On SOLEIL storage ring, three beamlines are dedicated to electron beam diagnostics: two in the X-ray range and one in the visible range. The visible range beamline uses the synchrotron radiation which is emitted in one of the ring dipoles and further extracted by a slotted mirror operated in surf-mode (surfing on the upper part of the synchrotron layer). The radiation in the visible range is then transported towards a diagnostic hutch in the experimental hall, allowing electron beam imaging at the source point onto a standard CCD camera. In the perspective of prototyping works for the eventually forthcoming upgrade of SOLEIL, and for the on-going commissioning of a new Multipole Injection Kicker (MIK), we recently installed in this hutch two new branches ended by two new cameras (a KALYPSO system and a standard CMOS camera). We report in this paper the optimization we performed on the mirror mode of operation, as well as on spectral filtering, polarization selection, image plane location, fast acquisition tools, to improve the resolution and increase the speed of our initial transverse beam size measurement at source point

Bence, A.

Bründermann, E.

Caselle, M.

Ebersoldt, A.

Hubert, N.

Labat, M.

Patil, M. M.

Pédeau, D.

English

KITopen

FAST MEASUREMENTS OF THE ELECTRON BEAM TRANSVERSESIZE AND POSITION ON SOLEIL STORAGE RINGM. Labat∗, A. Bence, N. Hubert, D. Pédeau, Synchrotron SOLEIL, Gif-sur-Yvette, FranceM. M. Patil, M. Caselle, A. Ebersoldt, E. Bründermann,Karlsruhe Institute of Technology, Karlsruhe, GermanyAbstractOn SOLEIL storage ring, three beamlines are dedicatedto electron beam diagnostics: two in the X-ray range andone in the visible range. The visible range beamline usesthe synchrotron radiation which is emitted in one of the ringdipoles and further extracted by a slotted mirror operatedin surf-mode (surfing on the upper part of the synchrotronlayer). The radiation in the visible range is then transportedtowards a diagnostic hutch in the experimental hall, allowingelectron beam imaging at the source point onto a standardCCD camera. In the perspective of prototyping works forthe eventually forthcoming upgrade of SOLEIL, and forthe on-going commissioning of a new Multipole InjectionKicker (MIK), we recently installed in this hutch two newbranches ended by two new cameras (a KALYPSO systemand a standard CMOS camera). We report in this paper thefirst results obtained on those branches.INTRODUCTIONSOLEIL storage ring presently delivers synchrotron radi-ation to 29 beamlines. The stability of the delivered photonbeams, a main figure of merit for users, relies on an accuratemonitoring of the electron beam orbit and electron beamsize / emittance at users source points. The electron beamsize / emittance is measured independantly by two pinholecamera (PHC) systems, located at two different places ofthe ring. One of the PHC measurement is used in a feed-back loop to maintain the vertical emittance within +/-5 %.However, with typical exposure times of 50–100 ms, thosesystems cannot follow fast beam dynamics features at theturn-by-turn scale for instance.The installation of a new injection magnet and the per-spective of an upgrade for SOLEIL recently revealed theneed for an additionnal beam size diagnostic with a fasterresponse.We therefore decided to upgrade our visible range MRSV(Moniteur de Rayonnement Synchrotron Visible) beamlinewith two new branches. This work presents the preliminarresults obtained on those branches.EXPERIMENTAL SETUPThe experimental layout of the MRSV beamline is pre-sented in Fig. 1. The synchrotron radiation of one of thering dipoles (ANS-C01) is extracted using a slotted mirroroperated in surf–insertion mode: the mirror is surfing ontop of the synchrotron radiation layer to catch visible range∗ marie.labat@synchrotron-soleil.frphotons. Because of heat load issues, we can’t operate themirror in slot–insertion mode, i.e. centered on the beam axiswith synchrotron radiation X–rays going through the slot, athigh current. The synchrotron radiation is then transportedvia a set of mirrors down to a hutch in the experimental hall.On the beam path inside the tunnel are succesivly placed: amotorized slit to define the horizontal collection angle 𝜃𝑥of the beamline and a spherical lens for refocussing. At thearrival on the optical bench in the hutch, the synchrotronradiation is splitted into several branches.Initially, three branches were implemented (see pink boxesin Fig. 1): one with a fast diode for filling pattern measure-ment, one with a Streak Camera for bunch length measure-ment and one with a standard ethernet camera for a