Photo Detachment Diagnostics for H- High Current Applications

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Photo Detachment Diagnostics for H- High
Current Applications
NonDestructive Emittance Measurements at the
Front End Test Stand FETS at 3MeV
C. Gabor, J. Pozimski, A. Letchford, C. Prior
The Front End Test Stand Collaboration
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Overview

• Introduction
• Photo detachment, different emittance principle
• Experimental Results, Measured at the Ion
  Source
• Development Rig (ISDR)
• "Simulation" of the emittance principle with a
  slit
• Interpretation of shown results
• Summary
• Outlook, further development

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PhotoDetachment Principle
? driven by neutralization of photo
neutralization ? peak cross section s4.010-17
cm2 at a photon wavelength of l830 nm

• Only a small portion of H- will be neutralized
  (laser beam ltlt ion beam)
• Magnetic field splits the ion beam into 3
  different beamlets (ions, neutrals
• produced by photo and residual gas
  interaction

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Different Emittance Measurement Principle
Comparison of slitslit and pepperpot emittance
measurement principles Photo detachment emittance
based on "slit point" principle
Envellope of the ion beam
Cross section of the ion beam
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Ion Source Development Rig (ISDR)
Pepperpot emittances Beam profiles at
various zpositions Slit slit emittances Seperat
e, movable slit in combination with scintillator
Beam parameters H- _at_ 50mA 17 keV
extraction voltage 35 keV total beam
energy field gradient n1.4 (w), extractor
terminated
Slit height 0.3mm Slit length
110mm Drift in between 107mm
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Collimated Ion Beam (Pseudo Coloured)
1.) Beam distribution at 260mm 2.) Collimated
beam parts after a separation drift of 107mm
? 0.6mrad angle resolution
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Comparison with 4dim Beam Distribution
xy-distribution xx'
emittance yy' emittance
x /- 60mm x
/-60mm y /-60mm
y /- 100 mm x' /-
100mrad y' /- 100mrad
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Summary / Outlook

• Presented emittance measurement method is a
  general princip
• and not restricted to photo neutralization
• Additional information about beam distribution
  are possible
• Demonstrated at ISDR where curved patterns
  indicate nonlinear
• forces (probably mainly in the sector
  magnet)
• Outlook maybe helpfull to reduce the ion
  source emittance
• further data analysis
• What I didn't mention .....
• Photo detachment beam profile monitor
  (tomography 2dimtime)
• Further development of "slit point principle"
  by introducing a
• movable scintillator along z to computate a
  4dim phase space
• distribution using Baysian statistics
  (Maximum Entropy), in co-
• operation with the RAL Computer Science ?
  LINAC 2008)

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Additional Slides(not presented at the talk)
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Influence of Nonlinear Electromagnetic Fields
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Transform function of an emittancewith
Saberration
Drift length 100mm
Cut of (neutralized) particles drifted pattern
Integrated beam profile I(y')
2 dimensional phase space projection (Emittance)
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Technical Design of an emittance scanner
Slit slit emittance scanner
Moving rod
Vacuum bellows
Mounting flange
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Slit Slit Emittance Measurement
Emittance scan at 260mm, standard resolution
(0.5mm 2.5mrad) H- (17kV ext) (18kV PA)
35keV total beam energy
Why you don't see the features with the slitslit
scanner even if the resolution of the instrument
should be good enough to show features with
10mrad ?