A powerful transverse feedback system ("Damper") has been installed in LHC. It will stabilise coupled bunch instabilities in a frequency range from 3 kHz to 20 MHz and at the same time damp injection oscillations originating from steering errors and injection kicker ripple. The transverse damper can also be used as an exciter for purposes of abort gap cleaning or tune measurement. The power and lowlevel systems layouts are described along with results from the hardware commissioning. The achieved performance is compared with earlier predictions and requirements for injection damping and instability control

Baudrenghien, P

Gorbachev, E V

Höfle, Wolfgang

Killing, F

Kojevnikov, I

Kotzian, G

Lebedev, N I

Louwerse, R

Makarov, A A

Montesinos, E

Rabtsun, S V

Rossi, V

Schokker, M

Thepenier, E

Valuch, D

Zhabitsky, V

CERN Document Server

LHC-PROJECT-REPORT-114804Sep2008P. Baudrenghien, E. V. Gorbachev1), W. Hofle, F. Killing, I. Kojevnikov, G. Kotzian, N. I. Lebedev1), R. Louwerse, A. A. Makarov1), E. Montesinos, S. V. Rabtsun1), V. Rossi,M. Schokker, E. Thepenier, D. Valuch, V. Zhabitsky1)EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsLHC TRANSVERSE FEEDBACK SYSTEM AND ITS HARDWARECOMMISSIONINGA powerful transverse feedback system (“Damper”) has been installed in LHC. It will stabilise coupledbunch instabilities in a frequency range from 3 kHz to 20 MHz and at the same time damp injectionoscillations originating from steering errors and injection kicker ripple. The transverse damper can also beused as an exciter for purposes of abort gap cleaning or tune measurement. The power and lowlevel systemslayouts are described along with results from the hardware commissioning. The achieved performance iscompared with earlier predictions and requirements for injection damping and instability control.Large Hadron Collider ProjectAbstractLHC Project Report 1148CERN,CH-1211 Geneva 23Switzerland1)  JINR, Dubna, RussiaCERN, Geneva, SwitzerlandPresented atEPAC'08, 11th European Particle Accelerator Conference, Genoa, Italy - June 23-27, 2008LHC TRANSVERSE FEEDBACK SYSTEM ANDITS HARDWARE COMMISSIONINGP. Baudrenghien, W. Ho¨fle∗, F. Killing, I. Kojevnikov, G. Kotzian, R. Louwerse, E. Montesinos,V. Rossi, M. Schokker, E. Thepenier, D. Valuch, CERN, Geneva, SwitzerlandE.V. Gorbachev, N.I. Lebedev, A. A. Makarov, S.V. Rabtsun, V.M. Zhabitsky, JINR, Dubna, RussiaAbstractA powerful transverse feedback system (“Damper”) hasbeen installed in LHC. It will stabilise coupled bunch insta-bilities in a frequency range from 3 kHz to 20 MHz and atthe same time damp injection oscillations originating fromsteering errors and injection kicker ripple. The transversedamper can also be used as an exciter for purposes of abortgap cleaning or tune measurement. The power and low-level systems layouts are described along with results fromthe hardware commissioning. The achieved performanceis compared with earlier predictions and requirements forinjection damping and instability control.INTRODUCTIONThe powerful transverse feedback system (“Damper”)for the Large Hadron Collider (LHC) is a joint project ofthe European Organization for Nuclear Research (CERN)and the Joint Institute for Nuclear Research (JINR) [1]. Toa large extent this project is based on the system in theSPS [2] which has operated successfully for many years,facing in recent years, the additional challenge from theelectron cloud effect [3].The LHC will provide high intensity proton and leadion beams. The ultimate intensities after injection into theLHC will be about 4.8 · 1014 particles for the proton beamwith an energy of 450 GeV and 4.1 · 1010 ions for the208Pb82+ beam with an energy of 177 GeV/u. These inten-sities can lead to coherent transverse instabilities. The the-oretical prediction for the instability rise time τ inst, dom-inated at injection energy by the resistive wall effect, isabout 18.5 ms