The critical and delicate process of dumping the beams of the LHC requires very low particle densities within the $3 \mu$s of the dump kicker rising edge. High beam population in this so-called 'abort gap' might cause magnet quenches or even damage. Constant refilling due to diffusion processes is expected which will be counter-acted by an active abort gap cleaning system employing the transverse feedback kickers. In order to assess the feasibility and performance of such an abort gap cleaning system, simulations and measurements with beam in the SPS have been performed. Here we report on the results of these studies

Goddard, B

Höfle, Wolfgang

Koschik, A

Kotzian, G

Kramer, Daniel

Kramer, T

CERN Document Server

LHC-PROJECT-REPORT-116005Sep2008A. Koschik, B. Goddard, W. Höfle, G. Kotzian, D. Kramer, T. KramerEUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsABORT GAP CLEANING USING THE TRANSVERSE FEEDBACKSYSTEM: SIMULATION AND MEASUREMENTS IN THE SPS FOR THELHC BEAM DUMP SYSTEMThe critical and delicate process of dumping the beams of the LHC requires very low particle densitieswithin the 3 μs of the dump kicker rising edge. High beam population in this so-called ’abort gap’ mightcause magnet quenches or even damage. Constant refilling due to diffusion processes is expected which willbe counter-acted by an active abort gap cleaning system employing the transverse feedback kickers. In orderto assess the feasibility and performance of such an abort gap cleaning system, simulations andmeasurements with beam in the SPS have been performed. Here we report on the results of these studies.Large Hadron Collider ProjectAbstractLHC Project Report 1160CERN,CH-1211 Geneva 23SwitzerlandCERN, Geneva, SwitzerlandPresented atEPAC'08, 11th European Particle Accelerator Conference, Genoa, Italy - June 23-27, 2008ABORT GAP CLEANING USING THE TRANSVERSE FEEDBACKSYSTEM: SIMULATION AND MEASUREMENTS IN THE SPS FOR THELHC BEAM DUMP SYSTEMA. Koschik∗, B. Goddard, W. Ho¨fle, G. Kotzian, D. Kramer, T. KramerCERN, 1211 Geneva 23, SwitzerlandAbstractThe critical and delicate process of dumping the beamsof the LHC requires very low particle densities within the3μs of the dump kicker rising edge. High beam populationin this so-called ’abort gap’ might cause magnet quenchesor even damage. Constant refilling due to diffusion pro-cesses is expected which will be counter-acted by an activeabort gap cleaning system employing the transverse feed-back kickers. In order to assess the feasibility and perfor-mance of such an abort gap cleaning system, simulationsand measurements with beam in the SPS have been per-formed. Here we report on the results of these studies.INTRODUCTIONAbort gap cleaning has been successfully applied at theTevatron using an electron lens [1] and at RHIC [2,3] usingkick pulses applied to stripline kickers.Also for the LHC beam dump system (LBDS) [4] a so-called beam abort gap is needed, a particle-free gap in thebunch filling scheme which can accommodate the field risetime of the beam dump kicker system, see Fig.1.Particles filling this abort gap would see a sweepingkicker field (3μs rise time) and may be lost at various po-sitions in the machine, possibly causing magnet quenchesor damage. Hence one important aspect is to keep the par-ticle density below a critical level defined by the quenchlimit. Non-negligible intensities can be accumulated in theotherwise particle-free abort gap by re-filling due to longi-tudinal diffusion processes (eg. by intra-beam scattering orRF noise) [5].Therefore an active abort gap cleaning system has beenforeseen for the LHC [6]. A gated signal on the transversefeedback kickers will be used to resonantly excite the parti-cles in the abort gap to large amplitudes and intercept themin a controlled way on the LHC collimators.Previous SPS studies demonstrated a proof of princi-ple [7]. In the present study in the SPS, we performedquantitative measurements of the cleaning time dependenton different beam parameters in order to benchmark simu-lations. Since the LHC is a non-linear machine, particularinterest was put on the dependence on chromaticity ξ anddetuning arising from non-linearities in the machine.In a linear machine the performance of an abort gapcleaning system can be roughly estimated by the averagetime τclean needed to dispose of particles from the abort gap∗ alexander.koschik@cern.chFigure 1: Illustration of the LHC abort gap.