Ruggiero, F

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ELECTRON CLOUD IN THE SPSSUMMARYF. Ruggiero, SL Division, CERN, Geneva, Switzerland1 INTRODUCTIONThe session summarised here was devoted to electron cloudeffects in the SPS and LHC. Eight talks were presented,covering recent experimental observations in the SPS withLHC type beams with 25 ns bunch spacing, measurementsof surface properties, simulation results, potential remediesfor the SPS vacuum system, and alternative filling schemesfrom the PS.• The first talk by F. Ruggiero [1] gave a short introduc-tion to some key concepts relevant to the discussionof electron cloud effects and their implications for PS,SPS and LHC operation.• This was followed by a talk given by W. Ho¨fle [2] whoreported about strong perturbations on pick-up sig-nals of the SPS transverse feedback system (damper),observed with LHC type beams and attributed to theelectron cloud effect. He also presented observationswith a solenoid magnetic field and encouraging testswith new electronics, working at a multiple of thebunch frequency.• SPS vacuum observations and, in particular, the de-pendence of the measured strong pressure rise on var-ious beam parameters were reported in the third talkby J.M. Jimenez [3]. Also electron currents collectedby a dedicated, shielded pick-up as well as first indi-cations of some conditioning effect were discussed.• The last talk on SPS experimental observations byG. Arduini [4] focussed on a fast instability, accom-panied by emittance blow-up and beam losses. Theeffect is more pronounced in the horizontal plane andaffects mainly the tail of the batch.• N. Hilleret [5] gave an overview of secondary elec-tron yield (SEY) measurements for pure metals ver-sus technical surfaces, performed by an electron gunand a collector cage, with very low primary electroncurrents. He discussed the effect of oxide layers, ad-sorbed water, and special surface treatments, includ-ing bake-out, freon processing, glow discharge, lowemissivity coatings, electron and photon dosing.• Simulation results for SPS and LHC were presentedby F. Zimmermann [6], who explained how the elec-tron cloud build-up is modelled and what are the po-tential implications for the heat load on the LHC beamscreen and for beam stability in the SPS and LHC.• I.R. Collins [7] discussed possible remedies, includingsurface conditioning by photon scrubbing and electronbombardment.• Finally R. Cappi [8] reported about methods to pro-duce gaps in the bunch train of the LHC beam via RFmanipulations in the PS.2 PHYSICAL MECHANISM OF THEELECTRON CLOUD BUILD-UPIn the LHC, photoelectrons created at the pipe wall are ac-celerated by proton bunches up to 200 eV and cross thepipe in about 5 ns. Depending on the bunch spacing, a sig-nificant fraction of secondary electrons is lost in betweentwo successive bunch passages, but slow secondary elec-trons survive until the next bunch and are again acceleratedup to several keV (see Fig. 1 in Ref. [1]). This non res-onant mechanism may lead to an electron cloud build-upwith implications for beam stability, emittance growth, andheat load on the cold LHC beam screen.The average number of secondary electrons generatedwhen a primary electron hits the pipe wall with a given in-cidence angle depends on the chemical composition of thesurface and on its roughness [5]. It is described by a uni-versal curve characterised by two material parameters: themaximum secondary emission yield δmax and the energyat which the yield is maximum (see Fig. 11 in Ref. [6]).Each bunch passage can be considered as the amplifica-tion stage of a photo-multiplier: a minimum gain is re-quired to compensate for the electron losses and this cor-responds to a critical secondary electron yield δcr, typi-cally around 1.3 for nominal LHC beams (see Fig. 18 inRef. [6]). When δmax > δcr, the electron cloud is ampli-fied at each bunch passage and reaches a saturation value,determined by space charge repulsion.The electrons are not trapped in the proton beam poten-tial, but form a time-dependent cloud extending up to thepipe wall. In field-free regions this cloud is almost uniform,while in the dipole magnets the electrons spiral along thevertical field lines, with typical Larmor radii ranging froma few µm in the LHC to a few hundreds µm in the SPS,and tend to form two stripes at about 1 cm away from thebeam axis, where the average energy gain corresponds tothe maximum SEY (see Figs. 29 and 30 in Ref. [6]). Sincethe vertical dimensions of the LHC beam screen and of theSPS vacuum chamber are very similar, the correspondingcritical SEY is the same for both machines. However themechanism that triggers the electron cloud build-up is dif-ferent. In the LHC at 7 TeV, each proton generates 10−39Chamonix Xphotoelectrons/m, while in the SPS the critical photon en-ergy is not sufficient to create photoelectrons and the pri-mary yield is dominated by ionization of the residual gas:at 10 nTorr the latter is only 10−7 electrons/m per pro-ton [6]. This is one reason why electron cloud effects werenot anticipated in the SPS, the other reason being that it wasreasonable to expect a substantial surface conditioning ofthe SPS vacuum chamber after so many years of operation,especially with leptons (photon scrubbing). However theSPS vacuum chamber is often vented and this may createoxide and/or condensed water layers with high SEY (seeFigs. 