The use of Smith-Purcell radiation to measure electrons longitudinal profiles
has been demonstrated at several facilities in the picosecond and
sub-picosecond range. There is a strong interest for the development of non
intercepting longitudinal profile diagnostics for high current proton beams. We
present here results of simulations on the expected yield of longitudinal
profile monitors using Smith-Purcell radiation for such proton beams.Comment: Presented at IPAC 2014 - THPME08

Barros, Joanna

Delerue, Nicolas

Kittelmann, Irena Dolenc

Thomas, Cyrille

Vieille-Grosjean, Mélissa

English

arXiv

THPME089 - Work financially supported by the Université Paris-Sud (programme "attractivité") and the French ANR (contract ANR-12-JS05-0003-01) - http://accelconf.web.cern.ch/AccelConf/IPAC2014/papers/thpme089.pdfInternational audienceThe use of Smith-Purcell radiation to measure electrons longitudinal profiles has been demonstrated at several facilities in the picosecond and sub-picosecond range. There is a strong interest for the development of non intercepting longitudinal profile diagnostics for high current proton beams. We present here results of simulations on the expected yield of longitudinal profile monitors using Smith-Purcell radiation for such proton beams

Barros, J.

Delerue, N.

Vieille-Grosjean, M.

Dolenc Kittelmann, I.

Thomas, C.