τcleanT0 1π |q − ν| ·arcsin[2 hcoll.θˆ√βcoll. βkick.π |q − ν|]. (1)θˆ is the peak kick angle applied by the cleaning pulses,βkick., βcoll. are the β-functions at the kicker and collima-tor position, T0 the revolution time, q < 1 is the tune lineclosest to the excitation frequency and hcoll. is the collima-tor half gap. The cleaning pulses are modulated with a fre-quency f = ν/T0 which should be close to the (fractional)tune of these particles or the corresponding image fre-quency line. The factor 2 accounts for the fact that the aver-age effective angle applied√〈θ2〉 is only half the peak kickθˆ. In the case the excitation is exactly at the tune frequencyline, ν = q, the oscillation amplitude will grow linearly andthe collimator is reached after 2 hcoll./(θˆ√βcoll.βkick.) turns.Eq.(1) assumes a sinusoidal beating of the excitation withthe particle betatron oscillation.In a non-linear machine however, one has to resort toparticle tracking simulations to get an estimate for τclean.Using the SPS as LHC test bed, we carried out a measure-ment and simulation campaign in order to verify the signif-icance of the simulations results.SIMULATION SETUPIn order to reproduce the actual processes of the real ma-chine a full 6D tracking code has to be used. We have cho-sen the latest MADX version as it allows 6D, element-by-element tracking with aperture and the machine descrip-tion of the SPS (and LHC) is readily available in form ofsequence files, strength files and aperture definitions. Op-tics parameters can be varied and changed easily via matchand twiss commands.For the purpose of modelling the feedback system, theMADX code was upgraded by implementing time (ie. turnby turn) varying parameters in the tracking module. A typ-ical simulation run then comprises the following steps:1. Set optics and beam parameters (eg. nominal SPS op-tics, LHC beam & working point);2. Set ξ chromaticity;3. Set k3 octupole strength (non-linearities);4. Set transverse damper excitation program;5. Generate particle distribution (according to givenbeam parameters);6. Track particles over x turns (with time-varying kicksfrom the feedback kicker) and record loss rate, inte-grated loss and loss pattern.No magnetic field errors or misalignment errors weretreated in this study, since we are primarily interested inthe first order (dominant) effects.Given the simple estimation in Eq.(1) the expected ’lin-ear’ cleaning time in the SPS is around 250 turns, as-suming a peak kick angle per turn of θˆ = 2.89μrad,βkicker = 39 m, βcoll. = 81 m and the tune is QV = 26.18.The aperture limit is the TIDV with a half gap size ofhcoll. = 20.4 mm. Simulation results reproduce this be-haviour for this linear case.MEASUREMENT SETUPBeam conditions We used an LHC type beam in the SPS,single bunch (captured) with an intensity of 5.0×1010 pro-tons, at a momentum of 26 GeV with the following mea-sured characteristics:Long. emittance: εn, long. = 0.26 eVs,4σ bunch length = 2.0 ns (3MV);Trans. emittance: εn, trans. = 1.0μm;Tunes: QH = 26.13, QV = 26.18;Chromaticity: ξH = +0.02, ξV = +0.03 (nominal);Coupling: no coupling effects (via tune meas.).An orbit bump of −8 mm at the TIDV was used to con-centrate losses at this location.Tests with up to 4 × 12 bunches at 25 ns bunch spac-ing were also carried out and show an increased cleaningefficiency compared to [7] due to the optimized