5 and 6 in Ref. [5]). In addition, synchrotron radia-tion masks reduce or prevent photon scrubbing.3 ELECTRON CLOUD EFFECTS IN THESPS: EXPERIMENTAL EVIDENCEHere is a rather convincing list of SPS observations sup-porting the conclusion of an electron cloud build-up withLHC type beams:• A similar threshold bunch intensity Nb = 2.5 ÷5 × 1010 is observed for damper pick-up signals, dis-tributed pressure rise and beam instability, when thebunch spacing is 25 ns. This is in good agreement withelectron cloud simulation results, assuming a maxi-mum SEY δmax  1.9 (compare the simulated elec-tron cloud build-up of Fig. 23 in Ref. [6] with thesignals measured at the damper pick-ups, shown inFigs. 4a and 5b of Ref. [2], or with the observed rela-tive pressure rise of Fig. 7 in Ref. [3]).• Ion effects are excluded, since they would depend onthe integrated charge over several bunches, while noeffect is observed with different bunch spacings andthe same total batch intensity [2]. Moreover the ob-served threshold bunch intensity has a weak depen-dence on the residual gas pressure, contrary to ion ef-fects and in agreement with electron cloud simulations(see Fig. 26 in Ref. [6]).• There is direct evidence of negative charge (elec-trons), collected by a dedicated pick-up with a shield-ing grid, correlated with beam intensity and bunchpattern (see Fig. 14 in Ref. [3]).• There is no correlation of the damper pick-up signalswith local orbit, beam losses, or previous operationwith lepton beams.• The threshold intensity for damper pick-up signalsdoubled by applying a 100 Gauss solenoid field.First observations of anomalies in the behaviour of theSPS damper with LHC type beams date back to Septem-ber 1998. In June 1999 the problem persisted and could beattributed to baseline jumps in the signals of all the eightelectrostatic pick-ups, as shown in Figs. 1 and 2 of Ref. [2].This phenomenon occurs only beyond a threshold bunchintensity Nb ∼ 4 ÷ 5 × 1010 of the 2 µs LHC batch with25 ns bunch spacing and the baseline drift starts after thepassage of some 30-40 bunches. No baseline drift was ob-served with 130 ns bunch spacing and Nb ∼ 2.5 × 1011,nor with 5 ns bunch spacing and Nb ∼ 1010, in agreementwith simulation results (see Fig. 28 in Ref. [6]).A modest solenoid field was effective in curing the base-line jumps, but limited to 100 Gauss (20 A) by heatingproblems. Above an LHC batch intensity of 5× 1012 pro-tons, the solenoid field was insufficient. This is qualita-tively understandable in view of the keV energies acquiredby electrons near the beam axis and is in marked contrastto the results of multipacting tests, performed with a multi-wire chamber and a 100 W wide-band amplifier [7]. Inthe last case, the maximum electron energies are limited toabout 100 eV and a week solenoid field completely sup-presses the electron cloud build-up.The observations of the damper pick-up signals showthat the baseline jumps are not present at multiples of the40 MHz bunch frequency. An effective solution to providea clean signal for the damper in the year 2000 is therefore tomix the beam position ∆-signals with a beam synchronousRF reference signal at a multiple of the bunch frequency,for example 120 MHz, down to baseband for further pro-cessing (see Fig. 8 of Ref. [2]).3.1 Do we understand all SPS observations?In spite of the experimental evidence for an electron cloudeffect in the SPS and of the significant agreement withsimulations, there are some observations that still requireanalysis and understanding. For example, the threshold forbaseline jumps in the damper pick-up signals decreased byabout 30% during summer 1999 and then increased duringthe ion run in autumn, going back to its original value ofthe beginning of the 1999 run. Also the influence of a biasvoltage applied to the pick-up electrodes is not completelyunderstood, since the optimum voltage changed from dayto day [2].Moreover, violent baseline jumps in the damper pick-upsignals are observed at injection or when the beam is trans-versely kicked. The reason is unclear, although there maybe some relation with the narrow vertical strips shown byelectron cloud simulations in the dipole magnets. A tenta-tive conclusion is that transverse RF beam shaking duringsurface conditioning might be beneficial and improve theuniformity of the beam scrubbed surface.A fast horizontal instability with a rise time of 20-25turns is observed in the SPS above a threshold LHC batchintensity of 4× 1012 protons (see Fig. 10 of Ref. [4]). Theobservations have been performed with couplers equippedby a 200 MHz receiver, not affected by the electron cloudinduced baseline distortions of electrostatic pick-ups, andbeam oscillations have been monitored at six consecutiveslices along the batch. The instability mainly affects thebatch tail, saturates and leads to emittance blow-up andbeam losses. There is a slower instability also in the ver-tical plane, possibly associated with non-optimum adjust-ment of the transverse feedback gain. Vertical oscillationsChamonix X10in the range 400 to 800 MHz are observed with a verti-cal wide-band pick-up and may be associated with singlebunch activity. This fast instability might be interpretedas a single bunch, beam break-up instability caused by theshort range wakefield in the electron plasma; preliminaryestimates [6] assuming an electron cloud density of 1011electrons/m3 lead to a rise time of 500 µs, or 20 turns, veryclose to the observed instability rise time. If this instabilitymechanism is confirmed, it can not be easily cured by thedamper. On the other hand it should disappear after surfaceconditioning, since the electron cloud build-up would thenbe suppressed.3.2 Cures against the electron cloud build-upLaboratory measurements indicate that electron bombard-ment at a few hundred eV is one of the most effective meansto reduce the SEY of a technical surface. For example anelectron dose of about 1 mC/mm2 leads to a significant re-duction of δmax and to a positive shift of the energy cor-responding to the maximum yield (see Fig. 16 of Ref. [5]).An electron dose of 5×10−5 C/mm2 is the observable limitfor surface conditioning [7]. With the present 5% dutycycle, an observable surface conditioning by the electroncloud bombardment is expected within a 12 hour runningperiod: this prediction seems in agreement with SPS vac-uum observations (see Fig. 10 of Ref. [3]), although onlythree gauges out of twenty show a modest pressure de-crease between 7 and 13%. It appears that a substantiallyhigher duty cycle will be needed in the year 2000 for sur-face conditioning.Other possible remedies against the electron cloud build-up include clearing electrodes, plasma conditioning withmemory effect (e.g. with Freon11), ex-situ coating withlow emissivity materials (e.g. TiN or TiZrV) and ex-situ orin-situ bake-out [7]. The latter is not practical for the SPSdipole vacuum chambers.An alternative solution is to produce gaps in the LHCbatch by means of RF manipulations in the PS [8]. Forexample, starting from seven PS Booster bunches and ap-plying three subsequent bunch splittings yields a modifiedLHC bunch train consisting of 56 bunches with gaps offour missing bunches every eight bunches. As shown bythe simulation of Fig. 39 in Ref. [6], the electron cloudbuild-up would be suppressed for a maximum SEY of 1.5.For a higher maximum SEY of 1.9, one should resortto a reduced fill pattern consisting of a sequence of fourLHC bunches followed by four missing bunches, etc. (seeFig. 41 in Ref. [6]. Alternatively the bunch spacing couldbe 50 ns and the bunch intensity√2 higher than the nom-inal intensity. This solution would still provide the designluminosity to three of the four LHC experiments, but theevent multiplicity would be doubled. However two beamswith 50 ns bunch spacing would not collide at IP8, sinceLHCb is longitudinally displaced by 3/2 of an RF bucket.Satellite bunches with a few per cent of the nominal inten-sity could then be added at the empty nominal positions toprovide the required low luminosity. As shown in Fig. 37of Ref. [6], there would still be suppression of the electroncloud build-up.4 DISCUSSION AND CONCLUSIONSDuring the discussion, F. Caspers pointed out that onephysical mechanism not included in the simulations is thepossible ‘magnetron effect’ associated with the electron cy-clotron motion in the magnetic field of the dipoles. Thelatter has a frequency of about 28 GHz/Tesla and, underspecial resonance conditions, may give rise to a coherentRF modulation of the beam. The incoherent radiation dueto the electron cyclotron motion can not exceed the kineticenergy of the electron cloud and is therefore already in-cluded in the estimated heat load on the LHC beam screen.The electron cloud build-up is a single pass effect and, asshown by the SPS experience, it can be triggered by a fewprimary ionization electrons. Therefore, for a maximumSEY above the critical value, the build-up may take placealso in the transfer lines and in the LHC at injection.4.1 Heat load on the LHC beam screenAn effective solution to reduce the maximum SEY belowits critical value is to condition the LHC screen surface, ei-ther by synchrotron radiation photons or using electrons [5]accelerated by a special proton beam, with increased bunchspacing or weak satellite bunches, thus clearing slow sec-ondary electrons. To keep the heat load due to electroncloud build-up in the