Pepperpot Emittance Measurements of the FETS Ion Source

Simon Jolly

Imperial College

3rd October 2007

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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.

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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.

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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.

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Vacuum bellows

Camera

Moving rod

Shutter

Mounting flange

Pepperpot head

Bellows

Tungsten mesh

Beam profile head

FETS Pepperpot Design

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Pepperpot Installation

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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

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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.

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Pepperpot Emittance Extraction

Pepperpot image spots: hole positions (blue) and beam spots (red)

Emittance profiles

Y

X

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Pepperpot GUI and Data Analysis

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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

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Pepperpot/Profile Comparison

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Pepperpot vs. Slit-Slit: 11kV X Emittance

0.39 mm mrad

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Pepperpot vs. Slit-Slit: 11kV Y Emittance

0.45 mm mrad

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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.

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Pepperpot 11kV: 119/250 Cut

250 cut119 cut

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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.

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Ce-Quartz Decay: 1000 Images

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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.