The JLab IR-FEL can do the job

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Electron Driver Capabilities and Proposed JLab
Test Facility
Reza Kazimi JLab
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Outline

• Existing CEBAF injector capabilities and
  limitations.
• JLAB FEL Injector
• Injector Test Facility
• Thanks to Carlos Hernandez, Joe Grames, and
  Matt Poelker for discussions and some of the
  slides

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INJECTOR
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Layout of the CEBAF Injector
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Measured Bunch length Along the Injector
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CEBAF Injector Beam Parameters
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What limits the high current?

• High average current
• More RF power needed for acceleration
• High average current
• Less Cathode life
• High charge/bunch
• High space charge
• beam blow up in transverse and
  longitudinal

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INJECTOR
The JLab FEL is an Energy Recovery Linac, Fourth
Generation Light Source.
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Electron bunches are generated when the GaAs
photocathode is illuminated by pulses of green
light from a drive laser
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The JLab FEL is driven by a 350 kV DC GaAs
electron gun
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JLab FEL photo-injector operational performance

• 135pC bunch charge
• 9 MeV/c
• Measured Normalized transverse emittance e8 p mm
  mrad
• Average current up to 9 mA
• Measured rms bunch length 3.4 ps
• Measured rms energy spread 18 keV
• Longitudinal Emittance 61 ps-keV
• Photocathode lifetime operating at 5 mA CW and
  135pC bunch charge is about 550 Coulombs or 50
  hours per re-cesiation

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Challenges for High Current electron driver

• Longer Cathode life time
• 1 mA beam requires 86 C/day
• 10 mA, 860 C/day
• Higher voltage electron gun.
• Need shorter distance between the gun and first
  acceleration

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Injector Test Facility
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• What does Test Cave have?
• Room 70 long,12 wide 10-12 high
• Thick concrete walls for shielding

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Other resources at Test Cave

• Some of the other resources at test Cave
• LCW, e.g., cooling the Faraday cup or magnets
• Compressed air, e.g., pneumatic viewers
• Compressed LN2 room temperature boil-off, e.g.,
  vacuum work
• 240V electrical power, e.g., for heater power
  supplies
• 120V electrical power, e.g., wall power items
• laser room interlock w/ magnetic locks, keycode
  access,
• Interlock to fire system
• PSS room interlock (for low energy operations)
• two CARMS
• two RF waveguide feedthroughs
• high voltage shed and cable for 500 kV
  feedthrough
• laser clean room
• two trim card magnet racks and cabling
• two iocs, support for CAMC
• it's own fiefdom (server), ITS
• storage cage
• basic shop

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Lifetime versus Laser Spot Size

• Imperfect vacuum limits photocathode lifetime -
  damage from ion backbombardment
• Can we increase operating lifetime by merely
  increasing the laser spot size? Same number
  electrons, same number ions, but distributed over
  larger area.

• Exceptionally high charge lifetime, gt1000C at
  beam current to 10mA!
• Lifetime scales with laser spot size but simple
  scaling not valid. Factor 10 instead of factor 20.

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Experimental Setup
Faraday Cup
Laser (1 W _at_ 532 nm)
High Voltage (100 kV)
NEG pipe
Activation (Cs/NF3, 5 mm)
Spot Size Adjustment
350 mm
1500 mm
Load lock (GaAs on puck)
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SMALL vs. LARGE Laser Spot (BP vs. LL)
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• High Voltage Chamber
• Side ceramic design
• load chamber at ground potential
• No moving parts at HV

Side View

• Activation Chamber
• Mini-stalk heater
• Mask selects active area
• UHV IP supplies gauge activation
• Keyed eared pucks

Load Locked Gun
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A possible next generation gun design

• Will be based on CEBAF/Cornell load-lock systems
• Will explore CEBAFs new approach of inverted
  insulator

Picture courtesy of Matt Poelker
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Compact Injector

• The aim is to put together an accelerator which
  produces 10 MeV, few mA CW electron beam.
• Among many other uses, it could be used as a
  driver for the positron production.

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10 MeV Teststand (option 1)
100/350 keV
10 MeV
1 mA 10MeV
¼ Cryo
Diagnostics Spectrometer, Mott, FC,
Photo-Cathode Gun
Buncher Warm cavity
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10 MeV Teststand (option 2)
10 MeV
100/350 keV
1 mA 10MeV
Small Chicane
¼ Cryo
Diagnostics Spectrometer, Mott, FC,
Photo-Cathode Gun
Buncher Warm cavity
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Standard five cell cavity
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Modified five cell cavity for low energy entrance
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Summary

• The CEBAF injector optics has been tested during
  the G0 operation up to 1.4 pC/bunch which is 0.7
  mA _at_ 0.5 GHz and 2.1 mA _at_ 1.5 GHz
• Jlab FEL injector has operated up to 135
  pC/bunch, 9 mA average current at 9.1 MeV/c.
• 1 mA polarized beam with 200 Coulomb cathode life
  time has been achieved in the test cave.
• 10 mA unpolarized beam with life time of
  thousands of Coulomb has been achieved in the
  test cave.
• Inverted gun design can provide higher voltage
  gun for CEBAF load locked system.
• A Compact injector could provide an independent
  driver for positron.
• Production and transport of 1 mA polarized beam
  in CEBAF machine needs to be demonstrated before
  it could be used for Positron production.

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Production at 11 GeV
Convert 1.1 GeV Electron to Positron
Compact High Current Electron Injector
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12 GeV CEBAF
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The End
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Synchronous Photoinjection
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Photocathode lifetime operating at 5 mA CW and
135pC bunch charge is about 550 Coulombs or 50
hours per re-cesiation
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The quantum efficiency drops during average
current operation when the electron beam ionizes
residual gas in the gun vacuum chamber.
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A single GaAs wafer delivered over 7000 Coulombs
and over 900 hours of CW beam at currents ranging
from 1 to 8 mA. This wafer was activated into a
photocathode a total of 9 times in 36 months of
operation with an average of 6 re-cesiations per
activation.
Front-end view of the GaAs photocathode being
illuminated by the drive laser while delivering 5
mA of CW electron beam
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The FEL and the GTS guns are identical in design
and dimensions except for two features

• The anode plate in the GTS gun is used as a
  mirror for reflecting off the drive laser and
  illuminating the photocathode at a 40 degree
  angle.

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Three Proposed plans
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Layout of the Injector
Make 10 MeV Electrons Convert to
Positrons Insert Before the Modules
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Layout of the Injector
Make 10 MeV Electrons Accelerate to 65
MeV Convert to Positrons
500 keV dump
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10 MeV Teststand
10 MeV
100/350 keV
1 mA 10MeV
Small Chicane
¼ Cryo
Diagnostics Spectrometer, Mott, FC,
Photo-Cathode Gun
Buncher Warm cavity