FLUTE (Ferninfrarot Linac- Und Test-Experiment) is a compact linac-based test facility for accelerator and diagnostics R&D located at the Karlsruher Institute of Technology (KIT). A new accelerator diagnostics tool, called the split-ring resonator (SRR), was tested at FLUTE, which aims at measuring the longitudinal bunch profile of fs-scale electron bunches. Laser-generated THz radiation is used to excite a high frequency oscillating electromagnetic field in the SRR. Electrons passing through the 20 µm x 20 µm SRR gap are time-dependently deflected in the vertical plane, leading to a vertical streaking of the electron bunch. During the commissioning of the SRR at FLUTE, large series of streaking attempts with varying machine parameters and set-ups were investigated in an automatized way. The recorded beam screen images during this experiment have been analyzed and evaluated. This contribution motivates and presents the automatized experiment and discusses the data analysis

Andrian, Ivan

Assmann, Ralph

Bisoffi, Giovanni

Fabris, Alessandro

Haerer, Bastian

McIntosh, Peter

Mueller, Anke-Susanne

Nabinger, Matthias

Nasse, Michael

Ruprecht, Robert

Schaefer, Jens

Schmelzer, Thiemo

Smale, Nigel

Vinicola, Giulia

English

KITopen

SPLIT-RING RESONATOR EXPERIMENTS AND DATA ANALYSIS ATFLUTEJ. Schaefer ∗, B. Haerer, M. Nabinger, M. Nasse, R. Ruprecht, T. Schmelzer, N. Smale, A.-S. MuellerKarlsruher Institute of Technology, Karlsruhe, GermanyAbstractFLUTE (Ferninfrarot Linac- Und Test-Experiment) is acompact linac-based test facility for accelerator and diagnos-tics R&D located at the Karlsruher Institute of Technology(KIT). A new accelerator diagnostics tool, called the Split-ring resonator (SRR), was tested at FLUTE, which aims atmeasuring the longitudinal bunch profile of fs-scale electronbunches. Laser-generated THz radiation is used to excite ahigh frequency oscillating electromagnetic field in the SRR.Electrons passing through the 20 µm × 20 µm SRR gap aretime-dependently deflected in the vertical plane, leading toa vertical streaking of the electron bunch. During the com-missioning of the SRR at FLUTE, large series of streakingattempts with varying machine parameters and set-ups wereinvestigated in an automatized way. The recorded beamscreen images during this experiment have been analyzedand evaluated. This contribution motivates and presents theautomatized experiment and discusses the data analysis.INTRODUCTIONThe technically valuable THz light can be created by elec-tron accelerators in an unmatched intensity by the CoherentSynchrotron Radiation (CSR) [1] effect. This effect requiresa bunch length as short as the desired radiation wavelength.The development of techniques for creating and diagnosingsuch short bunches is of great interest. The SRR is designedfor the measurement of the electron bunch length with afs-resolution.The SSR has been installed and evaluated at FLUTE [2](KIT, Germany). In this experiment, the RF photo-injectorcreated electron bunches of 10 pC with 5 MeV. A solenoidfocuses the bunch onto the SRR gap, after which the bunchesare drifting 1 m further downstream to a YAG screen. Simul-taneously, a THz laser pulse excites an electric field that oscil-lates vertically with 240 GHz inside the 20 µm times 20 µmSRR gap. Consequently, while the bunch passes through theSRR gap a vertical kick is applied with the kick direction andstrength changing within the passing time. This principleis sketched in Fig. 1. The bunch then produces a streakedimage on the YAG screen, where the bunch length can beresolved from the vertical beam size.EXPERIMENTAL CHALLENGESTo identify the streaking effect, ideally we would com-pare screen images of consecutive bunches with and withoutthe influence of the streaking field. Blocking the incident∗ jens.schaefer2@kit.eduFigure 1: Working principle of the SRR experiment, in themiddle the SRR with its gap of 20 µm × 20 µm × 80 µm.THz laser pulse prevents the cause of streaking without af-fecting the rest of the experiment. However, the pointingstability before FLUTE’s major upgrade [3] is insufficientfor comparing individual images, see Fig. 2.Figure 2: Bunch stability at SRR position, horizontal bunchposition over 1h with 95% confidence level spans 100 µm.The orange lines refers to the 20 µm SRR gap size.As a countermeasure, the bunch size was chosen largerthan the SRR gap size. In addition, we did not only comparedsingle screen images, but averaged over 50 shots, whichprovided much more stable images. The screen image inFig. 