For measuring the temporal profile of the coherent synchrotron radiation (CSR) at the KIT storage ring KARA (Karlsruhe Research Accelerator) an experimental setup based on electro-optical spectral decoding (EOSD) is currently being implemented. The EOSD technique allows single-shot, phase-sensitive measurements of the far-field radiation on a turn-by-turn basis at rates in the MHz range. Therefore, the resulting THz radiation from the dynamics of the bunch evolution, e.g. the microbunching, can be observed with high temporal resolution. This far-field setup is part of the distributed sensor network at KARA. Additionally to the information acquired from the near-field EOSD spectral decoding and the horizontal bunch profile monitor, it enables to monitor the longitudinal phase-space of the bunch. In this contribution, the characterization of the far-field setup is summarized and its implementation is discussed

Bründermann, Erik

Caselle, Michele

Funkner, Stefan

Mai, Carsten

Müller, Anke-Susanne

Nasse, Michael J.

Niehues, Gudrun

Patil, Meghana M.

Sax, C.

Steinmann, Johannes Leonhard

Weber, Marc

Widmann, Christina

English

KITopen

IMPLEMENTING ELECTRO-OPTICAL DIAGNOSTICS FOR MEASURINGTHE CSR FAR-FIELD AT KARAC. Widmann∗, E. Bründermann, M. Caselle, S. Funkner, M. J. Nasse, G. Niehues, M. M. Patil, C. Sax,J. L. Steinmann, M. Weber, A.-S. Müller, Karlsruhe Institute of Technology, Karlsruhe, GermanyC. Mai, Center for Synchrotron Radiation (DELTA), TU Dortmund University, Dortmund, GermanyAbstractFor measuring the temporal profile of the coherent syn-chrotron radiation (CSR) at the KIT storage ring KARA(Karlsruhe Research Accelerator) an experimental setupbased on electro-optical spectral decoding (EOSD) is cur-rently being implemented. The EOSD technique allowssingle-shot, phase-sensitive measurements of the far-fieldradiation on a turn-by-turn basis at rates in the MHz range.Therefore, the resulting THz radiation from the dynamicsof the bunch evolution, e.g. the microbunching, can be ob-served with high temporal resolution.This far-field setup is part of the distributed sensor networkat KARA. Additionally to the information acquired fromthe near-field EOSD spectral decoding and the horizontalbunch profile monitor, it enables to monitor the longitudinalphase-space of the bunch.In this contribution, the characterization of the far-fieldsetup is summarized and its implementation is discussed.INTRODUCTIONIn the short-bunch mode at the KIT storage ring KARA(Karlsruhe Research Accelerator), the so-called low-𝛼𝑐mode, the electron bunch interacts with itself and its emittedradiation [1]. This leads to varying microstructures in thebunch, the micro-bunching instability, and intense emissionoutbursts of coherent synchrotron radiation (CSR) in theTHz range. One aspect of studying the microbunching isto understand the dynamics and to be able to predict themwith simulations [2–6]. The second aspect is to be able tocontrol the THz emission by tuning accessible parametersin the storage ring [7]. For both it is necessary to track theevolution of the longitudinal phase space of the bunch withsophisticated methods. Single-shot measurement techniquesideally being able to take a snapshot of the bunch at everyturn or even of several bunches each turn are necessary.In the short-bunch mode, the electron bunch length isreduced to some picoseconds (ps) at a beam energy of≤1.3 GeV and a current of some mA. The revolution fre-quency is 2.7 MHz (for a detailed parameter list, see e.g. [3]).Several diagnostic tools for the observation of the longitudi-nal phase space in low-𝛼𝑐 mode were developed, all part ofthe distributed sensor network at KARA [5].Distributed Sensor Network at KARAThe distributed sensor network at KARA consists of syn-chronized sensors measuring the longitudinal bunch profile,∗ christina.widmann@kit.eduFigure 1: Location of a selection of sensors of the distributedsensor network at KARA.the energy spread and the temporal profile of the THz pulses(Fig. 1).The longitudinal bunch profile is measured using electro-optical spectral decoding (EOSD) of the near-field of theelectron bunch. With this near-field EOSD setup the