1. Laser-based Beam Diagnostics for the RAL Front End Test Stand
Thanks for the introduction
Grateful to Jon and the group for opportunity to give this seminar
Going to be talking about…
David Lee

2. 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
First I’ll introduce the FETS
Then I’ll talk a bit about beam diagnostics, motivating the need for non-intrusive beam diagnostics and describing how that can be done using lasers for H− beams
And go on to describe the design and status of the FETS laser profile measuring device
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3. The Front End Test Stand
Laser-based Beam Diagnostics for the RAL Front End Test Stand • 2
David Lee • 28/01/2009

4. The Front End Test Stand
Beam dump and laser
emittance measurement upstream (not shown)
MEBT and chopper
RFQ
Magnetic LEBT
H− ion source
Test Stand for first part (the Front End) of next-generation proton accelerator chain – beam quality (emittance) defined here. Mention losses (context of ISIS losses?)
Give context (what can be used for)
Describe components briefly
Ion source H− (injection into ring) 65 keV 60 mA 2 ms
LEBT Low Energy Beam Transport 3 Solenoids Matches
RFQ Bunches and Accelerates whilst constantly focussing
MEBT Matches the beam from the RFQ to the downstream linac
Chopper Removes some bunches to minimise injection losses
Laserwire Mention!
Laser profile monitor
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5. Neutrino Factory
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6. Neutrino Factory
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7. Status
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8. Status MENTION Time frame for first ion beam
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9. Status
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10. Status
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11. Beam Diagnostics
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David Lee • 28/01/2009

12. 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
EXPLAIN What you mean by profile and projection
NEXT Non-intrusive beam diagnostics
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13. Non-intrusive diagnostics
Allows for online monitoring of the beam
Keeps users happy
MENTION Scintillator Example
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14. Non-intrusive diagnostics
Allows for online monitoring of the beam
Keeps users happy
Beam dynamics not affected by the instrument
Keeps accelerator group happy
Residual gas ion
energy analyser
RGIE spectrum (beam potential distribution)
depending on position of emittance scanner
Ion source
~10 keV dc Helium beam
Beam potential affects space charge and so the beam dynamics
dl’ / dW [μA / (eV m)]
200V
0.365T 0V
0.406T
Allison scanner
in / out
Beam dump 0V
W / eV
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15. Non-intrusive diagnostics
Allows for online monitoring of the beam
Keeps users happy
Beam dynamics not affected by the instrument
Prevents the beam from damaging the instrument
Keeps accelerator group happy
Plastic
Ruby
P46
MENTION Activation (particularly above ~3 MeV)
NEXT Laser-based H− diagnostics
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16. Ionisation Cross Section
MENTION Outer electron loosely bound ~0.75 eV (inner electron ~13.6 eV)
MENTION Non-relativistic (no Doppler shift)
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17. Ionisation Cross Section
MENTION Negligible momentum transfer to electrons
MENTION Laser parameters
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18. Laser-based H− diagnostics
To measure
Beam profiles
Longitudinal emittances
use photo-detached electrons
What do we do once the electron is detached?
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19. Laser-based H− diagnostics
To measure
Beam profiles
Longitudinal emittances
use photo-detached electrons
Collect the detached electrons − − − −
MENTION Number of electrons detached
MENTION How do longitudinal emittance − − − − − − − − − − − − − − − − − − − − − − − − − −
Photo-ionise some of the H- ions
Separate species using a dipole magnet
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20. Laser-based H− diagnostics
Collect the detached electrons − − − − − − − − − − − − − − − − − − − − − − − − − − − − − −
Photo-ionise some of the H- ions
Separate species using a dipole magnet
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21. Laser-based H− diagnostics
Collect the detached electrons − − − − − − − − − − − − − − − − − − − − − − − − − − − − − −
Photo-ionise some of the H- ions
Separate species using a dipole magnet
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22. Laser-based H− diagnostics
Collect the detached electrons − − − − − − − − − − − − − − − − − − − − − − − − − − − − − −
Photo-ionise some of the H- ions
Separate species using a dipole magnet
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23. Laser-based H− diagnostics
Collect the detached electrons − − − − − − − − − − − − − − − − − − − − − − − − − − − − − −
Photo-ionise some of the H- ions
Separate species using a dipole magnet
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24. Laser-based H− diagnostics
Collect the detached electrons − − − − − − − − − − − − − − − − − − − − − − − − − − − − − −
Photo-ionise some of the H- ions
Separate species using a dipole magnet
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25. Laser-based H− diagnostics
To measure
Transverse emittances
use photo-ionised neutrals
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26. Laser-based H− diagnostics
To measure
Transverse emittances
use photo-ionised neutrals − −
Use a dipole to separate out the particles neutralised by residual gas interactions
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27. Laser-based H− diagnostics
To measure
Transverse emittances
use photo-ionised neutrals − − − − −
Photo-ionised neutrals −
Photo-ionise some of the H- ions in the dipole
Residual gas neutrals
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28. Laser-based H− diagnostics
To measure
Transverse emittances
use photo-ionised neutrals −
Image with scintillator and CCD − −
CCD − −
Photo-ionised neutrals −
Photo-ionise some of the H- ions in the dipole
Residual gas neutrals
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29. Laser-based H− diagnostics
Image with scintillator and CCD − −
CCD − −
Photo-ionised neutrals
MENTION Scan laser across to get (eg.) y information
(Can also be clever and extract x-x’ information too) −
Photo-ionise some of the H- ions in the dipole
Residual gas neutrals
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30. The Front End Test Stand Laser Profile Monitor
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David Lee • 28/01/2009

