In the low-alpha operation mode of the ANKA synchrotron light source, coherent synchrotron radiation (CSR) is emitted from short electron bunches. Depending on the bunch current, the radiation shows bursts of high intensity. These bursts of high intensity THz radiation display a time evolution which can be observed only on long time scales with respect to the revolution period. In addition, long range wake fields can introduce a correlation between the bunches within a bunch train and thus modify the observed behaviour. A novel detection system consisting of an ultra-fast superconducting THz detector and data acquisition system was used to investigate correlations visible on the bursting pattern and to study the interactions of very short pulses in the ANKA storage ring

Balzer, B. M.

Caselle, M.

Hiller, N.

Hofherr, M.

Ilin, K. S.

Judin, V.

Kehrer, B.

Marsching, S.

Müller, A.-S.

Naknaimueang, S.

Nasse, M. J.

Raasch, J.

Scheuring, A.

Schuh, M.

Schwarz, M.

Siegel, M.

Smale, N. J.

Steinmann, Johannes L.

Thoma, P.

Weber, M.

Wuensch, S.

English

KITopen

STUDIES OF BUNCH-BUNCH INTERACTIONS IN THE ANKA STORAGERINGWITH COHERENT SYNCHROTRON RADIATION USING ANULTRA-FAST TERAHERTZ DETECTION SYSTEM∗A.-S- Müller† , V. Judin, M. Balzer, M. Caselle, N. Hiller, M. Hofherr, K.S. Ilin,B. Kehrer, S. Marsching, S. Naknaimueang, M.J. Nasse, J. Raasch, A. Scheuring, M. Schuh,M. Schwarz, M. Siegel, N.J. Smale, J. Steinmann, P. Thoma, M. Weber, S. Wuensch,Karlsruhe Institute of Technology, Karlsruhe, GermanyAbstractIn the low-alpha operation mode of the ANKA syn-chrotron light source, coherent synchrotron radiation(CSR) is emitted from short electron bunches. Depend-ing on the bunch current, the radiation shows bursts of highintensity. These bursts of high intensity THz radiation dis-play a time evolution which can be observed only on longtime scales with respect to the revolution period. In ad-dition, long range wake fields can introduce a correlationbetween the bunches within a bunch train and thus modifythe observed behaviour. A novel detection system consist-ing of an ultra-fast superconducting THz detector and dataacquisition system was used to investigate correlations vis-ible on the bursting pattern and to study the interactions ofvery short pulses in the ANKA storage ring.INTRODUCTIONAt the ANKA storage ring, a special user operation modewith short bunches that emit coherent synchrotron radia-tion (CSR) in the far infrared is offered on regular basissince 2004 [1]. In the course of a constant optimisationprocess to better adapt to the needs of the user commu-nity, continuous studies with CSR are needed to further theunderstanding of the underlying accelerator physics. Thisusually involves a substantial effort in long term data tak-ing. With the new, ultra-fast detection system used forthe studies presented in this paper, however, the processis substantially simplified and accelerated. Online analy-ses and parameter tuning are now well within reach. Avery important aspect of these studies is the analysis of thetemporal behaviour of the emission of bursting coherentsynchrotron radiation (CSR) above the micro bunching in-stability threshold [2, 3]. In previous measurements, clearindication for a correlation between adjacent bunches hasbeen observed [4]. This could be caused, for example, bythe presence of long range longitudinal wake fields as in-dicated in earlier studies [5]. Numerical studies also con-firmed the influence of one bunch on the bursting dynamics∗Work supported by the Initiative and Networking Fund of theHelmholtz Association (Grand No. VH-NG-320), the German Fed-eral Ministry of Education and Research (Grant Nos. 05K10VKC and05K2010), and by the Helmholtz International Research School for Tera-tronics (HIRST)† anke-susanne.mueller@kit.eduFigure 1: Consecutive turns of an inhomogeneous fillingpattern consisting of three trains filled with 33 buncheseach monitored with the YBCO THz detector.of a second bunch [6].DETECTOR AND DATA ACQUISITIONSYSTEMFor the studies presented in this paper, several detectorsystems were used. One such system, the Hot ElectronBolometer (HEB), which has been described in detail else-where [7] is a detector system based on a superconduct-ing NbN ultra-fast bolometer [8] with an intrinsic responsetime of 100 ps to resolve radiation from single bunches. Itis sensitive in the spectral range from 10 to 150 cm−1 (0.3to 4.5 THz) [9].Recently, a breakthrough with respect to response timeswas achieved with a system based on the