Blomley, E.

Brosi, M.

Bründermann, E.

Kehrer, B.

Müller, A.-S.

Schuh, M.

Schönfeldt, P.

Steinmann, J. L.

English

KITopen

FILLING PATTERN MEASUREMENTS USING DEAD-TIMECORRECTED SINGLE PHOTON COUNTINGB. Kehrer∗, E. Blomley, M. Brosi, E. BründermannP. Schönfeldt, M. Schuh, J. L. Steinmann and A.-S. MüllerKIT, Karlsruhe, GermanyAbstractTime-correlated single photon counting (TCSPC) is aversatile tool for various accelerator diagnostics aspects.Amongst others it allows a precise determination of the fill-ing pattern. At the visible light diagnostics port at the Karl-sruhe Research Accelerator (KARA), the KIT storage ring,a Single-Photon Avalanche Diode (SPAD) in combinationwith a histogramming device (PicoHarp) is used. To com-pensate for possible dead-time effects, a correction schemewas developed and tested successfully. The compensation in-creases the dynamic range in which accurate measurementsare possible and avoids distortion of the measured fillingpattern. This contribution presents the experimental setup,as well as a series of benchmark measurements.INTRODUCTIONA precise determination of the filling pattern in a stor-age ring is important for several reasons. For some time-resolved studies using synchrotron radiation, it must be en-sured that there are no spurious bunches in the storage ring(bunch purity). Another motivation are studies of currentdependent effects in multi-bunch operation, e.g. during themicro-bunching instability. This instability occurs when theelectron bunches are compressed longitudinally until theyemit coherent synchrotron radiation (CSR) and start to in-teract with their own radiation. At KARA this instabilityis studied extensively [1–3], pushing the need for a precisedetermination of the filling pattern.A versatile tool for all kinds of accelerator diagnosticsis the incoherent synchrotron radiation as its intensity isproportional to the emitting charge.TIME-CORRELATED SINGLE PHOTONCOUNTINGThe temporal distribution of the incoherent synchrotronradiation represents the filling pattern and can be used formeasurements in various ways. One possibility is the usageof fast photodetectors and DAQ systems with a sufficientelectrical bandwidth to resolve single bunches. At KARA,the bunch spacing is 2 ns. In this case, averaging over manyturns has to be applied to increase the signal-to-noise ratio.The technique of time-correlated single photon counting(TCSPC) is a cost-efficient alternative [4]. There, the lightintensity is lowered to be able to distinguish between singlephotons. The temporal profile of the light signal is thenreconstructed by histogramming the arrival times of the∗ benjamin.kehrer@kit.edusingle photons relative to a fixed reference signal, e.g. arevolution trigger. Being originally a tool for spectroscopystudies, TCSPC can also be used for precise filling patternmeasurements at synchrotron light sources (e.g. at Diamond[5] and BESSY-II [6]). At KARA, we also make use of thistechnique since 2011 [7] and upgraded our setup stepwisesince then [8].EXPERIMENTAL SETUPOur experimental setup is located at the visible light diag-nostics port [8] where we use synchrotron radiation from adipole magnet. As detector we use a single photon avalanchediode (SPAD) from idQuantique (id100-20 ULN, [9]) and aPicoHarp 300 as histogramming device [10]. We operate theSPAD with a wavelength of 400 nm as for short wavelengthsthe penetration depth of light in silicon is reduced. Thisreduces the probability to create electron-hole pairs outsideof the depletion zone which then diffuse into the depletionzone where they trigger an avalanche after a statistically fluc-tuating delay [11]. As a consequence, the diffusion tail inthe histogram peaks is minimized [8, Fig.2]. In addition, asmall iris lens is mounted directly in front of the detector.Together with the small diameter of the sensor (20 µm), thisleads to a very small angular acceptance. Thus it acts as anoptical discriminator which blocks almost all backgroundlight and allows to operate the setup without any additionalshielding.As KARA has a revolution time of 368 ns, the PicoHarpis operated with a bin width of 8 ps to cover one revolutionwith the available 65536 bins.Data AnalysisThe aim of the data analysis is to determine the relativedistribution of the current on the 184 RF buckets from theraw histogram. The whole data analysis (as well as the dead-time correction scheme discussed in the following section)is written in Python.As first step, the first 368 ns of the histogram are groupedin 184 intervals for the 184 RF buckets. For each interval, thelocal maximum is determined and the bins of the adjacentbins are summed. Monte-Carlo studies showed, that thebest performance is achieved, if the 25 preceding and the 25successive bins are taken into account. This is illustrated inFig. 