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Pepperpot Emittance Measurements of the FETS Ion Source
Simon Jolly
Imperial College
3rd October 2007
3/10/07 Simon Jolly, Imperial College 2
The Pepperpot Emittance Scanner• Current slit-slit scanners give high resolution emittance
measurements, but at fixed z-position, with x and y emittance uncorrelated.
• Correlated, 4-D profile (x, y, x’, y’) required for accurate simulations.
• Pepperpot reduces resolution to make correlated 4-D measurement.
• Moving stage allows measurement at different z-locations: space charge information.
• Possible to make time-resloved measurements within a single pulse.
• Added bonus: make high resolution x-y profile measurements.
3/10/07 Simon Jolly, Imperial College 3
Pepperpot Principle
H- Ion Beam
Tungsten screen
Copper block
Quartz screen
H- Beamlets
Fast CCD Camera
• Beam segmented by tungsten screen.
• Beamlets drift ~10mm before producing image on quartz screen.
• Copper block prevents beamlets from overlapping and provides cooling.
• CCD camera records image of light spots.
• Calculate emittance from spot distribution.
3/10/07 Simon Jolly, Imperial College 4
Pepperpot Components• Pepperpot head:
– Tungsten intercepting screen, 50m holes on 3mm pitch in 41x41 array.
– Tungsten sandwiched between 2mm/10mm copper support plates.
– Quartz scintillator images beamlets.• Camera system:
– PCO 2000 camera with 2048 x 2048 pixel, 15.3 x 15.6 mm CCD.– Firewire connection to PC.– 105 mm Micro-Nikkor macro lens.– Bellows maintains light tight path from vacuum window to
camera.• Main support:
– Head and camera mounted at either end of 1100 mm linear shift mechanism, with 700 mm stroke.
– All mounted to single 400 mm diameter vacuum flange.
3/10/07 Simon Jolly, Imperial College 5
Vacuum bellows
Camera
Moving rod
Shutter
Mounting flange
Pepperpot head
Bellows
Tungsten mesh
Beam profile head
FETS Pepperpot Design
3/10/07 Simon Jolly, Imperial College 6
Pepperpot Installation
3/10/07 Simon Jolly, Imperial College 7
Diagnostics Vessel
Beam Current Toriod
X and Y Slit-Slit Emittance Scanners
Movable Scintillator with Interchangeable Pepperpot or
Profile Head
Buffer Gas Delivery System
Diagnostic DipoleBeam Shutter
ISDR Diagnostics
3/10/07 Simon Jolly, Imperial College 8
Pepperpot Data ImageRaw data Calibration image
Colour enhanced raw data image, 60 x 60 mm2.
Calibration image: use corners of 126 x 126 mm square on copper plate to give image scaling, tilt and spot spacing.
3/10/07 Simon Jolly, Imperial College 9
Pepperpot Emittance Extraction
Pepperpot image spots: hole positions (blue) and beam spots (red)
Emittance profiles
Y
X
3/10/07 Simon Jolly, Imperial College 10
Pepperpot GUI and Data Analysis
3/10/07 Simon Jolly, Imperial College 11
Position Variation for 13 kV Extract
0 mm
100 mm
200 mm
300 mm
x = 1.36y = 1.47 mm mrad
x = 1.82y = 1.96 mm mrad
x = 1.65y = 1.78 mm mrad
x = 1.90y = 2.04 mm mrad
3/10/07 Simon Jolly, Imperial College 12
Pepperpot/Profile Comparison
3/10/07 Simon Jolly, Imperial College 13
Pepperpot vs. Slit-Slit: 11kV X Emittance
0.39 mm mrad
3/10/07 Simon Jolly, Imperial College 14
Pepperpot vs. Slit-Slit: 11kV Y Emittance
0.45 mm mrad
3/10/07 Simon Jolly, Imperial College 15
Emittance Cut Optimisation
• As with Slit-Slit scanner, pepperpot emittance measurement is sensitive to cut level.
• Have to impose some sort of cut due to inherent 100 count noise from camera and background noise.
• Need to optimise cut level to give consistent emittance measurement.
3/10/07 Simon Jolly, Imperial College 16
Pepperpot 11kV: 119/250 Cut
250 cut119 cut
3/10/07 Simon Jolly, Imperial College 17
Scintillator Problems• Pepperpot rapidly
became “scintillator destruction rig”.
• Scintillator requirements:– Fast (down to 500ns
exposure).– High light output.– Survives beam (<1
micron stopping distance).
• High energy density from Bragg peak causes severe damage.
• Finally chose Ce-Quartz.
3/10/07 Simon Jolly, Imperial College 18
Ce-Quartz Decay: 1000 Images
3/10/07 Simon Jolly, Imperial College 19
Conclusions
• 2-D profiles with high resolution (70 m)• Medium resolved 4-D emittance measurements
(3 mm, 7 mrad).• Both data can be combined to produce 4-D data
with high resolution.• Clear correlation between pepperpot, profile and
slit-slit emittance measurements.• Emittance measurements at different z-positions
allow investigation of space charge forces.• Able to output data into GPT and LINTRA
simulations.