AWAKE Electron Spectrometer

Simon Jolly, Lawrence Deacon, Matthew Wing

28th January 2015

28/01/15

Spectrometer SpecificationsWakefield accelerated electrons ejected collinear with proton beam: need to separate the 2 and measure energy of electron beam only.

Must be able to resolve energy spread as well as energy: spectrometer must accept a range of energies, probably 0-5 GeV.

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towards proton beam dump

Spectrometer Layout

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2 GeV Beam, 1.86 T Field

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Spectrometer Status• Lawrence’s BDSIM simulations have shown we expect to get enough light from GadOx

scintillator:– Signal allows Alexey’s input distributions to be resolved.– Screen will easily survive lifetime of experiment.– Background particles being included to check signal well above background levels.

• MBPS now replaced with HB4 C-magnet: cheaper and better!• Alexey’s beam dynamics simulations have demonstrated how much we will gain from including

quadrupole doublet upstream of dipole:– Now part of baseline design.– Extends distance from plasma cell to spectrometer dipole to ~5 m.

• No longer purely UCL but shared with CERN-BI:– Effort generally split at screen.– UCL look after beam dynamics simulations, backgrounds, vacuum vessel specs, windows, resolution.– CERN-BI fellow (Bart Biskup) looking after optical elements, readout, camera (already purchased by UCL),

DAQ.

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Beamline between plasma cell and spectrometer magnet

Diagnostics

Plasma cell

Spec Dipole

Quad doublet

Input Energy

• Input: witness electron bunch file consisting of 12,688 electrons injected in a length 120 mm +/- a few mm behind the laser pulse.

• Repeated this file to fire a total of 1.7065543x107 ~ 6% of the number predicted by plasma wakefield simulations (30% injection efficiency -> “final Ne = 3x108” — Alexey Petrenko)

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Measured Energy: Electrons at Screen

• The positions of the primary electrons were recorded at the screen.

• These positions were then used to reconstruct the energy spectrum.

• The energies are binned with the bin widths of the camera pixels (variable width bins).

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Measured Energy: Electrons at Screen

• The two histograms plotted on the same axes.

• However, the electrons will not be detected directly:– Phosphor screen.– Camera.

• We simulate the photon production in the screen and count photons in the camera lens.

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Measured Energy: Photons at Camera Lens

• Sampling plane placed 4 m away from the screen.

• Position at the screen of optical photons within the lens diameter (50 mm) recorded.

• Total of 2.29x105 photons in lens acceptance.

• Energy reconstructed from these positions (perfect image assumed).

• Rescaled using conversion factor (photons per electron at screen) to plot the e- spectrum (magenta on plot).

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Spectrometer Dipole Options

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MBPS C-Type HB4

Weight 15 t 8.5 tPower consumption

60 kW 15 kW rms & 24 kW cycled

Integrated field (B*L)

1.9 T*m 1.3 T*m rms & 1.6 T*m cycled

Max. magnetic field

1.65 T 1.2 T rms & 1.5 T cycled

Horizontal aperture

52 cm 32 cm

Vertical aperture 11 cm 8 cmIron length 1 m 1 mTotal length 1.7 m 1.6 mTotal width 1.2 m 1.3 mCurrent 545 A 400 A rms & 500 A cycledResistance 195 mΩ 94 mΩInductance 663.0 mH 205.2 mH

Transverse Magnetic Fields

• HB4 field maps simulated by Alexey Vorozhtsov in Opera: many more currents possible than measurement (field shape is current dependent).

• Above: HB4. Field drops to 90% at x ~14cm.

• Below: MBPS. Field drops to 90% at x ~24 cm.

• So MBPS field width ~1.6 times that of HB4.

• But HB4 has larger dynamic aperture for spectrometer since beam does not get collimated on side yoke.

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Longitudinal Field

• HB4 simulated: (top): field 90% at z = 50cm

• MBPS measured (below): field 78% at z = 50cm

• HB4 closer to ‘top hat’.• Simulations compare

650 A (1.43T) HB4 field with MBPS 650 A field scaled to 1.43 T (all field values multiplied by scaling factor to make peak field 1.43 T).

• Default quadrupole doublet (QF0, QD0) coefficients k1 are set to +/- 4.678362 m-2 (would focus 1.32 GeV witness beam on screen).

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Resolution Studies

• Simulations looked at differences in spectrometer resolution between two magnets.

• HB4 (top) shows fractionally worse high energy performance than MBPS (bottom) due to lower field, but slightly better low energy performance.

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HB4 Resolution for Different Fields

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Possible Screen Locations• With HB4, default option is 45

degree screen at edge of yoke: range 200 MeV – >1.6 GeV

• Could move screen inside yoke: drops min energy to ~100 MeV.

• Altering screen angle to 20 degrees provides smaller vacuum chamber: need to check effect on resolution.

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Backgrounds

• Received input particles from the end of the plasma cell from the simulation by Pablo Ortega et. al.

• Particles including the following: electrons, positrons, protons, photons, neutrons.

• These were the output of a FLUKA simulation of 15 million protons from the SPS. Baseline number = 3x1011 so scaling factor = 20,000.

• Accelerated witness electrons from Alexey Petrenko: 1.4x105, from 1x106 injected into the plasma cell.

• Baseline number of electrons in the witness beam before injection = 1.3x109 so scaling factor = 1,300.

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Signal-to-Background

• x = 0 is the centre of the 1 m wide screen: negative x is high energy side (screen viewed from downstream).

• Photon signal and background integrated in y in +/- 5mm steps.

• Shows peak signal to background ratio of ~1300.

• Many more background particles produced within beampipe end up close to beam axis, increasing S/B on high energy side.

• We can clearly see our high energy peak…

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All particles

Particles originating outside beampipe

Future Plans

• Need to continue simulations as a matter of urgency:– 100% of Lawrence’s time for 2 years: ~£200k?

• CERN (Bart) looking after design of light collection system, but UCL covering hardware costs:– ~£50k for optical line components?

• Camera and lenses already purchased by may need to budget for spare:– £50k for replacement camera + lenses.

• DAQ simple (camera to PC over remote desktop) but needs to be integrated with complete AWAKE DAQ system:– Peter Sherwood’s time covered by UCL CG.

• CERN covering cost of vacuum vessel fabrication, magnet installation and power supplies.

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