1.After a normalisation of the 184 values to a sum of 1, thisrelative filling pattern is multiplied with the average beamcurrent measured using a DC current transformer (DCCT)[12] before and after the histogram acquisition. For the error9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW PublishingISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-WEPAL02706 Beam Instrumentation, Controls, Feedback, and Operational AspectsT03 Beam Diagnostics and InstrumentationWEPAL0272219ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.Figure 1: Top: Raw data histogram, Center: Zoom into theraw data histogram (indicated by the grey bar), the light bluearea below the peaks indicates the bins used for integratingthe counts to calculate the filling pattern. Bottom: Integratedcounts for the 184 RF buckets.on the bunch currents, two contributions are assumed: Thefirst is the counting error per bin σi assuming Poisson’sstatistics σi =√ni with ni as number of counts per bin.The second is the error of the beam current measurementfrom the DCCT given by the quadratic sum of the standarddeviation of the beam current and the measurement error ofthe DCCT of 0.5 µA [13].Monte-Carlo studies are used to determine the error onthe bunch currents. There, the data analysis is repeated(a) 1.0 × 105 photons/s (b) 1.3 × 103 photons/sFigure 2: Comparison of two datasets with a high (2a) anda low photon rate (2b). Top: Raw data histograms. Cen-ter: Corresponding bunch currents. Bottom: Bunch currenterrors.many times and the input parameter (raw data histogram andbeam current) are varied according to their error as standarddeviation.Two measurements for the same filling pattern with dif-ferent photon rates are shown in comparison in Fig. 2. Thereduction of the photon rate was achieved by inserting aneutral density filter into the optical beam path.A reduction of the photon rate from 1.0 × 105 to 1.3 ×103 increases the error on the individual bunch currents byapproximately one order of magnitude. Thus, the photon rateshould be as high as possible to achieve a good resolution.On the other hand, a high count rate can lead to a distortionof the measured histogram due to dead-time effects. Tostudy and handle these distortions, a dedicated dead-timecorrection scheme was developed and tested.Dead-Time Correction SchemeBoth, the SPAD and the PicoHarp, have a certain dead-time during which they cannot process incoming events.When a photon is detected by the SPAD and an avalanchehas been triggered and detected, the bias voltage of the pn-junction is reversed to quench the avalanche before it isrestored and the next photon can be detected. This quench-ing takes a certain amount of time in which the detector isblind. The dead-time of the PicoHarp, in which the deviceis insensitive to logical pulses from the SPAD, is definedby its electronics. Both dead-times were measured, for thePicoHarp it is (86.8 ± 0.2) ns while for the SPAD it is (37 ±1) ns.Under certain conditions, the effect of the dead-time in-duced distortion can be estimated from the measured valuesand compensated as well. Based on the ansatz discussedin [14], we adjusted the proposed correction scheme accord-ing to the needs of a synchrotron light source. The schemewas already briefly introduced in [3]:The real number of photons per bin νi can be calculatedto beνi =nii−b∏j=i−nτd, PH(1 − n jN) ≈ ni1 −i−b∑j=i−nτd, PHn jN(1)with the parameters given by Table 1.This method is based on the following assumptions:• The dead-time of the PicoHarp dominates, Monte-Carlo studies showed that the dead-time of the SPADis a second order effect and neglected.• The probability pi = ni/N for an event to be detected inbin i is small (typically 1 × 10−4), allowing to replacethe product in Eq. 1 by a sum. This also leads to ahigher numerical stability as higher order terms areneglected.• There is only one photon emitted per bunch and turnand thus the correction scheme only takes the photonsfrom the previous bunches into account (sum ends ati − 50).9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW PublishingISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-WEPAL027WEPAL0272220ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.06 Beam Instrumentation, Controls, Feedback, and Operational AspectsT03 Beam Diagnostics and InstrumentationTable 1: Parameter for the Dead-Time CorrectionVariable Meaning Valueνi Real number of photons / binni, n j Measured counts / binτd, PH PicoHarp dead-time 86.8 nsnτd, PH Bins per PicoHarp dead-time 10850b Bins per bunch 50TAcq Acquisition time Typically 30 sTRev Revolution time 368 nsN Number of excitation cycles TAcq/TRevTEST MEASUREMENTSTo test the dead-time correction scheme, two histogramswere taken with different photon rates. From the one with thelower rate (1.9×104 counts/s), the reference filling pattern iscalculated. In the histogram with the high count rate (1.2 ×106 counts/s), dead-time effects induce a distortion whichis corrected using the correction scheme discussed above.From the distorted as well as the corrected histograms, fillingpatterns are determined, which are finally compared to thereference pattern. This comparison is illustrated in Fig. 3.There, the top panel shows three filling patterns: The firstone, plotted in blue, is the reference filling pattern recordedFigure 3: Top: Reference (blue), dead-time distorted (green)and dead-time corrected (red) filling pattern. Bottom: Dif-ference between reference and distorted (blue) and referenceand