Signal Quality Of The LHC AC Dipoles And Its Impact On .
BNL-90819-2010-CPSignal quality of the LHC AC dipoles and its impacton beam dynamicsR. MiyamotoBrookhaven National Laboratory Upton, NYM. Cattin, J. Serrano, R.TomasCERN, Geneva SwitzerlandPresented at the First International Particle Accelerator Conference (IPAC'10)Kyoto, JapanMay 23-28, 2010Collider-Accelerator DepartmentBrookhaven National LaboratoryP.O. Box 5000Upton, NY 11973-5000www.bnl.govNotice: This manuscript has been authored by employees of Brookhaven Science Associates, LLC underContract No. DE-AC02-98CH10886 with the U.S. Department of Energy. The publisher by accepting themanuscript for publication acknowledges that the United States Government retains a non-exclusive, paid-up,irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow othersto do so, for United States Government purposes.This preprint is intended for publication in a journal or proceedings. Since changes may be made beforepublication, it may not be cited or reproduced without the author’s permission.
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SIGNAL QUALITY OF THE LHC AC DIPOLES AND ITS IMPACT ONBEAM DYNAMICS R. Miyamoto† , BNL, Upton, NY, USAM. Cattin, J. Serrano, R. Tomas, CERN, Geneva, SwitzerlandAbstractThe adiabaticity of the AC dipole might be compromisedby noise or unwanted frequency components in its signal.An effort has been put to characterize and optimize the signal quality of the LHC AC dipoles. The measured signal isused in realistic simulations in order to evaluate its impacton beam dynamics and to ultimately establish safe marginsfor the operation of the LHC AC dipoles.INTRODUCTIONAn AC dipole produces a sinusoidally oscillating dipolemagnetic field, excites a large sustained transverse motionin a ring, and provides clean signals to beam position monitors (BPMs) for beam optics measurements [1]. Figure 1 isan AC dipole excitation of a 3.5 TeV LHC beam, showingits sustained coherence. Benefit of the AC dipole has beendemonstrated in BNL AGS and RHIC [2, 3, 4], CERN SPS[5], and FNAL Tevatron [6]. If strength of the AC dipoleis adiabatically changed, excitations are produced with nosignificant emittance growth [1, 7, 8, 9], allowing multiple measurements with one beam unlike single-turn kickers. This nondestructive nature is particularly useful for aslow cycled LHC. Total of four AC dipoles (one per transverse plane per beam) have been installed in LHC [10] andused as the primary probe to beam optics above the injection energy [11].x - xmean @mmD1.00.5 Σ0.0-0.5Q, δ Qd Q , (nonlinear) detuning, and chromaticity Q0 . Emittance is also affected by signal quality of theAC dipole. In operations of the AC dipole, we adjust theamplitude and δ (sometimes chromaticity as well) to keepthe emittance growth to a negligible level (a few percents orless) to allow multiple measurements with one beam1 . Operational conditions of the LHC AC dipole was studied indetail [12] but, since then, the top energy has been limitedto 3.5 TeV and we have acquired knowledge of LHC andits AC dipoles. Hence, to assure non destructive operationsof the LHC AC dipoles, we perform detailed studies of theemittance growth due to the AC dipole for the present operational conditions at 3.5 TeV. We also report an effort toimprove the signal quality of the LHC AC dipoles.SIGNAL QUALITY OF LHC AC DIPOLESThe AC dipole magnets in LHC can be also used as twotypes of single-turn kickers. The magnet is connected tothe AC dipole generator and generators of high voltagepulses for the kicker modes through a relay and the relayis controlled by a Programmable Logic Controller (PLC).Originally, the relay was closed with 230 V and 50 Hz ACvoltage provided by the PLC. However, we observed therelay chopped the sine wave of the AC dipole generatorand produced 100 Hz sidebands around the main frequency(3 kHz). To overcome this problem, the relay driver hasbeen modified and now the original 230 V and 50 Hz ACsource is used only for a short time to close the relay andanother 12 DC source maintains it closed2 . Figure 2 showsa schematic of the new relay driver. This solution is adaptedsince the necessary energy to close the relay is more important than that to maintain it closed.-ΣAC dipole generator15002000Revolution numberFigure 1: A typical AC dipole excitation of a 3.5 TeV LHCbeam recorded by one BPM in arc (βx ' 180 m).Relative emittance growth due to one AC dipole excitation is determined by three parameters of the AC dipoleand two machine parameters [8]: number of turns for theAC dipole to reach its maximum strength nr , the excitationamplitude in unit of the initial RMS beam size a/σ, separation of the AC dipole’s driving tune Qd and (machine) tune Thiswork partially supported by the US Department of Energythrough the US LHC Accelerator Research Program (LARP).† miyamoto@bnl.govSolenoid saver230V AC100012V DC500Relay-1.00Control PLCMagnetFigure 2: Schematic of the new relay driver.Figure 3 shows a measured current spectrum of the LHCAC dipole. Here, a sine wave is fitted to the data and the fit1 Obviously, the amplitude must be kept under the aperture too to avoidbeam losses.R2 The new relay driver is called ”Solenoid Saver ” and based on acircuit produced by ROSS ENGINEERING INC.
