Beutner, B.

Gerth, Ch.

Hensler, O.

Obier, F.

Scholz, M.

Yan, Minjie

FLUTE Collaboration

KITopen

ONLINE LONGITUDINAL BUNCH PROFILE AND SLICE EMITTANCEDIAGNOSTICS AT THE EUROPEAN XFELCh. Gerth∗, B. Beutner, O. Hensler, F. Obier, M. Scholz, M. Yan†Deutsches Elektronen-Synchrotron DESY, Hamburg, GermanyAbstractThe longitudinal current profile and slice emittance areimportant bunch parameters for the operation of an X-rayfree-electron laser. At the European XFEL, dedicated di-agnostic sections equipped with transverse deflecting RFstructures (TDS) have been installed for the control andoptimisation of these parameters. Travelling-wave TDS incombination with fast kicker magnets and off-axis screensallow for the study of individual bunches without affectingthe other bunches in the super-conducting linear acceleratorwhich can generate bunch trains of up to 2700 bunches at 4.5MHz within 600 microsecond RF pulses at a repetition rateof 10 Hz. The measurement of the slice emittance is realisedin a static F0D0 lattice equipped with four individual screenstations.Variations of the longitudinal bunch profile or slice emit-tance along the bunch train may lead to degraded FEL per-formance for parts of the train which reduces the effectiveavailable number of bunches for FEL operation. By gradu-ally adjusting the timing, individual bunches along the bunchtrain can be measured in order to optimise the overall beamparameters for all bunches in the train. In this paper, wedescribe in detail the diagnostic concept and present firstmeasurement results of the projected and slice emittancealong the bunch train.INTRODUCTIONThe European XFEL, which is currently under commis-sioning, will deliver ultra-short X-rays in the photon energyrange 0.25 keV to 25 keV [1]. The superconducting linearaccelerator that will drive 3 FEL undulator beamlines willgenerate beam energies of up to 17.5GeV. As is illustratedin Fig. 1, the RF accelerating field is pulsed at a frequencyof 10Hz with a flat-top duration of up to 600 µs, in which atrain of electron bunches is accelerated. The bunch repeti-tion rate can be up to 4.5MHz (equal to a bunch spacing of220 ns), which corresponds to a maximum of 2700 bunchesper bunch train. In future, it is envisioned to operate theaccelerator with constant bunch filling patterns that are ad-vantageous in terms of operation stability. The photon pulsepattern can be chosen by sending unwanted electron buncheswith fast kicker magnets into a local dump upstream of theFEL undulator beamlines.Variations during the RF flat-top duration of either thelaser pulse properties of the photo-cathode laser or the RFparameters of the accelerator modules in the injector may∗ christopher.gerth@desy.de† Current address: Karlsruher Institut für Technologie, 76131 Karlsruhe,Germany, minjie.yan@kit.eduRF amplitudeelectron bunchest 0.1 s600 μs .....min. 220 ns.....Figure 1: RF timing structure and electron bunch pattern ofthe European XFEL.result in variations of beam matching or emittance of thebunches in the bunch train. This in turn may lead to substan-tial variations along the bunch train of the photon pulse ener-gies of the X-rays that are delivered to the user experiments.Hence, there is a strong demand for diagnostic techniqueswith which electron bunch parameters such as the longi-tudinal current profile or slice emittance can be measuredalong the bunch train. This has been realised by utilisinga transverse deflecting RF structure (TDS) in combinationwith kicker magnets and off-axis screens.The RF field of the TDS induces a time-dependent trans-verse deflection of the electrons of one bunch in the bunchtrain by which the longitudinal bunch shape is transformedto the transverse plane. A fast kicker magnet downstream ofthe TDS is then used to deflect this bunch onto an off-axisscreen without affecting the remaining bunches in the bunchtrain. The diagnosed bunch is taken out of the bunch trainbefore the FEL undulators while the remaining bunches con-tinue for the generation of FEL radiation. By utilising fourkicker magnets with four off-axis screens in a static F0D0lattice, the longitudinal bunch profile and slice emittancecan be measured. Finally, these bunch parameters can bemeasured along the bunch train by properly adjusting thetiming of the TDS, kicker magnets and camera systems ofthe screen stations. In this paper, we present the results ofprojected and slice emittance measurements obtained in theinjector section of the European XFEL.DIAGNOSTIC SECTIONThree dedicated beamline sections equipped with TDShave been designed for the measurement of the longitudi-nal profile and slice emittance as well as the longitudinalphase space [2]. A schematic layout of the TDS diagnosticssections, which are located downstream of the laser heatersystem in the injector