The JLab IR-FEL can do the job - PowerPoint PPT Presentation

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The JLab IR-FEL can do the job

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INJECTOR Electron bunches are generated when the GaAs photocathode is illuminated by pulses of green light from a drive laser The JLab FEL is driven by a 350 kV DC ... – PowerPoint PPT presentation

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Title: The JLab IR-FEL can do the job


1
Electron Driver Capabilities and Proposed JLab
Test Facility
Reza Kazimi JLab
2
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

3
INJECTOR
4
Layout of the CEBAF Injector
5
Measured Bunch length Along the Injector
6
CEBAF Injector Beam Parameters
7
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

8
INJECTOR
The JLab FEL is an Energy Recovery Linac, Fourth
Generation Light Source.
9
Electron bunches are generated when the GaAs
photocathode is illuminated by pulses of green
light from a drive laser
10
The JLab FEL is driven by a 350 kV DC GaAs
electron gun
11
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

12
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

13
Injector Test Facility
14
  • What does Test Cave have?
  • Room 70 long,12 wide 10-12 high
  • Thick concrete walls for shielding

15
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

16
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17
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18
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.

19
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)
20
SMALL vs. LARGE Laser Spot (BP vs. LL)
21
  • 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
22
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
23
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.

24
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
25
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
26
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27
Standard five cell cavity
28
Modified five cell cavity for low energy entrance
29
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.

30
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31
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32
Production at 11 GeV
Convert 1.1 GeV Electron to Positron
Compact High Current Electron Injector
33
12 GeV CEBAF
34
The End
35
Synchronous Photoinjection
36
Photocathode lifetime operating at 5 mA CW and
135pC bunch charge is about 550 Coulombs or 50
hours per re-cesiation
37
The quantum efficiency drops during average
current operation when the electron beam ionizes
residual gas in the gun vacuum chamber.
38
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
39
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.

40
Three Proposed plans
41
Layout of the Injector
Make 10 MeV Electrons Convert to
Positrons Insert Before the Modules
42
Layout of the Injector
Make 10 MeV Electrons Accelerate to 65
MeV Convert to Positrons
500 keV dump
43
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
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