Electro-optical (EO) methods are nowadays well-proven diagnostic tools, which are utilized to detect THz fields in countless experiments. The world’s first near-field EO sampling monitor at an electron storage ring was developed and installed at the KIT storage ring KARA (Karlsruhe Research Accelerator) and optimized to detect longitudinal bunch profiles. This experiment with other diagnostic techniques builds a distributed, synchronized sensor network to gain comprehensive data about the phase-space of electron bunches as well as the produced coherent synchrotron radiation (CSR). These measurements facilitate studies of physical conditions to provide, at the end, intense and stable CSR in the THz range. At KIT, we also operate FLUTE (Ferninfrarot Linac- und Test-Experiment), a new compact versatile linear accelerator as a test facility for novel techniques and diagnostics. There, EO diagnostics will be implemented to open up possibilities to evaluate and compare new techniques for longitudinal bunch diagnostics. In this contribution, we will give an overview of results achieved, the current status of the EO diagnostic setups at KARA and FLUTE and discuss future prospects

Bründermann, Erik

Caselle, Michele

Funkner, Stefan

Müller, Anke-Susanne

Nasse, Michael J.

Niehues, Gudrun

Patil, Meghana M.

Ruprecht, Robert

Schuh, Marcel

Weber, Marc

Widmann, Christina

English

KITopen

ELECTRO-OPTICAL DIAGNOSTICS AT KARA AND FLUTE– RESULTS AND PROSPECTSG. Niehues∗, E. Bründermann, M. Caselle, S. Funkner, M. J. Nasse, M. M. Patil,R. Ruprecht, M. Schuh, M. Weber, C. Widmann, A.-S. MüllerKarlsruhe Institute of Technology, Karlsruhe, GermanyAbstractElectro-optical (EO) methods are nowadays well-provendiagnostic tools, which are utilized to detect terahertz (THz)fields in countless experiments. The world’s first near-fieldEO sampling monitor at an electron storage ring was devel-oped and installed at the KIT storage ring KARA (KarlsruheResearch Accelerator) and optimized to detect longitudinalbunch profiles. This experiment with other diagnostic tech-niques builds a distributed, synchronized sensor networkto gain comprehensive data about the longitudinal phase-space of electron bunches as well as the produced coherentsynchrotron radiation (CSR). These measurements facilitatestudies of physical conditions to provide, at the end, intenseand stable CSR in the THz range. At KIT, we also operateFLUTE (Ferninfrarot Linac- und Test-Experiment), a newcompact versatile linear accelerator as a test facility for noveltechniques and diagnostics. There, EO methods will be im-plemented to open up possibilities to evaluate and comparenew techniques for longitudinal bunch diagnostics. In thiscontribution, we will give an overview of results achieved,the current status of the EO setups at KARA and FLUTEand discuss future prospects.INTRODUCTIONWhile diagnostics is crucial for evaluation, verificationand control, new accelerators and also advanced operationmodes of accelerators lead to challenges for diagnostic meth-ods. For electron accelerators, real-time measurements ofbunch lengths from extended to ultra-short demand diag-nostics covering a broad range of time scales. Nevertheless,only single-shot measurements (every bunch or every turnin case of a storage ring) give access to the full informationof individual bunches and their temporal evolution, and thuspush the possibilities for online control. Here, in partic-ular high-repetition rates for single-shot diagnostics are abottle-neck.For storage rings operating with short pulses, the mi-crobunching instability has been studied to gain insightson the emission and control of strong CSR bursts in the THzfrequency range (e.g. [1]). In this context, electro-optical(EO) methods have been utilized to measure the longitudi-nal bunch profiles. The so called EO near-field diagnosticsis based on the Pockels-effect: the presence of an electricfield, here from the relativistic electron bunch, induces achange in birefringence of a nearby EO crystal. Therebythe longitudinal electron bunch profile is encoded onto the∗ gudrun.niehues@kit.edupolarization of a probe laser pulse passing through the EOcrystal, changing from linear to ellliptical. With polariza-tion optics, this polarization modulation is converted into anintensity modulation. To enable single-shot measurementsthe electro-optical spectral decoding (EOSD) method basedon chirped laser pulses, which was originally applied in THztime-domain spectroscopy (THz-TDS) [2], can be utilized.EOSD was first applied in a linear accelerator to detectthe near-field of electron bunches [3]. At KARA, the world’sfirst near-field EO sampling monitor at an electron storagering was developed and installed [4]. To cope with the highrepetition rate of the storage ring (in the MHz range) andmeasure the bunch profile on a turn-by-turn basis, advanceddetection methods like the photonic-time stretch mode [5]or modern ultra-fast line cameras (KALYPSO [6, 7]) arerequired. An EOSD system using KALYPSO for