section and downstream of the secondand third bunch compressor chicane, is depicted in Fig. 2.Each section comprises a TDS, four kicker magnets, fourscreen stations with off-axis and on-axis screens, one dipole6th International Beam Instrumentation Conference IBIC2017, Grand Rapids, MI, USA JACoW PublishingISBN: 978-3-95450-192-2 doi:10.18429/JACoW-IBIC2017-TUPCC034 Time-Resolved Diagnostics and SynchronizationTUPCC03153ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.magnet and one screen station in the dispersive section be-hind the dipole magnet [3]. The four screen stations arelocated in the subsequent drift sections of a F0D0 cell struc-ture, which allows for projected emittance measurements [4].When the TDS is switched on, slice emittance and longitu-dinal profile measurements can be performed. Longitudinalphase space measurements can be carried out in the disper-sive section only when the dipole magnet is switched on and,as a consequence, the whole bunch train is deflected to thelocal dump and FEL operation is interrupted.TDSkicker magnetsdipolescreen stationslocaldump {FODO cellmain beamline Figure 2: Schematic layout of the 3 TDS diagnostic sectionsat the European XFEL.The TDS are disk-loaded S-band RF waveguide struc-tures designed and manufactured by the Institute for NuclearResearch of the Russian Academy of Sciences (Moscow,Russia). They are operated in travelling wave 2pi/3 mode ata frequency of 2.997GHz1. The TDS in the injector sectioncomprises 16-cells and has a filling time below 110 ns, i.e.the RF pulse is short enough to act only on one bunch withinthe bunch train without affecting the other bunches.The screen stations [5] are equipped with both full on-axis and half off-axis scintillator screens (LYSO:Ce) whichare mounted perpendicular to the beam axis and imaged atan angle of 45◦ to the beam axis with CCD cameras in aScheimpflug configuration. As a result, the screen stationsprovide a constant spatial (rms) resolution of 10 µm for theconfiguration with a magnification of 1 : 1 over the entirescreen. The distance from the edge of the off-axis screen tothe beam axis amounts to 6mm.As is illustrated in Fig. 3, the kicker magnets consist ofa ceramic vacuum chamber, which has been sputtered atthe inside with a layer of 1 µm thick stainless steel, and twoflat copper bars outside the vacuum beam pipe at oppositesides with a length of 350mm. One high-voltage pulser (upto 20 kV) is directly attached to each kicker magnet. Thepulsers generate half cycles of a sine wave with a pulseduration of tp = 380 ns at 10Hz which enables deflection ofsingle bunches onto the corresponding off-axis screens fordiagnostics while the remaining bunches in the bunch trainare not affected and proceed to the undulator beamlines forthe generation of FEL radiation.1 The frequency of the TDS f = 332/144 · 1.3GHz= 2.997GHz is gener-ated from the master oscillator frequency of 1.3GHz and is an integermultiple of the bunch repetition rate of 4.5MHz.connection to the pulsercopper bar beam axisFigure 3: Schematics of a kicker magnet. The high-voltagepulser is not shown.This is demonstrated in Fig. 4 for the operation of 2bunches, of which the second bunch is kicked. On the lefthand side the image of the two bunches on an on-axis screenis shown while on the corresponding image of the off-axisscreen on the right only the kicked bunch is visible (andreflections on the edge of the scintillator).A bunch with a certain bunch number within the bunchtrain can be chosen for diagnostics via the timing systemby adjusting appropriately the trigger delays of the TDS,kicker magnets and camera systems. The diagnosed bunchexperiences a betatron oscillation due to the kicker magnetand may get lost along the accelerator. This bunch is markedby the timing system and taken out of the bunch train bythe fast kicker magnets upstream of the FEL undulator sec-tions or, in case of beam loss, ignored by the fast machineprotection system, which would inhibit the generation offurther electron bunches. For an emittance measurement,the kicker magnets are toggled subsequently and typically20 beam images are recoded per screen. One emittance mea-surement takes less than 20 secods as the screens do not haveto be moved. By scanning the bunch number, the projectedor slice emittance as well as the longitudinal bunch profilecan be measured along the bunch train. The diagnostic sec-tions also serve as matching sections, in which the beam isroutinely matched to the design accelerator optics for trans-port into the subsequent beamline sections equipped withaccelerating modules.On-axis screen200 200 300 400 500 600X axis [pixel]100200300400500600700Y axis [pixel]Off-axis screen100 200 300 400 500 600X axis [pixel]100200300400500600700Y axis [pixel]Figure 4: Beam operation with two bunches: