At the visible light diagnostic (VLD) port at the Karlsruhe Research Accelerator (KARA), it is possible to measure the energy spread of electron bunches by measuring the horizontal bunch profile of the incoherent synchrotron radiation. KALYPSO, a MHz-rate line-array detector has been used to measure the bunch profile. Recently, the KALYPSO system has been upgraded to a version incorporating a microstrip sensor based on TI-LGAD. The performed measurements have shown that the overall sensitivity of the system was significantly improved, which enables measurements at low bunch charges. In this contribution, a brief overview of the upgraded setup and preliminary measurement results will be presented

Boscardin, Maurizio

Bründermann, Erik

Caselle, Michele

Centis Vignali, Matteo

Funkner, Stefan

Goffing, Christian

Mueller, Anke-Susanne

Niehues, Gudrun

Paternoster, Giovanni

Patil, Meghana

Steinmann, Johannes

Widmann, Christina

English

KITopen

STATUS AND UPGRADE OF THE VISIBLE LIGHT DIAGNOSTICS PORTFOR ENERGY SPREAD MEASUREMENTS AT KARAM. M. Patil, E. Bründermann, M. Caselle, S. Funkner, C. Goffing, G. Niehues, J. L. Steinmann,C. Widmann, A.-S. Müller, Karlsruhe Institute of Technology, Eggenstein Leopoldshafen, GermanyM. Centis Vignali, M. Boscardin, G. Paternoster, Fondazione Bruno Kessler, Trento, ItalyAbstractAt the visible light diagnostic (VLD) port at the KarlsruheResearch Accelerator (KARA), it is possible to study the en-ergy spread of electron bunches by measuring the horizontalbunch profile of the incoherent synchrotron radiation. KA-LYPSO, an MHz-rate line-array detector, has been employedto measure the bunch profile. Recently, the KALYPSO sys-tem has been upgraded to a version of a microstrip sensorbased on TI-LGAD. The measurements have shown thatthe system’s overall sensitivity was significant - at least bya factor of 20 improved, enabling the study of bunch pro-files at low bunch charges. This contribution will present anoverview of the upgraded setup and preliminary results.INTRODUCTIONSeveral operation modes are implemented at the Karls-ruhe Research Accelerator (KARA) for generating intensesynchrotron radiation. One of which is the short-bunch, low-𝛼𝑐 mode. In this mode of operation, the electron bunchesare squeezed to a bunch length of a few ps. Due to this, aself-interaction with the emitted radiation occurs, resultingin short bursts of THz radiation. The formation of micro-structures on the longitudinal density profile of the electronbunch characterizes this self-interaction behaviour [1–3].This phenomenon is called the microbunching instability. Tostudy this instability at KARA, several diagnostic methodshave been implemented, including streak camera, electro-optical spectral decoding (EOSD) for longitudinal bunchprofile measurements [4, 5], THz detectors (e.g. Schottkydiodes) for measuring the THz intensity [6] and measuringthe transverse bunch profile of the electron bunch for energyspread studies [7].The transverse bunch profile has been acquired by measur-ing the incoherent synchrotron radiation (ISR) emitted at a5° port of a dipole-bending magnet at a dispersive section ofthe storage ring. As the radiation is emitted in a dispersivesection, the horizontal bunch size is coupled with the relativeenergy spread of the electron bunch given by the Eq. (1).Measuring the ISR provides an opportunity to study the ef-fect of the microbunching instability on the transverse bunchprofile and hence the energy spread [8]. The horizontal pro-file is measured by a line array camera called KALYPSO.The following section describes the optical setup used tostudy the horizontal bunch profile.𝜎𝛿 =1𝐷𝑥√︃𝜎2𝑥 − 𝛽𝑥𝜖𝑥 . (1)Figure 1: Optical setup for measuring the transverse bunchprofile. Taken from Ref. [9].The 𝜎𝛿 is the relative energy spread, 𝜎𝑥 is the horizontalbunch size, 𝐷𝑥 is the horizontal dispersion, 𝜖𝑥 is the hori-zontal emittance and 𝛽𝑥 is the horizontal beta function ofthe electron bunch.EXPERIMENTAL SETUPThe experimental setup used for studying the transversebunch profile is shown in Fig. 1. A beam elevator aligns theISR beam to the optical axis on the optical table. The beamis split using a wavelength splitter and the spectral rangefrom 400 nm to 550 nm is focused onto a line array detectorusing cylindrical lenses. The previous measurement setuphad a fast-gated camera (FGC) instead of KALYPSO [10].Although the FGC had good sensitivity, it could resolve abunch with a framerate of 450 kHz in a single-bunch op-eration mode, the sensor size only allowed for the acqui-sition of 80 bunch profiles. To overcome this limitation,KALYPSO has replaced the FGC. KALYPSO (KArlsruheLinear arraY