A Compact Transverse Deflecting System (Compact-TDS) designed for longitudinal electron bunch diagnostics in the femtosecond regime is presently undergoing commissioning at the Karlsruhe Institute of Technology (KIT). This technique, based on THz streaking using a resonator structure, demands a high level of electron beam controllability and stability at the micrometer scale. To meet these requirements, the linear accelerator FLUTE (Ferninfrarot Linac- Und Test-Experiment) has undergone major upgrades in 2023, incorporating a new RF system equipped with a klystron, RF photoinjector and solenoid magnet.
					In this contribution, we present first experiments conducted with the Compact-TDS at FLUTE, utilizing the upgraded RF setup

Boine-Frankenheim, O.

Bruendermann, Erik

Dehler, M.

Feurer, T.

Fuchs, Matthias

Glukhov, S.

Hayati, M.

Ischebeck, R.

Laabs, M.

Malygin, Anton

Mayer, Katharina

Moser, M.

Mueller, Anke-Susanne

Nabinger, Matthias

Nasse, Michael Johannes

Noll, Marvin

Ollmann, Z.

Ruprecht, Robert

Schaefer, Jens

Schlott, V.

Schmelzer, Thiemo

Smale, Nigel John

Steinmann, Johannes

English

KITopen

COMMISSIONING AND EXPERIMENTS WITH A COMPACTTRANSVERSE DEFLECTING SYSTEM AT FLUTEM. Nabinger∗, E. Bründermann, M. Fuchs, A. Malygin, K. Mayer, M. J. Nasse, M.-D. Noll,R. Ruprecht, J. Schäfer, T. Schmelzer, N. J. Smale, J. L. Steinmann, A.-S. MüllerKarlsruhe Institute of Technology, Karlsruhe, GermanyM. M. Dehler, R. Ischebeck, M. Moser, V. Schlott, Paul Scherrer Institute, Villigen, SwitzerlandT. Feurer, M. Hayati, Z. Ollmann, University of Berne, Berne, SwitzerlandS. Glukhov, O. Boine-Frankenheim, Technical University of Darmstadt, Darmstadt, GermanyM. Fritzsche, M. Laabs, Dresden University of Technology, Dresden, GermanyAbstractA Compact Transverse Deflecting System (Compact-TDS)designed for longitudinal electron bunch diagnostics in thefemtosecond regime is presently undergoing commissioningat the Karlsruhe Institute of Technology (KIT). This tech-nique, based on THz streaking using a resonator structure,demands a high level of electron beam controllability andstability at the micrometer scale. To meet these requirements,the linear accelerator FLUTE (Ferninfrarot Linac- Und Test-Experiment) has undergone major upgrades in 2023, incor-porating a new RF system equipped with a klystron, RFphotoinjector and solenoid magnet.In this contribution, we present first experiments con-ducted with the Compact-TDS at FLUTE, utilizing the up-graded RF setup.INTRODUCTIONLast year’s Nobel Prize in Physics was awarded for theestablishment of attosecond physics [1], which clearly high-lights the current relevance of deeper understanding andapplications on ultra-short time scales. Many acceleratorfacilities worldwide are also extensively active in variousfields using bunch lengths in the femtosecond range and be-low [2,3]. Reliable, yet space- and cost-saving diagnosticsof particle distributions on such time scales play a crucialrole in this endeavor.Therefore, a novel method for determining the longitu-dinal bunch profile of sub-10 femtoseconds is being inves-tigated at KIT. It is intended to serve as a cost-effectiveand compact alternative to conventionally applied streakingmethods [4]. The proposed Compact-TDS is currently beingtested and commissioned at the LINAC-based test facilityFLUTE. Its concept aims to elevate the standard streakingfrequencies from GHz to the THz range, while employinga resonator structure to amplify the electric field. As illus-trated in Fig. 1, an example of such a resonator structurecould be an Inverse Split-Ring Resonator (ISRR), whichholds the promise of achieving streaking field strengths ofseveral MV/m [5]. For a detailed explanation of the workingprinciple shown in Fig. 1, we direct readers to [6].The Compact-TDS offers several advantages: One ex-ample is, since the same laser can be used for generating∗ matthias.nabinger@kit.eduFigure 1: Working principle of the Compact-TDS diagnos-tics THz-based streaking experiment at FLUTE. The sketchillustrates an ISRR