single-shotlongitudinal bunch profile monitoring for electron bunchlengths is in operation at KARA, as well as at the EuropeanXFEL [8]. New accelerators aiming for shorter electronbunches (e.g. KIT FLUTE [9] and cSTART [10]) are callingfor EO diagnostics covering shorter time scales.This paper gives a short overview of the distributed sensornetwork, describes results and developments of EO diagnos-tics at KARA. We also discuss prospects for EO diagnosticsat FLUTE. Finally, we give an outlook on new techniquesand approaches for EO diagnostics for electron accelerators.DISTRIBUTED DIAGNOSTICS AT KARAFigure 1: Displayed is the KARA layout with the diagnosticstations of the distributed sensor network. Adpated fromimage by U. Herberger, KIT IBPT.12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW PublishingISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-MOPAB293MC6: Beam Instrumentation, Controls, Feedback and Operational AspectsT03 Beam Diagnostics and InstrumentationMOPAB293927ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2021).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOIFigure 2: Schematic illustration of EO far- and near-field detection systems: The preparation of the laser pules is for bothsystems implemented by chirping fs pulses mapped to the ps range. For EO far-field diagnostics, the modulation of thelaser pulses is induced by leading emitted CSR through a crystal, which changes its birefringence, resulting in a change ofthe laser pulse polarization. For EO near-field diagnostics the birefringence is induced by the electric field of the electronbunch passing close to the EO crystal (inside the vacuum pipe). The analysis of this polarization change is performed forboth methods by converting the polarization modulation into intensity modulation, followed by detection using a gratingand a fast linear camera and DAQ system (KALYPSO).At KARA, we operate a distributed sensor network cover-ing various synchronous measurements at three beam linesand one setup in the ring. Figure 1 displays these diagnosticstations around the KARA ring.For electron accelerators, EO diagnostics can be employedat the near-field as well as the far-field (see Fig. 2). A near-field EO setup is installed in the ring. Two infrared (IR)beam lines enable CSR intensity measurements with fastTHz detectors and fast read-out systems (KAPTURE) [11].Here, at these beam lines, CSR spectral measurements usingEO far-field setups are currently in commissioning [12].Furthermore, in a dispersive section of KARA - at the visiblelight diagnostics (VLD) port - horizontal beam profiles canbe measured using the ultra-fast line camera KALYPSOenabling determination of the energy spread [13].Synchronous, turn-by-turn measurements at the diagnos-tic stations have been performed to detect the longitudinalbunch profile and horizontal bunch size as well as the emittedCSR intensity [1, 14]. These measurements reveal correla-tions between the emitted CSR intensity, deformations inthe longitudinal bunch profile and the horizontal bunch sizeduring the microbunching instability.EO DIAGNOSTICS AT KARAFor near-field measurements, the modulation of the laserpulses takes place in the vacuum pipe, close to the electronbeam, where the relativistic Coulomb field of the electronbunch is leaked into the EO crystal. For far-field measure-ments, the modulation takes places at a beam line guidingthe emitted CSR through the EO crystal. The preparationof the chirped laser pulses and analysis (cf. Fig. 2) are verysimilar for the EO near- and far-field setups.At KARA, first experiments of far-field CSR detectionusing EO sampling has been applied averaging over 105 in-dividual traces [15]. At the moment, new setups for EO far-field measurements are in commissioning using a 1560 nmfiber laser [12]. Such EO far-field measurements enable thedetection of the electric field in amplitude and phase andFigure 3: A 2D reconstruction example of the electron bunchphase space density during a microbunching instability cycleat KARA using experimental data measured with the EOnear-field diagnostics setup (for details see [16]).thus full spectral information of CSR in the frequency andtime domain. In contrast, direct detectors, such as fast THzdetectors, measure the CSR intensity only. The latter type ofmeasurements were performed at KARA with the ultra-fastread-out DAQ KAPTURE [1], which can be operated atrepetition rates of hundreds of MHz. Both types of measure-ments are complementing each other to cover a broad rangeof applications. While the near-field EO sampling monitor atKARA has been demonstrated for the first time in 2013 [4],improvements have been achieved and are ongoing. Thebottleneck of commercial cameras for single-shot detectionof the chirped laser pulses is the rather low repetition rate (inthe kHz regime). Here, KALYPSO, the KIT-built ultra-fastline camera dedicated for MHz repetition rate spectroscopy,opened up new possibilities [17]. Turn-by-turn single-shotmeasurements allowed synchronous measurements with var-ious other detectors [1, 14].Recently, using an approximation of the electron bunch dy-namics by a rigid rotation of the phase space density duringhalf of a synchrotron oscillation period, we could demon-strate a robust method to reconstruct the phase space den-12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW PublishingISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-MOPAB293MOPAB293ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2021).