laser-based beam diagnostics for the ral front end test stand

Laser-based Beam Diagnostics for the RAL Front End Test Stand

David [email protected]

28.01.2009

mailto:[email protected]

David Lee • 28/01/2009

Outline

• The Front End Test Stand (FETS)• Beam diagnostics

– Beam profile measurements– Non-intrusive beam diagnostics

• Laser-based H− diagnostics

• FETS laser profile monitor• Conclusion

Laser-based Beam Diagnostics for the RAL Front End Test Stand • 1

The Front End Test Stand

Laser-based Beam Diagnostics for the RAL Front End Test Stand • 2 David Lee • 28/01/2009

David Lee • 28/01/2009

The Front End Test Stand

Magnetic LEBT

RFQ

MEBT and chopper

H− ion source

Laser profile monitor


Beam dump and laseremittance measurementupstream (not shown)

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Neutrino Factory


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Status


Beam Diagnostics

David Lee • 28/01/2009Laser-based Beam Diagnostics for the RAL Front End Test Stand • 6

David Lee • 28/01/2009

Beam Profile Measurements

• Why are beam profile measurements of interest?– First half of the beam emittance– Gives information about the charge density

• And so the beam’s self field / space charge

– Will the beam fit through the beam pipe?– What is the beam halo like?– …

• How are they typically done?– Scintillator – Wire scanner– …


David Lee • 28/01/2009

Non-intrusive diagnostics

• Allows for online monitoring of the beam– Keeps users happy


David Lee • 28/01/2009



• Beam dynamics not affected by the instrument– Keeps accelerator group happy

RGIE spectrum (beam potential distribution)depending on position of emittance scanner

W / eV

dl’

/ d

W [

μA

/ (

eV

m)]

Residual gas ionenergy analyser

Ion source

0V

200V

0.365T 0.406T Beam dump0V

Allison scannerin / out


David Lee • 28/01/2009



• Beam dynamics not affected by the instrument• Prevents the beam from damaging the instrument

– Keeps accelerator group happy

Plastic RubyP46


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Ionisation Cross Section


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• To measure– Beam profiles– Longitudinal emittances

use photo-detached electrons

Laser-based H− diagnostics


David Lee • 28/01/2009

• To measure– Beam profiles– Longitudinal emittances

use photo-detached electrons

Photo-ionise someof the H- ions

Separate species usinga dipole magnet

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Collect the detached electrons

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David Lee • 28/01/2009

Photo-ionise someof the H- ions

Separate species usinga dipole magnet

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Collect the detached electrons

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• To measure– Transverse emittances

use photo-ionised neutrals



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Use a dipole to separate out the particles neutralised by residual gas interactions


David Lee • 28/01/2009



Photo-ionise some of the H- ions in the dipole

Residual gasneutrals

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Photo-ionised neutrals


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Image with scintillator and CCD

CCD


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CCD


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Image with scintillator and CCD



The Front End Test StandLaser Profile Monitor

Laser-based Beam Diagnostics for the RAL Front End Test Stand • 12 David Lee • 28/01/2009

David Lee • 28/01/2009

The Benefit of Multiple (>2) Projections

David Lee • 28/01/2009Laser-based Beam Diagnostics for the RAL Front End Test Stand • 13

David Lee • 28/01/2009

The Benefit of Multiple (>2) Projections


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Laser-scanning setup

• To get multiple projections, need to be able to pass laser through beam at variety of angles

• To do this, use movable mirrors mounted in vacuum vessel


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Laser-scanning setup


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Optics


A simple, one lens setup will be used to begin with

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Optics


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The Detector

FaradayCup

Magnet

Copperaccelerating

sheath

H−, H0

Electrons


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The Detector

Beamdirection

• Constructed here in the workshop

• Compact; the whole assembly is~80x100x250 mm


David Lee • 28/01/2009

Simulations

• Detector’s E and B fields simulated using a electromagnetic finite element program – CST EM Studio

• Particle tracking was performed by the General Particle Tracer package

• Input distribution from a pepperpot correlated emittance measurement of the ion source


David Lee • 28/01/2009

Simulations

FaradayCup

Magnet yoke

Copperacceleratingsheath


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Dipole Field Map

• To confirm the simulation results the dipole had its field (bending component; By) mapped


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Dipole Field Map


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Longitudinal Acceptance

• Electrons from residual gas interactions form a background to our measurement– Problem made worse by proximity to ion source

• 20 ml per minute of hydrogen gas

• To reduce this background, the longitudinal acceptance of the detector can be reduced byintroducing an additional electrode