excitationprograms used.Beam instrumentation Knowledge of the beam condi-tions is of vital importance in order to compare mea-surement with simulation. During the experiments tune,orbit (BPM), intensity (BCT, FastBCT) and loss (BLM)data were taken. Additionally wirescanner and wallcur-rent monitor data for measuring transverse emittance andbunch length were stored. A special beam loss monitor(LHC type), installed on the TIDV, was capable to provideloss data with a 40μs resolution over a period of 1.7 s inpost-mortem operation. A trigger signal from the trans-verse damper system synchronized the start of the BLMacquisition with the start of the damper excitation signalwith a precision of one machine turn.0 10 20 30 40 50 60 70 80 903434.234.434.634.83535.235.435.635.836Time − [ms]Frequency − [kHz]MD 2007/11/05 − Frequency Program  16:23:1017:55:0118:02:3318:13:2218:15:2618:45:0119:07:2319:12:12Figure 2: Illustration of the excitation programs used.Excitation signal Measurements were done in the verti-cal plane. Only one vertical damper V1 (BDV 214.55) wasused, providing a maximum kick angle of 2.89μrad perturn. A programmable arbitrary waveform generator (Ag-ilent 33250A) was used to provide the excitation signal,which was always applied around (1− q)× frev, ie. 35 kHzwith the fractional tune of q = 0.18 and the revolution fre-quency frev = 43.3 kHz at 26 GeV.Different frequency programs were explored, Fig.2:• Fixed frequency;• Swept frequency;• Modulated frequency (FM);• FM on a carrier that is itself swept.The maximum frequency deviation and variation for theswept and modulated cases was adjusted to provide op-timum cleaning efficiency, and relates directly to the fre-quency spread of the particles in the beam.A swept frequency should be the optimum when the tunedepends on the oscillation amplitude, where the excitationfollows the change of average tune of the particle distribu-tion as the oscillation amplitude increases. Therefore mod-ulated programs were introduced to provide optimum exci-tation in synchronization with the longitudinal motion. Thetransverse tune changes with momentum during the syn-chrotron motion, and the synchrotron frequency decreaseswith increasing momentum, hence the idea of sweeping themodulation frequency.A different approach is to use band-limited noise with awidth matched to the tune spread of the particle distribu-tion. This method was not tried in the experiments. It isexpected to provide slower but more regular cleaning in adiffusion-like process. A combination of coherent excita-tion alternated with band limited noise has the potential ofcombining the advantages of both types of excitation.SPS MEASUREMENTSWe measured the cleaning time (= loss time) and clean-ing efficiency (= intensity drop) while varying the verticalchromaticity ξV from +0.03 to +0.93 and the octupolesstrengths k3 of both families (LOD, LOF) in the range of0 to +50 units, see also Figs.3–4.-20 0 20 40 60 80 100012345x 107 Abort gap cleaning MD 2007/11/05 15:05:41time [ms]lost protonsChromaticity low (ξH = +0.02, ξV = +0.03)Octupoles offfixed frequency excitation-20 0 20 40 60 80 100051015x 106 Abort gap cleaning MD 2007/11/05 17:57:33time [ms]lost protonsChromaticity medium (ξH = +0.02, ξV = +0.43)Octupoles ON (k3 = +20.0)optimized frequency sweep-20 0 20 40 60 80 1000246810x 106 Abort gap cleaning MD 2007/11/05 18:16:57time [ms]lost protonsChromaticity high (ξH = +0.02, ξV = +0.93)Octupoles ON (k3 = +20.0)frequency modulationFigure 3: Measured loss rates at TIDV.For an optimized fixed frequency excitation and negli-gible chromaticity (ξV = +0.03), octupoles switched off,we find a cleaning time of 5 ms (220 turns) which corre-sponds again very well to the estimate made with Eq.