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI928MC6: Beam Instrumentation, Controls, Feedback and Operational AspectsT03 Beam Diagnostics and InstrumentationFigure 4: Simplified overview of the FLUTE layout. The green arrow displays the position of first choice for an EO near-fieldsetup, blue arrows mark additional positions where EO near-field setups can be installed.sity using single-shot EO measurements of the longitudinalbunch profile [16]. To calculate this 2D longitudinal phasespace density, filtered back-projections of sinograms (revo-lution plots) are used enabling phase space tomography ofthe electron bunch. Figure 3 shows an example of such a 2Dphase space density plot during a cycle of the microbunchinginstability at KARA.EO DIAGNOSTICS AT FLUTEFLUTE, the new compact versatile linear accelerator atKIT serves as a test facility for novel techniques and diagnos-tics [9]. EO near-field setups for FLUTE have been designedin a flexible way, so that an exchange of the EO arm (and thusEO crystal) can be conducted without the need of breakingthe vacuum in the FLUTE beam pipe. Furthermore, win-dows in the beam cube enable online optical inspection ofthe EO crystal. Thus, experiments with novel modulationtechniques can be performed.Moreover, it is possible to place near-field setups at var-ious positions of FLUTE. Figure 4 gives an overview ofFLUTE, where a green arrow shows the first choice for anEO near-field setup and the blue arrows denote possibleadditional positions. Systematic studies at different posi-tions (and eventually using more than one setup at the sametime) enable investigations of the electron bunches along theaccelerator as well as comparisons of EO techniques anddevices.SUMMARY AND OUTLOOKIn this paper, we showed a broad range of applications ofEO diagnostics at KIT, at the storage ring KARA as wellas at the linear accelerator FLUTE. EO diagnostics is asingle-shot, non-destructive, online tool enabling variousapplications. The combination of near-field and far-field EOmeasurements at electron accelerators opens up the perspec-tive to gain comprehensive information on the phase space ofthe electron bunches, and to gain time and frequency domaininformation of the emitted CSR pulses.In particular, the usage of 1.5 µm laser technology for EOdiagnostics can facilitate developments due to the broaderavailability of such lasers compared to 1 µm lasers.While the development in the direction of shorter bunchesenforces diagnostics and new challenges on smaller timescales, EO methods can also be applied for diagnostics onlarge time scales and frequency ranges in the GHz range. Forexample, new highly efficient silicon photonic modulatorsare currently under development at KIT [18].ACKNOWLEDGEMENTSS.F. was funded by BMBF contract No. 05K16VKA, C.W. by BMBF contract number 05K19VKD. G.N. and E.B.acknowledge support by the Helmholtz President’s strategicfund IVF “Plasma Accelerators”. M.M.P acknowledges thesupport by the DFG-funded Doctoral School “KarlsruheSchool of Elementary and Astroparticle Physics: Scienceand Technology”.Special thanks to S. Schott and IBPT vacuum group forthe excellent design and technical support.REFERENCES[1] M. Brosi et al., “Synchronous Measurements of ElectronBunches Under the Influence of the Microbunching Instabil-ity”, in Proc. 10th Int. Particle Accelerator Conf. (IPAC’19),Melbourne, Australia, May 2019, pp. 3119–3122. doi:10.18429/JACoW-IPAC2019-WEPTS015[2] Z. Jiang and X. C. Zhang, “Electro-optic measurement of THzfield pulses with a chirped optical beam”, Applied PhysicsLetters, vol. 72, pp. 1945–1947, 1998. doi:10.1063/1.121231[3] I. Wilke et al., “Single-shot electron-beam bunch length mea-surements”, Physical Review Letters, vol. 88, p. 124801, 2002.doi:10.1103/PhysRevLett.88.124801[4] N. Hiller et al., “Electro-optical Bunch Length Measurementsat the ANKA Storage Ring”, in Proc. 4th Int. Particle Accel-erator Conf. 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IPAC2021, Campinas, SP, Brazil JACoW PublishingISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-MOPAB293MOPAB293ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2021).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI930MC6: Beam Instrumentation, Controls, Feedback and Operational AspectsT03 Beam Diagnostics and Instrumentation

Electro-Optical Diagnostics at KARA and FLUTE – Results and Prospects

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Electro-Optical Diagnostics at KARA and FLUTE – Results and Prospects

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