Electron Spectrometer Baseline. Simon Jolly 26 th March 2014

Transcription

1 Electron Spectrometer Baseline Simon Jolly 26 th March 2014

2 Spectrometer Requirements AWAKE will pass 450 GeV protons through plasma to create wakefield MeV(?) electrons injected into plasma to witness proton driven plasma wakefield. Protons and electrons collinearly ejected from plasma: must be separated before measuring electron energy. Spectrometer must measure energy and energy spectrum of electrons to infer behaviour of proton driven wakefield. 26/03/14 Simon Jolly, UCL, AWAKE Spectrometer Review 2

3 Spectrometer Limits What is the energy of the proton beam? 450 GeV. What is the beam stay clear for protons? 7 cm (14 cm diameter). What is the mean/maximum/minimum energy of the accelerated electron beam? We don t know What is the energy spread of the electron beam? We don t know What is the size/emittance of the electron beam? We don t know How stable is the electron beam pulse-to-pulse (and therefore how repeatable is the measurement? We don t know Although judging by previous plasma wakefield experiments, it s not likely to be repeatable at all 26/03/14 Simon Jolly, UCL, AWAKE Spectrometer Review 3

4 Spectrometer Specifications As such, we need to measure the energy and energy spread of an electron beam with unknown energy, energy spread and emittance, that is likely to have large variations pulse-topulse, to (better than) 1% This is not easy! In fact, it s impossible Large pulse-to-pulse variation means we need to capture as many energies as possible in each shot: must be able to resolve energy spread as well as energy and accept a range of energies, probably 0-5 GeV. 26/03/14 Simon Jolly, UCL, AWAKE Spectrometer Review 4

5 Spectrometer Options Several ways to measure energy in single shot. Direct measurement of particle energy through absorption and conversion to scintillation light (calorimetry): Best way of collecting all energy from individual particles. Very difficult to measure more than one particle without highly segmented calorimeter. Penetration depth in material of known density (dosimetry/sampling calorimetry): Can measure multiple particles at once: measure number of particles or temperature rise at given depth. Can only cope with a few particles with sampling calorimeter and energy resolution low (5%). Need lots of particles for dosimetry measurement. Use magnet to introduce energy-position correlation: Can measure many particles at once. How many depends on detection technique. Tracking gives fewer particles with greater capture efficiency. Scintillator screen allows many more particles but needs more for light output. Susceptible to magnet aberrations: energy measurement depends on optics of system. Like other plasma wakefield experiments, we chose option 3 26/03/14 Simon Jolly, UCL, AWAKE Spectrometer Review 5

6 Magnet Options (1) In an ideal world with a pointlike electron beam with zero emittance all we need is a smoothly varying magnetic field with a single maximum: make sure low energy beam gets bent more than high energy beam. We, of course, are not in an ideal world Browne-Buechner spectrometer design provides one-to-one correspondence between position on screen and energy: Corrects for divergence by changing path length such that all energies focussed to a point. Energy range limited to 0.5 E E 0. For E 0 = 3 GeV, need 3 m magnet radius with ~5 m screen C. P. Browne and W. W. Buechner, Rev. Sci. Instrum. 27, 899 (1956). 26/03/14 Simon Jolly, UCL, AWAKE Spectrometer Review 6

7 Magnet Options (2) Next best option would be to use circular magnet: Without specific Browne- Buechner screen, you won t get rid of divergence, so there will be some smearing from emittance. Electrons still leave magnet at 90 angle so no issues with sharp edges of fields. Would need custom magnet design. Still requires big magnet to reach integrated fields we require. Difficult to support such large fields with this magnet geometry. 26/03/14 Simon Jolly, UCL, AWAKE Spectrometer Review 7

8 Magnet Options (3) C-magnet would give larger dynamic range for given field. Smaller than circular magnet. Beam can escape one side. Half of magnet not wasted. Difficult to get 1.5 T fields due to missing half support. No longer have 90 exit angle so need to be careful about fields in these regions. 26/03/14 Simon Jolly, UCL, AWAKE Spectrometer Review 8

9 Magnet Options (4) But: we already have a magnet, don t we? Yes, with a but MBPS dipole available for free from CERN: free should not be underestimated 1.82 T field with 1 m effective length but only 300 mm wide. Field starts to fall off rapidly outside good field region: reduces reliable energy range at exit. Danger of running through nonlinear field regions, particularly for low energy trajectories where spatial spread is larger (but convolution with emittance larger also). 26/03/14 Simon Jolly, UCL, AWAKE Spectrometer Review 9

10 Square Dipole Trajectories 26/03/14 Simon Jolly, UCL, AWAKE Spectrometer Review 10

11 MBPS Magnet 26/03/14 Simon Jolly, UCL, AWAKE Spectrometer Review 11

12 MBPS Magnet: Good Field Region? 300 mm 1000 mm 26/03/14 Simon Jolly, UCL, AWAKE Spectrometer Review 12

13 Multiple Magnets Meeting last week at with Allan Gillespie and Silvia Cipiccia from Strathclyde, plus Jim Clarke from Daresbury. Circular dipole unfeasible due to size and necessary field strength. Concerns about the convolution of emittance with energy measurement for single square dipole: leads to smearing of energy and reduction in resolution. 2 magnet design acts like a drift and cancels out issues from magnet edge effects: you take out whatever you put in, with just the position spread left. Don t try and measure low energies with high energy spectrometer 26/03/14 Simon Jolly, UCL, AWAKE Spectrometer Review 13

