Electron cloud effects have been recently observed in the CERN SPS in the presence of LHC type proton beams with 25 ns bunch spacing. Above a threshold intensity of about 4 X 10^12 protons in 81 consecutive bunches, corresponding to half of the nomina

Arduini, Gianluigi

Baglin, V

Brüning, Oliver Sim

Cappi, R

Caspers, Friedhelm

Collier, Paul

Collins, I R

Cornelis, Karel

Garoby, R

Gröbner, Oswald

Henrist, Bernard

Hilleret, Noël

Höfle, Wolfgang

Jiménez, J M

Laurent, Jean Michel

Linnecar, Trevor Paul R

Mercier, E

Pivi, M

Ruggiero, F

Rumolo, Giovanni

Scheuerlein, C

Tückmantel, Joachim

Vos, L

Zimmermann, Frank

CERN Document Server

ELECTRON CLOUD EFFECTS IN THE CERN SPS AND LHCG. Arduini, V. Baglin, O. Bru¨ning, R. Cappi, F. Caspers, P. Collier, I.R. Collins, K. Cornelis,R. Garoby, O. Gro¨bner, B. Henrist, N. Hilleret, W. Ho¨fle, J.M. Jimenez, J.-M. Laurent, T. Linnecar,E. Mercier, M. Pivi, F. Ruggiero, G. Rumolo, C. Scheuerlein, J. Tuckmantel, L. Vos, F. ZimmermannCERN, Geneva, SwitzerlandAbstractElectron cloud effects have been recently observed in theCERN SPS in the presence of LHC type proton beams with25 ns bunch spacing. Above a threshold intensity of about4 × 1012 protons in 81 consecutive bunches, correspond-ing to half of the nominal ‘batch’ intensity to be injectedinto the LHC, an intense electron bombardment gives riseto a strong perturbation of the transverse feedback pick-upsignals and to significant pressure rises. Also the intensityand emittance along the bunch train is affected. We com-pare experimental results with simulations of the electroncloud build-up and discuss possible solutions.1 INTRODUCTIONIn 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. For the nominal bunch spacing of 25 nsa significant fraction of secondary electrons is lost in be-tween two successive bunch passages, but slow secondaryelectrons with energies below 10 eV survive until the nextbunch and can be again accelerated up to several keV. Thisnon resonant, single pass mechanism may lead to an elec-tron cloud build-up if the maximum secondary electronyield (SEY) δmax of the pipe wall is larger than a criticalvalue, typically around 1.3 for nominal LHC beams [1].Then the electron cloud is amplified at each bunch passageand reaches a saturation value determined by space chargerepulsion, with implications for beam stability, emittancegrowth, and heat load on the cold LHC beam screen.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 where the average energy gaincorresponds to the maximum of the SEY. Since the verticaldimensions of the LHC dipole beam screen and of the SPSvacuum chamber are very similar (about 2 cm half height),the corresponding critical SEY is the same for both ma-chines. However the mechanism that triggers the electroncloud build-up is different. In the LHC at 7 TeV, the gener-ated synchrotron radiation creates 10−3 photoelectrons/mper proton, while in the SPS (and in the LHC at injectionenergy) the primary electron production is dominated byionization of the residual gas: assuming an ionization crosssection of 2 Mbarn, at 10 nTorr the primary yield is only6.4× 10−8 electrons/m per proton. This is one reason whyelectron cloud effects were not anticipated in the SPS, theother reason being that it was reasonable to expect a sub-stantial surface conditioning of the SPS vacuum chamberafter so many years of operation, especially with leptons(photon scrubbing). However the SPS vacuum chamber isoften vented and this creates oxide and/or condensed wa-ter layers with a high SEY [3]. In addition, tungsten syn-chrotron radiation masks located at the extremities of theSPS dipole chambers reduce or prevent photon scrubbing.2 EXPERIMENTAL EVIDENCEThere is a rather convincing list of SPS observations sup-porting the conclusion of an electron cloud build-up withLHC type beams [2]. In particular, a similar thresholdbunch intensity Nb = 2.5 to 5 × 1010 is observed fordamper pick-up signals, distributed pressure rise and beaminstability, when the bunch spacing is 25 ns. This is ingood agreement with electron cloud simulation results [2],assuming a maximum SEY δmax  1.9: compare the simu-lated electron cloud build-up of Fig. 1 with the signals mea-sured at the damper pick-ups, shown in Fig. 2a, or with theobserved relative pressure rise of Fig. 3. As for the LHC,the simulated electron cloud density in the SPS grows sig-nificantly after about 30-40 bunch passages and reaches asaturation value of a few 109 electrons/m, consistent withthe observed pressure rise and with the charge depositionmeasured on the damper pick-up. Ion effects are excluded,since they would depend on the integrated charge over sev-eral bunches, while