or 208 turns [4] with a significant contri-bution of the LHC collimators at collision energy [5]. TheLHC Damper will stabilize the beam against coupled bunchinstabilities as well as damp the transverse oscillations ofthe beam originating from steering errors and kicker ripple.It will also be used for the purposes of tune measurementsimilar to the SPS system [6] and for abort gap cleaning [7].GENERAL DESCRIPTIONThe LHC Damper has 4 feedback systems on 2 cir-culating beams (one feedback system per beam and plane).Each system is a classical bunch-by-bunch transverse feed-back system (TFS, see Fig. 1) [8]. It consists of 2 pick-ups(PU), 4 damper kickers (DK) and an electronic feedback∗Wolfgang.Hofle@cern.chpath with appropriate signal transmission from PU to DK.The DK corrects the transverse momentum of a bunch inproportion to its displacement from the closed orbit at thePU location. The digital signal processing unit (DSPU) en-sures a resonance condition and an optimal phase advanceψPK to achieve the optimum damping. The mixing of sig-nals from 2 pick-ups allows adjustment of the betatron os-cillation phase advance from the “virtual” PU to the DK toan odd multiple of π/2. The total delay τdelay in the sig-nal processing of the feedback path from PU to DK adjuststhe timing of the signal to match the bunch arrival time. Itequals τPK, the particle flight of time from PU to DK, plusan additional delay of q turns:τdelay = τPK + qTrev , (1)where Trev is the revolution period of a particle in the syn-chrotron. The PU and DK are installed at locations withhigh β-functions. For vertical oscillations in the LHC (seeFig. 1), the delay τdelay is slightly smaller than one beamrevolution period Trev = 88.93 μs and qV = 0. For thehorizontal systems, kicker downstream of the PU, an addi-tional delay of one turn (qH = 1) is added. The delay τdelayis then slightly larger than one turn.The damping time τd = 40 Trev of the LHC feedbackwas chosen to limit the emittance growth due to injectionerrors [9, 10]:Δ=e2inj2σ2F ; F =(1 +τdecτd− τdecτinst)−2, (2)where einj  4 mm at β = 185 m (3.5σ)is the maximumassumed amplitude of a beam deviation from the closedorbit due to displacement and angular errors at injection; σis the initial RMS beam size; τdec  750 Trev = 68 ms isthe assumed decoherence time. These parameters lead toΔ/ < 2.5% the maximum admissible emittance blow-upin the LHC allocated to injection dipole errors [4]. Thus,the LHC TFS gain is g = 2Trev/τd = 0.05 and the overalldamping time 1/τd − 1/τinst of the injection oscillationsbecomes about 50 turns or 4.4 ms.The gain g and the maximum injection error e inj yieldthe maximum deflection θmax = 2 μrad required for theproton beam with energy 450 GeV and the location of thekickers at βK  100 m. The deflection θmax is deliveredby a set of electrostatic kickers with an aperture of 52 mm.The total required deflecting length of 6 m is divided into4 kickers to limit the capacitive loading of the power ampli-fiers. The nominal voltage up to 1 MHz is Vmax ± 7.5 kV.Beam 1Beam 2Outer wall of the LHC tunnelDK3DK2DK1PU1PA2PA3PA4PU2DK4BPUwith 4 ADCsPA1DA8DA2DA6DA3DA4DA7DA5DA1Comb Filter400.8 MHz+_BAHybridDSPU+_BADelaydelayDACVVVVV VH H HH V VH HH H H HV VV V H HPOINT 4Figure 1: Layout of the LHC Damper and block diagram of the transverse feedback system for vertical oscillations.The main instability that the feedback has to handle is theresistive wall instability for which the lowest frequency inthe LHC is about 8 kHz (QH = 64.28 and QV = 59.31). Forpurposes of abort gap cleaning the unwanted beam shouldbe coherently excited at frequencies of about 3 kHz and8 kHz which correspond to the non-integer parts of thetune. Consequently a lower cut-off frequency of 1 kHzwas chosen for the feedback loop. The highest frequencymust be sufficient to damp the dipole mode of two neigh-bouring bunches. For the nominal bunch spacing of 25 nsfmax = 20 MHz. Coherent oscillations at higher frequen-cies are assumed to be suppressed by Landau damping. Thepulse response must cope with the minimum gap betweenbatches in the LHC (995 ns).FEEDBACK ELECTRONICSThe Low Level part consists of two VME modules. Foreach PU a hybrid generates the difference and sum sig-nals (see Fig. 1). Strip-line comb filters generate 400.8MHz wavelets which are then processed by the Beam Po-sition module. Its RF front-end consists of I/Q demodula-tors with the LO at the RF frequency, that produce (I,Q)pairs. After sampling at the bunch synchronous frequencyof 40.08 MHz (16 bit ADCs) the signals, an FPGA calcu-lates the transverse position of each bunch. These signalsare then passed, via a 1 Gbps serial link, to the second VMEmodule, the Digital Signal Processing Unit [11]. This lattermodule receives the transverse position data streams mea-sured in two pick-ups. After deserialization the data areprocessed by an FPGA clocked at 80.16 MHz, with the fol-lowing functionalities: Closed orbit rejection (notch filter),pick-up vector sum, one-turn delay (with 20 ps resolution),phase equalisation of the non-linear response of the ampli-fier and control of the loop bandwith, low pass cut-off at20 MHz. Finally a 14 bit DAC provides the output signal.Both modules include on board memory to acquire criticalsignals for observation and post-mortem.ELECTROSTATIC KICKERSThe electrostatic kicker [1] consists of: a) a vacuum tankof stainless steel 304 L, 1.6 m length, 100 mm internaldiameter and 14 mm wall thickness for optimal shieldingof electromagnetic fields at low frequency and for mechan-ical stability; b) an electrode module with two electrodes(shaped from copper strips as 90◦ arcs) and 3 ceramic-metal rings (metallization by a thin layer of rhenium toevacuate any charges) to hold the electrodes and align ofthe electrode module inside the vacuum tank; c) two highvoltage feedthroughs; d) two couplers capacitively coupledto the electrodes to damp high order modes which can beexcited by the beam and lead to instability. The estimatedFigure 2: Kickers and amplifiers in the LHC tunnel.power loss to each electrode from the circulating ultimatebeam current is 2 W/m. Tests under vacuum have shownthat the temperature reaches 70◦C when the electrodes areheated with 10 W/m [1]. This allows considerable mar-gin for extra heat loss due to electron cloud. Tests of thekickers confirmed their compliance with design specifica-tions. Tolerances on the 100 mm tank internal diameterare in the range of 0 . . . + 0.054 mm, camming actions ofmain flanges (152 mm) do not exceed 0.016 mm, the in-ternal surface smoothness obtained is Ra = 0.4 μm. Stan-dard vacuum cleaning procedures were used with a bake-out limited to < 200◦C due to the copper electrodes. Thepressure limits (S = 30 	/s for hydrogen) range from2·10−10 Torr to 1.7·10−9 Torr for the eight installed kickermodules. These results are better than expectations basedon an outgasing rate of 4 · 10−12 mbar ·	/s and a surfacearea estimated at 2 · 104cm2, with an expected limit pres-sure of 2.6 · 10−9 Torr.POWER AND DRIVER AMPLIFIERS16 power amplifiers (PA) are installed directly under16 electrostatic kickers (DK) in the LHC tunnel on ei-ther side of Point 4 (see Fig. 1 and Fig. 2). Each pairof electrodes is driven in counter phase by one widebandpower amplifier, consisting of two 30 kW grounded cath-ode tetrodes operated in class AB (push-pull). At low fre-quency the amplifier works on a relatively large impedance(∼ 1 kΩ) leading to a large kick voltage. At higherfrequency the capacitance of the kicker plates shunts theimpedance and consequently less kick strength is available.Every power amplifier is driven by two solid state driveramplifiers (DA) operating in class A. The maximum gainof the power amplifiers has been measured to be between39 dB with a RS–2048–CJC tetrode (Thales R©). The poweramplifier