Bründermann, E.

Hertle, E.

Hiller, N.

Judin, V.

Müller, A.S.

Schuh, M.

Schönfeldt, P.

Schütze, P.

Steinmann, J.L.

Steinmann, J. L.

Müller, A. S.

KITopen

SPECTRAL ANALYSIS OF MICRO-BUNCHING INSTABILITIES USINGFAST THz DETECTORSJ. L. Steinmann, E. Hertle, N. Hiller, V. Judin, A.-S. Müller, P. Schönfeldt, M. Schuh, P. Schütze,Karlsruhe Institute of Technology (KIT), Karlsruhe, GermanyE. Bründermann∗, Ruhr-Universität Bochum (RUB), Bochum, GermanyAbstractMicro-bunching instabilities occur at synchrotron lightsources when the particle density rises due to compression ofthe electron bunches. They lead to powerful bursts of coher-ent synchrotron radiation (CSR) in the THz range at the costof very unstable intensity and spectral properties, highly fluc-tuating on a millisecond time scale. For interferometry thischanging source demands a long averaging time to achieve areasonably high signal-to-noise ratio or balancing by the useof an additional reference detector. In this study we presentmeasurements taken by a Martin-Puplett-interferometer inthe bursting regime with ultra-fast THz-detectors.INTRODUCTIONANKA Synchrotron Radiation FacilityThe "ANKA Synchrotron Radiation Facility" is a syn-chrotron light source located at the Karlsruhe Institute ofTechnology (KIT) that provides dedicated low-αc operationin single- and multi-bunch mode to users since 2003 [1]. It isa 500MHz ramping machine that can be operated between500MeV and 2.5GeV. For stability reasons and to achievethe shortest bunches, low-αc operation is mostly performedat 1.3GeV. All experiments presented in this paper havebeen carried out at the ANKA-IR1 beamline, a dedicatededge radiation beamline [2].Low-αc Operation at ANKASince the zero-current bunch length is proportional tothe square root of the momentum compaction factor αc ,the bunches can be squeezed by lowering the αc-value viacontrolling the beam optics. Short bunches in the order ofa few pico seconds create CSR for the long wavelengthsin the THz regime. When the emitted electric fields of Nelectrons add up coherently, the radiated power scales withN2. This leads to a power gain by a factor of N, comparedto incoherent synchrotron radiation (ISR). Depending onthe bunch current there are 108 to 1010 electrons in a bunch,which is very beneficial for all users performing experimentsin the long wavelength spectral range.The power gain is partially constrained, because the lowfrequencies in the microwave regime are shielded by theANKA vacuum chamber of 32mm height. The radiatedspectrum of shielded CSR by a Gaussian bunch can be cal-culated [3]. The expected spectra for bunches of differentlength and a bunch current of 1mA ( =ˆ 2.3 × 109 electrons)in the ANKA vacuum chamber are shown in Fig. 1. The plot∗ on leave to KIT10-2210-2010-1810-1610-1410-1210-10Power Spectrum / a.u.100 GHz 1 THz 10 THzFrequency100 µeV 1 meV 10 meV 100 meVPhoton Energy1 10 100 1000Wavenumbers / cm-1 1 ps 3 ps 8.5 ps 12 psFigure 1: Calculated CSR Spectrum for Gaussian buncheswith different bunch length. Low frequencies are shieldedby the ANKA vacuum chamber of 32mm height.shows that for long bunches above 12 ps practically no CSRis observable. However, for shorter bunches an increase ofradiated power by orders of magnitude is observed.Micro-Bunching Instabilities at Low-αcOperationThe bunch compression is limited by micro-bunching in-stabilities that occur at high particle densities. Commonly,ANKA is operated for users in low-αc mode - for stabil-ity reasons - below the so-called bursting-threshold in thesteady-state regime. In this regime the maximum bunchcurrent for stable operation at a given bunch length is lim-ited. However, the unstable bursting regime is of interestfor several reasons: Future low emittance storage rings willbe operated at low values of αc and therefore might have todeal with micro-bunching instabilities. Micro-bunches emithigher frequencies than purely Gaussian-shaped bunches,and, therefore, can provide high power for experiments.Unfortunately, the shape of the micro-bunches is unstableand the radiated power changes rapidly because of a changingspectrum. In Fig. 2 the intensity of a light pulse in theTHz range is plotted over time. Each point represents theamplitude at one revolution. The amplitude of the pulse isbursting almost regularly depending on the bunch current.FAST THz-DETECTORSMost experiments use rather slow but sensitive detectorsand average the underlying bursting signals. Also balanceddetection, where a second detector balances out the instabil-ities, is used [4]. The frequency of the appearing intensitybursts corresponds to hundreds of Hertz and the needed aver-aging time is rather high, thus, the acquisition speed limited.THPME124 Proceedings of IPAC2014, Dresden, GermanyISBN 