evolu-tion of the longitudinal bunch profile in low-𝛼𝑐 operationmode at KARA could be studied in detail [2] and a 2D imageof the longitudinal phase space could be reconstructed [8].The energy distribution is measured indirectly by mea-suring the horizontal bunch size in a dispersive section ofKARA [5]. The bunch profile is imaged on a line array cam-era. The energy spread can be determined from the changein the horizontal bunch size.For both diagnostic setups KALYPSO, a fast line arraydetector developed at KIT, is used [9,10]. KALYPSO fea-tures a frame rate up to 12 MHz allowing measurements insingle-bunch mode turn-by-turn. Depending on the type ofline array used, KALYPSO has a resolution of 256 to 1024pixels.For measuring the THz pulse profile fast THz detec-tors with KAPTURE-2 as readout system is used [3].KAPTURE-2 has 8 channels for read-out with a samplingrate of up to 1 GSa/s [11]. Therefore, also in multi-bunchmode a tracking of the emitted THz radiation is possible,but it is not possible to resolve the pulse shape for pulsesthat are shorter than the impulse response of the Schottkybarrier diode used as a detector.To increase the temporal resolution of the THz bunchprofile in single-shot measurements an alternative setup us-ing EOSD is currently under commissioning at KARA. Inthis far-field EOSD setup we use KALYPSO as detector, i.e.the setup will be capable to capture THz bunch profiles insingle-bunch mode turn-by-turn.12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW PublishingISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-MOPAB294MC6: Beam Instrumentation, Controls, Feedback and Operational AspectsT03 Beam Diagnostics and InstrumentationMOPAB294931ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2021).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOIEOSD Far-Field Setup at KARAEOSD is a method originally applied in THz spectroscopy[12]. An ultra-short laser pulse is overlapped collinearly withthe THz pulse inside an electro-optical (EO) crystal (see alsoFig. 2). The electric field of the THz pulse changes the bire-fringence of the EO crystal due to the Pockels-effect. Thisresults in a change of the polarization of the laser pulse fromlinear to elliptical. The change is proportional to the elec-trical field strength. By analyzing this polarization changethe electrical field strength of the THz pulse can be deter-mined. Using a chirped laser pulse, i.e. a laser pulse witha defined correlation of the temporal delay of the spectralcomponents, the temporal shape of the electrical field ofthe THz pulse is encoded on the spectral components ofthe laser pulse. Therefore, the temporal profile of the THzpulse can be analyzed in single-shot measurements using anoptical spectrometer. This method is applied to measure thefar-field of the CSR at synchrotrons [13, 14].Figure 2: Principle of EOSD of a THz pulse with a chirpedlaser pulse: THz and laser pulse are overlapped collinearlyin the EO crystal. The electrical field of the THz pulsegenerates a modulation in the intensity of the laser pulsespectrum, which can be analyzed in a spectrometer.Using this EOSD technique and KALYPSO as a detector,the CSR emitted at KARA can be measured single-shotwith the revolution frequency of 2.7 MHz. Two KALYPSOversions based on InGaAs will be integrated, first a line arraywith 512 pixels with a pitch of 25 µm. This version will bereplaced by a line array with 1024 pixels operating up to12 Mfps in the future.EOSD FAR-FIELD DEMONSTRATORFor tests independent of the accelerator KARA a firstdemonstrator setup was designed. The test setup not onlygives the opportunity to align large parts of the system inde-pendently of the accelerator, but also to test optical compo-nents including different EO crystals. A commercial lasersystem with a free space output of the laser beam and anintegrated THz emitter is used [15]. The emitter providesTHz pulses of more than 4.5 THz bandwidth, but weakerin intensity than the THz pulses emitted at KARA in short-bunch operation at maximum beam current. All parametersof the laser system and the EO section of the setup are listedin Table 1.Table 1: Parameters of the Commercial Laser System andthe