is subtracted from the data so that we can clearly observeunwanted frequency components and the noise level. Thesampling frequency is chosen to be revolution frequency ofthe LHC beam, frev ' 11 kHz, to observe what is seen bythe beam. We may see that the modification suppresses thecomponents near the main frequency, 3 kHz 0.267frev .The simulations in the next section indicate that influencesof the remaining frequency components are negligible. Thelevel of the noise floor corresponds to RMS white noise ofσnoise 0.74 A in time domain. This is only 0.04% of themaximum current (1.7 kA) and the simulations in the nextsection indicate that the white noise is also below the levelaffecting emittance.Spectrum of Hdata - fitL @AD10Before the modificationAfter the equency @ frevDFigure 3: Measured current spectrum of the LHC ACdipole before and after the modification (main frequencycomponent subtracted). Numbers represent orders of harmonics. The level of the noise floor corresponds to RMSwhite noise of 0.74 A (0.04% of the maximum current).EMITTANCE GROWTH SIMULATIONSAs discussed previously, we must choose proper amplitude and δ to ensure the adiabaticity of the AC dipole.To study margins of amplitude and δ in the current operational conditions, we perform simulations of emittancegrowth. We also study an influence of the signal quality onemittance, based on the measurement in the previous section.In the following simulations, we observe one dimensional motion of ten thousand particles at the location of theAC dipole. No other structure is considered and the mapbetween the AC dipole kicks depends only on tune, synchrotron tune, linear chromaticity, and nonlinear detuning.Table 1 summarizes parameters of our simulation. In thetable, RMS beam size, RMS momentum spread, tune, andsynchrotron tune are the design values. Chromaticity offive units is a typical value at present operations. Detuninghas not been measured at 3.5 TeV yet. The listed value is anestimate from magnet measurements [13]. The amplitudeand δ of the AC dipole are typical values of the presentoperations. The speed of the ramp up and ramp down hasbeen determined by simulations [8, 12] and the noise levelis from measurements in the previous section. We knowthe adiabaticity of the LHC AC dipoles is preserved forthese values by experience. However, for a smaller δ and/or a larger amplitude, we have occasionally observednon-adiabatic behaviors such as beam losses and emittancegrowths.Figure 4 shows the simulated emittance growth as function of δ . In the simulation, the AC dipole field is adjusted depending on δ so that the amplitude is kept to aconstant 1.5σ. Other parameters are fixed to the valueslisted in Table 1. The red data points represent the casewhen tune moves toward driving tune for higher amplitudes(“bad side”) and the blue data points represent the oppositecase (“good side”). Clearly, it is ideal to use the AC dipoleon the good side, but the separation of horizontal and vertical tunes is only 0.01 for the collision lattice of LHC andso that may not be always possible. Three local maximashown in the range δ 0.007 are caused when drivingtune is on one of synchrotron sidebands. We could suppress magnitude of this effect if we could lower chromaticity. We note that δ s of these local maxima are differentfor the good and bad sides due to detuning. Hence, avoiding these δ s is not trivial and we need to know the value ofdetuning. The simulation predicts the emittance growth becomes significant when δ is around 0.006-0.007 and thisagrees to our experimental experiences.100Operation pointTable 1: Parameters for the emittance growth simulation.10RMS beam size in arc [mm]RMS momentum spread (normalized)Fractional tuneSynchrotron tuneChromaticityNonlinear detuning (for 3σ amplitude)0.4251 10 40.310.00195 5 10 4AC dipoleExcitation amplitudeSeparation of Qd and QTurns to ramp up and ramp downRMS noise (w.r.t. the maximum amplitude)Εfin Εini -1LHC (3.5 TeV)10.1Bad sideGood sideAmp 1.5ΣQ’ 5DQH3ΣL 5 10-4Σnoise 0.05%0.010.00110-40.0000.0050.0100.0150.020È È1.5σ0.00622500.05%Figure 4: Simulation of emittance growth vs. δ in a typical condition at 3.5 TeV. “Bad” and “good” sides denotethe direction of detuning. Local maxima are due to synchrotron sidebands.