978-3-95450-132-83530Copyright©2014CC-BY-3.0andbytherespectiveauthors06 Instrumentation, Controls, Feedback & Operational AspectsT03 Beam Diagnostics and Instrumentation1.00.50.0109876543210Time / ms1.00.50.01.00.50.0Pulse Amplitude / V1.5 mA0.7 mA0.47 mAfs =  15 kHzVRF = 4x400 kVFigure 2: Detectors sensitive in the unstable part of the syn-chrotron spectrum observe a changing source power. Fre-quently occurring micro-bunches lead to intensity fluctua-tions, characteristic for different bunch charges.The interest in time-resolved measurements makes the use offast detectors and matched fast electronic readout mandatory.Slowly pulsing light sources such as the new linac-basedTHz light source at KIT named FLUTE [5], operating at arepetition rate of 10 Hz, and synchrotron operation in single-bunch mode can significantly benefit from faster detectors:The average power is low compared to the peak power, sothat the signal-to-noise ratio (SNR) can be improved by or-ders of magnitude, if we measure only during the short lightpulse interval and neglect the noise in-between.Fast Schottky detectors have an adequate SNR even atroom temperature when measuring high power pulses. Weuse ultra-wide-band quasi-optical zero-bias Schottky-barrierdiode detectors from ACST [6]. These room temperaturedetectors are equipped with a log-spiral antenna to receiveradiation between 50GHz and 2 THz. Their speed is limitedby an internal 4GHz amplifier, which allows to resolve con-secutive bunches at ANKA. The peak sensitivity of 450V/Wis reached at 70GHz. The noise equivalent power (NEP) issufficient to detect CSR in the bursting regime, but it is in-sufficient to measure ISR. In addition, the impulse responseis linear.Fast THz detectors are optimized for speed, but their fre-quency response is often not constant. We normalized theresults by the detector’s spectral sensitivity to minimize theeffect of different detector responsivity and to allow com-parison even during micro-bunching, which can alter thespectral intensity distribution significantly.MARTIN-PUPLETT INTERFEROMETRYConventional beam splitters used in Michelson interfer-ometers limit broadband measurements in the THz range,in contrast to a Martin-Puplett interferometer (MPI), whichuses wire-grids as beam splitters that split the beam by polar-ization [7]. If the time resolution of the detector is faster thanthe time scales which correspond to the bursting frequencies,balancing would be desirable to compensate for the unstablesource power.The MPI provides two cross-polarized beams at differ-ent outputs that can be recorded simultaneously and givecomplementary interferograms (IFG), which can enable bal-ancing. By calculating the difference interferogram of thehorizontally IH and vertically IV polarized signals and nor-malization by the sum according to (1), small fluctuationscan be damped and the signal-to-noise ratio improved [8].IB =IH − IVIH + IV(1)BURSTING STUDIES WITH MPIMeasurements have been done in step-scan and rapid-scanmode. While in step-scan mode the movable mirror is drivenin small steps and data is taken at a stable position beforemoving to the next step, in rapid-scan mode the mirror movescontinuously, allowing faster data acquisition. The experi-mental set-up is shown in Fig. 3. A polarizer ensures thatFigure 3: Set-up of the Martin-Puplett interferometer.linearly polarized light enters the interferometer. The beamis divided by a wire-grid beam splitter, reflected at rooftopmirrors and recombined again. Another wire-grid, mountedat 45 degrees, divides the horizontal and vertical polarizedbeam components to illuminate two Schottky detectors.Interferometry in the bursting regime is very challenging.The repetition rate of the bursts is usually between 400 and600Hz. This means that the interferogram is taken overseveral bursts that overlay the interferogram and have to becompensated by balancing.Step-Scan ModeIn step-scan mode data was recorded for several 100msfor both Schottky diodes at every step. Fig. 4 shows the ac-quisition of the interferogram. First, a time signal at a mirrorposition is recorded while the amplitude of the pulse seenby the Schottky diode is measured every turn and recordedover time (Fig.4 top trace). The height of the burst dependson the form factor of the micro-bunches and is not easilypredictable. This height of intensity fluctuations can’t befully compensated by balancing. We Fourier-transformedthe raw signal and determined the amplitude of the mainbursting frequency (usually around 500Hz). This is used asa point for the interferogram corresponding to the currentProceedings of IPAC2014, Dresden, Germany THPME12406 Instrumentation, Controls, Feedback & Operational AspectsT03 Beam Diagnostics and InstrumentationISBN 978-3-95450-132-83531 Copyright©2014CC-BY-3.0andbytherespectiveauthorsmirror position. Then the next point is measured. When thefull interferogram is acquired, it is processed and Fourier-transformed as usual FTIR-interferograms [9] to get thespectrum.0.1 10 FFT / a.u.100  Hz2 3 4 5 61 kHz2 3 4 5 610 kHz2 3 4 5 6100 kHz20015010050Amplitude / mV302520151050Time / ms0.50.0-0.5IFG / a.u.