Linnecar, Trevor Paul R

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LIMITATIONS ON LHC BEAM INTENSITY IN THE SPS IN 2000T.Linnecar, CERN, Geneva, SwitzerlandAbstractThe intensity of the LHC beam in the SPS is at presentlimited both by known hardware restrictions and theunexpected phenomenon of the electron cloud. Theformer are progressively being removed by upgrades inthe various systems or will disappear when the machineno longer accelerates leptons. The latter has added newrestrictions. The possibilities for intensity increase in2000 and the compatibility of LHC beam MDs withlepton acceleration are discussed.1  INTRODUCTIONFirst steps were taken to accelerate the LHC batches of84 bunches at the end of 1998. The intensity in thesebatches was low, ~2x1012, and the time during the MDwas spent on adjusting basic parameters such as RF, tuneand chromaticity. During 1999 the intensity in a singlebatch (84 bunches) from the PS and accelerated in theSPS was slowly increased as different limitations due tothe local batch intensity were observed and overcome.The final intensity accelerated in a batch was ~4x1012,which corresponds to half the nominal bunch intensityrequired.At the end of 1999 it was possible to inject andaccelerate three batches from the PS, as is foreseen in thefinal transfer scheme for LHC, and begin to studylimitations coming from total intensity. The maximumintensity accelerated in three batches was ~8x1012,corresponding to ~1/3 of the nominal LHC requirement.Known limitations to the intensity are due toequipment that must be upgraded in performance, such asthe 200 MHz travelling wave (TW) system or thetransverse damping system, and to equipment that will beremoved from the ring when LEP stops and leptons areno longer accelerated in the SPS. To make progress in theMDs it has been necessary to make modifications to someequipment which prevents lepton acceleration - thus someMDs have been incompatible with LEP filling.The increase in intensity has led to an unexpectedlimitation - the electron cloud effect. The electronsproduced affect the electronics of equipment such as thedamper and also cause beam instability. The pressure riseassociated with this effect causes problems for equipmentsuch as the 352 MHz super-conducting RF cavities.In 2000 certain foreseen hardware upgrades will beintroduced. This in principle will allow an increase inintensity. However the electron cloud effect will still existand this may very well be the intensity limitation in thenear future.2. KNOWN LIMITATIONS2.1 Super-conducting cavity limitations frombatch structure and impedanceA) Beam LoadingThe PS beam has a residual modulation at ~8 MHz,Fig.1, due to incomplete debunching of the beam on theflat top at 26 GeV/c. Increasing the debunching timedecreases the modulation.Fig.1 Injected batch of 84 bunches with residualstructure at ~8 MHz.Fig.2 Batch structure at injection with increased de-bunching time in the PS.By chance, this modulation, close to the 9th harmonicof the bunch spacing frequency, produces a line falling inthe bandwidth of the super-conducting (SC) cavities. Thecavities are held at zero voltage by strong RF feedbackand to do this the power amplifier must compensate thebeam-induced voltage. The extra modulation trips thepower amplifiers. Two steps were taken to overcome this. First thecoupling factor to the cavity was decreased from 700,000to 45,000. More beam current can be compensated in thisway but it is no longer possible to produce the highvoltage for lepton acceleration. Hence LEP filling isexcluded. Secondly the debunching time in the PS wasincreased. This reduces the residual structure, Fig.2, but33Chamonix Xlarger emittance bunches, 0.5eVs, are produced due to themicrowave instability in the PS.B) ImpedanceThe beam also becomes unstable at high energy due tothe SC cavity impedance, Fig.3. Active damping is usedto lower the impedance seen by the beam as this is moreeffective than passive damping; both short-delay RFfeedback and one-turn delay feedback are used.Fig.3 352 MHz beam component (linear) along thecycle, 1 s/div. At top energy the beam becomes unstable.The bandwidth of the cavity with feedback is ~400 kHzand this makes the local intensity, averaged over severalPs, important. The initial limit to batch intensity was~5x1012 - equivalent to a fixed target beam of ~5.5x1013.This limit was pushed to more than 5x1012 protons in onebatch by increasing the gain of the single turn loop.Tests were made to try and predict the final intensitylimitation due to the supra impedance. One, two or threebatches were injected into the machine and the gain of theone-turn feedback was decreased until instability wasobserved. The main RF feedback was left at nominalgain; the coupling was 45,000. Extrapolating from theresults of this experiment, the maximum stable intensityfor a nominal gain setting of four