corrected filling pattern (red), respectively. The greyarea depicts the bunch current error of the reference fillingpattern.with a low count rate while the green and the red curve showtwo filling patterns recorded with a high count rate. Forthe red curve, the dead-time correction was applied. Asthe effect of the distortion and the dead-time correction arerelatively small, they are illustrated in the bottom panel inmore detail. It shows the bunch current differences betweenthe reference, distorted and the corrected filling pattern, re-spectively.While the dead-time distortion of the histogram leads tobunch current differences of up to 50 µA, they are reducedsignificantly by the correction to values of below 10 µA. Thecorrected and the reference filling pattern then coincidesmostly within the uncertainty of the reference filling patternwhich is given by the estimated bunch current error usingMonte-Carlo studies. This is indicated by the grey area.This shows that the dead-time correction scheme is ableto reduce the distortion induced by the PicoHarp dead-timesignificantly.SUMMARYThe technique of time-correlated single photon countingis a useful tool for precise measurements of the filling patternof a storage ring. At KARA, we use a setup based on a SPADand a PicoHarp 300 as histogramming device. To preventdistortions of the measured filling pattern due to dead-timeeffects, we successfully implemented and tested a dead-timecorrection scheme. This increases the dynamic range of thesystem e.g. for bunch purity studies as a higher photon ratecan be handled and furthermore it allows a reduction of theacquisition time.ACKNOWLEDGEMENTSEdmund Blomley acknowledges the support by the DFG-funded Doctoral School “Karlsruhe School of Elemen-tary and Astroparticle Physics: Science and Technology(KSETA)“. Miriam Brosi, Patrik Schönfeldt and JohannesSteinmann acknowledge the support by the “Helmholtz In-ternational Research School for Teratronics (HIRST)”.This work has been supported by the German Federal Min-istry of Education and Research (Grant No. 05K16VKA)and the Helmholtz Association (Contract No. VH-NG-320).REFERENCES[1] M. Brosi et al., “Fast mapping of terahertz bursting thresh-olds and characteristics at synchrotron light sources”, Phys.Rev. Accel. Beams 19 (11 Nov. 2016), p. 110701, doi:10.1103/PhysRevAccelBeams.19.110701.[2] A.-S. Müller et al., “Experimental Aspects of CSR in theANKA Storage Ring”, ICFA Beam Dynamics Newsletter 57(2012), pp. 154–165.[3] M. Brosi et al., “Studies of the micro-bunching instabilityin multi-bunch operation at the ANKA storage ring”, Proc’IPAC’17, Copenhagen, Denmark, May 2017, pp. 3645–3648,doi: 10.18429/JACoW-IPAC2017-THOBA1.9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW PublishingISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-WEPAL02706 Beam Instrumentation, Controls, Feedback, and Operational AspectsT03 Beam Diagnostics and InstrumentationWEPAL0272221ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.[4] W. Becker, “Advanced time-correlated single photon countingtechniques”, Springer Science & Business Media, 2005, doi:10.1007/978-3-319-14929-5.[5] C.A. Thomas et al., “Bunch purity measurement for Di-amond”, NIM A, Vol. 566, pp. 762 - 766, 2008, doi:10.1016/j.nima.2006.07.059.[6] K. Holldack et al., “Bunch fill pattern at BESSY monitoredby time-correlated single photon counting”, Proc’ Optics East2007, pp. 677118-1 – 677118-8, doi: 10.1117/12.734226.[7] B. Kehrer et al., “Filling Pattern Measurements at the ANKAStorage Ring”, Proc. IPAC’11, San Sebastian, Spain, Septem-ber 2011, paper TUPC087, pp. 1209 -1211.[8] B. Kehrer et al., “Visible Light Diagnostics at the ANKA Stor-age Ring”, Proc. IPAC’15, Shanghai, China, May 2015, pp.866-868, doi: 10.18429/JACoW-IPAC2015-MOPHA037.[9] idQuantique website: http://idquantique.com.[10] PicoQuant website: http://picoquant.com.[11] S. Cova et al., “Towards picosecond resolution with single-photon avalanche diodes”, Review of Scientific Instruments52, 1981, pp. 408-412, doi: 10.1063/1.1136594.[12] U. Raich, “Beam diagnostics”, CAS - CERN Accel-erator School and KVI: Specialised CAS Courseon Small Accelerators, Zeegse, Netherlands, 2005,doi:10.5170/CERN-2006-012.297.[13] BERGOZ Instrumentation, “Parametric current transformeruser’s manual”, Rev. 3.1, 1998.[14] M. Patting et al., “Dead-time effects in TCSPC data analysis”,Proc. SPIE 6583, Photon Counting Applications, QuantumOptics, and Quantum Cryptography, 658307 (11 May 2007),doi: 1117/12.722804.9th International Particle Accelerator Conference IPAC2018, Vancouver, BC, Canada JACoW PublishingISBN: 978-3-95450-184-7 doi:10.18429/JACoW-IPAC2018-WEPAL027WEPAL0272222ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2018).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.06 Beam Instrumentation, Controls, Feedback, and Operational AspectsT03 Beam Diagnostics and Instrumentation

Filling Pattern Measurements Using Dead-Time Corrected Single Photon Counting

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Filling Pattern Measurements Using Dead-Time Corrected Single Photon Counting

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