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-500V


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David Lee • 28/01/2009


-500V


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Readout Electronics

• Microprocessor controlled integrate and hold ADC with serial output to PC

• LabVIEW-based DAQ

• In the process of testing and fine-tuning production version


David Lee • 28/01/2009

Readout Electronics


David Lee • 28/01/2009

Reconstruction

• Currently envisaged that the profiles will be reconstructed using the Algebraic Reconstruction Technique (ART) algorithm– Maximum entropy (MENT) also under consideration


Original Profile Reconstructed Profile

David Lee • 28/01/2009

Conclusion

• A novel laser-based beam diagnostic that is able to measure the full profile of the FETS H− beam has been designed and constructed

• It is currently being installed at RAL• And should be ready to make measurements on

the first beam in a few weeks time


Spare Slides


David Lee • 28/01/2009

ISIS Upgrades

ISIS

(0.24 MW)

Diamond

3 GeV RCS

(1 MW)

400-800 MeV Linac

(2-5 MW)


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For scale: CERN


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H− Injection

H− from Linac

Circulating protons

Stripping foil


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FETS Ion Source

Mounting Flange 10mm

Mica

Copper Spacer

Ceramic

H- Ion Beam Extract Electrode

Cathode

Anode

Penning Pole Pieces

Discharge Region

Aperture Plate

Source Body

53.7mm

35kV

17kV

PlatformGround

Platform DC Power Supply

Pulsed Extract Power Supply

Post Extraction

Acceleration Gap

Laboratory Ground

Extraction Electrode, Coldbox and Analysing Magnet all Pulsed

35keV H- Beam

+-

+-

18kV


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FETS LEBT

• Matches beam from the ion source to the RFQ• 3 solenoid construction• Solenoids and their power

supplies complete• Beam pipe design complete

D DDD SS S


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Solenoid Focussing

• Second order focussing– Field gradient starts rotation

of particles about axis

– This rotation then produces a focussing force towards the axis

F, v, B


David Lee • 28/01/2009

FETS RFQ

• Cold model complete– RF tests underway to verify

simulation• mostly complete

• Mechanical design study starting– Integrated with simulation

work– Lots to consider

• How to manufacture

• Cooling

• …


David Lee • 28/01/2009Simon Jolly, Imperial College

RFQ Focussing

• RF field causes positive / negative charges on pairs of vanes.

• Since field varies with time, alternate focussing / defocussing mimics a FODO lattice.

Standard Quad

RFQ E-field

RFQ vane tips

David Lee • 28/01/2009Simon Jolly, Imperial College

RFQ Acceleration/Bunching

• RFQ vane tips modulated longitudinally.– Vane tip modulations produce longitudinal field:

acceleration and bunching.

Single vane

= distance moved by particle in one oscillation

+

-

Alternate modulation gives acceleration

David Lee • 28/01/2009

FETS MEBT/Chopper (I)

• Matching between RFQ and DTL• Houses beam chopper to remove some bunches

of the beam


David Lee • 28/01/2009

FETS MEBT/Chopper (II)


David Lee • 28/01/2009

Laser-based e± diagnostics

http://www.pp.rhul.ac.uk/~lbbd/


David Lee • 28/01/2009

Laser Characterisation

• Need a compromise between– Having as high a resolution as possible

• Lots of bins in histogram

– But not having an over-focussed beam• That would distort the measurement


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H- profile comparison

Initial particle distribution H- distribution just past magnet


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H- phase space comparison

Initial particle distribution H- distribution just past magnet


David Lee • 28/01/2009

H- phase space comparison


David Lee • 28/01/2009

Residual gas interactions

• Assuming partial pressures of – H2: 5x10-3 Pa

– N2: 5x10-5 Pa

we loose ~2% of the beam per metre(or ~1.8x1013 ions per metre)

• So we need to minimise the fraction of these collected to minimise the background


David Lee • 28/01/2009

Stray fields

• The ion source dipole field leaks into the diagnostic vessel

• Scott Lawrie (RAL) has designed shielding for thisDeflections (with no E-field)– Average deflection: 3.88 mm (with B-field)– Average deflection: 0.0252 mm (no B-field)

• Deflections (with E-field)– Average deflection: 2.44 mm (with B-field)– Average deflection: 0.044 mm (no B-field)

• Shielding sufficient


David Lee • 28/01/2009

ART


laser-based beam diagnostics for the ral front end test stand

Documents

beam diagnosticsdavid

beam pipe

beam halo

end test standdavid

beam profile measurementswhy

showndavid lee

online monitoring

h ionsseparate species