is:  No. batches     N/batch   N total1   ~10 x 1012    ~1013      2   ~7.5 x 1012 ~1,5 x 10133   ~6.6 x 1012 ~2 x 1013The gain can probably be increased a further factor 1.5before reaching loop stability limits. There is also thepossibility of increasing beam stability with the 800 MHzLandau system. The SC impedance should not be aproblem next year up to the nominal intensities.3.2 Limitations from the TW cavity impedanceand the local intensity in the batchA one one-turn feedback system is applied around allTW cavities taken together to reduce the impedance seenby the beam. The bandwidth of the cavities makes themsusceptible to the local intensity in the batch and there is astrong transient beam loading voltage induced at thepassage of each batch.  The characteristics of thistransient voltage depend upon the cavity and feedbackfrequency response together. At injection, the inducedvoltage, comparable to and approximately in quadraturewith the required capture voltage, modifies the beamdistribution, which then alters the induced voltage. Above~3x1012 losses occur leading finally to a stable situationafter some 10 ms. A strong ~1 MHz intensity modulationalong the batch is produced. This is shown in Fig.4.Fig.4 Structure along the batch due to the beam/cavitytransient instability at injection.During the year this situation has been improved byraising the bandwidth in the feedback system and also byusing a higher capture voltage (which nonetheless leadsto emittance blow-up from filamentation). The presentlimitation is ~ (5-6)x1012/batch. Next year there will beone feedback per cavity with increased bandwidth, andalso a feed-forward system will be added to each cavity -hopefully this intensity limitation will disappear.Preliminary tests have been made on the feedback / feed-forward combination and these look promising [1].3.3 The damperThe damper is at present limited in bandwidth and doesnot have the final power required to correctly damp theinjection oscillations [2]. These limitations lead toincreased transverse emittance, but do not give particleloss. The main problems associated with the dampercome from the electron cloud [3].4 UNEXPECTED LIMITATIONS - THEELECTRON CLOUD4.1 Transverse damperThe effect of the electron cloud was first observed inDec. '98 when for intensities > 2.5x1012 / batch spurioussignals were seen at the output of the pick-up electronics.It wasn't until the beginning of 1999 that the effect wasfully understood. Fig.5 shows the signal observed fromthe individual plates of the pick-up during the passage ofthe batch. The bandwidth of the sampled signal is nothigh enough to completely resolve each bunch. Above thethreshold intensity for this effect the signal becomesChamonix X34seriously perturbed from about half-way along the batch.At intensities of 5-6x1012, particle loss at the end of thebatch occurs.Fig.5. The signal observed from the two electrodes ofthe damper pick-up during the passage of the batch.Horizontal scale 2 Ps/div.This perturbation comes from the electrons producedby the electron cloud effect. Various techniques weretried to alleviate the problem. Biasing of the plates provedto have a large effect but, as intensity dependent, wasdelicate to use. A solenoidal field of ~100 Gauss,produced by 20 A in a coil of eighty turns wound aroundthe pick-up, protected the pick-up to ~5x1012 /batch byforcing the electrons to travel along the pick-up axis. Theeffect is essentially low frequency and is stronglyperturbing since the electronics works at base-band fromthe lowest betatron line (~kHz). With a bunched beam theinformation repeats at multiples of the bunch spacingfrequency and so going to a higher frequency could solvethe problem. Fig.6 shows the resulting difference signalobtained by demodulation at 200 MHz - a single batch isobserved over five turns.  The bottom trace shows theequivalent perturbed base-band signal.Next year the pick-ups will all be equipped withelectronics working at 120 MHz as this seems theoptimum frequency for signal to noise ratio [3].4.2 Super-conducting CavitiesThe pressure rise (up to 10 times or more according tointensity) leads to interlock trips on the super-conductingcavities.  