3 shows the shadow of the SRR averaged over imagesgenerated by 50 electron bunches to ensure that the bunchindeed hits the SRR gap. With such a large beam size, onlythe bunch core travels through the SRR gap and is ableto show streaking when unblocking the THz laser pulse.In theory this should be a sufficient change to be visible.However, manually observing averaged images, no streakingsignal could be detected.Figure 3: The YAG image shows the average of 50 framesof bunches, the c-shaped SRR blocking the beam.2D SCAN EXPERIMENTA detailed evaluation of the experimental setup has iden-tified two mandatory conditions for successful streaking.First, the temporal synchronisation between the incidentTHz pulse and the electron bunch. And secondly, the spatialaiming of the THz pulse at the resonator, which influencesthe power coupling and finally the strength of the streakingfield. Although both parameter were aligned carefully [4,5],a systematic error cannot be excluded with certainty. A 2Dscan was set up around the assumed correct alignment inboth parameter axes, the scan progress is shown in Fig. 4.Figure 4: Parameters of the temporal-spatial 2D scan duringa 20 h experiment. The light shades indicate a possible win-dow for streaking per parameter. Their overlap is indicatedby the black (dashed) boxes.1st Axis: THz DelayTo manipulate the temporal synchronisation, the THz laserpulse was delayed by extending the path of the drive laser.After the THz laser pulse reaches the SRR, the streakingfield builds up and then decays within only a few cycles.Therefore, a bunch is only affected by the THz pulse if thesynchronisation is within a narrow window of 8 ps aroundan ideal synchronization.The whole scan range considers 250 ps in 188 steps aroundthe assumed ideal configuration of 0 ps. The 8 ps windowspans 6 out of 188 scan steps, it is sketched in Fig. 4 by thehorizontal light blue bar. Streaking is therefore only possiblewithin 6 consecutive scan steps during each loop of the THzdelay.2nd Axis: Longitudinal SRR PositionA misplaced longitudinal (in electron flight direction)position of the SRR shifts the resonator perpendicular to theexciting THz laser pulse. A misalignment leads to a reducedintensity of the streaking field and eventually to a reducedvertical streaking in the beam screen image.The THz pulse is focused to a waist diameter (FWHM)of 𝑑 ≈ 1 mm. During the scan, the SRR is moved by amotorized stage two steps around the pre-aligned positionwith 350 µm step width, see Fig. 4 in red.The correct alignment is assumed to have an ideal cou-pling of the THz pulse and consequently to the full streakingamplitude on the beam screen. A misalignment of 350 µmalready leads to a reduced THz intensity at the resonator andfinally to a reduced streaking amplitude. The attenuationhere is yet unknown. A misalignment of 700 µm or evenmore will most certainly prevent measurable streaking.In the scan plot the light red box shows a window of apossible correct alignment. The adjacent two lighter redboxes show where streaking might be possible with anattenuated amplitude due to a 350 µm misalignment. Thespacing between these clusters is roughly 4 h.Combining the expectations for both axes leads to onlyone region in the scan that satisfies the criteria of both, tem-poral and spatial aiming. This forms the main cluster of sixconsecutive data points that show streaking with the full am-plitude, shown by the solid black box in Fig. 4. Dependingon the attenuation level of a 350 µm misalignment, streakingmight also be detectable in the two adjacent, dashed clusters,again with 6 consecutive scan steps.Finally, the position of the black (dashed) boxes indicatewhere streaking is expected, given that the pre-aligned timingand position are correct. Else this expectation pattern ofmain and secondary cluster shifts in time but will keep itsshape.NIPD ANALYSISThe large data set acquired with this scan method requiresan automated evaluation of the screen images that is sensitiveto streaking. Because of the large bunch size w.r.t. the SRRgap size only the bunch core will be streaked. Depending onthe streaking field strength it is possible, that the bunch coremay not be streaked enough to tower over the entire bunchfootprint. Then the bunch screen image will not increasein the vertical plane but the charge distribution within thebunch will become rearranged.In each scan step two averaged images are recorded, withand without THz, each averaged over 50 single images. Thequantity NIPD (normalized integrated absolute value of pixelcount difference) was defined to give a measure of suchcharge rearrangement. NIPD is a scalar value that is zero ifthe two images are identical and monotonously increases bycharge displacement.The idea behind NIPD can be explained with Fig. 5: itshows a simulated data set of one scan step with the two aver-aged images, with THz off (left) and with THz on includingsuccessful streaking (center). The difference image (right)is calculated by subtracting their pixel count. The absolutevalue of all pixel in the difference image is then summedover a given region of interest (ROI) and normalized to thenumber of pixel (𝐼 · 𝐽).NIPD =𝐼,𝐽∑︁𝑖, 𝑗 [diff.image]𝑖, 𝑗 𝐼 · 𝐽 (1)Where 𝑖, 𝑗 are the column and row of the pixel in the differ-ence image. Without the normalization, the sum equals theP1-matrix norm. To calculate NIPD from the simulation, theROI of the experiment, a 5.62 mm2 square was chosen, seealso Fig. 3. Although for readability the simulated screenimages are shown only within a 1.122 mm2 square. The gran-ularity of the difference image reflects the real resolution ofthe diagnostic camera that reads out the YAG screen. Thesimulated pixel count was upscaled to indicate not the num-ber of simulated macro particles but the