used as the resonator structure.both the electrons, as well as the THz pulse driving theresonator structure, a synchronization between the bunchand the streaking field is intrinsically given. Moreover, theused THz frequencies enable resonance structures with di-mensions in the order of millimeters and less, thus makingthe system extremely compact compared to other methods.However, due to the small structures, the method requiresa very precise control and stability of the electron beam.For instance, in case of the ISRR used at FLUTE, the beamhas to pass a gap size of only 20 µm, where it can interactwith the oscillating streaking field. In order to meet thesedemands of machine stability, the entire FLUTE RF systemhas been upgraded recently.FLUTE RF SYSTEM UPGRADEOver the course of 2023, the entire FLUTE RF systemhas undergone a major upgrade. The main changes for theFigure 2: Photographs of the new RF photoinjector (left)and 10 MW RF unit (right) installed at FLUTE.Figure 3: Comparison of RF stability performance for the old(up to 2022) and new (2024) FLUTE RF system (light anddark blue, respectively). Top: Normalized RF power 𝑃/𝑃avgversus time for an interval of 2 hours. The measurementswere taken directly at the output of the specified RF unit.Bottom: Horizontal electron beam position versus time foran interval of one minute. The beam position was evaluatedfrom the camera image of a YAG screen in the resonatorplane with comparable machine settings.5 MeV-section of FLUTE, where the Compact-TDS experi-ment is currently installed, are shown in Fig. 2: a new RFphotoinjector with a new solenoid magnet, as well as a new10 MW RF unit consisting of a klystron and modulator driv-ing the RF photoinjector have been successfully installed.In October 2023, the first electron beam was measured us-ing the new setup. Also, the RF system upgrade includesthe integration of an additional 37 MW RF unit driving theLINAC of FLUTE and the possibility to operate at repetitionrates up to 50 Hz. For more details of the RF system design,we refer to [7].Figure 3 illustrates a comparison of the RF stability per-formance for the old and new FLUTE RF systems, withmeasurements taken in 2022 and 2024, respectively. Thetop diagram shows a 2-hour-measurement of the RF powerat the directional coupler directly after the RF unit used atthe time. To observe the percentage fluctuation, the poweris normalized by the average power over that time inter-val, which was approximately 9.5 MW in both cases. Thecomparison demonstrates that the 1-𝜎-fluctuation of the RFpower was reduced by roughly a factor of 10 from the old(𝜎old = 0.36 %) to the new system (𝜎new = 0.04 %).The improved RF stability directly translates to a highershot-to-shot electron beam stability: As an example, wepresent in the bottom diagram of Fig. 3 the standard devi-ation of the horizontal beam position measured over oneminute. It clearly shows a decrease by one order of magni-tude for the new system compared to the old one. It shouldbe noted, that the beam position was observed at a scintil-lating YAG screen in the resonator plane for comparablemachine settings, which were assumed to be optimal forexperiments with the Compact-TDS setup at the particulartime. As discussed in [8], the beam position fluctation withthe old FLUTE RF system was in the order of hundreds ofmicrometers, which is one order of magnitude larger thanthe gap size of the ISRR. With the new system, however, thebeam fluctuation could be improved to the extent that it ison the order of the resonator gap size (for Fig. 3, the peak-to-peak amplitude of the beam position measured with the newsystem is ≈ 40 µm). The increased electron beam stabilityallows for more precise handling of the experimental setupfor its further enhancement.EXPERIMENTAL SETUPIn order to decrease dark current and the electron bunchsize at the resonator structure, a pinhole aperture was re-cently installed in the Compact-TDS experimental vacuumchamber in front of the resonator structure. The componentwas manufactured by LT Ultra