Brosi, M.

Caselle, M.

Hertle, E.

Hiller, N.

Kopmann, A.

Müller, A.S.

Schwarz, M.

Schönfeldt, P.

Steinmann, J.L.

Weber, M.

English

Müller, A. S.

Steinmann, J. L.

KITopen

ONLINE STUDIES OF THz-RADIATION IN THE BURSTING REGIME ATANKAM. Brosi∗, M. Caselle, E. Hertle, N. Hiller, A. Kopmann, A.-S. Müller, P. Schönfeldt,M. Schwarz, J. L. Steinmann, M. Weber, KIT, Karlsruhe, GermanyAbstractThe ANKA storage ring of the Karlsruhe Institute ofTechnology (KIT) operates in the energy range from 0.5 to2.5GeV and generates brilliant coherent synchrotron radia-tion in the THz range with a dedicated bunch length reducingoptic. The producing of radiation in the so-called THz-gap ischallenging, but this intense THz radiation is very attractivefor certain user experiments. The high degree of compres-sion in this so-called low-alpha optics leads to a complexlongitudinal dynamics of the electron bunches. The resultingmicro-bunching instability leads to time dependent fluctua-tions and strong bursts in the radiated THz power. The studyof these fluctuations in the emitted THz radiation providesinsight into the longitudinal beam dynamics. Fast THz de-tectors combined with KAPTURE, the dedicated KArlsruhePulse Taking and Ultrafast Readout Electronics system de-veloped at KIT, allow the simultaneous measurement of theradiated THz intensity for each bunch individually in a multi-bunch environment. This contribution gives an overviewof the first experience gained using this setup as an onlinediagnostics tool.INTRODUCTIONANKA is a synchrotron radiation source located in Karl-sruhe, Germany, and is operated by the Karlsruhe Instituteof Technology. It consists of a 110.4m long electron storagering, which can operate at energies ranging from 0.5GeVup to 2.5GeV. Beside the standard user operation a specialoperation mode is provided to the research community. Thisso called low-alpha mode allows the reduction of the bunchlength down to a few picoseconds by making use of an adapt-able magnet optics. Additionally, the pattern in which theRF-buckets are filled can be chosen ranging from a singleelectron bunch up to custom filing patterns provided by theBunch-by-Bunch feedback system [1].The short bunch length of a few picoseconds in the low-alpha operation mode leads to an increase in the emittedpower in the lower THz band due to the coherent emissionof synchrotron radiation for wavelengths in the order of orlonger than the emitting structure. Partially, coherent syn-chrotron radiation (CSR) is also emitted for (slightly) shorterwavelength than expected by the bunch length. This hap-pens above a certain bunch current when a modulation of thelongitudinal phase space is induced by the CSR impedance.This effect in the longitudinal particle distribution is calledmicro-bunching instability [2]. Due to the temporally chang-ing/evolving nature of these substructures the emitted power∗ miriam.brosi@kit.eduin the THz regime fluctuates strongly with characteristic fre-quencies. The frequency of the fluctuations depends on thebunch current as well as on the parameters of the longitudi-nal beam dynamics, such as the RF-voltage, the synchrotronfrequency and the momentum compaction factor [3]. Thefluctuations of the radiated power in the THz regime areoften referred to as bursts, while the whole effect is calledbursting. For user experiments relying on stable intensity, itis necessary to know for each machine setting above whichbunch current the micro-bunching instability occurs andbursting starts.To detect and monitor bursting, the peak-intensity of theemitted THz pulse of each bunch at each turn is recordedusing a fast THz detector combined with a dedicated data ac-quisition system. The peak-intensity of each bunch, recordedin one measurement, for approximately 2.7 million consecu-tive turns, is called the bunch’s THz signal in the following.For the measurements described below, a zero-biased quasi-optical broadband Schottky diode detector from ACST [4]was combined with the KArlsruhe Pulse Taking and Ultra-fast Readout Electronics system (KAPTURE) [5], whichwas developed for this purpose. This combination opens upthe possibility to simultaneously measure the THz signal ofeach individual bunch in a multi-bunch fill, over an almostunlimited number of turns (see Fig. 1).Figure 1: The peak-intensity of the THz pulses of all 184RF-buckets is displayed over 130 thousand consecutive turnsand shows strong fluctuations caused by bursting. A typicalmeasurement takes one second and contains 2.7 millionturns. The graph to the right shows the filling pattern.This setup allows us to study the potential influence of amulti-bunch environment on the bursting behavior of eachbunch. Furthermore, if these multi-bunch influences areknown, then the information from all bunches