(1).The maximum cleaning efficiency that we could achievewas 97.3 %. When switching on octupole families as non-linearities the cleaning efficiency drops significantly, whilethe cleaning time is roughly doubled.At medium chromaticity settings of ξV = +0.43 andwith non-linearities (k3 = +20) no cleaning can beachieved with a fixed frequency excitation. With a sweepprogram we can however achieve 99.9 % efficiency. Thecleaning time increases to about 40 ms (1740 turns).We used a frequency modulation program to clean thevery stable beam at a chromaticity setting of ξV = +0.93,octupoles still powered at k3 = +20 units. A cleaningefficiency of 99.7 % was achievable, with the cleaning timein the order of 80 ms (3500 turns).Little dependence of cleaning efficiency and time onthe non-linearity settings (k3, octupoles) was observed, butconsiderable influence from chromaticity settings.The loss pattern around the SPS machine showed signif-icant losses also in other places than the TIDV, but whichfrom the time structure clearly result from the cleaning ef-forts. Modelling the actual orbit and aperture conditions ina simulation is beyond the scope of the present study notpossible, as the orbit from the SPS is not known with highenough precision.SPS SIMULATIONUsing the same optics setting, beam parameters as mea-sured and excitation programs as in the SPS experiments,we ran the MADX simulations. Within an error limit of 10–0 200 400 600 800 1000 1200 1400 16000100200300400500600Abort gap cleaning MD 2007/11/05 18:16:47.498000 SuperCycle 20521Time [ms]Intensity [108 protons]Cleaning←  Efficiency99.7%  BCT signalChromaticity high(ξH = +0.02, ξV = +0.93)Octupoles ON (k3 = +20.0)frequency modulationFigure 4: Measured cleaning efficiency.20 % we can reproduce the measured quantities of cleaningtime and efficiency very well. Only the loss pattern cannotbe reproduced due to aforementioned insufficient knowl-edge of actual orbit and aperture (misalignment) in the ma-chine.It is interesting to note that simulations done beforethe experiments predicted a dominant influence of thechromaticity on the cleaning efforts and less so the non-linearities from octupoles, which was entirely confirmedby the measurements.CONCLUSION AND OUTLOOKUsing the transverse feedback kickers to actively cleanthe abort gap has once more shown to be working well inthe SPS. It should be stressed however, that the measure-ments were done with captured beam and in particular thetune was known very well in order to excite the particles atthe right betatron frequency.In the LHC and especially with uncaptured beam the sit-uation has more unknown variables and the optimization ofthe excitation program will be more difficult. Therefore it isof vital importance to have an abort gap monitor working,to enable a precise measurement of the particle intensity inthe abort gap, their longitudinal distribution and their sus-ceptibility to the cleaning excitation program.The simulations have shown to give very good agree-ment with the SPS measurement results, hence they are amajor tool now for optimizing the abort gap cleaning sys-tem for LHC.REFERENCES[1] Zhang, X.-L. et al. PRST Accel. Beams 11:051002. 2008.[2] Drees, A et al. Abort Gap Studies and Cleaning during RHICHeavy Ion Operation. 20th PAC Conf., 2003, pp.1685.[3] Drees, A et al. Continuous Abort Gap Cleaning at RHIC. 9thEPAC Conference, Lucerne, Switzerland, 2004, pp.908.[4] O. Bru¨ning et al., editors. The LHC Design Report. Chap.17.CERN, Geneva, Switzerland, 2004.[5] E. Shaposhnikova et al. LHC abort gap filling by protonbeam. CERN-LHC-Project-Report-763, Aug 2004.[6] J.B. Jeanneret et al. Beam loss and collimation at LHC. AIPConf. Proc. 693:180–183. 2003. LHC-Project-Report-663.[7] W. Ho¨fle. Experience gained in the SPS for the futureLHC abort gap cleaning. Proceedings of 9th EPAC Conf.,Lucerne, Switzerland, Jul 2004, pp.575.

Abort Gap Cleaning using the Transverse Feedback System: Simulation and Measurements in the SPS for the LHC Beam Dump System

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