Precision Technology [9]and comprises two 5 mm thick copper plates, each with oneplane side of high flatness. On one side of one plate, threechannels are milled and the plates are tightly screwed to-gether (Fig. 4a). The resulting pinhole aperture featuresholes with 50, 100 and 200 µm diameter and was installedin a holder on two motor axes for horizontal and verticalposition adjustments (Fig. 4b). Due to the improved electronbeam stability, it was easily possible to get electrons throughFigure 4: Top: CAD-drawings of the recently installed pin-hole aperture. a) Manufacturing principle and b) the installa-tion on two motor axes with close-up look on the 3 differentavailable pinhole aperture sizes. c) Electron beam intensityas false-color representation measured on a YAG screen inthe resonator plane for all pinholes.Figure 5: a) Photograph of the interior of the experimentalchamber with the currently installed pinhole aperture andthe holder for the two resonators (ISRR and TSR), as wellas the diagnostics YAG screen. b) Microscope picture of theTSR glued onto the resonator holder with the ISRR in thebackground. c) Zoom of the TSR and dimensions of the slit.all of the pinholes, reducing the electron spot size down tothe dimensions of the resonator gap (Fig. 4c).Furthermore, an alternative resonator structure was addedto the Compact-TDS setup. This Tilted-Slit-Resonator (TSR)structure - proposed in [10] - was installed next to the exist-ing ISRR resonator by gluing. The motorized holder allowsto center either the TSR or the ISRR in the THz focus point,allowing to test the streaking effect from two different ge-ometries. Figure 5a shows a picture of the inside of theexperimental chamber, with microscopic views of the TSRin b and c. Besides the above mentioned pinhole aperture,the YAG screen and the ISRR, it can be seen that the addedTSR is installed with a relative angle of 45◦ to both the elec-tron and THz beam. A comparison of the properties andperformance between both resonator geometries was studiedby simulations in [11] and [12].The TSR structure was made by laser microfabrication[13] and has a slit length of 650 µm and a slit or gap heightof 30 µm. The main advantage of this alternative geome-try is that it serves as an additional natural aperture for theelectron beam: all electrons reaching the detection screenhave experienced streaking field, the rest is blocked by theresonator (in our case 100 µm thick molybdenum foil) if theelectron beam size is on the order of the slit dimensions.With the improved beam quality by the RF upgrade and pin-hole aperture, the TSR geometry could be scanned with veryhigh precision, significantly improving our knowledge of theelectron beam position relative to the resonator. However, asa spatial overlap with the THz beam is needed for the gener-Figure 6: Left: CAD-drawing of the newly designed holderfor resonator, YAG screen and THz detector array. Right:Microscopic picture of the 4x4 Schottky-diode based THzdetector array prototype.ation of a streaking field, an additional enhancement of thesetup is required to reduce the THz spot position uncertainty.For this purpose, a THz detector array is currently beingdeveloped and installed in collaboration with the DresdenUniversity of Technology. Figure 6 shows the new Compact-TDS resonator holder design with integrated THz detectorarray (left picture), which will allow an in-situ measurementof the THz spot location under vacuum. After calibrationwith a structurally identical THz generation setup [14], thearray can also be used for an estimation of the present THzpulse energy. A prototype of the 900 µm×900 µm array con-sisting of 4 × 4 antenna coupled Schottky-diodes is alreadyfabricated (Fig. 6, right picture) and successfully tested forultra high vacuum compatibility.A recent experimental campaign to search for an overlap ofelectron and THz beam at the TSR leading to an observablestreaking suffered from the large uncertainty of the THzspot location in the order of several millimeters. Therefore,the installation of the