LHC dipoles within the cooling ca-pacity of the cryogenic system (about 1 W/m per beam), itis foreseen a beam screen with ribbed surface and reducedreflectivity [9]. For example, with a reflectivity of 10%and nominal LHC beam parameters, the heat load in thedipoles goes from 5 W/m for an initial δmax = 2.3 downto 42 mW/m for a final δmax = 1.1, after surface condi-tioning (see Table 2 in Ref. [6]). It is clear that the nominalLHC beam can not be used during the initial conditioningphase, since the cryogenic limit would be largely exceeded.However, one can control the electron cloud build-up andoperate the LHC with a special ‘conditioning beam’. As-suming a tolerable heat load around 200 mW/m, the beamscrubbing time would be about 35 hours.To clarify the relative merits of a dipole screen with lowreflectivity, let me recall that synchrotron radiation photonstravel practically in synchronism with the proton bunchfrom which they are emitted, even after several reflections.These photons hit the LHC beam screen along a strip ex-tending a few mm above and below the horizontal plane.Photoelectrons created near the horizontal plane are harm-less in the dipole magnets, since their effective accelerationdepends only on the small vertical component of the beamelectric field. For low screen reflectivity only a few pho-tons are scattered away from the horizontal plane, towardsthe (flat) top and bottom regions, where they can create‘dangerous’ photoelectrons and contribute to the heat load.A low screen reflectivity does not significantly reduce the11Chamonix Xheat load unless δmax < δcr, as shown by the simulationin Fig. 32 of Ref. [6]. On the other hand, a low screenreflectivity may have the drawback that photon scrubbingtakes longer to condition the most sensitive, top and bottomregions.4.2 SPS strategy for the year 2000 and needsfor future MD’sIn conclusion, I report here a list of recommendations andcomments to study, understand and master electron cloudeffects in the SPS during the year 2000. Before the SPSstart-up:• Install a few vacuum chambers with special low-emissivity coatings.• Remove some of the synchrotron radiation masks?• Install solenoids and/or clearing electrodes in (some)straight sections, measure SEY in situ.• If possible, adopt a more strict policy for vacuumchamber opening. P. Collier remarked that this willnot be possible before 2-3 years.At the SPS start-up:• Fix baseline drift of damper pick-up =⇒ work at120 MHz.• Repeat measurements of beam emittance and instabil-ity, vary beam parameters (RF voltage, chromaticity,etc.) and develop minimum software for fast correla-tion plots.• Compare with simulations and draw conclusions forLHC.As soon as possible in the SPS:• Condition beam pipe surface by beam scrubbing =⇒need dedicated MD time (several hours) with highduty cycle.• Monitor electron cloud threshold (increase acquisitionrate on more vacuum gauges, measure treated cham-bers, etc.).• If threshold does not increase with beam scrubbing af-ter several A h =⇒ as a last resort, try local injectionof Freon?• Test suppression of electron cloud in straight sectionsby solenoid fields and/or clearing electrodes.As soon as possible for the PS and/or SPS:• Test satellite bunches at 3 ns distance from nominalLHC bunches. T. Linnecar reported that it may bepossible to generate such satellites in the SPS, usingthe 800 MHz cavities.• Test new PS schemes for LHC type beams with gapsand/or 50 ns spacing.• Repeat measurements of beam emittance and instabil-ity, vary residual gas pressure, bunch intensity, etc.5 REFERENCES[1] F. Ruggiero, “Electron cloud in the LHC: introduction andmotivation”, these proceedings.[2] W. Ho¨fle, “Observations of the electron cloud effect on pick-up signals in the SPS”, these proceedings.[3] G. Arduini, K. Cornelis, J.M. Jimenez, G. Moulard, M. Pivi,K. Weiss, “Electron cloud: SPS vacuum observations withLHC type beams”, these proceedings.[4] G. Arduini, “Observations in the SPS: beam emittance, insta-bilities”, these proceedings.[5] B. Henrist, N. Hilleret, E. Mercier, C. Scheuerlein “Ingredi-ents for the understanding and the simulation of multipact-ing”, these proceedings.[6] F. Zimmermann, “Electron cloud simulations for SPS andLHC”, these proceedings.[7] I.R. Collins, O. Gro¨bner, N. Hilleret, J.M. Jimenez, andM. Pivi, “Electron cloud: potential remedies for the vacuumsystem of the SPS”, these proceedings.[8] R. Cappi, “Solutions and cures–Missing bunches in the bunchtrain of the ‘PS for LHC’ beam”, these proceedings.[9] O. Bru¨ning, F. Caspers, I.R. Collins, O. Gro¨bner, B. Henrist,N. Hilleret, J.-M. Laurent, M. Morvillo, M. Pivi, F. Ruggiero,and X. Zhang, “Electron Cloud and Beam Scrubbing in theLHC”, CERN LHC Project Report 290 (April 1999), in Proc.1999 Particle Accelerator Conference (PAC99), New York,eds. A. Luccio and W. MacKay (IEEE, New York, 1999),pp. 2629-2631.Chamonix X12

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