AWAKE Electron Spectrometer

1. AWAKE Electron Spectrometer Simon Jolly, Lawrence Deacon, Matthew Wing 28th January 2015

2. Spectrometer Specifications towards proton beam dump Wakefield 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. protons Simon Jolly, UCL, AWAKE-UK Meeting

3. Spectrometer Layout Simon Jolly, UCL, AWAKE-UK Meeting

4. 2 GeV Beam, 1.86 T Field Simon Jolly, UCL, AWAKE-UK Meeting

5. 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. Quad doublet Diagnostics Plasma cell Beamline between plasma cell and spectrometer magnet Spec Dipole Simon Jolly, UCL, AWAKE-UK Meeting

6. 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) Simon Jolly, UCL, AWAKE-UK Meeting

7. 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). Simon Jolly, UCL, AWAKE-UK Meeting

8. 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. Simon Jolly, UCL, AWAKE-UK Meeting

9. 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). Simon Jolly, UCL, AWAKE-UK Meeting

10. Spectrometer Dipole Options Simon Jolly, UCL, AWAKE-UK Meeting

11. 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. Simon Jolly, UCL, AWAKE-UK Meeting

12. 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). Simon Jolly, UCL, AWAKE-UK Meeting

13. 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. Simon Jolly, UCL, AWAKE-UK Meeting

14. HB4 Resolution for Different Fields Simon Jolly, UCL, AWAKE-UK Meeting

15. 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. Simon Jolly, UCL, AWAKE-UK Meeting

16. 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 = 3x1011so scaling factor = 20,000. • Accelerated witness electrons from Alexey Petrenko: 1.4x105, from 1x106injected into the plasma cell. • Baseline number of electrons in the witness beam before injection = 1.3x109so scaling factor = 1,300. Simon Jolly, UCL, AWAKE-UK Meeting

17. Signal-to-Background All particles Particles originating outside beampipe • 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… Simon Jolly, UCL, AWAKE-UK Meeting

18. 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. Simon Jolly, UCL, AWAKE-UK Meeting