Smith-‐Purcell radia0on as a bunch length diagnos0c Nicolas Delerue LAL (CNRS and Université de Paris-‐Sud) In collabora0on with: -‐ E-‐203 (SLAC, JAI/Oxford,…) -‐ Synchrotron SOLEIL -‐ Joint France-‐Ukraine Laboratory (Kyiv and Kharkiv Na0onal Universi0es)
Work supported by seed funding from Université Paris-‐Sud, program « A=rac?vité » and by the ANR under contract ANR-‐12-‐JS05-‐0003-‐01.
Content • Introduc0on to Smith-‐Purcell radia0on • Model dependence & Pre-‐wave/near field effects • Smith-‐Purcell Radia0on as a bunch length Monitor • E-‐203 • SPESO • The future: single shot Nicolas Delerue, LAL (CNRS)
RREPS – St Petersburg -‐ 9th September 2015
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Smith-‐Purcell radia0on • Discovered experimentally in 1953 by Smith and Purcell. • Electrons passing near a gra0ng induce the emission of (visible) radia0on. • Radia?on is dispersed spectrally. • S.J. Smith and E.M. Purcell, Phys. Rev. 92, pg. 1069, (1953) • 300 keV electrons to emit in the visible wavelengths (d = 1.67 um) • Theta is the azimuthal angle. Nicolas Delerue, LAL (CNRS)
RREPS – St Petersburg -‐ 9th September 2015
% "1 λ = $ − cosθ ' n #β & 3
Explana0on: dipole radia0on • Smith-‐Purcell radia0on can be interpreted as the dipole radia0on of the current induced by the beam in the conduc0ng gra0ng. • This radia0on then interfere construc0vely with different wavelength propaga0ng in different direc0ons. Ishiguro and Tako, Op0ca Acta (GB) 8 1961 25
Nicolas Delerue, LAL (CNRS)
RREPS – St Petersburg -‐ 9th September 2015
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Explana0on: EM field at grazing angle • Another interpreta0on suggest that evanescent waves from the EM field arrive at grazing angle on the gra0ng and are then diffracted by the gra0ng. G. Toraldo di Francia, Nuovo Cimento, 16 (1960) 61 Nicolas Delerue, LAL (CNRS)
RREPS – St Petersburg -‐ 9th September 5 G. T2015 oraldo di Francia, Nuovo Cimento, 16 (1960) 61
Understanding gra0ng yields • The diffrac0on by a gra0ng is very complicated! Neither model seem to fully take into account the gra0ng theory… • Anomalies… • Polarisa0on… % "1 λ = $ − cosθ ' n #β &
Nicolas Delerue, LAL (CNRS)
RREPS – St Petersburg -‐ 9th September 2015
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Smith-‐Purcell models: comparison • Several developments of both models since 1960… (including by people present at this conference). • Most recent comparison (mostly at low gamma): D.V. Karlovets and A. P. Potylitsyn Phys. Rev. ST Accel. Beams 9, 080701 (2006) Nicolas Delerue, LAL (CNRS)
RREPS – St Petersburg -‐ 9th September 2015
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Model comparison at high gamma • Comparison at gamma=200 (SOLEIL linac)
• RDR predicts a more intense signal with a larger drop at phi (polar) = 0º. • Overall slope is similar. Intensity within 50%. Nicolas Delerue, LAL (CNRS)
LAL + Kharkiv Na0onal University RREPS – St Petersburg -‐ 9th Collabora0on September 8 2015
Model comparison at high gamma • Comparison at gamma=40000 (FACET)
• Now SC predicts a more intense signal with a larger drop at phi (polar) = 0º! • Once again overall slope is similar. Intensity within 50%. Nicolas Delerue, LAL (CNRS)
RREPS – St Petersburg -‐ 9th Collabora0on September LAL + Kharkiv Na0onal University 9 2015
Model comparison at high gamma • To discriminate between the models one needs a good polar resolu0on and/or good detectors/ beam calibra0on. • There is no obvious measurement that could allow an easy discrimina0on between the two models. • Neither are available at the moment at these facili4es. => Within the experimental accuracy available both models predict compa0ble single electron yield. Nicolas Delerue, LAL (CNRS)
RREPS – St Petersburg -‐ 9th September 2015
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Near field (Pre-‐wave) effect • At large wavelength correc0ons must be applied to the gra0ng yield as the rays do not perfectly build an interference pamern: Near field / pre-‐wave effect. => Signal yield is modified • Most advanced study: D.V. Karlovets and A. P. Potylitsyn, JETP Lemers Vol 84 No 9 (2006).
