Reconstruction of longitudinal bunches profiles at FACET, SLAC ∗
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M. Vieille Grosjean ,N. Delerue, J. Barros, S. Jenzer 2 F.Bakkali Taheri, G. Doucas, I. Konoplev, A. Reichold 3 C.Clarke ∗
[email protected] 1 Laboratoire de l’Accélérateur Linéaire (LAL), Université Paris-Sud XI, F-91898 Orsay, France 2 John Adams Institute, Department of Physics, University of Oxford, OX1 3RH Oxford, UK 3 SLAC,Stanford, USA Work supported by seed funding from Université Paris-Sud, program “Attractivité” and by the ANR under contract ANR-12-JS05-0003-01.
Radiation de Smith-Purcell
Averaging over many pulses
• Smith-Purcell radiation is emitted when a charged particle passes close to the surface of a metallic grating. • If the bunch is short compared to the wavelength emitted then the radiation is emitted coherently. • Coherent emission involves the Fourier transform of the longitudinal profile of the electron bunch: dI dI ≈ Ne + Ne2 |F (ω)|2 G(σx , σy ) dΩdω Ne dΩdω SP
We use an averaging over a hundred pulses. We want to know if there is a good reproducibility between the pulses. The following images show the comparison between the profile reconstructed using one pulse per data set and the actual profile. E−203 preliminary Reconstructed time profile Comparison pseudo−single−shot/multi−shot Max Compression
E−203 preliminary Reconstructed time profile Comparison pseudo−single−shot/multi−shot Medium Compression 1.2 Multi shot profile Pseudo single−shot 1
1.2 Multi shot profile Pseudo single−shot
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Use of a single grating
Measuring the intensity emitted by Coherent Smith-Purcell radiation allows us to reconstruct the bunch spectrum. Then we can access the longitudinal profile of the bunch. E−203 preliminary Reconstructed time profiles 3 compressions
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• Medium bunch compression, the lowest frequencies dominate. Both 250 µm and 1500 µm grating profiles give good estimate of the actual one.
Toward a single-shot measurement • We use one grating instead of three and one pulse per data set instead of 200.
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Motivations to design a single-shot device
Plasma wakefield accelerators Reduce the number of pulses have a limited shot to shot stability. needed. Do faster measurements at conventional accelerators for a more accurate profile (when the accelerator is slightly unstable).
References • G. Doucas et al. First measurements of the longitudinal bunch profile of a 28.5 GeV beam using coherent Smith-Purcell radiation. Phys.Rev.ST Accel.Beams 12:032803,2009. • O. Grimm and P. Schmüser Principles of Longitudinal Beam Diagnostics with Coherent Radiation, TESLA FEL 2006-03
E−203 E−203 Reconstructed time profile Reconstructed time profile Comparison between actual profile and profile using one shot one grating max compression Comparison between actual profile and profile using one shot one grating med compression sigma= 221fs sigma= 130fs sigma= 621fs sigma= 749fs trms= 189fs trms= 118fs trms= 637fs trms= 632fs 1.2 1.2 All shots all gratings All shots all gratings One shot only 50um One shot only 1500um 1 1 0.8
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E−203 preliminary Rho vs wavelength
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
• Highly compressed bunch: the 50 µm grating profile is the closest from the actual one. Indeed the high frequencies contribute the most.
Reconstructions of profiles
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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 current [a.u.]
• FACET (Facility for Advanced Accelerator Experimental Tests) linear accelerator provides 23 GeV electron bunches, with a duration of hundred femtoseconds. • E-203 is placed behind the bunch compressor chicane, so that several compressions are available for the bunches. • 11 pyroelectric detectors measure the emitted radiation. An assembly of filters is placed in front of them to reject background. • 3 gratings with 3 different pitches to cover a range from 20 µm to 2000 µm and a blank piece mounted on a carousel. A motorized arm can expose each grating to the beam.
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E-203 installation at FACET, SLAC
• We use one grating instead of three. One measurement is two data sets: one for the grating with the corresponding filters and one for the background with the corresponding filters.
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Reconstructed profile close to the actual one.
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Conclusions and outlook At the moment we use 6 data sets and about 100 shots per grating to reconstruct a profile. To come closer from a single-shot monitor, we can reduce the number of pulses from 1200 to 2 while still reconstructing an accurate profile. To build a real single-shot device will require a modification of the geometry of the experiment to measure simultaneously the signal and the background. This is being investigated in the LINAC of the french synchrotron SOLEIL. We also plan to design a single-shot detector for a laser driven plasma wakefield accelerator.