coarseimaging of the beam at the source point. We added two newbranches to test higher resolution / higher speed imagingsystems for beam size retrieval.Figure 1: Layout of the MRSV beamline with its three initialbranches in pink, and its two new branches.BEAMLINE MODELINGTo achieve an accurate imaging, an accurate modelingof the beamline is mandatory. We used for that the SRW(Synchrotron Radiation Workshop) code [1]. In Fig. 2 isfirst presented the simulated effect of using the extractionmirror in surf rather than in full (or slot) –insertion mode.Because of the disymmetry of the extraction, there is nearlyno more difference between the polarization components ofthe radiation.After a first attempt of comparison between measured andsimulated intensity distributions in the image plane of theMRSV beamline, we realized that the beamline was sufferingfrom strong stigmatism: the image plane in 𝑦 was few tensof centimeters downstream the image plane in 𝑥. Becausethe focussing lens in the tunnel is spherical, we immediatlysuspected the extraction mirror to introduce this stigmatism.To verify this interpretation, we simulated the effect of a10th Int. Beam Instrum. Conf. IBIC2021, Pohang, Rep. of Korea JACoW PublishingISBN: 978-3-95450-230-1 ISSN: 2673-5350 doi:10.18429/JACoW-IBIC2021-TUPP1703 Transverse Profile and Emittance MonitorsTUPP17235ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2021).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI(a) Full–insertion mode(b) Surf–insertion modeFigure 2: Intensity distribution in the image plane simulatedwith SRW using an extraction mirror in (a) full–insertion and(b) surf–insertion mode. Polarization is (left) Horizontal,(middle) Vertical and (Right) Total, using (top line) single–electron, (bottom line) multi–electron mode. Other param-eters: 𝜆=400 nm, 𝑓𝑥𝑦=3.21 m, 𝐷=6.984 m, 𝜃𝑥=3.5 mrad,𝜃𝑦=10 mrad.double focussing: an astigmatic one from the spherical lenstogether with a stigmatic one (only along vertical dimension)from the extraction mirror. The result is an effective focallength 𝑓𝑥 along 𝑥 and 𝑓𝑦 along 𝑦 where 𝑓𝑥 ≠ 𝑓𝑦. Fitting the ex-perimental observations of the intensity distributions versus𝐷 the distance from lens to camera for several wavelengthsled to 𝑓𝑥=3.21 m and 𝑓𝑦=3.4 m at 400 nm. The good agree-ment between model and simulation confirmed that underheat load, the extraction mirror becomes slightly convex inthe 𝑦 direction, introducing an additionnal focussing in thisdirection. In Fig. 3 is presented the result of the simulatedeffect of this stigmatic focussing on the beam distribution inthe 𝑥 image plane. The image distribution is further distortedalong 𝑦.In Fig. 4, we finally present the simulated and measuredeffect of the distance 𝐷 from lens to camera for a wide hori-zontal collection angle 𝜃𝑥. Even though images are displayedin logarythmic scale, the agreement between simulation andexperiment remains reasonnable.Figure 3: Intensity distribution in the image plane simulatedwith SRW using an extraction mirror in surf–insertion modewith𝑓𝑥=3.21 m and 𝑓𝑦=3.4 m. Other parameters as in Fig. 2.Figure 4: Intensity distribution in the image plane simu-lated with SRW (first line in single–electron, second line inmulti–electron mode) and experimentally measured (thirdline) versus 𝐷. Extraction mirror in surf–insertion modewith 𝑓𝑥=3.21 m and 𝑓𝑦=3.4 m, 𝜃𝑥=𝜃𝑦=10 mrad, 𝜆=400 nm,vertical polarization. Images in logarythmic scale. Camerafor experiment: Basler ace1920.FIRST NEW BRANCH: NB#1Confident in the modeling of the beamline, we then usedSRW simulations to optimize the distance 𝐷, the observationwavelength 𝜆 and the horizontal collection angle 𝜃𝑥 to getthe highest contrast between the single–electron spot size(PSF) and the multi–electron spot size. The higher indeedwill be this contrast, the more sensitive should be the finalbeam size measurement. The best result was obtained atthe shortest reachable wavelength (400 nm) with the maxi-mum collection angle (10 mrad) in the 𝑥 image plane. Weimplemented the first new branch NB#1 accordingly. The ra-diation is spectrally filtered using a bandpass filter centeredat 400 nm with 10 nm bandwidth, for the sake of simplictythe vertically polarized component is retained using a po-larizing beamsplitter cube (see Fig. 1), while an ace192010th Int. Beam Instrum. Conf. IBIC2021, Pohang, Rep. of Korea JACoW PublishingISBN: 978-3-95450-230-1 ISSN: 2673-5350 doi:10.18429/JACoW-IBIC2021-TUPP17TUPP17ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2021).