The Karlsruhe Research Accelerator (KARA) at KIT features an electro-optical (EO) near-field diagnostics setup to conduct turn-by-turn longitudinal bunch profile measurements in the storage ring using electro-optical spectral decoding (EOSD). Within the Future Circular Collider Innovation Study (FCCIS) an EO monitor using the same technique is being conceived to measure the longitudinal profile and center-of-charge of the bunches in the future electron-positron collider FCC-ee. This contribution provides an overview of the EO near-field diagnostics at KARA and discusses the development and its challenges towards an effective beam diagnostics concept for the FCC-ee

Brosi, M.

Bründermann, E.

Funkner, S.

Härer, B.

Müller, A-S.

Niehues, G.

Patil, M. M.

Reissig, M.

Ruprecht, R.

Widmann, C.

English

KITopen

DEVELOPMENT OF AN ELECTRO-OPTICAL LONGITUDINALBUNCH PROFILE MONITOR AT KARATOWARDS A BEAM DIAGNOSTICS TOOL FOR FCC-eeM. Reißig∗, M. Brosi† , E. Bründermann, S. Funkner, B. Härer, G. Niehues, M. M. Patil, R. Ruprecht,C. Widmann, A.-S. Müller, Karlsruhe Institute of Technology, Karlsruhe, GermanyAbstractThe Karlsruhe Research Accelerator (KARA) at KIT fea-tures an electro-optical (EO) near-field diagnostics setup toconduct turn-by-turn longitudinal bunch profile measure-ments in the storage ring using electro-optical spectral de-coding (EOSD). Within the Future Circular Collider Inno-vation Study (FCCIS) an EO monitor using the same tech-nique is being conceived to measure the longitudinal profileand center-of-charge of the bunches in the future electron-positron collider FCC-ee.This contribution provides an overview of the EO near-fielddiagnostics at KARA and discusses the development and itschallenges towards an effective beam diagnostics conceptfor the FCC-ee.INTRODUCTIONAn electro-optical near-field monitor can be used as a non-destructive method to measure the longitudinal bunch profileat high resolution and high repetition rate. The KarlsruheResearch Accelerator (KARA) at KIT has been the first elec-tron storage ring to successfully perform single-shot bunchprofile measurements using electro-optical spectral decoding(EOSD) in 2013 [1]. Since then, the setup has been improvedto enable observations of the longitudinal electron bunchdynamics on a turn-by-turn basis using the KIT-developedultra-fast line camera KALYPSO (KArlsruhe Linear arraYdetector for MHz rePetition-rate SpectrOscopy) [2]. With atomographic imaging technique, it is also possible to observethe dynamics of the longitudinal Phase-Space Distribution(PSD) including formation of microstructures caused by themicrobunching instability [3]. As a result, EOSD is a promis-ing technique for new particle accelerators like the futurecircular collider FCC-ee. It can be used as a non-destructivebeam diagnostic tool to perform longitudinal bunch profilemeasurements with sub-picosecond resolution and the poten-tial to observe phase-space dynamics on a microsecond timescale. In context of the Future Circular Collider InnovationStudy (FCCIS), an adaption of the KARA EO setup is underinvestigation to be optimized for FCC-ee beam parameters.REQUIREMENTS FOR FCC-eeThe FCC-ee is an electron-positron collider planned asa successor to the LHC at CERN with a circumference ofaround 100 km that operates at 4 different energies for pre-cision measurements of different elementary particles [4].∗ micha.reissig@kit.edu† Now at MAX IV Laboratory, Lund, SwedenTable 1: Comparison of KARA in Low-Alpha Mode andExtrema of FCC-ee Parameters (Based on 2 IP layout [5])KARA FCC-eelow-alpha mode extremaBeam energy / GeV 1.3 182.5Bunch charge / nC 2.2 38.0Bunch length / mm 3 12.1It starts with a centre-of-mass energy at the Z-boson poleof 45.6 GeV and increases later to the WW threshold, ZHproduction peak and finally the t ̄t threshold at 182.5 GeV. Inorder to achieve high luminosities, the accelerator holds upto 12 000 bunches in which the number of particles is heldconstant with top-up injection [5]. This requires monitoringof the bunch profile and its center of charge for every bunch.EOSD has the potential to fulfill