the secondbunch is kicked by a kicker magnet. Left: Image of on-axisscreen with both bunches visible. Right: Image of off-axisscreen with only the kicked bunch visible.6th International Beam Instrumentation Conference IBIC2017, Grand Rapids, MI, USA JACoW PublishingISBN: 978-3-95450-192-2 doi:10.18429/JACoW-IBIC2017-TUPCC03TUPCC03154ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.4 Time-Resolved Diagnostics and Synchronization0 200 400 600 800 1000 1200 1400 1600 1800Bunch number11.21.41.61.8Norm. emittance [m]hor.vert.0 200 400 600 800 1000 1200 1400 1600 1800Bunch number11.522.5BMAGhor.vert.Figure 5: Intra-train emittance measurement for individualbunches along the train. Top: Projected normalized emit-tances. Bottom: Optics mismatch amplitude (BMAG) withrespect to design optics.EMITTANCE MEASUREMENTSThe layout of the diagnostic sections and time structure ofthe electron bunches allow for novel phase space measure-ments of individual bunches along the train. In Fig. 5 anexample of such a train emittance scan is shown. Each datapoint represents a transverse phase space measurement withthe four screen stations in the F0D0 lattice. For each mea-surement a bunch is selected by the kicker magnet timingand sequentially kicked onto the four off-axis screens, and20 beam images are recorded per screen station. A symmet-ric F0D0 optics with a phase advance of 76◦ per cell waschosen to give best results for the reconstruction of the pro-jected emittances in both the x and y plane [6]. Phase spaceproperties such as the beam Twiss parameters or transverseemittance are determined by a least square method usingknown beam transport functions [7]. The beam momentsare varied at a reference point upstream of the F0D0 screenstations to best reproduce the measured beam spot sizes onthe screens. A single emittance measurement for a certainbunch number takes less than 20 s.For a train emittance scan, this procedure is repeated forbunches with different bunch numbers in the train. For theresults shown in Fig. 5, every 20th bunch of the about 450 µslong train has been selected for a measurement. The mea-surements have been carried out in the injector section aton-crest operation with a beam energy of 130MeV, bunchcharge of 0.5 nC and bunch repetition rate of 4.5MHz. Inthis particular example, the projected emittance and opticsmismatch is not constant along the bunch train. The ver-tical emittance and optics mismatch exhibit a strong slope0102030β(m)0 5 10 15 20050100150s (m)∆µ(degree)xyTDS Screen QuadrupoleFigure 6: Accelerator optics for slice emittance measure-ments. Top: Schematic beamline layout; the quadrupolestrengths are indicated by the height of the bars. Middle:Beta-functions. Bottom: Phase advances.up to about bunch number 200. The horizontal emittanceand optics mismatch increase slowly along the bunch train.This behaviour could be attributed to thermal effects in apart of the photo-cathode laser system, which could easilybe corrected after being identified. This illustrates the util-ity of such a diagnostics setup which allows for optimisingthe phase space properties of each bunch in order to estab-lish uniform FEL performance for all bunches in the train.The measurement time for this train emittance scan (mea-surement for every 20th bunch number) amounted to about15minutes.Dispersion may be generated in the emittance measure-ment plane at the screen locations, when the kicker magnetsare utilized for online emittance measurements. For relativeenergy spreads below σδ ∼ 10−3, as is the case for on-crestoperation, the relative error contribution in the calculatedemittance is below 1%. A detailed error analysis of statisticaland systematic errors that may contribute in the emittancemeasurement has been carried out in Ref. [6].SLICE EMITTANCE MEASUREMENTSBy utilizing a TDS, the longitudinal bunch profile is trans-formed to one transverse plane while in the perpendicularplane the spot sizes can be measured for the longitudinalslices and slice emittances can be evaluated with the leastsquare fit technique described in the latter section.Figure 6 depicts an example of an accelerator optics opti-mised for a slice emittance measurement in the TDS diag-nostic section with the beta-functions (middle) and phaseadvances (bottom) for both the emittance measurement planex and TDS streak plane y. For a good longitudinal resolution6th International Beam Instrumentation Conference IBIC2017, Grand Rapids, MI, USA JACoW PublishingISBN: 978-3-95450-192-2 doi:10.18429/JACoW-IBIC2017-TUPCC034 Time-Resolved Diagnostics and SynchronizationTUPCC03155ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.0.40.60.81600Norm. emittance [m] 1.21.4500400Bunch number3004200 3Slice index2100 10-11-2-30.50.60.70.80.911.11.2Figure 7: Nomalised horizontal slice emittance