Longitudinal coupled-bunch oscillations in the CERN Proton Synchrotron have been studied in the past years and they have been recognized as one of the major challenges to reach the high brightness beam required by the High Luminosity LHC project. In the frame of the LHC Injectors Upgrade project in 2014 a new wide-band Finemet cavity has been installed in the Proton Synchrotron as a part of the coupled-bunch feedback system. To explore the functionality of the Finemet cavity during 2015 a dedicated measurement campaign has been performed. Coupled-bunch oscillations have been excited with the cavity around each harmonic of

the revolution frequency with both a uniform and nominal filling pattern. In the following the measurements procedure and results are presented

Damerau, H.

Migliorati, Mauro

Sterbini, G.

Ventura, Letizia

English

Longitudinal coupled-bunch oscillations in the CERN Proton Synchrotron have been studied in the past years and they have been recognized as one of the major challenges to reach the high brightness beam required by the High Luminosity LHC project. In the frame of the LHC Injectors Upgrade project in 2014 a new wide-band Finemet cavity has been installed in the Proton Synchrotron as a part of the coupled-bunch feedback system. To explore the functionality of the Finemet cavity during 2015 a dedicated measurement campaign has been performed. Coupled-bunch oscillations have been excited with the cavity around each harmonic of the revolution frequency with both a uniform and nominal filling pattern. In the following the measurements procedure and results are presented

Damerau, Heiko

Sterbini, Guido

CERN Document Server

Excitation of Longitudinal Coupled-bunch Oscillations with the Wide-band Cavity in the CERN PS

Longitudinal coupled-bunch oscillations in the CERN Proton Synchrotron have been studied in the past years and they have been recognized as one of the major challenges to reach the high brightness beam required by the High Luminosity LHC project. In the frame of the LHC Injectors Upgrade project in 2014 a new wide-band Finemet cavity has been installed in the Proton Synchrotron as a part of the coupled-bunch feedback system. To explore the functionality of the Finemet cavity during 2015 a dedicated measurement campaign has been performed. Coupled-bunch oscillations have been excited with the cavity around each harmonic of
the revolution frequency with both a uniform and nominal filling pattern. In the following the measurements procedure and results are presented