these requirements, but adetailed investigation is needed to adapt the setup at KARAto the demanding FCC-ee beam parameters.A comparison of the most interesting parameters forEOSD measurements of KARA and FCC-ee are presentedin Table 1, where the extrema out of all operation modes ofFCC-ee are shown.EO NEAR-FIELD SETUP AT KARAThe EO near-field setup at KARA is driven by a 1030 nmlaser producing chirped ultra-short pulses in the range ofsome 10 fs. The modulation and detection of the laser pulse,which enables the measurement of the electron bunch profile,can be divided in three major steps as shown in Fig. 1.e bunchEOcrystalchirped laser pulsepolarizingbeam splitterλ/2λ/4gratingultra-fastline cameraencoded bunchprofilelaser beamdumpI0Idetect123Figure 1: Scheme of the EO setup. Adapted from [3].At first, the bunch profile is encoded in the polarization ofthe laser pulse. This is caused by the Pockels effect, wherethe birefringence of the crystal changes proportional to the13th Int. Particle Acc. Conf. IPAC2022, Bangkok, Thailand JACoW PublishingISBN: 978-3-95450-227-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2022-MOPOPT025MOPOPT025ContentfromthisworkmaybeusedunderthetermsoftheCCBY4.0licence(©2022).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI296MC6: Beam Instrumentation, Controls, Feedback and Operational AspectsT03: Beam Diagnostics and Instrumentationapplied electric field. As a result, a laser pulse changes itspolarization when it propagates through the crystal, whilean electron bunch is flying by. The crystal is oriented suchthat the modulation Γ of the polarization is proportional tothe electric field 𝐸𝑦 and is given byΓ = 2𝜋𝑑𝜆 𝑛30𝑟41𝐸𝑦. (1)The modulation therefore also depends on the crystal width𝑑, the refractive index 𝑛0, the Pockels coefficient 𝑟41 and thewavelength 𝜆 of the laser [6, 7].In a second step, the polarization modulation is convertedto an intensity modulation. An arrangement of a 𝜆/4 wave-plate, a 𝜆/2 waveplate and an polarizing beam splitter aresetup in a near-crossed configuration [1]. In this setting, thelaser intensity 𝐼detect on the detector is very small. But if thepolarization is modulated in the crystal by Γ, the intensity𝐼detect increases according to𝐼detect =12𝐼0 (1 − cos (Γ − 4𝜃)) , (2)where 𝜃 describes the angle of the 𝜆/2 waveplate relativeto the crossing angle and 𝐼0 is the initial laser intensity [1].The last part belongs to the data acquisition. For EOSD,this is a spectrometer with KALYPSO as an ultra-fast linecamera to measure the spectrum of individual pulses [2].Since the laser pulse is chirped, each laser wavelength isencoded to a position in the laser pulse and therefore thespectrum also corresponds to the longitudinal bunch profile.A photodiode can also be used instead of the spectrometer toperform electro-optical sampling (EOS) measurements [8].With EOS, the sampled wakefield over multiple turns can bescanned by shifting the time delay of the laser pulse, whilemeasuring the laser pulse intensity. EOS is also used inpreparation of EOSD measurements to find temporal overlapof the laser pulse with the E-field of the electron bunch.SIMULATION SETUPFor a complete simulation of the EO near-field measure-ments, the first step is to simulate the electric fields insidethe crystal. For this task, a simplified 3D-model of the crys-tal with its holder inside the beam pipe was created in CSTStudio Suite [9] and its wakefield solver was used to simulatethe electric field of an electron bunch travelling through it.The laser first propagates anti-parallel to the electron beamthrough the crystal, until it is reflected back and movingdownstream. For bunch profile measurements, only the firstpeak of the downstream signal is important and it should notoverlap with a peak of the upstream signal. The simulatedup- and downstream sum of the electric field along the laserpath inside the crystal is presented in Fig. 2. It is simulatedfor typical beam parameters at KARA in low-alpha single-bunch operation and shows that in this case, the first peakof the downstream signal has just a slight overlap with theupstream signal.The