is equipped with an RF voltage divider sensingthe tetrode anode RF voltage. The higher order mode cou-plers (HOM) can also be used to measure accurately thevoltage at the kicker plates. The HOM couplers consist ofa 50 Ω vacuum feedthrough with a small plate attached thatcapacitively couples to the kicker deflecting plates. Thecoupling capacitance of 6.4 pF and the 50 Ω loading at theHOM form a high pass with a cut-off of fHP = 50 MHz.The first two higher order modes are at 89 MHz and at176 MHz with an R/Q  2...3 Ω and a damped Q of 256and 110, respectively. The transfer function from kickervoltage to the voltage measured at the HOM port whenloaded with 50 Ω is F = jf/fHP/(1+jf/fHP). Fig. 3 shows0.1 1 10 20-40-30-20-10-30frequency in MHzrelativegainindBFigure 3: Power amplifier frequency characteristics: roll-off of gain for kicker voltage (solid) and tetrode anode volt-age (dashed).the relative gain versus frequency measured at the anode ofthe tetrode (dashed) and at the HOM port, corrected for thehigh pass response (solid). The latter represents the volt-age seen by the beam as a function of frequency. At theanode of the tetrode the gain characteristic exhibits a reso-nance at ∼ 25 MHz caused by the inductance of the con-nection to the kicker and the kicker capacitance. This res-onance (notch in gain curve) is not seen on the kicker volt-age transfer function. The measured phase response usingthe HOM ports and correcting for the high pass character-istics is shown in Fig. 4 together with the phase responseas measured at the anode of the tetrode. Below 3 MHzthe phase responses measured via the HOM ports and onthe anode voltage dividers perfectly match. The mismatchabove 3 MHz is again caused by the resonance. The phaseresponse will be compensated by an FIR in the digital sig-nal processing part by adding phase at higher frequency inorder to achieve an overall linear phase and constant groupdelay [11].Power amplifiers and kickers are now all installed in theLHC tunnel and the system is undergoing extensive test-ing in the run-up for beam commissioning. The designspecifications have all been met, the available peak volt-age, 10.5 kV at up to 100 kHz, has exceeded the designvalue of 7.5 kV, giving a comfortable operational margin.0.1 1 10 20-90-450frequency in MHzphaseresponse(degrees)Figure 4: Phase response of power amplifier kicker ensem-ble, kicker voltage (solid), tetrode anode voltage (dashed).ACKNOWLEDGMENTSThe authors thank T. Linnecar (CERN) for the helpfulassistance and support for the project and the many col-leagues of CERN and LPP (JINR) groups, in particular forvacuum, survey and installation.REFERENCES[1] E. Gorbatchev et al. PAC’01, June 2001, Chicago, Ill. (USA),pp. 1237–1239 (2001).[2] R. Bossart, L. Burnod, J. Gareyte, B. de Raad, V. Rossi,IEEE, Transactions on Nuclear Science, NIM, vol. NS-26,No. 3, pp. 3284–3286 (1979).[3] W. Ho¨fle, Chamonix XI, January 2001, CERN-SL-2001-003-DI, CERN, Geneva, pp. 117–124 (2001).[4] O. S. Bru¨ning et al. (Ed.) The LHC Design Report vol. I,CERN-2004-003, CERN, Geneva (2004).[5] E. Me´tral et al., PAC’07, June 2007, Albuquerque, NM(USA), pp. 2003-2005 (2007).[6] R. Bossart, V. Rossi, SPS-Improvement-Report-No-156,CERN-SPS-ABM-RB-jf, CERN, Geneva (1979).[7] A. Koschik et al., WEPP060, EPAC’08, Genova, June 2008,WEPP060, These Proceedings (2008).[8] L. Vos. IEEE, Transactions on Nuclear Science, NIM A,vol. 391, p. 56 (1996).[9] L. Vos. EPAC ’98, June 1998, Stockholm, pp. 1365–1367(1998).[10] V.M. Zhabitsky. Physics of Particles and Nuclei Letters,JINR, Dubna, 2008, vol. 5, No. 1(143), pp. 49–53 (2008).[11] P. Baudrenghien, G. Kotzian, W. Ho¨fle, V. Rossi, THPC122,EPAC’08, Genova, June 2008, These Proceedings (2008).

LHC Transverse Feedback System and its Hardware Commissioning

LHC Transverse Feedback System and its Hardware Commissioning

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