array is planned for the near future toenable more effective experiments with the Compact-TDS.CONCLUSION AND OUTLOOKFirst experiments were conducted for the longitudinalbunch diagnostics Compact-TDS at KIT, using the upgradedRF system at FLUTE. The enhancements significantly in-creased the electron beam quality by an order of magnitude,allowing for a precise alignment to a newly installed pinholeaperture and an alternative resonator structure. Experimentalruns with the TSR revealed the need for an in-situ possibilityto verify the THz spot location, which is planned to be re-alized by adding a THz detector array to the Compact-TDSsetup in order to observe an electron bunch streaking.ACKNOWLEDGMENTSM. Nabinger acknowledges the support by the DFG-funded Doctoral School ”Karlsruhe School of Elemen-tary and Astroparticle Physics: Science and Technology”(KSETA), the IPAC Student Grant and funding by the BMBFErUM-Pro project Compact TDS (FKZ 05K22VK3).REFERENCES[1] “Light as fast as electrons”, Nat. Phys., vol. 19, no. 11, p.1520, Nov. 2023. doi:10.1038/s41567-023-02305-y[2] J. Roensch-Schulenburg et al., “Generation of Ultra-shortElectron Bunches at FLASH”, in Proc. 34th Int. Free-Electron Laser Conf. (FEL2012), Nara, Japan, Aug. 2012, pp.610–613, paper THPD33.[3] V. Schlott et al., “Overview and Status of SwissFEL Di-agnostics”, in Proc. 4th Int. Beam Instrumentation Conf.(IBIC’15), Melbourne, Australia, Sep. 2016, pp. 12–16, paperMOBLA03.[4] P. Krejcik, R. Akre, L. Bentson, and P. Emma, “A Trans-verse RF Deflecting Structure for Bunch Length and PhaseSpace Diagnostics”, in Proc. 19th Particle Accelerator Conf.(PAC’01), Chicago, IL, USA, Jun. 2001, paper WPAH116,pp. 2353–2355.[5] M. Yan et al., “Design of a Time-resolved Electron Diag-nostics Using THz Fields Excited in a Split Ring Resonatorat FLUTE”, in Proc. 5th Int. Beam Instrumentation Conf.(IBIC’16), Barcelona, Spain, Sep. 2016, pp. 475–478.doi:10.18429/JACoW-IBIC2016-TUPG56[6] M. Nabinger et al., “Efficient terahertz generation by tilted-pulse-front pumping in lithium niobate for the split-ring res-onator experiment at FLUTE”, in Proc. 12th InternationalParticle Accelerator Conf. (IPAC’21), Campinas, Brazil, May2021, pp. 4299–4301.doi:10.18429/JACoW-IPAC2021-THPAB251[7] A. Malygin et al., “Status of the FLUTE RF system up-grade”, in Proc. 13th International Particle Accelerator Conf.(IPAC’22), Bangkok, Thailand, May 2022, pp. 2452–2453.doi:10.18429/JACoW-IPAC2022-THPOST008[8] J. Schäfer et al., “Split-ring resonator experiments and dataanalysis at FLUTE”, in Proc. 14th International Particle Ac-celerator Conf. (IPAC’23), Venice, Italy, May 2023, pp. 4760–4763. doi:10.18429/JACoW-IPAC2023-THPL122[9] LT Ultra-Precision Technology GmbH, Herdwangen-Schönach, Germany, https://www.lt-ultra.com/impressum/[10] L. Zhao et al., “Terahertz Streaking of Few-FemtosecondRelativistic Electron Beams”, Phys. Rev. X, vol. 8, no. 2, pp.021061, June 2018. doi:10.1103/PhysRevX.8.021061[11] S. Glukhov et al., “Simulations of the compact transverse-deflecting system for ultra-short electron bunch diagnos-tic”, in Proc. 14th International Particle Accelerator Conf.(IPAC’23), Venice, Italy, May 2023, pp. 4887–4890.doi:10.18429/JACoW-IPAC2023-THPL192[12] S. Glukhov et al., “Resonator Design Optimization for a Com-pact Transverse-deflecting System”, presented at the 15th Int.Particle Accelerator Conf. (IPAC’24), Nashville, TN, USA,May 2024, WEPG53, this conference.[13] Innotech Photonics GmbH, Kaiserslautern, Germany, https://innotech-photonics.de/[14] M.. Nabinger et al., “Characterization and optimization oflaser-generated THz beam for THz based streaking”, in Proc.14th International Particle Accelerator Conf. (IPAC’23),Venice, Italy, May 2023, pp. 4149–4153.doi:10.18429/JACoW-IPAC2023-THPA079

Commissioning and experiments with a compact transverse deflecting system at FLUTE

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Commissioning and experiments with a compact transverse deflecting system at FLUTE

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