VENTURA, LETIZIA

MIGLIORATI, Mauro

Archivio della ricerca- Università di Roma La Sapienza

EXCITATION OF LONGITUDINAL COUPLED-BUNCH OSCILLATIONSWITH THEWIDE-BAND CAVITY IN THE CERN PSL. Ventura∗, M. Migliorati, Sapienza Universita’ di Roma and INFN-Roma1, Rome, ItalyH. Damerau, G. Sterbini, CERN, Geneva, SwitzerlandAbstractLongitudinal coupled-bunch oscillations in the CERNProton Synchrotron have been studied in the past years andthey have been recognized as one of the major challengesto reach the high brightness beam required by the HighLuminosity LHC project. In the frame of the LHC InjectorsUpgrade project in 2014 a new wide-band Finemet cavityhas been installed in the Proton Synchrotron as a part of thecoupled-bunch feedback system. To explore the functionalityof the Finemet cavity during 2015 a dedicated measurementcampaign has been performed. Coupled-bunch oscillationshave been excited with the cavity around each harmonic ofthe revolution frequency with both a uniform and nominalfilling pattern. In the following the measurements procedureand results are presented.INTRODUCTIONOne of the major limitations for the LHC-type beams inthe Proton Synchrotron (PS) are longitudinal coupled-bunch(CB) oscillations which are present after transition duringbeam acceleration and at flat-top energy [1]. Two majorissues related to CB instability have been observed: (1) un-equal allocation of charge distribution between the differentbunches caused by the asymmetry of the beam splitting(h = 21→ 42→ 84) at extraction energy; (2) limited repro-ducibility of the bunch length along the batch. Extensivebeam tests were performed with the existing analog CB feed-back system [2] driving a spare acceleration cavity to studythe parameter space in view of the increased intensity forLHC-type beams and to define the requirements for the newfeedback [3]. To cover all possible CB oscillation modessimultaneously a Finemet cavity [4], the first wide-bandcavity in the PS, has been installed together with a new digi-tal low level radio frequency (LLRF) system, based on thehardware developed for the PS 1-turn delay feedback (FB)system [5]. During 2015 machine development studies havebeen performed to commission the new prototype LLRF andthe wide-band longitudinal damper. Both filling patterns,nominal (where only 18 bunches are accelerated in h = 21,leaving a gap of three empty buckets for extraction) anduniform (h = 21 with 21 bunches) have been studied, lowerand upper synchrotron frequency sidebands (LSB, USB)of the revolution frequency, f rev, have been excited, andmultiple scans in frequency, voltage and oscillation modenumber have been performed. In this paper we will presentan overview of the new FB system, as well as the excitationfirmware setup and measurements results.∗ letizia.ventura@uniroma1.itTHE LONGITUDINAL COUPLED-BUNCHFEEDBACKThe design of the new wide-band cavity, driven by solidstate amplifiers, is based on the frequency characteristics ofthe Finemet magnetic alloy. With a frequency range from0.4 − 5.5 MHz and a maximum RF voltage, VRF, of about 5kV, the cavity is ment to damp CB oscillations rather thento target acceleration. It is also the first time that transistorpower amplifiers are operated in the radiation environmentclose to the beam in the PS. Of the 6 available gaps, 4 wereoperational in 2015, with a maximum peak voltage of 3.5kV. Due to the symmetry of the spectral lines of the excitedmodes, a bandwidth of 5 MHz, corresponding to half of theaccelerating frequency, fRF = h f rev, allows to damp all theoscillation modes at h = 21 [6]. A simplified model of thesignal processing implemented is shown in Fig. 1 for oneharmonic.DACCavity drivesin(hCB frevt)cos(hCB frevt)fs side-band filterADCWall current monitorcos(hFBfrevt +φ)sin(hFBfrevt + φ)fs side-band filterAmplitude, VFCsinLow freq. DDSAmplitude, VFCcosExcitation frequency, fexcSidebandselectionFigure 1: Coupled-bunch feedback system signal processingand excitation setup.The beam phase is detected with a wall current monitorand down-converted through a local oscillator at harmonichFB; in order to filter the synchrotron frequency, fs, side-bands, a sharp notch is necessary to separate them fromf rev harmonics, which are attenuated by more than 40 dBcompared to sidebands. Additionally, the phase of the trans-fer function changes by 180◦ at the f rev harmonics. Dueto the symmetry of the beam spectrum the signal can thenbe up-converted at harmonic hCB so that the sum of down-and up-conversion harmonics becomes hFB + hCB = 21, theRF harmonic. In order to explore the functionality of theFinemet