Nicolas Delerue, LAL (CNRS)
RREPS – St Petersburg -‐ 9th September 2015
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Pre-‐wave signal intensity Gamma=40000 d=1.5mm
Nicolas Delerue, LAL (CNRS)
RREPS – St Petersburg -‐ 9th Collabora0on September LAL + Kharkiv Na0onal University 12 2015
Pre-‐wave signal intensity Gamma=40000 d=1.5mm I/Rˆ2
Nicolas Delerue, LAL (CNRS)
RREPS – St Petersburg -‐ 9th Collabora0on September LAL + Kharkiv Na0onal University 13 2015
Pre-‐wave signal intensity Gamma=40000 d=1.5mm I/Rˆ2
Nicolas Delerue, LAL (CNRS)
RREPS – St Petersburg -‐ 9th Collabora0on September LAL + Kharkiv Na0onal University 14 2015
Pre-‐wave/near field correc0ons comparisons • The E-‐203 collabora0on uses a different method to calculte the correc0ons but the results are similar.
Nicolas Delerue, LAL (CNRS)
RREPS – St Petersburg -‐ 9th September 15 2015 Collabora0on LAL + Kharkiv Na0onal University
Coherent SP radia0on • Like many other radia0ve phenomena in EM, SP radia0on can also be coherent at wavelengths sufficiently longer than the bunch length (form factor). • This means that for sufficiently short bunches the signal intensity is propor0onal to the square of the beam charge. ⎛ dI ⎞ ⎛ dI ⎞ 2 ⎜ ⎟ (Ω, ω ) = ⎜ ⎟ (Ω, ω ) ⋅ [ N e + N e ( N e + 1) | F(ω ) | ] ⎝ dΩdω ⎠ N e ⎝ dΩdω ⎠ sp
Nicolas Delerue, LAL (CNRS)
RREPS – St Petersburg -‐ 9th September 2015
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Coherent SPR as a longitudinal profile diagnos0c • Because Coherent SPR encodes the Fourier transform of the longitudinal profile, it can be used as a diagnos0c. • Such diagnos0c requires a measurement of the SPR spectrum. ⎛ dI ⎞ ⎛ dI ⎞ 2 ⎜ ⎟ (Ω, ω ) = ⎜ ⎟ (Ω, ω ) ⋅ [ N e + N e ( N e + 1) | F(ω ) | ] ⎝ dΩdω ⎠ N e ⎝ dΩdω ⎠ sp Nicolas Delerue, LAL (CNRS)
RREPS – St Petersburg -‐ 9th September 2015
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Aqer data taking: Profile reconstruc0on techniques ⎛ dI ⎞ ⎛ dI ⎞ 2 ⎜ ⎟ (Ω, ω ) = ⎜ ⎟ (Ω, ω ) ⋅ [ N e + N e ( N e + 1) | F(ω ) | ] ⎝ dΩdω ⎠ N e ⎝ dΩdω ⎠ sp
• The measured signal is the square of the Fourier transform of the bunch. • To reconstruct the profile, the phase of each Fourier components must be recovered. • Several techniques are available; we rely mostly on Hilbert or Kramers Kronig phase recovery technique. Nicolas Delerue, LAL (CNRS)
RREPS – St Petersburg -‐ 9th September 2015
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Kramers Kronig phase recovery technique • The Kramers Kronig phase recovery technique assumes 2 2 2 G ( ) N |F ( )| e that the system has the minimum phase difference. • The phase is recovered from the rela0on between the G(⇤) = (⇤) exp [i⇥(⇤)] amplitudes of the observables.
⇤(⌅) =
2⌅ 0
Nicolas Delerue, LAL (CNRS)
ln [⇥(⌅)/⇥(⌅0 )] d⌅ 2 2 ⌅0 ⌅
RREPS – St Petersburg -‐ 9th September 2015
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Performance of the Hilbert and Kramers Kronig phase recovery techniques −5
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We simulated more than a 1000 profiles and showed that the reconstruc0on performances were acceptable for both Hilbert and Kramers Kronig methods. Hilbert is faster and easier to implement in Matlab. IPAC’14 THPME088 / arXiv.org 1407.0741; Full paper in prepara0on.
Nicolas Delerue, LAL (CNRS)
RREPS – St Petersburg -‐ 9th Collabora0on September LAL + Kyiv Na0onal University 20 2015
E-‐203 at FACET • FACET at SLAC is a test facility offering access to 100s femtoseconds long electron bunches. • Electrons energy is 22.5 GeV (gamma=45000) • Aim of the E-‐203 collabora0on: development of a longitudinal profile monitor in the fs range.