31. The Benefit of Multiple (>2) Projections
Before describing the device itself I’d like to motivate one of it’s design features: namely, the ability to measure more than 2 (x and y) projections of the beam
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David Lee • 28/01/2009

32. The Benefit of Multiple (>2) Projections
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33. The Benefit of Multiple (>2) Projections
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34. The Benefit of Multiple (>2) Projections
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35. 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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36. Laser-scanning setup MENTION longitudinal length of vacuum vessel
MENTION Start with only x-motors
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37. Laser-scanning setup MENTION Mirror location
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38. Optics A simple, one lens setup will be used to begin with
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39. Optics
MENTION 0.09% difference in laser beam size at edge of ion beam compared to centre
NEXT Detector
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40. Copper accelerating sheath
The Detector
Faraday
Cup
Electrons
Magnet
H−, H0
EXPLAIN Copper sheath and Mention range of detector – up to hundreds (~350) of MeV.
MENTION Holes are for pumping
Copper accelerating sheath
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41. The Detector Constructed here in the workshop
Compact; the whole assembly is ~80x100x250 mm
Beam direction
NEXT Simulation
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42. 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
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43. Simulations Faraday Cup Magnet yoke Copper accelerating sheath
Cross section through detector
Electron Trajectories are in black
Coloured lines are isopotential
Copper accelerating sheath
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44. Dipole Field Map
To confirm the simulation results the dipole had its field (bending component; By) mapped
MENTION Daresbury
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45. Dipole Field Map MENTION Results include hysteresis
NEXT Longitudinal acceptance
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46. Dipole Field Map
EXPLAIN Good agreement between results and simulation, particularly in key central region
MENTION Larger discrepancies near the pole pieces go to ~6%. Not too bothered due to location of plane
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47. 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 by introducing an additional electrode
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48. Longitudinal Acceptance
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49. Longitudinal Acceptance
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50. Longitudinal Acceptance
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51. Longitudinal Acceptance
-500V
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52. Longitudinal Acceptance
MENTION Negative field lines
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53. Longitudinal Acceptance
-500V
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54. Longitudinal Acceptance
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55. Longitudinal Acceptance
NEXT Electronics
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56. 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
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57. Readout Electronics Signal ~ pC, noise < fC NEXT Reconstruction
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58. Reconstruction
Currently envisaged that the profiles will be reconstructed using the Algebraic Reconstruction Technique (ART) algorithm
Maximum entropy (MENT) also under consideration
MENTION 5 Projections, 2 mm laser beam, Overlapping (50%?)
Original Profile
Reconstructed Profile
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59. 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
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60. Spare Slides
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61. ISIS Upgrades Diamond ISIS (0.24 MW) 3 GeV RCS 400-800 MeV Linac
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62. For scale: CERN
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63. For scale: CERN
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64. For scale: CERN
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65. For scale: CERN
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66. H− Injection H− from Linac Stripping foil Circulating protons
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67. FETS Ion Source 35kV 17kV Platform Ground Platform DC Power Supply
53.7mm
35kV
17kV
Platform
Ground
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
Mounting Flange
10mm
Mica
Copper Spacer
Ceramic
H- Ion Beam
Extract Electrode
Cathode
Anode
Penning Pole Pieces
Discharge Region
Aperture Plate
Source Body
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68. 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 S
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69. 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
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70. FETS RFQ Cold model complete Mechanical design study starting
RF tests underway to verify simulation
mostly complete
Mechanical design study starting
Integrated with simulation work
Lots to consider
How to manufacture
Cooling …
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71. Simon 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.
RFQ E-field
Standard Quad
RFQ vane tips
Simon Jolly, Imperial College

72. RFQ Acceleration/Bunching
RFQ vane tips modulated longitudinally.
Vane tip modulations produce longitudinal field: acceleration and bunching.
Alternate modulation gives acceleration
Single vane
 = distance moved by particle in one oscillation + -
Simon Jolly, Imperial College

73. FETS MEBT/Chopper (I) Matching between RFQ and DTL
Houses beam chopper to remove some bunches of the beam
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74. FETS MEBT/Chopper (II)
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75. Laser-based e± diagnostics
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76. 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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77. H- profile comparison Initial particle distribution
H- distribution just past magnet
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78. H- phase space comparison
Initial particle distribution
H- distribution just past magnet
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79. H- phase space comparison
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80. 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
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81. Stray fields
The ion source dipole field leaks into the diagnostic vessel
Scott Lawrie (RAL) has designed shielding for this Deflections (with no E-field)
Average deflection: 3.88 mm (with B-field)
Average deflection: mm (no B-field)
Deflections (with E-field)
Average deflection: 2.44 mm (with B-field)
Average deflection: mm (no B-field)
Shielding sufficient
With and w/out shielding?
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82. ART
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83. ART
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84. ART
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