high-temperaturesuperconductor YBa2Cu3O7−δ (YBCO) due to a very fastelectron energy relaxation time of the order of single pi-coseconds. The new THz detection system allows to mon-itors the CSR THz pulses directly in the time domain.The THz detector operates at high critical temperatures(TC ≈ 84K) in a cryogen-free cooling system which isoperated fully automatically. Fig. 1 shows the YBCO de-tector response to the ANKA synchrotron radiation in theTHz frequency range in the low-αc mode (1.3GeV). Thefilling pattern with three trains consisting of 33 buncheswith a repetition rate of 2.7MHz is clearly visible. Due toa very broad dynamic range of the YBCO THz detector ofmore than 30dB [10] the detector response scales linearlywith the input THz power, thus recording the increase ofemitted THz power from the first to the third train.Proceedings of IPAC2013, Shanghai, China MOPEA01902 Synchrotron Light Sources and FELsA05 Synchrotron Radiation FacilitiesISBN 978-3-95450-122-9109 Copyrightc ○2013byJACoW—ccCreativeCommonsAttribution3.0(CC-BY-3.0)Figure 2: THz signals measured with the HEB detectorsystem and the fast DAQ for fs = 6.25 kHz and calcu-lated bunch length at zero current of about 3.7 ps. The 3trains were filled to cover a wide range of bunch currents(see Fig. 1) with some bunches above and some below thebursting threshold around 0.146 mA.A novel real-time and high accuracy data acquisition sys-tems complements the experimental setup. The system isdescribed in detail in [11]. This system allows the quasi-instantaneous recording of THz signals from all 184 buck-ets in the ANKA storage ring for many consecutive revolu-tions. The first version of the electronics can acquire datain blocks of roughly 10000 turns. Figure 2 shows an exam-ple for a dataset acquired with this system. Knowing thecurrent distribution over the bunches, it is thus possible todetermine the threshold for bursting emission with a singledata block (see Fig. 3). With the new detection and ac-quisition setup it is also possible to analyse each bunch in amulti-bunch filling pattern individually as shown in Fig. 4.BURSTING INSTABILITY AT LOWCURRENTIn single bunch studies, the HEB detector system wasused to study the temporal behaviour of the emission ofsynchrotron radiation down to very low single bunch cur-rents. From Fig. 5 the well-established threshold for thetransition from bursting to steady state emission of CSR[2, 3] can be seen to occur at a current of about 60μAbunch current / mA0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16avg. THz Signal / a. u.-0.100.10.20.30.40.50.60.70.8Instant bursting threshold determinationFigure 3: Quasi-instantaneous determination of the burst-ing threshold based on the dataset displayed in Fig. 2.The observed threshold is slightly below the predictionof 0.146 mA. This shift which might caused by the abovementioned long range wake field interactions.Time / a.u.0 1000 2000 3000 4000 5000 6000 7000 8000amplitude / a. u.-1000100200300400500600700bucket# 22Freq. / kHz0 20 40 60 80 100 120 140Modulation amplitude / a. u.-310-210-110110210310410510bucket# 22Figure 4: Example of an ’extracted’ analysis on bucket No.22 of the dataset displayed in Fig. 2. The upper plot showsthe temporal evolution of the THz signal, the lower plotthe corresponding frequency spectrum. The latter displaysprominent lines at 2 and 5 fs.which is in excellent agreement with the calculated thresh-old value. At very low currents, around 44μA, anothertransition into bursting emission is observed. This could bethe instability described in the bunched beam theory [12].In this work, a local reduction of the threshold current forbursting instability is predicted for a shielding parameter ofχ = σzb√ρb = 0.25 where ρ is the bending radius, b thevacuum chamber height, and σz the bunch length at zerocurrent. For the ANKA storage ring this corresponds to abunch length of 2.02 ps. The bunch length at zero currentfor the data presented in Fig. 5 is 2.0 ps, the bunch lengthextrapolated from streak camera measurements for the on-set of the low current bursting is (2.4±0.5)ps and thereforein excellent agreement with the theoretical prediction.BUNCH-BUNCH CORRELATIONSWith the high data acquisition rates provided by the newDAQ system, a completely different type of analysis be-comes possible for the first time: In order to verify theexistence of long range interactions, for example causedFigure 5: Spectrogram of the turn-by-turn THz signal mea-sured with the HEB detector system as a function of thesingle bunch