modulation Γ of a laser pulse following this pathis calculated based on the sum of the electric field of the0.0 0.5 1.0 1.5 2.0 2.5Time / ns1012Sum of E-field / MV/mFigure 2: Upstream (grey line) and downstream sum (blueline) of the E-field along the laser path inside the crystal. Thefirst downstream peak does not overlap with the upstreampeak, which is important to avoid distortions of the EOSDsignal.moving probe and Eq. (1). The conversion of the polarizationmodulation to an intensity modulation follows Eq. (2), with𝜃 = 4.6°. This angle of the waveplate is optimized to enablethe dependency of 𝐼detect to be as linear as possible [1].0 10 20 30 40 50Time / ps0.00.51.01.52.02.5Modulation0.000.020.040.06Laser intensity / a.u.Figure 3: The resulting modulation (orange line) is calcu-lated by dividing the modulated laser signal (dotted greyline) by an unmodulated laser signal (dashed grey line). Themodulated laser signal corresponds to the first peak of thedetected laser signal 𝐼detect. A comparison to the shape of theoriginal Gaussian bunch profile (black line) shows a slightasymmetry at the beginning of the pulse.In the last step, the laser intensity 𝐼detect can be used toemulate the result of an EOSD measurement by dividing themodulated signal of the first peak by an unmodulated signal,where the laser pulse does not overlap with the electronbunch. The resulting modulation is shown in Fig. 3. Itadditionally shows a Gaussian profile with 𝜎 = 3 mm, whichcorresponds to the shape of the original simulated bunchprofile. A comparison to the modulation signal shows anasymmetry of the modulation, which leads to a deviationto the Gaussian distribution towards the beginning of thesignal. However, the overall shape of the bunch is visibleand the deviation can be taken into account during analysis.13th Int. Particle Acc. Conf. IPAC2022, Bangkok, Thailand JACoW PublishingISBN: 978-3-95450-227-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2022-MOPOPT025MC6: Beam Instrumentation, Controls, Feedback and Operational AspectsT03: Beam Diagnostics and InstrumentationMOPOPT025297ContentfromthisworkmaybeusedunderthetermsoftheCCBY4.0licence(©2022).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOIMAJOR CHALLENGES AT FCC-EEA comparison of the polarization modulation Γ at KARAand FCC beam properties is presented in Fig. 4. It shows apeak modulation of ΓFCC-eeZ ≈ 100°, which is very high incomparison to ΓKARA ≈ 9°. According to Eq. (2), 𝐼detect isapproximately linearly proportional to small Γ ranges, butwith the large bunch charge at FCC-ee this is not the case.As a result, the laser intensity 𝐼detect does not correspond tothe bunch profile, because it is distorted by cos(Γ − 4𝜃). Tomitigate this issue, the distance of the crystal to the electronbunch could be increased, or the crystal properties can beadjusted, for example by using a smaller crystal.0.0 0.5 1.0 1.5 2.0Time / ns250255075100Phase modulation  / degreesFigure 4: Comparison of the simulated phase modulation Γat KARA low-alpha beam parameters (orange line) and FCC-ee at Z-boson pole energy (blue line). The large modulationΓ at FCC-ee parameters leads to a non-linear behavior of𝐼detect(Γ) (see Eq. (2)). This should be avoided to preventdistortions of the EOSD signal.The second major challenge is the bunch length at FCC-ee,which increases to 𝜎FCC-eeZ = 12.1 mm during the operationat Z-boson pole energy. A larger bunch causes the upstreamsignal to increase, because the temporal overlap of the up-stream laser pulse in the crystal with the electric field of theelectron bunch increases with bunch length. Since the EOSand EOSD signal is a result of the sum of up- and down-stream signal, but only the first peak of the downstreamsignal represents the electron bunch profile, a large upstreamsignal distorts the measurement signal. To avoid the up-stream signal, a transmission setup, where the laser is notreflected back through the crystal, could be considered. Sucha setup is more challenging because it requires an additionalmirror in front of the crystal, but it would allow to reducethe crystal size and therefore decrease