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI236 03 Transverse Profile and Emittance MonitorsCMOS camera from Basler is used to record the images witha 5.6 𝜇m pixel pitch.We then investigated on the beam size retrieval methodfrom the experimental images. For nearly Gaussian PSFprofiles, beam size retrieval can simply be achieved by fit-ting the beam profiles in the image plane with Gaussianfunctions, and then quadratically substract the widths of thePSF to those fitted widths. But in our case of imaging witha surf–inserted mirror, a chromatic focussing and a largehorizontal collection angle, the PSF functions are far frombeing Gaussian. We therefore tried, as a first step, to fit thebeam profiles in the image plane with the convolution of theSRW simulated PSFs with Gaussian functions of width 𝑆𝑥,𝑆𝑦. (𝑆𝑥, 𝑆𝑦 would correspond to the real beam size in theimage plane, i.e. beam size at source point 𝜎𝑥, 𝜎𝑦 magnifiedby 𝑀 the beamline magnification). The results were notsatisfactory in the 𝑦 direction because images are recordedin the 𝑥 image plane, not in the 𝑦 plane. We then tried tocompute a pseudo–PSF for the 𝑦 plane, making the deconvo-lution of the SRW simulated beam profile with a Gaussianfunction of width 𝑆𝑦 = 𝑀×𝜎𝑦, where 𝜎𝑦 is the beam verticalsize at source point used as input in the simulation. How-ever, this method was still leading to beam size at the sourcepoint several times larger than the expected one. This wasessentially due to the fact that we could not reach a perfectagreement between SRW simulation and measurement ofthe beam distribution in the image plane probably becauseof complicated mirror surface distortions. We finally triedto convolute the previous pseudo–PSF with a Gaussian errorfunction. The width of this function was manually adjustedto match the beam size retrieved with the beam size expected.Beam size retrieval with this method is illustrated in Fig. 5.Up to now, this is the best method we could set.In order to test the sensitivity of our mesurement, we thensimultaneously recorded the vertical beam sizes measured onNB#1, PHC#1 and PHC#3 as a function of the vertical emit-tance. The results are shown in Fig. 6. This measurementfirst reveal the high level of fluctuation on NB#1 (more than100% fluctuation in the beam size measurement). Whilethe exposure time for PHCs is in the 50–100 ms range, theexposure time on NB#1 is 30 𝜇s, i.e. nearly three orders ofmagnitude, so we expected of course more fluctuations onNB#1. But the main source of fluctuation is most probablydue to the radiation transport in air, over more than 10 me-ters and passing via differently air conditionned areas. Theamplitude of the fluctuations is indeed far above the possiblephysical ones, of the order of a few microns.SECOND NEW BRANCH: NB#2The polarizer at the entrance of the optical bench splitsthe synchrotron radiation in one vertically polarized compo-nent going straight to NB#1, and one horizontally polarizedcomponent deflected by 90 degrees. This component of thesynchrotron radiation is the one used for our second branchNB#2. The radiation on NB#2 is transported with mirrorsdown to a Kalypso camera [2]. This camera was providedFigure 5: Example of beam size retrieval from measuredimages. (data) experimental profiles corresponding to linecuts of 2 pixels at maximum intensity lcoation. (data w roi)data restricted to a ROI of ±1 mm. (psf) Point Spreaf Func-tion simulated with SRW and eventually corrected. (datafitted) Experimental profiles restricted to ROI fitted by con-volution of a Gaussian profile with the PSF function. (datadeconv.) Gaussian profile obtained from fit. 𝐷=6.994 m,𝜃𝑥=𝜃𝑦=10 mrad, 𝜆=400 nm, polarization is vertical.Figure 6: Vertical beam size measured versus vertical emit-tance by (green dot) PHC#1, (blue dot) PHC#3, (o) MRSVHR branch single shot, (•) MRSV HR branch average of the10 single shots. 𝐷=6.994 m, 𝜃𝑥=𝜃𝑦=10 mrad, 𝜆=400 nm,polarzation is vertical.by KIT. It consists of a linear array detector with an ultra-fast reading electronics. This system is described in detailsin [2]. To match the linear array geometry, the photon beamis refocussed with a pair of cylindrical lenses (𝑓=100 mm).To obtain a reasonnable resolution with the maximum flux,a bandpass filter of 80 nm bandwidth centered at 600 nm isused.10th Int. Beam Instrum. Conf. IBIC2021, Pohang, Rep. of Korea JACoW PublishingISBN: 978-3-95450-230-1 ISSN: 2673-5350 doi:10.18429/JACoW-IBIC2021-TUPP1703 Transverse Profile and Emittance MonitorsTUPP17237ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2021).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOIA typical example of a Kalypso image is presented inFig. 