-30 -20 -10 0 10 20 30Mirror path difference / ps12080400FFT of IFG(Power / a.u.)600500400300200100Frequency / GHz Datapoints Box filter length = 5Figure 4: Process to get the interferogram (IFG) and spec-trum during bursting in step-scan mode. For detailed expla-nation see text.Rapid-Scan ModeIn rapid-scan mode, the interferogram is modulated bythe bursts of radiation that lead to an unstable source power.Figure 5 shows a spectrum taken during bursting mode withthe MPI using balancing detection to partially compensatefor the unstable source power. In comparison, a spectrum atthe same machine parameters was recorded using a commer-cial Michelson interferometer (Bruker GmbH, Germany)while averaging 16 scans.250200150100500Intensity Michelson / a.u.2422201816141210864Wavenumbers / cm-1140120100806040200Intensity Martin-Puplett / a.u.700600500400300200Frequency / GHz Michelson; 4.2K Si Bolometer Martin-Puplett; Schottky Diodes Mylar Beam SplitterFigure 5: Spectra taken with Michelson and Martin-Puplettinterferometer at the same beamline and machine settings.Both have been normalized by the beam splitter respectivedetector response function.We used a 50 µm Mylar beam splitter and a 4.2K Sibolometer at the Michelson interferometer and room tem-perature Schottky detectors at the MPI. The Mylar beamsplitter has a limited bandwidth and a zero-pole at approx. 6wavenumbers. For comparison, we normalized the spectraby the beam splitter transfer function, but left out the pointsof the zero-pole location.The spectra are in good agreement, if we consider the lowNEP of fast room temperature Schottky diodes compared toa slow and sensitive bolometer at 4.2K. The bunch length forthis measurement was 8.5 ps. The measured spectrum showsstronger damping for lower frequencies in comparison tothe theoretical simulation (cf. Fig.1) due to a non-optimizedTHz transport in the ANKA-IR1 beamline. For this bunchlength there is no stable CSR visible, only the radiationemitted by micro-bunching instabilities.CONCLUSION AND OUTLOOKWe have shown that interferometry with fast THz detec-tors in the bursting regime is possible and can be used forspectroscopy. The emitted synchrotron radiation spectrum ischanging faster than the available scan speed, so that a fastermethod is preferred to study micro-bunching instabilities.We will use balanced detection with ultra-fast THz detectorsto balance out bursting instabilities on a bunch-by-bunchbasis, which is feasible with the new KAPTURE system(KArlsruhe Pulse Taking and Ultrafast Readout Electron-ics) [10]. It enables us to continuously acquire the signalof up to 4 detectors for every bunch during every revolu-tion. Signal balancing will be performed by an integratedFPGA that drives a balanced low frequency output with dras-tically improved signal-to-noise ratio, which is fed into thefollowing signal processing unit, for example, a spectrometersoftware.ACKNOWLEDGMENTWewould like to thank Y.-L. Mathis and his team from theANKA Infrared-Group for excellent support at the beamlines.Also we acknowledge the fruitful discussions with, and sup-port of, the ANKA THz group. This work is supported bythe Helmholtz International School for Teratronics.REFERENCES[1] A.-S. Müller et al., ICFA Beam Dynamics Newsletter 57,154–165 (2012).[2] Y.-L. Mathis et al., J. Biol. Phys., 29, 313–318 (2003).[3] A. Novokhatski, ICFA Beam Dynamics Newsletter 57, 127-144 (2012).[4] J. Barros et al., Rev. Sci. Instrum. 84, 033102 (2013).[5] M.J. Nasse et al., Rev. Sci. Instrum. 84, 022705 (2013).[6] C. Sydlo et al., Frequenz (2008).[7] E. Bründermann et al., Terahertz Techniques, Springer Seriesin Optical Sciences, Vol. 151 (2012).[8] L. Fröhlich, DESY-THESIS-2005-011 (2005).[9] P. Griffiths, J.A. de Hasseth, Fourier Transform Infrared Spec-trometry (2nd ed.), Wiley-Blackwell, (2007).[10]M. Caselle et al., IPAC’14, Dresden, Germany, THPME113,These Proceedings.THPME124 Proceedings of IPAC2014, Dresden, GermanyISBN 978-3-95450-132-83532Copyright©2014CC-BY-3.0andbytherespectiveauthors06 Instrumentation, Controls, Feedback & Operational AspectsT03 Beam Diagnostics and Instrumentation

Spectral analysis of micro-bunching instabilities using fast THz detectors

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Spectral analysis of micro-bunching instabilities using fast THz detectors

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