electron driver capabilities and proposed jlab test facility

Reza Kazimi, Positron Workshop 2009

Electron Driver CapabilitiesandProposed JLab Test Facility

Reza KazimiJLab


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


INJEC

TOR


Layout of the CEBAF Injector


Measured Bunch length Along the Injector


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


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


The JLab FEL is an Energy Recovery Linac, Fourth Generation

Light Source.

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 GaAs electron gun


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


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


Injector Test Facility


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


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


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.


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


SMALL vs. LARGE Laser Spot (BP vs. LL)

5

15

1500350

2≈ 18

Expectation:


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


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


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.


10 MeV Teststand (option 1)

Photo-CathodeGun Buncher

Warm cavity

¼ CryoDiagnostics:Spectrometer,Mott,FC, …

100/350 keV 10 MeV

1 mA10MeV


10 MeV Teststand (option 2)


Warm cavity


100/350 keV 10 MeV

1 mA10MeVSmall

Chicane


Diag.

Diag.


Standard five cell cavity


Modified five cell cavity for low energy entrance


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.


CHL-2CHL-2 Compact HighCurrent Electron

Injector

Convert 1.1 GeVElectron to Positron

Production at 11 GeV


CHL-2CHL-2

12 GeV CEBAF


Synchronous Photoinjection


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

(%

)


The quantum efficiency drops during average current

operation when the electron beam ionizes residual gas in the

gun vacuum chamber.


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


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


Three Proposed plans


Layout of the Injector

Make ~10 MeV ElectronsConvert to Positrons

Insert Before the Modules


Layout of the Injector

500 keV dump

Make ~10 MeV ElectronsAccelerate to ~65 MeVConvert to Positrons


10 MeV Teststand


Warm cavity


100/350 keV 10 MeV

1 mA10MeVSmall

Chicane

electron driver capabilities and proposed jlab test facility

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