HAL-IN2P3

ON THE SUITABILITY OF LONGITUDINAL PROFILEMEASUREMENTS USING COHERENT SMITH-PURCELL RADIATIONFOR HIGH CURRENT PROTON BEAMS∗J. Barros†, N. Delerue, M. Vieille-Grosjean, LAL, Orsay, FranceI. Dolenc Kittelmann, C. Thomas, ESS, Lund, SwedenAbstractThe use of Smith-Purcell radiation to measure electronslongitudinal profiles has been demonstrated at several facili-ties in the picosecond and sub-picosecond range. There is astrong interest for the development of non intercepting longi-tudinal profile diagnostics for high current proton beams. Wepresent here results of simulations on the expected yield oflongitudinal profile monitors using Smith-Purcell radiationfor such proton beams.INTRODUCTIONHandling high current proton beams is a challenge forbeam diagnostics in future proton accelerators. In order toachieve a high beam power, it is mandatory to understand thebeam dynamics and to determine the beam size and profilewith precision. In high power proton Linacs, such under-standing would allow a full characterization of the beamafter each Linac section. With a beam transverse size of2 to 3mm, intercepting profile diagnostics such as OTRscreens or Cherenkov monitors can be used only during tun-ing and specific beam operation modes. To determine thelongitudinal profile, Feschenko-type Bunch Shape Monitors(BSM [1]) can be used in the low energy sections of theLinac, however, BSM may not be sensitive to short bunchlengths. In the higher energy sections, it is highly advis-able to use non interceptive devices to avoid radiations ordamage to the equipment. Wall current monitors or othermethods based on detecting the fields at the vacuum chamberboundary could be used, but they are intrinsically limitedin resolution due to the rather low relativistic β [2] and donot have an adapted temporal resolution. Therefore, a di-agnostic based on Smith-Purcell radiation could be of highinterest. We will investigate here the possibility of usinga Smith-Purcell diagnostic for the measurement of protonbeams longitudinal profiles, using a numerical simulationcode presented hereafter.DESCRIPTION OF THE SIMULATIONTheoretical ConsiderationsSmith-Purcell radiation can be described by the "surfacecurrent" theory [4]: when charged particles travel over agrating, they induce an image charge on its surface andkeep pace with it. The corrugations of the grating causethe created current to accelerate, which in turn leads to the∗Work financially supported by the Université Paris-Sud (programme"attractivité") and the French ANR (contract ANR-12-JS05-0003-01)† barros@lal.in2p3.frFigure 1: Coordinate system used for the simulation.emission of radiation. In our coordinate system, we assumethat the protons travel perpendicularly to the grooves of ablazed grating. Due to the periodic structure of the grating,the emitted wavelength λ depends on the observation angleθ defined in Fig. 1. The grating shape and pitch definesits efficiency, that varies depending on θ and on the secondobservation angle called φ.Our simulation describes the Smith-Purcell intensity radi-ated per unit solid angle for a single particle, as described inthe equation (2a) of [3] for a grating of period l and lengthZ positioned at a distance x0 from the beam center:(dIdΩ)1= 2piq2Zln1λ3R2exp−4pix0λ√1 + β2γ2sin2θsin2φβγ(1)The term R2 represents the grating efficiency that containsthe contribution from each period of the grating, and is differ-ent for each grating shape [5]. Here we use echelette gratingswhose efficiency will be shown in the Results section.For a given bunch of NP particles, the emitted radiationhas a high degree of coherence when the radiation wave-length is comparable to the bunch length. In the equation(2), Scoh . and Sincoh . represent the coherent and incoherentcomponents of the emitted radiation respectively. Assumingthat the transverse and longitudinal profiles are not corre-lated, Scoh . can be expressed as the product two functions Tand G encoding the longitudinal (temporal) and transverseprofile respectively. σx and σy are the beam dimensions inx and y directions, and ω is the frequency.(dIdΩ)NP=(dIdΩ)1(NPSincoh . + N2PScoh .) (2)Proceedings of IPAC2014, Dresden, Germany THPME08906 Instrumentation, Controls, Feedback & Operational AspectsT03 Beam Diagnostics and InstrumentationISBN 978-3-95450-132-83439 Copyright©2014CC-BY-3.0andbytherespectiveauthorsScoh . =∫ ∞−∞Te−iωtdt2G2(σx ,σy ) (3)In the case where the coherent component prevails, theemitted radiation encodes the form factor of the bunch andits measurement allows to recover information about thebunch longitudinal profile. If we assume that the beam isgaussian in the transverse plane and G is normalized to 1,then the emitted energy is proportional to the form factor asshown in 4.(dIdΩ)NP'(dIdΩ)1N2P∫ ∞−∞Te−iωtdt2(4)This form factor corresponds to themodulus of the Fouriertransform of the temporal profile. In order to recover thefull profile the phase needs to be reconstructed. Severaltechniques are possible, and are documented in [7].In this work, we use a code package written by G. Dou-cas [3], that takes several beam parameters as an input andgenerates the a Smith-Purcell spectrum, which is a calcula-tion made from the Fourier transform of the temporal profileand chosen grating parameters.Beam ParametersThe parameters used for this study on proton beams aresummarized in Table 1. We assume that the beam transversedimensions would allow approaching a grating at a distancex0 = 10mm from the beam center.Table 1: Used Beam Parameters for the SimulationParameter Unit Valueγ - ≈3β - 0.92Transverse x size (FWHM) mm 2Transverse y size (FWHM) mm 2Number of protons per bunch - 1 × 109Expected bunch length (FWHM) ps 3Number of bunches per train - 1 × 106Frequency MHz 352Repetition rate Hz 14CHOICE OF MEASUREMENTPARAMETERSDetection ParametersThe choice of suitable grating parameters is made by firsttaking into account the preferred geometry for detection.Here we chose to maximize the signal at θ=90°. Our opticscollect light with a high numerical aperture, so that the signalgenerated by the code will be integrated over the wholecaptured solid angle. The optics are placed at 