Injector and Source Goals

1. Improve performance of CEBAF photoinjector (all Users)

2. Prepare for parity violation experiments (incl. Qweak, PRex)

3. Demonstrate high average current (EIC, FEL)

4. Demonstrate high bunch charge (ILC, CLIC, FEL)

5. Demonstrate high peak current (ILC, CLIC, light sources)

6. Make positrons at CEBAF

Common to all these Goals is a new gun design, with better

vacuum and higher operating voltage…

M.Poelker, P. Adderley, J.Clark, J. Grames, J. Hansknecht, M. Stutzman, R. Suleiman

Students: J. Dumas, A. Jayaprakash, J. McCarter, K. Surles-Law

Director’s Review, March 20, 2009

Higher Voltage Gun…• helps achieve ALL goals….

• More UP time at CEBAF, better beam quality for Parity Violation experiments

• Longer lifetime at high average current, good for FEL and positron source

• Emittance preservation at high bunch charge and peak current

High Voltage Issues:

• Field emission

• Electrode design:

reducing gradient and

good beam optics

• Hardware limitations at

CEBAF (Capture,

chopper)

Improve Vacuum

• Ion pumps

• NEG pumps

• Outgassing

• Gauges

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115KeV

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70KeV

Measurements at CEBAF/JLab PARMELA Simulation Results

Benchmarking PARMELA Simulation Results Against Beam-Based

Measurements at CEBAF/Jefferson Lab – work of Ashwini Jayaprakash, JLab

Message: Beam quality, including transmission, improves at higher gun voltage

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Reasonable agreement

“Inverted” Gun

e-

Present Ceramic

• Exposed to field emission

• Large area

• Expensive (~$50k)

Medical x-ray

technology

New design

New Ceramic

• Compact

• ~$5k

Want to move away from “conventional” insulator used on all GaAs photoguns

today – expensive, months to build, prone to damage from field emission.

neg modules

Field Emission – Most Important Issue

• Flat electrodes and small gaps not

very useful

• Want to keep gun dimensions

about the same – suggests our

200kV gun needs “quiet”

electrodes to 10MV/m

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Stainless Steel and Diamond-Paste Polishing

Good to ~ 5MV/m and 100kV.

Work of Ken Surles-Law, Jefferson Lab

5MV/m

100kV

Replace conventional

ceramic insulator with

“Inverted” insulator: no

SF6 and no HV

breakdown outside

chamber

Conventional

geometry: cathode

electrode mounted

on metal support

structure

Single Crystal Niobium:

• Capable of operation at higher voltage

and gradient

• Buffer chemical polish (BCP) much

easier than diamond-paste-polish

Work of Ken Surles-Law, Jefferson Lab

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BCP Niobium vs Stainless Steel

niobium

304 SS

304 SS #2

Thanks to P. Kneisel, L. Turlington, G. Myneni

Field emission studies,

Vacuum studies, etc.,

New designs,

Mods, ImprovementsCharacterize at Test Cave

Demonstrate Advances at Injector Test Cave

Tests to support new initiatives like EIC, ILC

Solve problems, answer important questions

For example:

• Lifetime vs laser spot size

• High current (mA) ops with high

polarization

• Charge limit studies….

pump speed test stand

Lifetime with Large/Small Laser SpotsTough to measure >1000 C

lifetimes with 100-200 C runs!

5

15

1500350

2≈ 18

Expectation:

“Further Measurements of Photocathode Operational Lifetime at Beam Current > 1mA using an Improved 100 kV DC High Voltage GaAs Photogun,” J. Grames, et al., Proceedings Polarized Electron Source Workshop, SPIN06, Tokyo, Japan

This result frequently cited in support of plans for eRHIC at >25mA

1mA at High Polarization*Parameter Value

Laser Rep Rate 499 MHz

Laser Pulselength 30 ps

Wavelength 780 nm

Laser Spot Size 450 mm

Current 1 mA

Duration 8.25 hr

Charge 30.3 C

Lifetime 210 C

#How long at 1mA? 10.5 days

High Initial QE

Vacuum signals

Laser Power

Beam Current

* Note: did not actually

measure polarization

# prediction with 10W laser

But QE not constant……when surface is damaged or dirty

Surface Charge Limit – a problem that can jeopardize

many of the new/proposed machines. Not just a problem for

pulsed machines (data above from CEBAF)