14 Spectrometer Chicane: 4 Magnets 26/03/14 Simon Jolly, UCL, AWAKE Spectrometer Review 14

15 2 Magnet Spectrometer (A. Gillespie) Advantages: It can have a very broad momentum "bite" (say +/- 50% of p 0 ) due to the reversed field in D2 and the rectangular pole shapes. It suffers from very few chromatic aberrations, because it is translationally-independent in the x- direction It is (relatively) compact for the same reasons Provided dipole D2 is made wide enough, it could accommodate rather large variations in electron momentum/energy, and could be modified later for lower electron energies (say MeV) simply by reducing the induction below 1.5 T and decreasing the distance lambda from 0.5 m (which is rather arbitrary in any case) The quadrupoles Q1 and Q2 can be used to adjust the H and V focussing conditions e.g. you could arrange point-to-parallel optics in the V (y) plane, and point-to-point optics in the H (x) plane, if the large momentum allows this (?). Disadvantages: The large (17.5 ) bend angles might produce V focussing at D1 exit and D2 entrance that is too big a variation across the large delta-p (+/- 50%), in which case theta must be decreased towards 10. The largish x and y emittances from the plasma channel may screw up the optics at the focal plane, reducing the momentum resolution. 26/03/14 Simon Jolly, UCL, AWAKE Spectrometer Review 15

16 Outcome of Daresbury Meeting 2 magnet design would give better energy resolution than single square magnet design. Adds in an extra variable to adjust: distance between magnets (fields must be matched). Second magnet either needs to be big or accept reduction in single-shot energy spread acceptance. Will need to do tracking simulations (MAD-X/GPT/ BDSIM) to characterise performance. Perhaps go for 2-stage approach: 1. Use single magnet with large momentum acceptance to get a general idea about energies produced from plasma. 2. Add second magnet to make precision measurements once we know what energy range we re looking at. 26/03/14 Simon Jolly, UCL, AWAKE Spectrometer Review 16

17 Experimental Layout Design really split into two sections: Dispersive part (magnet + input beam simulations). Collection part (screen and camera system). Can design the two largely independently (so long as you keep sizes in mind): Magnet simply has to deliver electron beam with monotonic energy-position spread to some point downstream. Electron position measurement doesn t need to know about how beam dynamics is produced upstream, merely what energy-position correlation is. The third part is unique to AWAKE: transitioning from vacuum pipe to air. 26/03/14 Simon Jolly, UCL, AWAKE Spectrometer Review 17

18 Windowed Beampipe 26/03/14 Simon Jolly, UCL, AWAKE Spectrometer Review 18

19 Windowed Beampipe: Camera Position 26/03/14 Simon Jolly, UCL, AWAKE Spectrometer Review 19

20 Vacuum Vessel 26/03/14 Simon Jolly, UCL, AWAKE Spectrometer Review 20

21 Light Tight Vessel 26/03/14 Simon Jolly, UCL, AWAKE Spectrometer Review 21

22 Off-Axis Camera + Focussing Mirror 26/03/14 Simon Jolly, UCL, AWAKE Spectrometer Review 22

23 Scintillator Screen Default scintillator choice is Lanex: Manufactured by Kodak. Used in Medical Physics as X-ray phosphor for imaging. Gd 2 O 2 S:Tb Gadolinium sensitiser, Terbium dopant activator/wavelength shifter. Phosphor grains on reflective backing. Properties don t seem to be well documented/studied Need to simulate light production (photons per MeV conversion efficiency) to ensure we have enough photons emitted in direction of camera. Procuring GadOx samples from Applied Scintillation Technologies: Variants of Medex screens (AST trade name for GadOx medical x-ray phosphor screens). More phosphor = higher light output = lower resolution. Need to compare results from Geant4 simulations with experimental tests using known source. Curlcontrol Backing Polyester support Phosphor Layer Clear overcoat 178 microns 50 to 305 microns 7.6 microns 26/03/14 Simon Jolly, UCL, AWAKE Spectrometer Review 23

24 Conclusions Simulation results for scintillator light output reasonably promising: Can collect light from all electron hits with camera at 0.7 m. Can still reconstruct energy spectrum with camera further away without focussing lens. Need to refine vacuum chamber dimensions: where is transition between vacuum and air for electrons? Magnet still a concern: Is MBPS going to give us large enough field region to measure sufficient energy range? Would we be better designing our own magnet ( 100k per magnet at least )? Would 2 magnet chicane spectrometer give better performance? Would certainly reduce energy acceptance Difficult to give precision measurement of GeV beam 26/03/14 Simon Jolly, UCL, AWAKE Spectrometer Review 24