no effect is observed with differentbunch spacings and the same total batch intensity. More-over the observed threshold bunch intensity has a weakFigure 1: Simulated electron cloud build-up in the SPS forfour different bunch populations, δmax = 1.9 and a primaryelectron yield (at the pipe wall) Ype = 2.5 × 10−7/m perproton, corresponding to a gas pressure p = 40 nTorr.259Proceedings of EPAC 2000, Vienna, Austriaa) without solenoid b) with solenoid (100 Gauss)Figure 2: Vertical pick-up signal (∆-signal) for a batch in-tensity of 5.5×1012 protons, in 81 bunches with 25 ns spac-ing, measured with high impedance FET amplifiers at theelectro-static pick-ups for the SPS damper system: a) with-out and b) with a solenoid field of 100 Gauss.                              	          	               	      			                        Figure 3: Relative pressure rise vs. number of bunches withNb = 7.5× 1010 protons in an LHC type batch, measuredat different gauges placed between two SPS dipoles.dependence on the residual gas pressure, contrary to ioneffects and in agreement with electron cloud simulations.There is also direct evidence of negative charge (electrons),collected by a dedicated pick-up with a shielding grid, cor-related with beam intensity and bunch pattern. Finally, amodest solenoid field was effective in curing the damperpick-up, but limited to 100 Gauss by heating problems (seeFig. 2b). Above an LHC batch intensity of about 5.5×1012protons, the solenoid field was insufficient. This is qualita-tively understandable in view of the keV energies acquiredby electrons near the beam axis.Early observations of anomalies in the behaviour of theSPS damper with LHC type beams in 1998 were confirmedin 1999 and the problem could be attributed to baselinejumps in the signals of all the eight electrostatic pick-ups.This phenomenon occurs only beyond a threshold bunchintensity Nb ∼ 4 to 5 × 1010 of the 2 µs LHC batch with25 ns bunch spacing and the baseline drift starts after thepassage of some 30-40 bunches (see Fig. 2a). No drift wasobserved with 130 ns bunch spacing and Nb ∼ 2.5× 1011,nor with 5 ns bunch spacing and Nb ∼ 1010, in agree-ment with simulation results shown in Fig. 4. The observa-tions of the damper pick-up signals show that the baselinejumps are not present at multiples of the 40 MHz bunchfrequency. An effective solution to provide a clean signalfor the damper, already successfully implemented for thehorizontal damper, is therefore to mix the beam position∆-signals with a beam synchronous RF reference signal ata multiple of the bunch frequency, for example 120 MHz,down to baseband for further processing [2].Figure 4: Simulated electron cloud build-up in the SPS fora bunch population Nb = 2 × 1010 and p = 40 nTorr,comparing bunch spacings of 5 ns and 25 ns. In both casesthe maximum SEY δmax = 1.9 exceeds the critical value,but the build up of the electron cloud is more regular andmuch faster for the 25 ns spacing than for the 5 ns spacing.-80-60-40-2002040600 1 2 3 4 5 6 7SLICE NUMBERGROWTH TIME [turns]Figure 5: Growth time of the horizontal instability ob-served at injection vs. position in the batch, for a batchintensity of 4 × 1012 protons. Slice 1 corresponds to thehead and slice 6 to the tail of the batch.A fast horizontal instability with a rise time of 20 to 25turns is observed in the SPS above a threshold LHC batchintensity of 4×1012 protons (see Fig. 5). The observationshave been performed with couplers equipped by a 200 MHzreceiver, not affected by the electron cloud induced base-line distortions of electrostatic pick-ups, and beam oscilla-tions have been monitored at six consecutive slices alongthe batch. The instability mainly affects the batch tail, sat-urates and leads to emittance blow-up and beam losses.There is a slower instability in the vertical plane too, alsoaffecting the tail of the batch. Vertical oscillations around700 MHz are observed with a wide-band pick-up and maybe associated with single bunch activity (this happens to beclose to the transverse oscillation frequency of the electronsinside the LHC proton bunches and there is also a knownSPS impedance source around the same frequency). Thisfast instability might be interpreted as a single bunch, beambreak-up instability caused by the short range wakefield inthe electron plasma; preliminary estimates [5, 6] assumingan electron cloud density of 5×1011 electrons/m3 lead to arise time of 500 µs, or 20 turns, very close to the observedinstability rise time. This is compatible with recent SPSmeasurements of single bunch head-tail phase shift, indi-cating that bunches near the tail of a batch are affected bya significant short range wakefield.Proceedings