detector for MHz rePetition-rate SpectrOscopy)is an ultra-fast line array camera capable of framerates up to12 MHz developed at KIT. KALYPSO uses microstrip sen-sors made of different semiconductor materials enabling itto measure radiation from 350 nm (near-UV) up to 5000 nm(mid-IR). Since the emitted ISR is in the visible region of thespectrum, a Si-based microstrip sensor is mounted on KA-LYPSO [11,12]. This KALYPSO system has already beenused in several measurement campaigns. However, throughall these measurements, it could not measure at low lightconditions, i.e. low bunch currents. The system’s sensitivityhad been too low to measure the ISR at bunch currents below0.71 mA (260 pC). This is a significant disadvantage sinceFigure 2: Comparison between the transverse profile mea-sured using standard Si microstrip versus TI-LGAD basedKALYPSO at the same bunch current of 1 mA.the onset of microbunching has been recorded at a thresholdof around 0.2 mA (73 pC) [1]. Hence, it has not been possi-ble to study this region of interest with the previous detectorsystems.Recently, the detector within the KALYPSO systemhas been upgraded by using a TI-LGAD (Trench IsolatedLow Gain Avalanche Detector), designed and fabricated atFBK, Trento [13]. LGADs are similar to the conventionalPN-junction microstrip, except they feature one additionaldopant layer introduced by diffusion under the standard im-plant - called the gain layer. This results in a highly localizedelectric field region around 300 kV cm−1. When biased, thisresults in a controlled avalanche mechanism in the sensor,thus accounting for its internal gain of a few tens (typically10-30). Moreover, LGADs are thin sensors with a thicknessof around 45 µm, hence having a short rise time of the or-der of around 50 ps when combined with suitable readoutelectronics. The trench isolation technique used in the fabri-cation of this sensor allows it to have a strip pitch down to50 µm. Hence, due to its internal gain characteristic, this sen-sor is a wholesome combination of timing resolution, spatialresolution, and sensitivity. The measurements performed bythis detector have shown an immense improvement in sensi-tivity, with the system having a dynamic range of 63.23 dBand SNR (signal-to-noise ratio) by a factor of at least 20.Combing this sensor with the KALYPSO front-end chipmakes an ultra-fast detector with a high dynamic range. Fig-ure 2 shows a comparison of the bunch profile of the ISRat a bunch current of 1 mA acquired with two KALYPSOfamilies at a framerate of 2.7 MHz.KALYPSO enables single-shot measurements to be ac-quired continuously for a long acquisition time. This allowsto study ultra-fast bunch dynamics. A clear signature ofthe microbunching instability (MBI) are short bursts of THzradiation called THz bursting. One example of such measure-ments is shown in Fig. 3. The machine parameters duringthe measurement are reported in Table. 1.The evolution of 80 000 consecutive transverse bunch pro-files were measured each in single-shot, spanning a period of29 ms. The transverse position and size of each bunch havebeen calculated by applying a Gaussian fit to the raw data.The sawtooth bursting seen during the THz bursting due toFigure 3: Incoherent synchrotron radiation measurement acquired with KALYPSO-LGAD. The first row corresponds to theraw data for 80 000 revolutions acquired. The second and third rows show the horizontal position and horizontal bunchsize calculated from a Gaussian fit. The inset shows a zoomed-in perspective of the highlighted red window, depicting thecoherent motion of the electron bunch.Figure 4: A spectrogram for transverse bunch size with logarithmic frequency axis, highlighting both the low frequenciescorresponding to the bursting dynamics and high frequencies corresponding to the synchrotron oscillations and theirharmonics.Table 1: KARA Beam Parameters in Short Bunch ModeUsed During the Transverse Profile Measurements in Fig. 3Energy (E) 1.3 GeVBunch current (𝐼𝑏) 0.1 to 3 mARF frequency ( 𝑓𝑅𝐹 ) 499.744 MHzSynchrotron frequency ( 𝑓𝑠) 7.05 kHzMomentum compaction factor (𝛼𝑐) 3.4 × 10−4RF Voltage (𝑉𝑅𝐹 ) 884.3 kVthe onset of the MBI can be seen in the calculated bunchprofile. The coherent motion of the electron bunch along itscenter of mass, resulting in an oscillation in the longitudinaland transversal position, can also be seen. The frequencyof this oscillation is the synchrotron frequency ( 𝑓𝑠), whichis here calculated from the Fast Fourier transform (FFT) ofthe bunch position and size leading to 7.05 kHz. The re-sult agrees with the beam position monitor measurements(BPM).One further advantage of the upgraded KALYPSO is thepossibility