expected pixel countper unit charge. This makes the simulated beam screen im-age comparable to the real YAG screen. The simulated dataFigure 5: Simulated beam screen images without (l) and with(c) streaking. The right plot show the calculated differenceimage from which NIPD is calculated.originates from a tracking simulation [6] with ASTRA [7]and yields a final result of NIPD = 57. Although the scenariodoes not accurately match the experiment beam parameter,the simulation clearly demonstrates the NIPD analysis andits sensitivity to charge displacement.The NIPD value rises with charge rearrangement fromstreaking but also from any other source. Instabilities andgeneral machine noise contribute to the difference imageand therefore increases NIPD. A real data set therefore willalways yield NIPD > 0. Slow drifts on the other hand donot further increase NIPD.EVALUATION OF SCAN DATAEvaluating every data set of the 20 h scan with the sameNIPD analysis leads to Fig. 6. For the experiment settingsof the scan recall Fig. 4. As discussed previously, streakingshould appear in a clear pattern of a main cluster and oneor two secondary clusters 4 h before and/or after the maincluster. The NIPD value should tower over the noise level byup to 57 for the main cluster and by an (unknown) attenuatedincrease for the secondary clusters. Inspecting the NIPDdistribution in Fig. 6 indeed shows a suspicious pattern closeto 0 h and 4 h. The NIPD value rises from 4 to 12 (0 h) in theseemingly main cluster (0 h) and from 4 to 8 in the seeminglysecondary cluster (4 h). Also increased NIPD values can befound in three rich clusters (black boxes). Unfortunately allthese irregularities could be traced back to acute instabilitiesin the RF power. Even including the irregular clusters, thedata set sums up to a skew normal distribution as expectedfrom a noise-dominated data set.Figure 6: NIPD measured during a 20 h scan (l). The pro-jected histogram (r) shows the expected skew normal distri-bution from noise. The boxed clusters of high NIPD valueoriginate from acute RF instabilities.CONCLUSIONThe Split-ring resonator experiment has been set-up andrun at FLUTE (KIT). The high experimental challengesrequired us to adapt our endeavours to scan two parameters:the temporal synchronisation and the longitudinal resonatorposition. A scan routine was developed and that led to anextensive amount of accruing data. For the scan data analysisNIPD was developed, a scalar measure that increases forcharge rearrangement from streaking but also from noise.The expectation on a streaking signal position and amplitudewithin the scan was carefully discussed and the scan dataevaluated. Unfortunately streaking could not be found thatprotrudes from the noise level.At the moment FLUTE is undergoing major upgradeson multiple frontiers in parallel that promises a more stableoperation in the near future. Also a new set of THz optics [8]will be installed that significantly reduce the losses of thestreaking source. These measures will lead to a reducednoise and increased signal level that hopefully will enableus to separate the streaking signal from its background.ACKNOWLEDGEMENTSJ. Schäfer, M. Nabinger and T. Schmelzer acknowledgethe support by the DFG- funded Doctoral School "KarlsruheSchool of Elementary and Astroparticle Physics: Scienceand Technology". This project has received funding from theEuropean Union’s Horizon 2020 Research and Innovationprogramme under GA no 730871 (ARIES) and from theHorizon Europe Research and Innovation programme underGrant Agreement No 101057511 (EURO-LABS).REFERENCES[1] P. Goldreich and D. A. Keeley, “Coherent Synchrotron Radia-tion”, Astrophys. J., vol. 170, p. 463, Dec. 1971.doi:10.1086/151233[2] M. J. Nasse et al., “FLUTE: A versatile linac-based THzsource”, Rev. Sci. Instrum., vol. 84, p. 022705, 2013.doi:10.1063/1.4790431[3] A. Malygin, O. Manzhura, A.-S. Müller, R. Ruprecht, M.Schuh, and N. J. Smale, “Status of the FLUTE RF System Up-grade”, in Proc. IPAC’22, Bangkok, Thailand, Jun. 2022, pp.2452–2453. doi:10.18429/JACoW-IPAC2022-THPOST008[4] M. Nabinger et al., “Efficient Terahertz Generation by Tilted-Pulse-Front Pumping in Lithium Niobate for the Split-RingResonator Experiment at FLUTE”, in Proc. IPAC’21, Camp-inas, Brazil, May 2021, pp. 4299–4302.doi:10.18429/JACoW-IPAC2021-THPAB251[5] J. Schäfer et al., “Laser alignment of internal componentsof the linear accelerator FLUTE”, in DPG-Frühjahrstagung(DPG’22), Mainz, Germany, Mar. 2022.[6] J. Schäfer et al., “Split Ring Resonator Experiment - Simula-tion Results”, in Proc. IPAC’21, Campinas, Brazil, May 2021,pp. 888–891. doi:10.18429/JACoW-IPAC2021-MOPAB280[7] K. Flöttmann, “ASTRA: A Space Charge Tracking Algorithm”,http://www.desy.de/~mpyflo[8] M. Nabinger et al., “Characterization and optimization of laser-generated THz beam for THz based streaking”, presented atthe IPAC’23, Venice, Italy, May 2023, paper THPA079, thisconference.

Split-ring resonator experiments and data analysis at FLUTE

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Split-ring resonator experiments and data analysis at FLUTE

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