EO Components Used in the EOSD Demonstrator SetupLaser systemcenter wavelength 1 560 nmspectral width 70 nmmin. pulse duration 90 fsaverage power 210 mWpulse repetition rate 62.5 MHzsynchronization to external source possibleEO componentsbandwidth THz emitter ≥4.5 THzEO crystal GaAs (110)crystal thickness 0.5 mm / 1.0 mmDesign and RequirementsThe setup consists mainly of four parts: the beam path ofthe THz pulse, the beam path of the laser pulse, the modula-tion section, where the temporal structure of the THz pulseis imprinted on the laser pulse spectrum, and the detectionpart. In Fig. 3, a picture of the demonstrator setup is shown.The major constraint designing the demonstrator setup isthe temporal overlap of the THz pulse and the laser pulse, i.e.the laser path length has to match the THz beam path. Aspropagation of the THz pulse in air over long distances dueto strong water absorption lines should be avoided, the laserpath of the setup has to be as compact as possible. Compact-ness is further a necessity, if at a later stage operation in awater-vapour-free atmosphere is considered, e.g. by usingdry air or nitrogen. For adjusting the temporal overlap adelay stage of 25 cm travel length is integrated in the setup.To set the required laser pulse length a grating stretcherwith a Treacy [16] grating compressor is used. This stretcherFigure 3: Picture of the optical setup of the EOSD far-fielddemonstrator. The laser beam path is shown in red, theTHz beam path in blue. In this configuration a commercialspectrometer is integrated for diagnostics, during the EOSDmeasurements an optical spectrometer with KALYPSO asdetector is integrated.12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW PublishingISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-MOPAB294MOPAB294ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2021).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI932MC6: Beam Instrumentation, Controls, Feedback and Operational AspectsT03 Beam Diagnostics and Instrumentationgenerates a negative chirp of the laser pulse. To achievean efficiency of more than 55% holographic transmissiongratings (600 lines/mm) optimized for the vertical polariza-tion of the laser pulse are used. Further advantages of thetransmission gratings are a flexible and compact stretchersetup. The grating distance can be set such that the laserpulse length is around 2 ps, which is needed for adjustingthe initial rough temporal overlap. Increasing the gratingdistance, a pulse length up to 8 ps is reached, which is nec-essary for sampling the whole THz pulse during th EOSDmeasurements. The laser pulse length is measured with acommercial autocorrelator [17].Alignment and SettingsThe THz beam is imaged from the THz emitter on thecrystal using two off-axis parabolic mirrors with a focusinglength of around 10 cm. For the focusing and collimation ofthe laser beam achromat lenses with a focal length of 15 cmare used. To ensure spatial overlap of the foci in the crystalan aperture at the crystal position was used for maximizingboth signals.Both, the laser and the THz beam, are polarized perpen-dicular to the table surface. The crystal is oriented withone axis parallel to the beam polarization. A 𝜆/4 waveplateis used to compensate for the intrinsic birefringence of thecrystal. The 𝜆/2 waveplate is adjusted such that the laserpolarization is perpendicular to the transmitted polarizationof polarizing beam splitter (PBS), i.e. the power of the beamtransmitted through the PBS is minimized. For the EOSDmeasurements a nearly crossed configuration of the PBSwith respect to the laser beam is used. Therefore, the 𝜆/2waveplate is rotated 5° from the minimum position to have alinear response of the signal to the electric field of the THzpulse. This angle might be adapted for optimizing of certainEOSD measurement configurations. As the output power ofthe laser system is 210 mW and the grating stretcher has anefficiency of slightly above 55 % in the current configurationaround 5 mW of the laser beam are transmitted by the PBS.In the current setup, the beam reflected from the PBSis guided to a beam dump. It is planned to use also theinformation of the second