200mm fromthe grating, and the opening of the detection cone is 100mmto increase the incoming signal on the detectors.Grating ParametersThe grating pitch is selected in order to make sure thatthe emission is mostly in the coherent regime, so that themeasurement would be sensitive to the beam longitudinalprofile (see Eq. (2)). The increase in signal seen on Fig. 2when the grating pitch increases, corresponds to the onsetof the coherent component. We chose a value of the gratingperiod that is a trade-off between the different angles toensure a good sensitivity at all angles: the three angularcomponents are in the coherent range. Keeping in mind thatthe insertion length needs to be as small as possible, and thatwe would prefer the maximum of the signal to be at 90°, wechose a grating period of 13mm. The grating parametersare summarized in Table 2.5 10 15 20012345Grating period (mm)Energy per bunch (nJ)   Theta=60° Theta=90° Theta=120°Figure 2: Evolution of the Smith-Purcell signal with differentgrating pitches.Table 2: Grating Parameters for the SimulationParameter ValueWidth 40mmNumber of periods 15Blaze angle 30°Pitch 13mmRESULTSKnowing the expected signal level, it is now necessary toverify that the device is sensitive to a longitudinal profilechange, which is a crucial feature for a profile measurementtool. The sensitivity to a variation in the bunch FWHMlength is shown in Fig. 3 for the chosen grating and opticsparameters. The coherent emission becomes progressivelypredominant as the bunch length decreases, which leads toa significant and measurable signal increase. This is shownin Fig. 3, where the wavelength at θ = 90°varies by morethan 20% for a bunch length varying from 3 to 10 ps, but asimilar change is also observed at other wavelengths showinga significant change of the power spectrum distribution.The signal level for the simulated case, as shown in Fig. 3,is of the order of 1 nJ per bunch, and needs to be comparedwith the performances of the detectors used. For example,THPME089 Proceedings of IPAC2014, Dresden, GermanyISBN 978-3-95450-132-83440Copyright©2014CC-BY-3.0andbytherespectiveauthors06 Instrumentation, Controls, Feedback & Operational AspectsT03 Beam Diagnostics and Instrumentation0 2 4 6 8 1000.511.52Bunch FWHM length (ps)Energy per bunch (nJ)   Theta=60° Theta=90° Theta=120°Figure 3: Simulated Smith-Purcell signal for a 13 mm grat-ing depending on the bunch length.if the detectors measure an averaged signal over several ms,which is the case of the pyroelectric sensors often used inexperiments involving electrons [6], they will detect theSmith-Purcell power produced by a whole train (around1 × 106 bunches), thus a higher signal by several orders ofmagnitude, up to mJ. However, these values do not take intoaccount the transmission of the different optical elementsthat would need to be installed in front of the detectors. Also,as seen in Fig. 4, the expected Smith-Purcell emission wouldoccur in the millimeter-wave range where the backgroundlevel needs to be evaluated to be able to distinguish it fromthe actual signal. If we use detectors that are sensitive onlyto signal variations, they will not impacted by the effectof the temperature. However, electromagnetic noise andother sources of radiations related to the beam can createbackground. Since the Smith-Purcell radiation is linearlypolarized, a way to decorrelate the signal and the backgroundcould then be to measure the two polarization components.2.87 7.64 14.1 20.6 25.4Wavelength (mm)104 39.3 21.2 14.5 11.8Frequency (GHz)20 40 60 80 100 120 140 16000.511.52Theta (deg.)Energy per bunch (nJ)  P1P2P1+P2Figure 4: Simulation of the expected signal for a 3 ps bunch.P1 and P2 are the two polarization components.The efficiency of the grating is given in Fig. 5 dependingon the φ angle. This indicates that most of the radiation  5e−10  1e−09  1.5e−09  2e−09302106024090270120300150330180 0   Theta=30° Theta=60° Theta=90° Theta=120°Figure 5: Efficiency of the grating depending on the φ angle,for different θ values. The emission source is the center ofthe figure, the detection optics would be placed on the righthalf of the figure.would be emitted along the x axis in a cone of approximately40°opening, allowing all signal to enter the 100mm optics.CONCLUSIONOur simulations indicate that with the parameters used forour simulation a longitudinal bunch profile monitor basedon Coherent Smith-Purcell radiation installed at near a highintensity proton beam would be sensitive to changes in thepulse length and could be useful in the tuning phase of suchaccelerator. However such device has never been tested atlow β proton beams and preliminary tests on a high intensityproton source would be useful.ACKNOWLEDGEMENTSThe authors acknowledge George Doucas from JohnAdams Institute in Oxford for his useful advice.REFERENCES[1] A.V. Feschenko, PAC’01, Chicago, p. 517.[2] S.Peggs (editor), ESS Technical DesignReport, (Lund, April 2013), chapter 4,http://europeanspallationsource.se/documentation/tdr.pdf[3] G. Doucas, V. Blackmore, B. Ottewell, C. Perry, P. G. Huggard,E. Castro-Camus, M. B. Johnston, J. Lloyd Hughes, M. F.Kimmitt, B. Redlich, A. van der Meer, Phys. Rev. ST Accel.Beams 9, 092801 (2006).[4] J. H. Brownell, J. Walsh, G. Doucas, Phys. Rev. ST Accel.Beams 57, 1075 (1998).[5] J. H. Brownell, G. Doucas, Phys. Rev. ST Accel. Beams 8,092301 (2005).[6] H.L. Andrews et. al., Phys. Rev. ST Accel. Beams 17, 052802(2014).[7] V. Blackmore, Ph.D. thesis, Oxford University, 2008.Proceedings of IPAC2014, Dresden, Germany THPME08906 Instrumentation, Controls, Feedback & Operational AspectsT03 Beam Diagnostics and InstrumentationISBN 978-3-95450-132-83441 Copyright©2014CC-BY-3.0andbytherespectiveauthors

On the suitability of longitudinal profile measurements using coherent Smith-Purcell radiation for high current proton beams

Hal-Diderot

On the suitability of longitudinal profile measurements using Coherent
  Smith-Purcell radiation for high current proton beams

On the suitability of longitudinal profile measurements using Coherent Smith-Purcell radiation for high current proton beams

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