awake electron spectrometer simon jolly, lawrence deacon, matthew wing 28 th january 2015
TRANSCRIPT
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.
Simon Jolly, UCL, AWAKE-UK Meeting 2protons
towards proton beam dump
Spectrometer Layout
28/01/15 Simon Jolly, UCL, AWAKE-UK Meeting 3
2 GeV Beam, 1.86 T Field
28/01/15 Simon Jolly, UCL, AWAKE-UK Meeting 4
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.
28/01/15 Simon Jolly, UCL, AWAKE-UK Meeting 5
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)
28/01/15 Simon Jolly, UCL, AWAKE-UK Meeting 6
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).
28/01/15 Simon Jolly, UCL, AWAKE-UK Meeting 7
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.
28/01/15 Simon Jolly, UCL, AWAKE-UK Meeting 8
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).
28/01/15 Simon Jolly, UCL, AWAKE-UK Meeting 9
Spectrometer Dipole Options
28/01/15 Simon Jolly, UCL, AWAKE-UK Meeting 10
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.
28/01/15 Simon Jolly, UCL, AWAKE-UK Meeting 11
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).
28/01/15 Simon Jolly, UCL, AWAKE-UK Meeting 12
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.
28/01/15 Simon Jolly, UCL, AWAKE-UK Meeting 13
HB4 Resolution for Different Fields
28/01/15 Simon Jolly, UCL, AWAKE-UK Meeting 14
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.
28/01/15 Simon Jolly, UCL, AWAKE-UK Meeting 15
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.
28/01/15 Simon Jolly, UCL, AWAKE-UK Meeting 16
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…
28/01/15 Simon Jolly, UCL, AWAKE-UK Meeting 17
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.
28/01/15 Simon Jolly, UCL, AWAKE-UK Meeting 18