to generate the so-called bursting spectrogramfrom the measured transverse profiles. A bursting spectro-gram is generated by calculating the spectrum of the bunchsize, individually for each bunch current and then combingthem to form one spectrogram [14]. Previously, this hasnot been possible due to the low sensitivity of the detec-tor. These spectrograms have been previously generated forthe THz intensity measurements and have helped to studybursting dynamics, especially along the bursting thresholdof the MBI [14,15]. The transverse bunch profiles (datasetconsists of at least 1 000 000 revolutions) are measured atregular intervals with a decaying bunch current to generatethis spectrogram. A DCCT (Direct Current Current Trans-former) is used to measure the bunch current.The FFT of the calculated bunch size versus the corre-sponding bunch current is displayed in Fig. 4. The leftsection of the figure corresponds to the lower bursting fre-quencies. These frequencies follow a pattern similar to thatobserved in CSR intensity spectrograms. The low burst fre-quencies increase and decrease along with the range of thedecaying bunch currents and vanish slightly at high bunchcurrents. Also visible is the constant synchrotron frequencyof 7.05 kHz that does not vary with beam current. The in-coherent synchrotron frequency, which corresponds to theincoherent motion of individual electrons, is lower than thecoherent synchrotron frequency and deviates to lower fre-quency values with increasing bunch current. The data qual-itatively agrees with the measurements performed with otherdetectors (e.g. Schottky diodes for THz intensity) [3].OUTLOOKThe overall performance of KALYPSO-LGAD has beentested. It has been shown that the system can resolve thebunch profile of the incoherent synchrotron radiation even atvery low bunch charges down to 0.05 mA (18 pC), even be-low the MBI threshold. The experiment has shown thatLGAD is advantageous not only for HEP (High EnergyPhysics) where it is primarily used for their timing reso-lution but also for photon science.ACKNOWLEGMENTSM. M. P. acknowledges the support by the DFG-fundedDoctoral School ”Karlsruhe School of Elementary and As-troparticle Physics: Science and Technology” (KSETA)and funding by the BMBF ErUM-Pro projects STARTRAC(FKZ 05K19VKD) and EVEBUD (FKZ 05K22VKB). Thisproject is partially conducted within the RD50 Collabora-tion.REFERENCES[1] M. Brosi et al., “Systematic studies of the microbunching in-stability at very low bunch charges”, Phys. Rev. Accel. Beams,vol. 22, p. 020701, 2019.doi:10.1103/PhysRevAccelBeams.22.020701[2] M. Brosi et al., “Synchronous Measurements of ElectronBunches Under the Influence of the Microbunching Instabil-ity”, in Proc. IPAC’19, Melbourne, Australia, May 2019, pp.3119–3122. doi:10.18429/JACoW-IPAC2019-WEPTS015[3] J. L. Steinmann et al., “Turn-by-Turn Measurements for Sys-tematic Investigations of the Micro-Bunching Instability”,in Proc. FLS’18, Shanghai, China, Mar. 2018, pp. 46–51.doi:10.18429/JACoW-FLS2018-TUP2WD03[4] S. Funkner et al., “High throughput data streaming of indi-vidual longitudinal electron bunch profiles”, Phys. Rev. Accel.Beams, vol. 22, p. 022801, 2019.doi:10.1103/PhysRevAccelBeams.22.022801[5] M. M. Patil et al., “Ultra-Fast Line-Camera KALYPSO for fs-Laser-Based Electron Beam Diagnostics”, in Proc. 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IPAC’15, Richmond, VA, USA, May 2015, pp. 872–875. doi:10.18429/JACoW-IPAC2015-MOPHA039[11] M. M. Patil et al., “Modern Ultra-Fast Detectors for OnlineBeam Diagnostics”, in Proc. IPAC’21, Campinas, Brazil,May 2021, pp. 4540–4544.doi:10.18429/JACoW-IPAC2021-FRXC03[12] L. Rota et al., “KALYPSO: A Mfps Linear Array Detectorfor Visible to NIR Radiation”, in Proc. IBIC’16, Barcelona,Spain, Sep. 2016, pp. 740–743.doi:10.18429/JACoW-IBIC2016-WEPG46[13] G. Paternoster et al., “Trench-Isolated Low Gain AvalancheDiodes (TI-LGADs)”, IEEE Electron Device Lett., vol. 41,pp. 884–887, 2020. doi:10.1109/led.2020.2991351[14] J. L. Steinmann, Diagnostics of short electron bunches withTHz detectors in particle accelerators, Karlsruhe: KIT Sci-entific Publishing, 2019. ISBN: 9783731508892[15] M. Brosi, “Depth Analysis of the Micro-Bunching Charac-teristics in Single and Multi-Bunch Operation at KARA”,Ph.D. thesis, Karlsruhe Institute of Technology (KIT), 2020.doi:10.5445/IR/1000120018

Status and upgrade of the visible light diagnostics port for energy spread measurements at KARA

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Status and upgrade of the visible light diagnostics port for energy spread measurements at KARA

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