7(a). Each vertical line of the image corresponds to onerecord of the light distribution along the linear array. In theexample, it is the vertical distribution of the beam which iscaptured. Along the image horizontal axis are displayed thesuccessive records. Each record can be externally or inter-nally triggered up to 3 MHz which offers outstanding op-portunities for machine physics studies. In the example, therecords were triggered by the turn-by-turn clock (846 kHz),so that each record corresponds to the imaging of the sameportion of the bunch train turn after turn.(a) Image(b) Normalized vertical intensity profileFigure 7: (a) Example of a Kalypso camera image in usersoperation at 500 mA. (b) Normalized projection of the (a)image along the horizontal axis to provide with the beamdistribution along the vertical axis.The Kalypso system is still under commissioning onNB#2. But we could already perform an interesting machinephysics experiment. We used indeed the Kalypso camera tofollow the effect of the injection on the stored beam duringthe operation of SOLEIL. To maintain a constant current,top–up injection is performed into the storage ring. Everyfew minutes, 4 kickers are used to create a bump on the storedbeam orbit, allowing the injection of few additionnal mA.Because the bump is not perfectly closed, we can observe onthe beam position monitors a transient displacement of thebeam in both the horizontal and the vertical planes. Launch-ing the acquisition of the Kalypso camera slightly before theinjection time, and using an external turn–by–turn trigger,we could record the images presented in Fig. 8. The top im-age shows the horizontal beam distribution and the bottomimage shows the vertical beam distribution. We can changethe imaging plane using a crossed or non–crossed periscopeon the photon beam path. In both planes, we clearly observethe kick given at the injection time, but we can also followthe evolution of the beam back to its equilibrium on a realturn–by–turn basis. The beam oscillates as expected in bothplanes at the synchrotron frequency with a damping timesignificantly shorter in the vertical than in the horizontalplane.Figure 8: Kalypso images recorded on MRSV beamline.Recording launched a few hundred turns before injectiontime. Imaging of (Top) the horizontal and (bottom) thevertical distribution of the beam.CONCLUSIONWe presented here recent results obtained on the two newbranches NB#1 and 2 of our visible range MRSV beamline.We made significant efforts to obtain a satisfactory mod-eling of our beamline relying on SRW simulations. Wemanaged to include the effect of the surf–insertion modeand of a stigmatic focussing induced by the mirror. Unfor-tunatly, some discrepancies remain between simulation andmeasurement, so that up to now, we can only retrieve thebeam sizes at the source point including, in addition to thePSF, an additionnal arbitrary –but constant– contribution tothe beam size. Next January, the extraction system for theMRSV beamline will be upgraded (new extraction mirrorand new cooling system). This should enable to improovethe accuracy of the imaging on NB#1 in surf–insertion mode,but also to work in the slot–insertion mode in users opera-tion.The preliminar results obtained on NB#2 are very encour-aging. The ultra–high repetition rate of the camera enabledalready to follow the beam dynamics at injection in onesingle image. By the end of 2021, we plan to improve thefocussing on the camera and to calibrate the magnificationon NB#2 to have an aboslute beam size measurement.Both NB#1 and 2 strongly suffer from a very high level offluctuations most probably due to the long in–air transport.We’ll try to upgrade the beamline to move it under-vacuumalong 2022.REFERENCES[1] O. Chubar, P. Elleaume, “Accurate and Efficient Computationof Synchrotron Radiation in the Near Field Region”, in Proc.6th European Particle Accelerator Conf. (EPAC’98), Stock-holm, Sweden, Jun. 1998, paper THP01G, p.1177-1179.[2] M. Caselle et al., “Ultrafast linear array detector for real-timeimaging”, Proc. of SPIE, vol. 10937, 1093704-1, 2019.10th Int. Beam Instrum. Conf. IBIC2021, Pohang, Rep. of Korea JACoW PublishingISBN: 978-3-95450-230-1 ISSN: 2673-5350 doi:10.18429/JACoW-IBIC2021-TUPP17TUPP17ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2021).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI238 03 Transverse Profile and Emittance Monitors

Fast Measurements of the Electron Beam Transverse Size and Position on SOLEIL Storage Ring

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Fast Measurements of the Electron Beam Transverse Size and Position on SOLEIL Storage Ring

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