Fig.7 shows the pressure measured in the cavityas a function of beam intensity during three separate MDs[4]. It is seen that the threshold varies from ~2x1012 to~4x1012, but for all three the pressure rises exponentiallywith the intensity. At 6x1012, the pressure is ~10-8 mbar.Later measurements with 2 and 3 batches in the machineshowed that the pressure increases linearly with thenumber of batches - the gap between the batches stops theexponential rise. The main item of concern is the couplerwhich even with zero volts in the cavity can carry pointsof high voltage at full reflection. Experience has shownthat the maximum pressure that can be safely tolerated is5x10-7 mbar. By extrapolation this will occur for a singlebatch of ~7.5x1012 protons, below nominal. It is possiblethat the coupler will need extra conditioning as thepressure rises and also that the surface of the cavity willbecome contaminated. This latter effect would lower thequality factor leading to a higher RF and cryogenicspower demand at high voltage during lepton acceleration.It is believed that this will not be a serious effect.Fig.6. Difference signal from the damper pick-up.Horizontal scale 100Ps/div. Upper trace signal obtained at200 MHz, bottom trace that obtained at base-band.4.3 Beam StabilityTransverse bunch oscillations have been seen, buildingup in amplitude from near the middle of the batch to theend of the batch [5]. The oscillation is strongest in thevertical plane. In fixed target operation similaroscillations have been observed in the horizontal planebut never in the vertical. The instability occurs atintensities where the pressure rise due to the electroncloud effect is observed.This does not cause particle loss but gives a verticalemittance at top energy four times the injected value forthe highest intensities [6].The damper is at the moment limited in bandwidth to afew MHz. Next year this will be increased to ~12 MHzand finally to >20MHz thus covering all possibleinstability modes. If the mode of instability causingemittance blow-up is dipole then next year some effectfrom the improved bandwidth may be seen as modefrequencies seem to be in the several MHz range.5. MULTI-BATCH LIMITATIONSThe limitations described up to now have been those seenwith a single batch. During the last two MDs of the year itwas possible to increase the total intensity rather than thelocal intensity by injection three batches from the PS.Both the RF and damper systems, as they are atpresent, have a sufficiently wide bandwidth that35Chamonix Ximpedance effects come mainly from the local batchintensity.2 3 4 5 6 7 8 9Beam Intensity E121.  1091.  1081.  107Vac.PressurembarSCC 352 MHzvacp.nb v00.00; 10.1.2000 11:41:1719991006199911011999111921Fig.7 Pressure rise observed in the super-conductingcavities as a function of beam intensity during threeseparate MDsHowever the wake-fields do couple the batches andconsequently the different effects are amplified as batchesare added.Fig.8 shows the result of injecting three batches, eachof intensity ~3.5x1012 into the ring. At the secondinjection, a similar transient phenomenon as observedwith a single batch is observed but it is linked to thestructure on the first batch. Similarly with the thirdinjection the modulation is linked to the second. Theresult is an intensity modulation along the three batches ata frequency this time of ~0.5 MHz.On the other hand, the electron cloud effect, which atthe highest intensities can lead to particle loss at the endof the batch, acts independently on each batch injected.The kicker gap decouples the batches.6. CONCLUSIONSx In 1999 the maximum total intensity in three batchesaccelerated to 450 GeV was ~8x1012, this is one third ofthe nominal LHC requirement.x The maximum intensity accelerated in a single batchwas  ~4x1012 corresponding to a bunch intensity of aboutone half the nominal.x The hardware upgrades should slowly removerestrictions on intensity during 2000.x The electron cloud becomes the main limitation tointensity increase. The damper hardware should be alrightwith the new pick-up electronics but the pressure rise inthe SC cavities may limit the intensity to below the LHCnominal value.x The vertical emittance increase is worrying. Is there aninstability and can the damper cure it?To study higher intensities it is interesting to injectbatches with missing bunches [7], if this restricts thepressure rise as expected. The beam spectrum of thesebatches is sufficiently close to nominal that progress withthe hardware upgrades can be continued.Fig.8. The intensity modulation produced by theinjection of three batches of ~3.5x1012 protons.AcknowledgementsThe limitations summarised here result from the workdone by members of the SL/HRF and SL/OP groups. Iwould like to thank E.Shaposhnikova for helpfuldiscussions.References1. P. Baudrenghien, Reducing the impedance of theTravelling Wave Cavities: feed-forward and one turnfeed-back, these proceedings2. W. Hofle, Active feedback for curing the transverseresistive wall instability in the SPS, these proceedings3. W. Hofle, Observations of the electron cloud effect onpick-up signals in the SPS, these proceedings4. T. Bohl private communication5. G. Arduini, Observations in the SPS: Beam emittance,instabilities, these proceedings6. K.Cornelis, Transverse instabilities, these proceedings7. R. Cappi, Solutions and cures - Missing bunches in thebunch train of the "PS for LHC" beam, these proceedingsChamonix X36

Limitations on LHC beam intensity in the SPS in 2000

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