cavity, excitation measurements with the feedbackin open loop were necessary. The initial part of the chain(down-conversion and fs filtering) has been substituted byan external perturbation. A low frequency numerical oscil-lator (DDS) in the firmware of the LLRF was used to exciteTUPOR028 Proceedings of IPAC2016, Busan, KoreaISBN 978-3-95450-147-21724Copyright©2016CC-BY-3.0andbytherespectiveauthors05 Beam Dynamics and Electromagnetic FieldsD06 Coherent and Incoherent Instabilities - Measurements and Countermeasuresthe beam with the Finemet cavity by changing parameterslike frequency fexc, voltage VFC and hCB. In the followingwe will report the open loop measurements results.EXCITATION MEASUREMENTS WITHTHEWIDE-BAND CAVITYDuring the measurement campaign we used an intensityof ∼ 1.3 · 1011 protons per bunch (in h = 84 with the nom-inal LHC-beam cycle) and VRF= 165 kV. These result infs ' 390 Hz and f rev ' 476 kHz. After transition crossing(γt = 6.1) the beam has been excited using the setup shownin Fig. 1. After having switched off the excitation, the lon-gitudinal position of the bunches has been recorded usinga 14 bit digitizer with 400 MHz sampling rate. The modeanalysis technique with circulant matrix approach [7] hasbeen adopted to extract the evolution of amplitude and phaseof the dipolar modes.As Sacherer [6], the dipolar CB oscillation modes occurat frequency:f = µ f rev ± fs (1)in the beam spectrum, where µ = 0 . . . h − 1 is the modenumber with the phase advance of 2piµ/nb , with nb num-ber of of bunches. The system illustrated in Fig. 1 allows,through the Finemet cavity, to inject an excitation RF signalat a frequency:fCB = hCB f rev ± fexc (2)where hCB = 0 . . . h − 1, and fexc is the parameter thatallows to select the synchrotron frequency sideband.Frequency Scan, fexcA preliminary set of measurements was necessary to ex-plore the sensitivity of the excitation with the new wide-bandcavity. Keeping the voltage, VFC, constant and choosing aspecific CB oscillation mode, hCB, the excitation frequencyfexc has been scanned.Time [ms]0 50 100 150Mode Amplitude [ns]00.511.522.5fexc = 440 Hz fexc = 390 HzFigure 2: Plot of hCB = 1 mode amplitude versus time fortwo different values of fexc.As expected, for fexc = 0 no excitation is observed sincewe are exciting the beam at the revolution frequency har-monic hCB f rev. During the frequency scan coupled-bunchoscillations appear as fexc approaches the regime of thesynchrotron frequency and actually a beating behavior isdetected for the mode amplitudes as shown in Fig. 2. Twodifferent cases are highlighted:• At fexc = 390 Hz (blue curve in figure), the oscillationmode amplitude grows almost linearly for about 25 msuntil a beating behavior appears due to the changingsynchrotron frequency during acceleration;• At fexc = 440 Hz (red curve in figure), a beating atapproximately fexc − fs is observed.From these observations we can conclude that the systembehaves like a driven harmonic oscillator.Oscillation Mode Scan, hCBDuring the commissioning of the system one importanttest was to verify that the Finemet cavity could excite inde-pendently all the dipolar modes. Figure 3 shows an exampleof the mode spectrum following the excitation of hCB = 16in the case of 21 bunches in h = 21. Lower and uppersidebands have been excited and a mode analysis has beenperformed to obtain the histogram plot which represents themode amplitude at the end of data recording. As expected,the beam spectrum shows a pure excitation of modes 5 and16, respectively.Mode Number , 70 2 4 6 8 10 12 14 16 18 20Mode Amp. [ns]00.511.52Mode Number, 70 2 4 6 8 10 12 14 16 18 20Mode Amp. [ns]00.511.5Figure 3: Mode amplitude for the uniform filling pattern,following the excitation of hCB = 16 LSB (top) and USB(bottom).A summary of the measured mode spectra following thescan of hCB for the USB only, for 21 bunches in h = 21, isshown in Fig. 4. The color coding is referring to the modeamplitude as shown in Fig. 3. The measurements confirmthat all modes can be excited individually [3].On the other hand with a batch of 18 bunches in h = 21,the excitation at fs-sidebands of f rev harmonics does notcorrespond to the excitation of a single CB mode, as visiblein Fig. 5. A spectrum containing several CB modes centeredaround µ = 18/21hCB is observed in this case.In addition to the mode scan, a linear fit on the evolution ofthe amplitude for each mode has been performed during theProceedings of IPAC2016, Busan, Korea TUPOR02805 Beam Dynamics and Electromagnetic FieldsD06 Coherent and Incoherent Instabilities - Measurements and CountermeasuresISBN 978-3-95450-147-21725 