Title: The JLab IR-FEL can do the job
1Electron Driver Capabilities and Proposed JLab
Test Facility
Reza Kazimi JLab
2Outline
- 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
3INJECTOR
4Layout of the CEBAF Injector
5Measured Bunch length Along the Injector
6CEBAF Injector Beam Parameters
7What 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
8INJECTOR
The JLab FEL is an Energy Recovery Linac, Fourth
Generation Light Source.
9Electron bunches are generated when the GaAs
photocathode is illuminated by pulses of green
light from a drive laser
10The JLab FEL is driven by a 350 kV DC GaAs
electron gun
11JLab 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
12Challenges 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
13Injector Test Facility
14- What does Test Cave have?
- Room 70 long,12 wide 10-12 high
- Thick concrete walls for shielding
15Other 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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18Lifetime 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.
19Experimental 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)
20SMALL 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
22A 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
23Compact 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.
2410 MeV Teststand (option 1)
100/350 keV
10 MeV
1 mA 10MeV
¼ Cryo
Diagnostics Spectrometer, Mott, FC,
Photo-Cathode Gun
Buncher Warm cavity
2510 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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27Standard five cell cavity
28Modified five cell cavity for low energy entrance
29Summary
- 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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32Production at 11 GeV
Convert 1.1 GeV Electron to Positron
Compact High Current Electron Injector
3312 GeV CEBAF
34The End
35Synchronous Photoinjection
36Photocathode lifetime operating at 5 mA CW and
135pC bunch charge is about 550 Coulombs or 50
hours per re-cesiation
37The quantum efficiency drops during average
current operation when the electron beam ionizes
residual gas in the gun vacuum chamber.
38A 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
39The 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.
40Three Proposed plans
41Layout of the Injector
Make 10 MeV Electrons Convert to
Positrons Insert Before the Modules
42Layout of the Injector
Make 10 MeV Electrons Accelerate to 65
MeV Convert to Positrons
500 keV dump
4310 MeV Teststand
10 MeV
100/350 keV
1 mA 10MeV
Small Chicane
¼ Cryo
Diagnostics Spectrometer, Mott, FC,
Photo-Cathode Gun
Buncher Warm cavity