm full size
PESP workshop at JLAB, Oct. 1-3, 2008
F. Zhou/SLAC
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Surface charge vs laser location
Same laser energy
10
10
2.5x10 e- production 10 mm full
size _at_ good location
1.4x10 e- production 10 mm
full size _at_ bad location
PESP workshop at JLAB, Oct. 1-3, 2008
F. Zhou/SLAC
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Charge limit vs laser energy
10 mm laser full size
PESP workshop at JLAB, Oct. 1-3, 2008
F. Zhou/SLAC
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Charge limit vs beam size

• Space charge limit (Child law)
• H qgf
• Wqjh
• WJH

• Space charge (Childs law)
• _at_ GTL gun
• Take beam parameters d5mm, Q3.75 nC, 300
  ns,
• Space charge negligible at current conditions
  thus surface charge limit dominates at smaller
  size with of doping at the
  surface layer.

PESP workshop at JLAB, Oct. 1-3, 2008
F. Zhou/SLAC
21
Surface charge vs pulse length (good location)
300 ns
And what about at 1 ns?
same laser energy 10 mm laser full size
F. Zhou/SLAC
PESP workshop at JLAB, Oct. 1-3, 2008
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Surface charge vs pulse length (bad location)
60 ns
300 ns
And what about at 1 ns?
same laser energy 10 mm laser full size
PESP workshop at JLAB, Oct. 1-3, 2008
F. Zhou/SLAC
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Whats the possible indications from the
measurements for ILC and CLIC surface charge
space charge?
ILC CLIC
Microbunch 300 ns ?1 ns Surface charge better Space charge worse 300 ns ? 100 ps Surface charge better Space charge much worse
Macropulse 1 ms (360 ns spacing) Accumulated surface charge may be much worse? 156 ns (0.5 ns spacing) Surface charge may accumulate
Current intensity (r1cm) Surface charge and space charge combined Surface charge may be serious in macropulse? Space charge serious Surface charge may accumulate in macropulse
PESP workshop at JLAB, Oct. 1-3, 2008
F. Zhou/SLAC
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Summary

• Combination of CTS GTL at SLAC is an unique
  diagnostic to characterize polarized
  photo-cathodes.
• Recent systematic measurements for one
  InAlGaAs/AlGaAs sample (sample 7-632) at both
  CTS GTL
• 0.3 QE at CTS against 0.7 QE of Russian data
  QE lifetime measured at GTL is 120-150 hrs.
• 82 (CTS) and 84 (GTL) of polarization against
  88 polarization of Russian data.
• Surface charge limit is observed, current
  intensity with
• _at_ of doping
  in surface.
• First observation of polarization dependence on
  surface charge limit.
• Need optimize parameters of InAlGaAs/AlGaAs to
  meet cathode critical requirements for linear
  collider sources.

0.06 A/cm
2
PESP workshop at JLAB, Oct. 1-3, 2008
F. Zhou/SLAC
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Future cathode RD at SLAC

• Measure another sample of InAlGaAs/AlGaAs and
  graded AlGaAs/GaAs cathode at GTL.
• Planned programs
• Study doping level in the structure of GaAs/GaAsP
  
• Gradient doping in the active layer
• Apply both techniques into GaAs/GaAsP
• Optimize InAlGaAs/AlGaAs cathode parameters
• Demonstrate charge production (surface charge and
  space charge) for the ILC once its laser ready.
  The ILC laser expected ready in the early of next
  year.

PESP workshop at JLAB, Oct. 1-3, 2008
F. Zhou/SLAC
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Doping level in GaAs/GaAsP

• Doping level at least affects
• Smearing band edge and broadening hole spectrum
• Spin relaxation in transport stage BAP process -
  one of major mechanisms -, exchange interaction
  between electrons and holes
• Spin relaxation in BBR
• Surface charge limit
• Plan to study doping level in surface and active
  layer of GaAs/GaAsP

PESP workshop at JLAB, Oct. 1-3, 2008
F. Zhou/SLAC
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Gradient doping in the active layer

• Electrons are accelerated when getting through
  band-bending regions
• High QE expected
• Much interest in gradient doping in the active
  layer of SL structure.

PESP workshop at JLAB, Oct. 1-3, 2008
F. Zhou/SLAC
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Future cathode RD (cont)

• Demonstrate CLIC-like beam production by simply
  modifying ILC laser
• ILC laser existing 76 MHz ML oscillator/25
  times3MHz
• CLIC-like laser directly use existing 76 MHz ML
  oscillator to generate CLIC-like beam (13 ns
  spacing).
• Real CLIC laser (need extra funds)
• The gun lab at SLAC is an ideal diagnostic to
  characterize critical parameters of a
  photocathode for both ILC and CLIC surface
  charge limit, polarization, QE, and QE lifetime.

PESP workshop at JLAB, Oct. 1-3, 2008
F. Zhou/SLAC
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• Thank colleagues at St. Petersburg and SVT
  Associates for the collaboration

PESP workshop at JLAB, Oct. 1-3, 2008
F. Zhou/SLAC