with differentcurrents in a multi-bunch fill can be used to speed up mea-surements like a characterization of different machine set-MOPHA042 Proceedings of IPAC2015, Richmond, VA, USAISBN 978-3-95450-168-7882Copyright©2015CC-BY-3.0andbytherespectiveauthors6: Beam Instrumentation, Controls, Feedback, and Operational AspectsT03 - Beam Diagnostics and Instrumentationtings. Measurements which normally required a full currentdecay over several hours can now be done within seconds.MULTI-BUNCH EFFECTSWith the combination of an ultra-fast THz detector andKAPTURE it is now possible to study potential influencesof a multi-bunch environment on the bursting behavior of abunch. In first measurements the bursting threshold of eachbunch in a multi-bunch fill was determined by measuringthe THz signal of all bunch individually during a full currentdecay. The results showed a small but significant spreadbetween the bursting thresholds of the different bunches.As the spread of the bursting threshold for all bunches ina multi-bunch environment is small, it will be neglected forthe following measurements and only considered as an erroron the overall detected bursting threshold.BURSTING THRESHOLDThe threshold current, below which the micro-bunchinginstability does not occur, is specific to the machine parame-ters. In the following paragraph, a fast measurement princi-ple using a multi-bunch measurement is demonstrated andused in a parameter sweep over the RF-voltage and magnetoptic.Measurement PrincipleThe general principle to determine the bursting thresholdemploys the dependency of the standard deviation of theTHz signal on the bunch current. Previously the mean ofthe signal has been used for this kind of measurements [6].Here we instead propose to use the standard deviation, whichhas proven to be more accurate. The bursting threshold isthen visible as a kink in the standard deviation. The THzsignal is constant below the threshold and with the burstingit starts to fluctuate over the turns, which naturally increasesthe standard deviation.The measurement time is shortened drastically by usingthe information from all bunches, instead of following thedecrease in current for one bunch over time. Consequently,to get useful information over the whole interesting currentrange a tailored filling pattern has to be used. In these mea-surements, the first two trains showed a linear increase /decrease in the bunch current over the bunches (first rising,second falling) and the third train had random current values.The minimal and maximal current were chosen in such a waythat they included all expected thresholds for the differentsettings in the scan. For such a measurement the standarddeviation of the THz signal of each bunch over the corre-sponding bunch current is shown in Fig. 2. The color ofthe points corresponds to the position of the bunch in thetrain. Figure 2 shows that bunches below the threshold at0.2mA show hardly any fluctuations compared to buncheswith higher current.This method was used to quickly map the threshold fordifferent settings of the RF-voltage and the magnet optics.The latter was varied stepwise by changing the current inFigure 2: The calculated standard deviation of the THzsignal of each bunch is plotted over the corresponding bunchcurrent. The current profile was patterned to resample thewhole current range.the quadrupole and sextupole magnets after a predefinedrecipe. For each optics step, the RF-Voltage was then rampedstepwise through all desired values.Figure 3: The standard deviation of the THz signal displayedover bunch current is shown for different settings of themagnet optics (with constant RF-voltage), which result indifferent synchrotron frequencies indicated by color [7].ResultsFigure 3 shows the resulting curves for a sweep over differ-ent magnet optics for constant RF-voltage. The synchrotronfrequency for each optic is indicated by the color of thecurve. A clear decrease of the threshold for a decreasingsynchrotron frequency is visible. This corresponds nicelyto the expected behavior, as a lower synchrotron frequencyindicates a lower momentum compaction factor and, there-fore, a shorter bunch length. For a shorter bunch lengththe critical charge density above which the micro-bunchinginstability still occurs is reached at a lower current.In Fig. 4 the different bursting thresholds of a combinedscan of the RF-voltage and magnet optic are plotted as afunction of the measured synchrotron frequency. The blackpoints corresponds to the measurement shown in Fig. 3where the RF-voltage was kept constant at 4 × 300 kV. Forthe other points, the color indicates the RF-voltage and thedashed lines connect measurements taken at the same mag-net optics. Again, the data follows the expected behavior:The threshold decreases for a reduction