Reza Kazimi, Positron Workshop 2009
Electron Driver CapabilitiesandProposed JLab Test Facility
Reza KazimiJLab
Reza Kazimi, Positron Workshop 2009
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
Reza Kazimi, Positron Workshop 2009
INJEC
TOR
Reza Kazimi, Positron Workshop 2009
Layout of the CEBAF Injector
Reza Kazimi, Positron Workshop 2009
Measured Bunch length Along the Injector
Reza Kazimi, Positron Workshop 2009
CEBAF Injector Beam Parameters
Beam energy 23-67 MeV
Beam current A 100 A, B 10-200 nA, C 100 A
Normalized rms emittance <0.5 mm mrad
Repetition rate 499 MHz/Hall (31.2 MHz for G0 exp)
Charge per bunch < 0.2 pC (1.4 pC for G0 exp)
Energy spread (rms) few tens of keV
Beam size (longitudinal) 60 microns (200 fsec)
Beam polarization 85% typical, 80% guaranteed
Reza Kazimi, Positron Workshop 2009
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
Reza Kazimi, Positron Workshop 2009
The JLab FEL is an Energy Recovery Linac, Fourth Generation
Light Source.
INJECTOR
Reza Kazimi, Positron Workshop 2009
Electron bunches are generated when the GaAs photocathode is illuminated by pulses of green light
from a drive laser
Reza Kazimi, Positron Workshop 2009
The JLab FEL is driven by a 350 kV DC GaAs electron gun
Reza Kazimi, Positron Workshop 2009
JLab FEL photo-injector operational performance:
• 135pC bunch charge•9 MeV/c•Measured Normalized transverse emittance ε=8 π 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
Reza Kazimi, Positron Workshop 2009
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
Reza Kazimi, Positron Workshop 2009
Injector Test Facility
Reza Kazimi, Positron Workshop 2009
• What does Test Cave have? – Room: 70’ long,12’ wide 10’-12’ high– Thick concrete walls for shielding
Reza Kazimi, Positron Workshop 2009
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
Reza Kazimi, Positron Workshop 2009
Reza Kazimi, Positron Workshop 2009
Reza Kazimi, Positron Workshop 2009
Lifetime versus Laser Spot Size
• Exceptionally high charge lifetime, >1000C at beam current to 10mA!
• Lifetime scales with laser spot size but simple scaling not valid. Factor 10 instead of factor 20.
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.
Reza Kazimi, Positron Workshop 2009
Load lock(GaAs on puck)
NEG pipe
Laser(1 W @ 532 nm)
Faraday Cup
High Voltage(100 kV)
Activation(Cs/NF3, 5
mm)
Experimental Setup
350 m 1500 m
Spot SizeAdjustment
Reza Kazimi, Positron Workshop 2009
SMALL vs. LARGE Laser Spot (BP vs. LL)
5
15
1500350
2≈ 18
Expectation:
Reza Kazimi, Positron Workshop 2009
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
Reza Kazimi, Positron Workshop 2009
A possible next generation gun design
• Will be based on CEBAF/Cornell load-lock systems• Will explore CEBAF’s new approach of inverted insulator…
Picture courtesy of Matt Poelker
Reza Kazimi, Positron Workshop 2009
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.
Reza Kazimi, Positron Workshop 2009
10 MeV Teststand (option 1)
Photo-CathodeGun Buncher
Warm cavity
¼ CryoDiagnostics:Spectrometer,Mott,FC, …
100/350 keV 10 MeV
1 mA10MeV
Reza Kazimi, Positron Workshop 2009
10 MeV Teststand (option 2)
Photo-CathodeGun Buncher
Warm cavity
¼ CryoDiagnostics:Spectrometer,Mott,FC, …
100/350 keV 10 MeV
1 mA10MeVSmall
Chicane
Reza Kazimi, Positron Workshop 2009
Diag.
Diag.
Reza Kazimi, Positron Workshop 2009
Standard five cell cavity
Reza Kazimi, Positron Workshop 2009
Modified five cell cavity for low energy entrance
Reza Kazimi, Positron Workshop 2009
Summary
• The CEBAF injector optics has been tested during the G0 operation up to 1.4 pC/bunch which is ~0.7 mA @ 0.5 GHz and 2.1 mA @ 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.
Reza Kazimi, Positron Workshop 2009
Reza Kazimi, Positron Workshop 2009
Reza Kazimi, Positron Workshop 2009
CHL-2CHL-2 Compact HighCurrent Electron
Injector
Convert 1.1 GeVElectron to Positron
Production at 11 GeV
Reza Kazimi, Positron Workshop 2009
CHL-2CHL-2
12 GeV CEBAF
Reza Kazimi, Positron Workshop 2009
Reza Kazimi, Positron Workshop 2009
Synchronous Photoinjection
Reza Kazimi, Positron Workshop 2009
Photocathode lifetime operating at 5 mA CW and 135pC bunch charge is about 550 Coulombs or 50
hours per re-cesiation
y = 4.1348e-0.0018x
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0 100 200 300 400 500 600
Extracted charge (Coulombs)
QE
(%
)
Reza Kazimi, Positron Workshop 2009
The quantum efficiency drops during average current
operation when the electron beam ionizes residual gas in the
gun vacuum chamber.
Reza Kazimi, Positron Workshop 2009
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.
Wafer 25 mm diaActive area 16 mm dia
Drive laser 8 mm dia
Front-end view of the
GaAs photocathod
e being illuminated by the drive laser while delivering 5 mA of CW electron
beam
Reza Kazimi, Positron Workshop 2009
The FEL and the GTS guns are identical in design and dimensions except for two features1. 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.
Incoming drive laser beam
Anode/mirror plate
12 inches
Reza Kazimi, Positron Workshop 2009
Three Proposed plans
Reza Kazimi, Positron Workshop 2009
Layout of the Injector
Make ~10 MeV ElectronsConvert to Positrons
Insert Before the Modules
Reza Kazimi, Positron Workshop 2009
Layout of the Injector
500 keV dump
Make ~10 MeV ElectronsAccelerate to ~65 MeVConvert to Positrons
Reza Kazimi, Positron Workshop 2009
10 MeV Teststand
Photo-CathodeGun Buncher
Warm cavity
¼ CryoDiagnostics:Spectrometer,Mott,FC, …
100/350 keV 10 MeV
1 mA10MeVSmall
Chicane