Nicolas Delerue, LAL (CNRS)
RREPS – St Petersburg -‐ 9th September 2015
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Experimental apparatus (schema0c)
Nicolas Delerue, LAL (CNRS)
RREPS – St Petersburg -‐ 9th September 2015
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Experimental apparatus: gra0ngs carousel 3 gra0ngs 1 blank piece of aluminium
Beam direc0on
Expected SP radia0on at FACET in the wavelength range 10 µm to 1 mm A carousel can rotate and offer three different gra0ngs or one blank to the beam. Rota0on is controlled remotely.
Nicolas Delerue, LAL (CNRS)
RREPS – St Petersburg -‐ 9th September 2015
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Detec0on of FIR SP radia0on Filters of many varie0es remove background radia0on. Suitable filters for each gra0ng are moved in front of silicon windows Winston cones collect the radia0on toward the pyroelectric detectors and provide addi0onal filtering Solid aluminium – absolute background measurement. No filter
~70mm
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Filters for different gratings & orders
Wire mesh: 117 µm, 175 µm; Δλ = 10-‐20 µm Wave guide array plates: 175 < λ < 1000; Mylar based thin films: 20 < λ < 117; Δλ = few µm Silicon based thin films : 10 < λ < 20; Δλ = few µm Nicolas Delerue, LAL (CNRS)
RREPS – St Petersburg -‐ 9th September 2015
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E-‐203 Results • Paper with profile reconstruc0on published in 2014. • Further data taking in 2015 to study the polar distribu0on of the signal and its polarisa0on. Nicolas Delerue, LAL (CNRS)
RREPS – St Petersburg -‐ 9th September 2015
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Posi0on sensi0vity • The beam-‐gra0ng is an important parameter of the measurement as the signal depends exponen0ally on it. • To improve the posi0on sensi0vity we have added a wire-‐ scanner to the experiment.
Nicolas Delerue, LAL (CNRS)
RREPS – St Petersburg -‐ 9th September 2015
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Polarisa0on • We have shown that the background is not polarized. • We made a preliminary measurement of the CSP polariza0on but this measurement does not agree with theory. • => measurement repeated in 2015. • Very preliminary analysis.
Nicolas Delerue, LAL (CNRS)
RREPS – St Petersburg -‐ 9th September 2015
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Polar distribu0on of the signal
• En 2015 we also modified the E-‐203 setup to measure the polar (phi) distribu0on of the signal by adding slits in front of our detectors. • Observa0on of a drop at phi=0 at some azimuthal angles but not all. Detailled analysis s0ll in progress….
Nicolas Delerue, LAL (CNRS)
RREPS – St Petersburg -‐ 9th September 2015
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SPESO • To overcome the E-‐203 limita0ons, a Smith-‐Purcell test stand has been installed at the end of the SOLEIL Linac (gamma=200). • A 5D robot can measure the radia0on emimed at different positon and angles. • Soon a polarisa0on measurement system will be added. Nicolas Delerue, LAL (CNRS)
RREPS – St Petersburg -‐ 9th September 2015
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SPESO: first results • Signal was difficult to find (bunch longer than expected). • We now have a system that can measure the SP signal. • Goal make 3D a map of SP emission (with polarisa0on). • However this will be limited by the detectors (Gunn diodes).
Nicolas Delerue, LAL (CNRS)
RREPS – St Petersburg -‐ 9th September 2015
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The future: single shot? • Further experiments are planned at SPARC and CLIO (french Free Electron Laser) to measure their bunch profile. • The addi0on of several gra0ngs should allow a larger wavelength coverage. • We also plan to study more in detail the gra0ng dependency.
Nicolas Delerue, LAL (CNRS)
RREPS – St Petersburg -‐ 9th September 2015
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Outlook • Smith-‐Purcell radia0on can be used as a bunch length diagnos0c. • Model uncertainty on the signal yield are comparable to the (large) experimental uncertain0es. • R&D is in progress to bemer understand the signal distribu0on. • The ul0mate aim would be to use it as a single shot device. Nicolas Delerue, LAL (CNRS)
RREPS – St Petersburg -‐ 9th September 2015
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Thank you!
Nicolas Delerue, LAL (CNRS)
RREPS – St Petersburg -‐ 9th September 2015
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Gra0ng effect
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E−203 Reconstructed time profile Comparison multi−shots all gratings/one grating medium compression sigma= 621fs sigma= 147fs sigma= 500fs sigma= 747fs 1.2 All gratings 50um 1 250um 1500um
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E−203 Reconstructed time profile Comparison multi−shots all gratings/one grating maximum compression sigma= 221fs sigma= 126fs sigma= 240fs sigma= 157fs 1.2 All gratings 50um 1 250um 1500um
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Nicolas Delerue, LAL (CNRS)
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