of EPAC 2000, Vienna, Austria260       	                                                            			                                  ! " #   $ Figure 6: Relative pressure rise vs. LHC beam time with abatch intensity above 3.8 × 1012 protons. The measuredpressure is practically the same for a reference vacuumchamber and for a special chamber treated with N2 dis-charge, both located in a field-free region of the SPS.05e+081e+091.5e+092e+092.5e+093e+093.5e+094e+094.5e+090 5e-07 1e-06 1.5e-06 2e-06dipole, SEY=1.6, N=7e10drift, SEY=1.6, N=7e10Figure 7: Simulated electron cloud build-up in the SPS forNb = 7× 1010, δmax = 1.6 and p = 40 nTorr: comparisonof field-free regions (top curve) and dipoles (bottom curve).3 POSSIBLE CURESLaboratory 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 above 1 mC/mm2 leads to a significant reduc-tion of δmax [3]. An electron dose of 5 × 10−5 C/mm2 isthe observable limit for surface conditioning, in agreementwith early SPS observations during 14 hours with a duty cy-cle of 3.4% [2, 4]. The effect of dosing has been confirmedby irradiation tests with synchrotron light from the EPAring [3]. A reduction of δmax down to a saturation valuebelow 1.3 is observed for a copper sample positively biasedat 100 V and for a primary photon dose around 10 22 pho-tons/m, corresponding to 30 hours of nominal LHC oper-ation at 7 TeV. In the SPS the accumulated photon doseduring lepton cycles after one year of LEP filling, with syn-chrotron radiation masks in place, is only 1021 photons/m.Recently a clear indication of surface cleaning has beenobserved in the SPS during an equivalent period of 60 hourswith 100% duty cycle and a batch intensity above 3.8×1012protons in 81 bunches: Fig. 6 shows a marked reduction ofthe relative pressure rise vs. LHC beam time. In spite ofthe reduced pressure rise after electron bombardment, it isnot clear which fraction of this reduction can be attributedto the SEY or to the molecular desorption yield of the vac-uum chamber wall. Comparing recent SPS measurementsof pressure rise vs. intensity to early measurements in 1999,Figure 8: Simulated suppression of electron cloud build-upin the SPS by low-intensity satellite bunches with interme-diate spacings from the main bunches, for Nb = 5× 1010,δmax = 1.9, and p = 40 nTorr. The full bunch length is3 ns both for the main and the satellite bunches.i.e. before surface cleaning, it appears that the thresholdLHC batch intensity for electron cloud build-up in the field-free regions has increased from about 4×1012 to more than5× 1012 protons. A possible interpretation of the observa-tion that surface conditioning is more effective in field-freeregions than in regions with strong dipole magnetic field isthat the electron cloud build-up in the dipoles is slower andreaches a lower saturation value, as shown in Fig. 7.Another possible solution is to produce gaps in the LHCbatch by means of RF manipulations in the PS [2]. Forexample, starting from 7 PS Booster bunches and apply-ing 3 subsequent bunch splittings yields a modified LHCbunch train consisting of 56 bunches with gaps of 4 miss-ing bunches every 8 bunches. Simulations show that theelectron cloud build-up would be suppressed for a maxi-mum SEY of 1.5. For a higher δmax of 1.9, one shouldresort to a reduced fill pattern consisting of a sequence of4 LHC bunches followed by 4 missing bunches. Alterna-tively the bunch spacing could be doubled. Such schemescould be used for the initial beam conditioning of the LHC.As shown in Fig. 8, low-intensity satellite bunches witha population around 1010 protons and intermediate spac-ings from the main bunches would be very effective againstelectron cloud build-up. In the SPS it may be possible togenerate satellite bunches at 3-5 ns distance from nominalLHC bunches, using the 800 MHz cavities.REFERENCES[1] O. Bru¨ning et al., CERN LHC Project Report 290 (1999), inProc. IEEE PAC99, New York, pp. 2629-2631.[2] “Session IV: Electron Cloud in the SPS”, in Proc. 10th Work-shop on LEP-SPS performance, Chamonix, 1999, CERN SL-2000-007 DI, pp. 110-157.[3] N. Hilleret et al., “The Variation of the Secondary ElectronYield of Technical Materials Submitted to the Impact of En-ergetic Photons and/or Electrons”, these Proc.[4] J.M. Jimenez et al., “Electron cloud: observations with LHCtype beams in the SPS”, these Proc.[5] K. Ohmi and F. Zimmermann, CERN SL-2000-015 AP.[6] G. Arduini et al., “Transverse instabilities of the LHC protonbeam in the SPS”, these Proc.261Proceedings of EPAC 2000, Vienna, Austria

Electron Cloud Effects in the CERN SPS and LHC

Electron Cloud Effects in the CERN SPS and LHC

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