Copyright©2016CC-BY-3.0andbytherespectiveauthors0 2 4 6 8 10 12 14 16 18 20Mode Number, 7Excitation harmonic, hCB02468101214161820012Mode Amplitude [ns]Figure 4: Summary of mode scan for 21 bunches in h = 21.On the vertical axis we have the CB mode excited and on thehorizontal axis the mode of oscillations observed followingthe excitation.Mode Number, 70 2 4 6 8 10 12 14 16Excitation harmonic, hCB02468101214161820012Mode Amplitude [ns]Figure 5: Summary of mode scan for 18 bunches in h = 21.On the vertical axis we have the CB mode excited and onthe horizontal axis the CB mode of oscillations observedfollowing the excitation.excitation, and the linear growth rate, α, has been evaluated.The results are summarized in Fig. 6 (bottom plot). Largerα are achieved for modes hCB < 10 which correspond to thefrequencywhere the cavity impedance is larger. Figure 6, topplot, represents the absolute value of the transfer function,|s21 |, of the Finemet cavity.Excitation Voltage Scan, VFCThe linearity of the Finemet cavity and the excitationsystem was verified by measuring the mode growth rate onthe uniform filling patter of 21 bunches on h = 21, as afunction of VFC. The result plotted in Fig. 7 shows that theexcitation is linear.CONCLUSIONAfter its installation in early 2014 an extensive machinedevelopment program has been carried out to qualify the newFinemet wide-band damper cavity and its digital coupled-bunch LLRF FB system. The complete prototype feedbackFrequency [MHz]0 2 4 6 8 10s 21 [dB]-100102010Mode Number, hCB0 5 15 20,[ns/ms]"10246810Figure 6: Absolute part of the Finemet cavity transfer func-tion in logarithmic units (top). Linear growth rate α versusCB oscillation mode (bottom) .VFC [kV]0 0.5 1 1.5 2 2.5 3 3.5,[ns/ms]00.050.10.150.20.25Figure 7: Voltage scan summary: excitation of single har-monic, hCB = 1, and observation of the growth rate fordifferent values of voltage, VFC, from the Finemet cavity.chain of pick-up, digital processing and Finemet kicker hasbeen installed and commissioned. Excitation measurementswith FB in open loop show that the Finemet cavity interactswith the beam as expected and that the different coupled-bunch oscillation modes can be excited individually. Firsttests at the end of 2015 confirm that coupled-bunch oscilla-tions can be damped by the Finemet cavity in closed loop [8].Full test and characterization will be carried out during 2016.ACKNOWLEDGEMENTThe authors acknowledge G. Arduini, S. Gilardoni, M.Haase, M. Paoluzzi, D. Perrelet and Y. Papaphilippou forthe fruitful discussions and the PS OP team for its supportduring the measurement campaign.REFERENCES[1] H. Damerau, S. Hancock, and M. Schokker, “Longitudinal per-formance with high-density beams for the LHC in the CERNPS”, in Proc. of HB 2010, Mohrschach, Switzerland, paperMOPD52, p. 193.TUPOR028 Proceedings of IPAC2016, Busan, KoreaISBN 978-3-95450-147-21726Copyright©2016CC-BY-3.0andbytherespectiveauthors05 Beam Dynamics and Electromagnetic FieldsD06 Coherent and Incoherent Instabilities - Measurements and Countermeasures[2] J.-L. Vallet, “Amortissement des instabilités longitudinales demodes couplés”, unpublished presentation, 2005.[3] H. Damerau et al., “Longitudinal coupled-bunch oscillationstudies in the CERN PS”, in Proc. of IPAC’13, Shanghai,China, paper TUPWA044, p. 1808.[4] M. Paoluzzi, M. Haase et al., “Studies on a wideband, solid-state driven RF system for the CERN PS Booster”, CERN,Geneva, Switzerland, Rep. CERN-ATS-2012-113, 2012.[5] D. Perrelet, “New CERN PS 10 MHz Cavity One-turn Feed-back Hardware and Beam Tests”, in Proc. of LLRF 2013, LakeTahoe, USA.[6] F. Pedersen and F. Sacherer, “Theory and performance of thelongitudinal active damping system for the CERN PS booster”,in Proc. of PAC’77, Chicago, USA, p. 1396.[7] L. Ventura et al., “Simulations and measurements of longitu-dinal coupled-bunch instabilities in the CERN PS”, in Proc.of IPAC’15, Richmond, USA, paper MOPJE072, p. 479.[8] H. Damerau et al., “Signal processing for the coupled-bunchfeedback in the CERN PS”, in Proc. of LLRF 2015, Shanghai,China.Proceedings of IPAC2016, Busan, Korea TUPOR02805 Beam Dynamics and Electromagnetic FieldsD06 Coherent and Incoherent Instabilities - Measurements and CountermeasuresISBN 978-3-95450-147-21727 Copyright©2016CC-BY-3.0andbytherespectiveauthors

Excitation of longitudinal coupled-bunch oscillations with the wide-band cavity in the CERN PS

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Excitation of longitudinal coupled-bunch oscillations with the wide-band cavity in the CERN PS

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