current with fs = 7.8 kHz. The frequenciesprominently involved in the bursting emission of CSR areclearly seen.MOPEA019 Proceedings of IPAC2013, Shanghai, ChinaISBN 978-3-95450-122-9110Copyrightc ○2013byJACoW—ccCreativeCommonsAttribution3.0(CC-BY-3.0)02 Synchrotron Light Sources and FELsA05 Synchrotron Radiation FacilitiesFigure 6: Quasi-simultaneous measurement of THz signalsfrom all bunches measured with the YBCO detector sys-tem and the fast DAQ for fs = 10.2 kHz. The burstingsignature is clearly visible in all three trains.0 20 40 60 80 100 120 140 160 180020406080100120140160180-0.2-0.100.10.20.30.4bucketbucketFigure 7: Matrix of correlation coefficients obtained ac-cording to Eq.(1) from 17 consecutive datasets similar tothe data shown in Fig. 6.by long range wake fields acting on following bunches, astatistical correlation analysis can be performed. The cor-relation coefficient between two buckets x and y is givenbyρ(x, y) =N∑i=1(xi − x̄)(yi − ȳ)√N∑i=1(xi − x̄)2 ·N∑i=1(yi − ȳ)2(1)where N is the number of acquired revolutions. The av-erage correlation coefficient as a function of the distancebetween bunches is a convenient means to quantify theamount of inter-bunch correlation.To study the influence of the bursting on the bunch-bunch correlation, it is particularly interesting to analysedatasets with all bunches above the bursting threshold (seeFig. 6). 17 consecutive such datasets were used to obtainthe matrix of correlation coefficients displayed in Fig. 7.The symmetry of the ρ(x, y) is clearly visible. The averagecorrelation coefficient as a function of the distance betweenbunches for the same measurements finally are shown inFig. 8. The data exhibit a clear, albeit small, correlation,decreasing with distance from the generating bucket in ac-cordance with earlier, indirect, observations by other meth-ods. The presence of long range wake fields in the ANKAstorage ring is also confirmed by recent measurements withelectro-optic methods [13, 14].0 5 10 15 20 25 30 35-0.0200.020.040.060.080.10.12distance / bucketscorrelation coefficientFigure 8: Correlation coefficients as a function of the dis-tance between the analysed buckets. A clear, albeit small,correlation, decreasing with distance from the generatingbucket, is observed. The curve is a fit of an exponential tothe coefficients to guide the eye.SUMMARYThe detection system consisting of an ultra-fast super-conducting THz detector and data acquisition system pro-vides new possibilities for the study of the dynamics ofshort bunches and CSR emission. Studies with a first ver-sion of the readout electronics allowed to confirm the ex-istence of long range correlations between bunches of onetrain. Furthermore, a sensitive THz detection system wasused to study the temporal behaviour of CSR emission atvery low currents. A bursting instability was observed inexcellent agreement with theoretical predications.ACKNOWLEDGMENTSincere thanks to all who have contributed to the resultspresented above and especially to M. Schmelling for inspir-ing discussions, and to the ANKA-IR group and M. Süpflefor hosting our experiments and for excellent technical sup-port.REFERENCES[1] A.-S. Müller, et al.,PAC’05, Knoxville, RPAE038[2] G. Stupakov and S. Heifets, PRSTAB, 5(5):5–8, 2002[3] Y. Cai, PRSTAB, 14(6):1–12, 2011[4] A.-S. Müller et al, ICFA Beam Dynamics Newsletter No. 57,pages 154–165, April 2012[5] A.-S. Müller et al., EPAC’04, Lucerne, WEPLT069[6] M. Klein, PhD thesis, KIT, 2012[7] A.-S. Müller, et al., PAC’09 Vancouver, TU5RFP027[8] Yu.P. Gousev, et al., J. Appl. Phys., 75:3695, 1994[9] A.D. Semenov, et al., IEEE Transactions on MicrowaveTheory and Techniques, 55(239), 2007[10] P. Probst, et al., Appl. Phys. Lett., 98(4):043504, 2011[11] M. Caselle, et al., WEOBB202, these proceedings[12] Y. Cai, IPAC2011 San Sebastian, FRXAA01[13] B. Kehrer et al., MOPME015, these proceedings[14] N. Hiller et al., MOPME014, these proceedingsProceedings of IPAC2013, Shanghai, China MOPEA01902 Synchrotron Light Sources and FELsA05 Synchrotron Radiation FacilitiesISBN 978-3-95450-122-9111 Copyrightc ○2013byJACoW—ccCreativeCommonsAttribution3.0(CC-BY-3.0)

Studies of Bunch-bunch Interactions in the ANKA Storage Ring with Coherent Synchrotron Radiation using an Ultra-fast Terahertz Detection System

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Studies of Bunch-bunch Interactions in the ANKA Storage Ring with Coherent Synchrotron Radiation using an Ultra-fast Terahertz Detection System

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