of the bunch length,which occurs due to a magnet optic with a lower momentumProceedings of IPAC2015, Richmond, VA, USA MOPHA0426: Beam Instrumentation, Controls, Feedback, and Operational AspectsT03 - Beam Diagnostics and InstrumentationISBN 978-3-95450-168-7883 Copyright©2015CC-BY-3.0andbytherespectiveauthorscompaction factor (indicated by the lower synchrotron fre-quency). The threshold also decreases for an increase of theRF-voltage leading to a shortening of RF-bucket (resultingin a higher synchrotron frequency).Figure 4: The bursting threshold for a combined scan isshown over the synchrotron frequency. The error bars in-clude the error of the algorithm for the automated thresholddetection, the bunch current measurement as well as theexpected spread of the threshold due to multi-bunch effects.BURSTING BEHAVIORNot only can the bursting threshold be determined from ashort multi-bunch measurement with a tailored filling pat-tern, it can also give hints about what bursting behavior isto be expected. Usually, a spectrogram is used to displaythe changes in the bursting occurring during the decay ofthe bunch current. Such a spectrogram shows the Fouriertransform of the THz signal over the bunch current, revealingcurrent ranges which show different bursting behaviors [3].In a short multi-bunch measurement with a tailored fillingpattern, the bunches show all the different bursting behaviorcorresponding to the different current ranges at the sametime. The resulting spectrogram has a limited resolutionon the current axis, due to the limited number of bunchesand hence current points, as shown in Fig. 5. So the currentresolution is traded for the short measurement time of onesecond.Figure 5: The fast Fourier transform of the THz signal ofeach bunch is plotted in rows sorted by the bunch current,resulting in a spectrogram showing the development of thebursting frequencies over the current normally obtained bya time consuming decay of one single bunch.However, the different dominant bursting frequencies andthe bursting regions are still visible and give a good im-pression on the bursting behavior to be expected at theseaccelerator settings.CONCLUSIONUsing the combination of a fast THz detector, like thezero biased quasi-optical broadband Schottky diode, andKAPTURE allows us to monitor all bunches in a multi-bunch fill simultaneously on a turn by turn basis.Multi-bunch fills were used to speed up time consumingmeasurements, such as the characterization of the burstingbehavior for different accelerator settings. It was shownthat it is possible to measure the bursting threshold as wellas a rough spectrogram of the bursting frequencies for onesetting of the RF-voltage and magnet optic within seconds.With this method it is possible to provide a map of the burst-ing threshold and of the behavior for different acceleratorsettings using only a single fill.ACKNOWLEDGMENTWe would like to thank the infrared group at ANKA andin particular M. Süpfle and Y.-L. Mathis for their supportduring the beam times at the IR1 and IR2 beam line. Fur-ther, we would like to thank the ANKA THz group for in-spiring discussions. This work has been supported by theGerman Federal Ministry of Education and Research (GrantNo. 05K13VKA), the Helmholtz Association (Contract No.VH-NG-320) and by the Helmholtz International ResearchSchool for Teratronics (HIRST).REFERENCES[1] E. Hertle et al., "First Results of the New Bunch-by-bunchFeedback System at ANKA", IPAC’14, Dresden, TUPRI074.[2] G. Stupakov and S. Heifets, “Beam instability and microbunch-ing due to coherent synchrotron radiation", Phys. Rev. STAB,2002, Vol. 5, Nr. 5, DOI:10.1103/PhysRevSTAB.5.054402.[3] M. Schwarz et al., “Bursting Patterns of Coherent SynchrotronRadiation in the ANKA Storage Ring”, IPAC’13, Shanghai,WEPEA011.[4] Zero-Bias Schottky Diodes, http://www.acst.de/.[5] M. Caselle et al., “Commissioning of an Ultra-fast Data Acqui-sition System for Coherent Synchrotron Radiation Detection",IPAC’14, Dresden, THPME113.[6] A.-S. Müller et al., "Studies of Bunch-bunch Interactions inthe ANKA Storage Ring with Coherent Synchrotron Radiationusing an Ultra-fast Terahertz Detection System", IPAC’13,Shanghai, MOPEA019.[7] V. Judin et al., "Studies of Bursting CSR in Multi-bunch Oper-ation at the ANKA Storage Ring ", IPAC’14, Dresden, MO-PRO063.MOPHA042 Proceedings of IPAC2015, Richmond, VA, USAISBN 978-3-95450-168-7884Copyright©2015CC-BY-3.0andbytherespectiveauthors6: Beam Instrumentation, Controls, Feedback, and Operational AspectsT03 - Beam Diagnostics and Instrumentation

Online Studies of THz-Radiation in the Bursting Regime at ANKA

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Online Studies of THz-Radiation in the Bursting Regime at ANKA

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