110010.1Operational region0.01Measured level0.1Εfin Εini -1Εfin Εini -110Bad sideGood sideÈ È 0.006Q’ 5DQH3ΣL 5 10-4Σnoise 0.05%0.01Bad sideGood sideÈ È 0.006Amp 1.5ΣQ’ 5DQH3ΣL 5 10-40.0010.00110-401234Amplitude @ΣDFigure 5: Simulation of emittance growth vs. amplitude ofthe AC dipole excitation.Figure 5 shows the simulated emittance growth for different values of the amplitude. The simulation predicts thatit is necessary to use the AC dipole on the good side to adiabatically produce excitations larger than the current operational range of 1-1.5σ. emittance growth.Figure 6 shows the simulated emittance growth as afunction of the noise level in the AC dipole field. In simulations of the red and blue data points, white noise ofthe given values are added to the pure AC dipole field.Whereas, the black and yellow data points represents simulations where the measured current of the LHC AC dipole,such as one shown in Fig. 3, is implemented to modela more realistic AC dipole field. Although the measuredAC dipole current includes some frequency components inaddition to white noise as shown in Fig. 3, the simulation based on the real signal and that with just white noiseagree. This indicates the emittance growth is mostly due towhite noise. For the bad side, the emittance growth is dominated by the detuning and insensitive to the noise level.On the other hand, for the good side, the emittance growthis within the acceptable level of a few percents even whenthe noise level is 0.4%, which is ten times of the measuredlevel. Hence, the LHC AC dipoles have a large margin inthe noise level.CONCLUSIONSA nondestructive instrument of an AC dipole has beenused as the primary probe to beam optics in LHC. To ensurenondestructive and safe operations of the LHC AC dipoles,the signal degradation caused by the relay of the AC dipolegenerator has been compensated and simulations of emittance growth are performed for the present operational conditions. The simulations indicate that the current conditionshave only small margins, particularly in driving tune, andthis agrees to our experiences. On the other hand, the simulations indicate signal quality of the LHC AC dipoles are anorder of magnitude better than the required level. As shownin our simulations as well as pointed out in [12], detuninghas a large impact on the emittance growth due to the AC10-40.00.20.40.60.81.0Σnoise Imax @%DFigure 6: Simulation of emittance growth vs. noise of theAC dipole field. In simulations of red and blue points,white noise is added to the pure AC dipole field. In simulations of black and orange points, a measured current ofan AC dipole, such as one in Fig 3, is used.dipole. Hence, if we could measure detuning and use it asan input, the presented simulations could be improved.ACKNOWLEDGMENTAuthors would like to thank to E. van der Bij and CERNBE-OP Group for their supports.REFERENCES[1] M. Bai et al., Phys. Rev. E 56, p. 6002 (1997).[2] M. Bai et al., “Measuring Beta Function and Phase Advancein RHIC with AC Dipole”, PAC’03, WPAB072, p. 2204.[3] M. Bai et al., “Measurement of Linear Coupling Resonancein RHIC”, PAC’03, WPAB073, p. 2207.[4] R. Tomás et al., Phys. Rev. ST Accel. Beams 8, 024001(2005).[5] F. Schmidt et al., “Completion of the Sextupole DrivingTerms Measurement at the SPS”, CERN AB Note 2003031-MD, 2003.[6] R. Miyamoto, “Diagnostics of the Fermilab Tevatron Usingan AC Dipole” (VDM Verlag, Germany, 2009).[7] O. Berrig et al., “Excitation of Large Transverse Beam Oscillations without Emittance Blow-up Using the AC-DipolePrinciple”, DIPAC’01, CT07, p. 82.[8] R. Tomás, Phys. Rev. ST Accel. Beams 8, 024401 (2005).[9] R. Miyamoto et al., “Initial Tests of an AC Dipole for theTevatron”, BIW’06, p. 402.[10] J. Serrano and M. Cattin, “The LHC AC Dipole system: anintroduction”, CERN BE Note 2010-14-CO, 2010.[11] R. Tomás, “LHC Optics Model Measurements and Corrections”, these proceedings, TUXMH02.[12] R. Tomás et al., “Reliable Operation of the AC Dipole in theLHC”, EPAC’08, WEPP026, p. 2575.[13] S. Fartoukh and O. Brüning, “Field Quality Specificationfor the LHC Main Dipole Magnets”, CERN LHC ProjectReport 501, 2001.
0 500 1000 1500 2000-1.0-0.5 0.0 0.5 1.0 Revolution number x-x mean @mm D Figure 1: A typical AC dipole excitation of a 3.5TeV LHC beam recorded by one BPM in arc (flx ’ 180m). Relative emittance growth due to one AC dipole exci-tation is determined by three parameters of the AC dipole and two machine parameters [8]: number of turns for the
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