the modulation Γ tohelp with the issue of high electric fields at FCC-ee.COMPARISON OF FCC-EE MODESThe different operation modes of FCC-ee come with largedifferences in bunch length and bunch charge [5]. To evalu-ate the effect on EOSD measurements, simulations have beendone of the phase modulation Γ for all FCC-ee operationmodes. The longest bunches are present during operation forZ-boson production with 𝜎FCC-eeZ = 12.1 mm and the short-est with 𝜎FCC-eet ̄t = 2.6 mm during operation at ̄t thresholdenergy. As discussed in the previous subsection, a transmis-sion setup would be better suited for long bunches, becausethe laser pulse is only propagating downstream through thecrystal and the unwanted upstream signal is avoided.The peak phase modulation amplitude ΓZ, max ≈ 100°,ΓWW, max ≈ 120° and ΓZH, max ≈ 150° are in a range thatone EOSD measurement setup could be sufficient to be usedfor all three modes with minor adjustments. For t ̄t modehowever, the peak amplitude is Γt ̄t, max ≈ 400° more thandouble of the other modes due to the higher charge densityof the electron bunches. Thus, the measurement setup needsto be adjusted, for example by increasing the distance of thecrystal to the beam or by decreasing the crystal size.SUMMARY AND OUTLOOKEOSD is a non-destructive method to measure a single-shot bunch profile and has been used at KARA since 2013.To investigate its potential as a bunch diagnostics tool forFCC-ee, a simulation environment has been set up to emulatethe complete measurement procedure from the electric fieldin the crystal to the optical components manipulating thelaser pulse. Simulations of the KARA measurement setupunder FCC-ee beam parameters provide a first insight tothe challenges, that an EOSD setup will face at FCC-ee.The two major differences are the longer bunches during Z-operation mode and the large electric fields especially duringt ̄t-mode operation. To mitigate these issues, a transmissionsetup is suggested, where the laser pulse is not reflectedback through the crystal. This would avoid issues with adisturbing upstream signal and would allow to decrease thecrystal size to reduce the impact of large electric fields.The next step is to develop a design for the EOSD trans-mission setup and perform preliminary tests in simulations.Since the current design of the FCC-ee beam pipe features acircular cross section with additional winglets to the sides, itshould be investigated, if the crystal could be placed insidea winglet. This would help to cope with the large electricfields and could reduce the impedance impact.ACKNOWLEDGEMENTSThe Future Circular Collider Innovation Study (FCCIS)project has received funding from the European Union’sHorizon 2020 research and innovation programme undergrant No 951754.M. R. and M. M. P. acknowledge the support by the Doc-toral School „Karlsruhe School of Elementary and Astropar-ticle Physics: Science and Technology“.C. W. acknowledges funding by BMBF contract number05K19VKD.REFERENCES[1] N. 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Schoenfeldt, “Simulation and Measurement of the Dynamicsof Ultra-Short Electron Bunch Profiles for the Generation ofCoherent THz Radiation”, Ph.D. thesis, Karlsruher Institut fürTechnologie (KIT), Karlsruhe, Germany, 2018.[9] CST Studio Suite 2021, https://www.3ds.com/products-services/simulia/products/cst-studio-suite13th Int. Particle Acc. Conf. IPAC2022, Bangkok, Thailand JACoW PublishingISBN: 978-3-95450-227-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2022-MOPOPT025MC6: Beam Instrumentation, Controls, Feedback and Operational AspectsT03: Beam Diagnostics and InstrumentationMOPOPT025299ContentfromthisworkmaybeusedunderthetermsoftheCCBY4.0licence(©2022).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI

Development of an Electro-Optical Longitudinal Bunch Profile Monitor at KARA Towards a Beam Diagnostics Tool for FCC-ee

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Development of an Electro-Optical Longitudinal Bunch Profile Monitor at KARA Towards a Beam Diagnostics Tool for FCC-ee

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