measuredalong the bunch train (in steps of 10 bunch numbers). Sliceindex 0 corresponds to the central slice withmaximum bunchcurrent.in the streak plane, the accelerator optics has been optimisedfor a large beta-function βy = 30m at the TDS, a phaseadvance of ∆µy = 90◦ from the TDS to the centre of theF0D0 lattice (i.e. the centre of the second and third screen)and a small phase advance of ∆µy = 30◦ per cell in theF0D0 lattice. For the reconstruction of the slice emittance,a phase advance of ∆µx = 76◦ per cell has been chosen inthe emittance plane as in the case of the projected emittancemeasurements.Figure 7 shows the results of online slice emittance mea-surements scanned along the bunch train. The slice emit-tance measurements have been carried out for bunch num-bers 1 to 671 in the train in steps of 10 bunch numbers, i.e. atotal of 67 measurements. For each emittance measurement,16 beam images have been recorded for each of the fourscreen stations. Each emittance measurement took about20 s and the total measurement time for the scan amounted toabout 25min. The data has been taken in the injector sectionat on-crest operation with a beam energy of 130MeV, bunchcharge of 0.5 nC and bunch repetition rate of 1.125MHz.The slice index 0 corresponds to the central slice of thebunch with maximum bunch current. The results for theslices in the head and tail of the bunch have been omittedfrom the plot for better illustration as they exhibit large errorbars due to a large beam optics mismatch and low imageintensities. All individual measurements display a simi-lar distribution of the normalised slice emittance along thebunch train for the slice indices plotted. The individual nor-malised slice emittances vary between 0.6 µm and 1.3 µmwith the minimum value at the central index 0 and increasingvalues towards bunch head and tail. Before the scan, thebeam optics was matched to the central slice of the bunchwith bunch number 1.CONCLUSIONThe combined utilization of a TDS with fast kicker mag-nets and off-axis screens is a versatile diagnostics imple-mentation to analyse individual bunches in a bunch trainwithout affecting the remaining bunches. In this paper wepresented the results of measurements of the projected andslice emittance along the bunch train. This is especiallyhelpful when it is required to achieve uniform FEL perfor-mance along the bunch train as it is the case for the EuropeanXFEL which operates in a 10Hz pulsed mode. Comparedto the maximum number of bunches in a bunch train (2700at a repetition rate of 4.5MHz), the bunches sacrificed fordiagnostics purpose are negligible, and the method can beregarded as semi-parasitic on-line diagnostics.ACKNOWLEDGEMENTThe European XFEL is a joint achievement of many con-tributing institutes (for details see http://www.xfel.eu). Theauthors wish to thank all people working in different partsof the project for their contributions.REFERENCES[1] W. Decking and H. Weise, “Commissioning of the EuropeanXFEL Accelerator“, in Proc. IPAC’17, Copenhagen, Denmark,May 2017, paper MOXAA1, pp. 1–6.[2] M. Röhrs and Ch. Gerth, “Electron beam diagnostics withtransverse deflecting structures at the European X-ray freeelectron laser“, in Proc. FEL’08, Gyeongju, Korea, Aug. 2008,paper MOPPH049, pp. 90–93.[3] J. Wychowaniak, Ch. Gerth, and M. Yan, “Design of Multi-setup TDS-based Electron Beam Diagnostics for the EuropeanXFEL“, in Proc. FEL’14, Basel, Switzerland, Aug. 2014, paperTHP075, pp. 909–912.[4] M. Yan et al., “Comparison of Quadrupole Scan and Multi-Screen Methode for the Measurement of Projected and SliceEmittance at the SwissFEL Injector Test Facility“, in Proc.FEL’14, Basel, Switzerland, Aug. 2014, paper THP088, pp.941–944.[5] Ch. Wiebers et al., “Scintillating Screen Monitors for theTransverse Electron Beam Profile Diagnostics at the Euro-pean XFEL“, in Proc. IBIC’13, Oxford, UK, Sept. 2013, paperWEPF03, pp. 807–810.[6] M. Yan, “Online diagnostics of time-resolved electron beamproperties with femtosecond resolution for X-ray FELs”,DESY, Hamburg, Germany, DESY-THESIS-2016-017, July2016.[7] M.G.Minty and F. Zimmermann, “Transverse BeamEmittanceMeasurement and Control”, inMeasurement and Control ofCharged Particle Beams, Springer-Verlag Berlin Heidelberg,2003, pp. 99–130.6th International Beam Instrumentation Conference IBIC2017, Grand Rapids, MI, USA JACoW PublishingISBN: 978-3-95450-192-2 doi:10.18429/JACoW-IBIC2017-TUPCC03TUPCC03156ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.4 Time-Resolved Diagnostics and Synchronization

Online Longitudinal Bunch Profile and Slice Emittance Diagnostics at the European XFEL

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Online Longitudinal Bunch Profile and Slice Emittance Diagnostics at the European XFEL

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