beam in a balanced detectionscheme as used at SOLEIL [18].For the characterization and rough alignment a commer-cial spectrometer or a photo diode are used. In the next stepKALYPSO with a spectrometer consisting of a reflectiongrating (600 lines/mm) and a cylindrical lens is integratedfor the EOSD measurements.IMPLEMENTATION AT KARAA second setup is currently designed for implementation atthe Infrared 2 (IR2) beamline at KARA to measure the CSRemitted by the storage ring. The requirements for this setupare slightly different than those of the EOSD demonstrator:The hard constraint in matching the path length for THzand laser pulse is not required anymore as there is no fixedcorrelation for the timing. Adjusting the temporal overlapcan be done using the synchronization of the laser system tothe timing signals at KARA. Therefore, a delay stage is notnecessary for the measurements at KARA.At the same time the setup has to be more stable than thedemonstrator setup to be capable of long-term operation and- as no permanent installation is possible at the moment -transport and installation at the beamline. For increasing thestability it was decided to use lockable mirror mounts anddefine single mirrors and apertures in the setup for alignmentafter moving the setup. Flip mirrors are included to haveaccess for characterizing the laser pulses at any time withoutchanges in the beam path for the EOSD setup.A spectrometer with KALYPSO as detector, a setup simi-lar to the one used in the demonstrator, is included for record-ing the EO-modulated laser spectra. Currently the optics forimaging of the spectrum on KALYPSO matching the widthof the detector is optimized.The main challenge for the installation will be the align-ment of the emitted CSR and the temporal overlap of it withthe laser pulse. Tests for the alignment are performed atthe moment at the demonstrator setup to prepare a goodworkflow for the implementation at the beamline.SUMMARYFor the implementation of a new EOSD setup for mea-suring the temporal CSR profile a demonstrator setup foroffline tests independent of the accelerator was set up. Afirst characterization of the system is finished, but tests andthe preparation for the EOSD measurements are ongoing.In parallel, an optimized setup is designed for the imple-mentation at the IR2 beamline at the storage ring KARA.First measurements are planned during the next months.ACKNOWLEDGEMENTSThis work was supported by BMBF ErUM-Pro project05K19 STARTRAC (Sophisticated Tools for Accelerator Re-search using Terahertz RAdiation Characteristics), C.W. wasfunded under contract No. 05K19VDK, C.M. under contractNo. 05K19PEC, S.F. under contract No. 05K16VKA.G.N. and E.B. acknowledge support by the Helmholtz Presi-dent’s strategic fund IVF “Plasma Accelerators”. M.M.P. ac-knowledges the support by the DFG-funded Doctoral School“Karlsruhe School of Elementary and Astroparticle Physics:Science and Technology”.REFERENCES[1] A.-S. Müller et al., “Characterizing THz Coherent Syn-chrotron Radiation at the ANKA Storage Ring”, in Proc.11th European Particle Accelerator Conf. (EPAC’08), Genoa,Italy, Jun. 2008, paper WEPC046, pp. 2091–2093.[2] S. Funkner et al., “High throughput data streaming of in-dividual longitudinal electron bunch profiles”, Phys. Rev.Accel. Beams, vol. 22, no. 2, p. 022801, Feb. 2019. doi:10.1103/PhysRevAccelBeams.22.022801[3] M. Brosi et al., “Systematic studies of the microbunchinginstability at very low bunch charges”, Phys. Rev. Accel.12th Int. Particle Acc. Conf. 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IPAC2021, Campinas, SP, Brazil JACoW PublishingISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-MOPAB294MOPAB294ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2021).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI934MC6: Beam Instrumentation, Controls, Feedback and Operational AspectsT03 Beam Diagnostics and Instrumentation

Implementing Electro-Optical Diagnostics for Measuring the CSR far-field at KARA

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Implementing Electro-Optical Diagnostics for Measuring the CSR far-field at KARA

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