This damaged (i.e., used) photocathode would have problem providing 0.5mA, even with “infinite” laser power

e-beam

• Condition to 600kV, operate at

500kV

• 3x bigger inverted insulators

• One insulator for HV: one for

cooling

• Niobium electrode – no

diamond paste polishing

• Use the CEBAF prep chamber

- demonstrated to work

Working with the FEL gun group, applying recent

lessons-learned to an inverted, load-locked gun at very

high voltage

Courtesy: M. Marchlick, G. Biallis, C. Hernandez-Garcia, D. Bullard, P. Evtushenko, F. Hannon, and others from JLab-FEL

Positrons at JLab: Strategy & Collaboration

Explore CEBAF and Dedicated Facility User Base (3 sessions) Two photon exchange effects in elastic and semi-inclusive (e, e’)

Measure directly Bethe-Heitler virtual Compton scattering

interference

NEW Physics with positrons

U-boson search

Coulomb distortion in the inelastic regime

C_3q Measurements

BES/Industry/University Applications

Positron Source & Accelerator Issues (2 sessions) International experts to discuss technical issues & challenges

JLab collaborations and working groups

Joint scientist position and R&D effort with Idaho Accelerator

Center

PhD student - Serkan Golge: e+ source design for CEBAF

PhD student - Jonathan Dumas: novel polarized e+ source

design

Stimulus proposal for Test Cave upgrade supports e+ program

3 day workshop next week (March 25-27) explores “Positrons at JLab”

Workshop is supported by Accelerator, Physics & Theory Divisions,

the Idaho Accelerator Center and the Laboratory for Physics & Cosmology, Grenoble

Source Property E-166 ExperimentPRL 100, 210801 (2008)

J. Dumas et al.Proc. Spin 2008

Electron beam energy 50 GeV - Undulator 10 MeV - Conversion

Electron beam polarization Unpolarized 85%

Photo Production Synchrotron Bremsstrahlung

Converter Target Tungsten Foil Tungsten Foil

Positron Polarization 80% (measured) 40% (Simulation)

PhD Thesis: Polarized Positrons for JLab, Jonathan DUMASAdvisors: Eric Voutier, LPSC and Joe Grames, JLab

ILC Polarized e+ Schemes/Demos(synchrotron/Compton polarized photon)

Conventional un-polarized e+ Scheme(bremsstrahlung photon)

High Polarization, High Current e- Gun(polarized bremsstrahlung photon)

OR

Positron Yield scales with Beam Power• Replace GeV-pulsed with MeV-CW

Reduce radiation budget• Remain below photo-neutron threshold

Bunch/Capture to SRF linac• Compact source vs. Damping Ring

Unique capabilities• First CW source with helicity reversal

T. Omori, Spin 2006

E = 50 GeV L = 1m E-166 Experiment

Proof of Principle Experiment: extendible to higher energy (& yield)

Precision ElectronSpectrometer (~3%)

Precision ElectronMott Polarimeter (~1%)

e- e+

e-PairBrem

CEBAF Electron SourceHigh-P (~85%), High-QE (~3mA/500 mW)e- bunch: 3mA @ 1497MHz demonstratedThesis: duty factor => low power, high peak

MeV-Accelerator Cryounit tested to ~8 MeV G0 setup 1.9mA @ 1497 MHz

e+ Spectrometer(or e- & no spin rotation) Transmission Polarimeter (MIT loan)

Conversion Target(Tilted/Normal Tungsten Foils)

= ±20

= ±10

= ±5

Collimators

Analyzer magnet

Spec.Dipole#2

Spec.Dipole#1

SweepDipole

e+

e- after targetnot shown

e-

converter

E = ±250 keV, = 2π

Geant4simulation

Geant4simulation

G4 Beamlinesimulation

…seems reasonable but model does not include Wien fields or quad magnets.

What’s next: add Wien fields to Parmela model, determine quad locations. Joe

Grames, Ashwini Jayaprakash working on this aspect of project.

D. Machie, K. Smith to work on design

Two-Wien Front End – Extra Spin Flip

The Source Group hosted two recent workshops: PESP2008 – Workshop on Polarized Electron Sources and PolarimetersJPOS09 – International Workshop on Positrons at JLab.