The electron cloud instability limits the performance of many existing proton and positron rings. A simulation study carried out with the HEADTAIL code revealed that the threshold for its onset decreases with increasing beam energy, if the 6D emittance of the bunch is kept constant and the longitudinal matching to the bucket is preserved. Experiments have been carried out at the CERN-SPS to study the dependence of the vertical electron cloud instability on the energy and on the beam size. The reduction of the physical transverse emittance as a function of energy is considered in fact to be the main reason for the unusual dependence of this instability on energy

Arduini, Gianluigi

Benedetto, E

Calaga, R

Métral, E

Papotti, G

Rumolo, G

Salvant, B

Shaposhnikova, E

CERN Document Server

CERN-AB-2008-01929Aug2008EXPERIMENTAL STUDY OF THE ELECTRON CLOUD INSTABILITY AT THECERN-SPSThe electron cloud instability limits the performance of many existing proton and positron rings. Asimulation study carried out with the HEADTAIL code revealed that the threshold for its onset decreaseswith increasing beam energy, if the 6D emittance of the bunch is kept constant and the longitudinalmatching to the bucket is preserved. Experiments have been carried out at the CERN-SPS to study thedependence of the vertical electron cloud instability on the energy and on the beam size. The reduction ofthe physical transverse emittance as a function of energy is considered in fact to be the main reason for theunusual dependence of this instability on energy.EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN   -   AB DepartmentGeneva, SwitzerlandAugust 2008AbstractCERN-AB-2008-0191) BNL, NY 11973, USA2)  EPFL, Lausanne, SwitzerlandG. Rumolo, G. Arduini, E. Benedetto, R. Calaga 1), E. Métral, G. Papotti, B. Salvant 2), E. ShaposhnikovaCERN, Geneva, SwitzerlandPresented atEPAC'08, 11th European Particle Accelerator Conference, Genoa, Italy - June 23-27, 2008EXPERIMENTAL STUDY OF THE ELECTRON CLOUD INSTABILITY ATTHE CERN-SPSG. Rumolo, G. Arduini, E. Me´tral, E. Shaposhnikova, E. Benedetto, G. Papotti,CERN, Geneva, SwitzerlandR. Calaga, BNL, NY 11973, USAB. Salvant, EPFL, Lausanne, SwitzerlandAbstractThe electron cloud instability limits the performance ofmany existing proton and positron rings. A simulationstudy carried out with the HEADTAIL code revealed thatthe threshold for its onset decreases with increasing beamenergy, if the 6D emittance of the bunch is kept constantand the longitudinal matching to the bucket is preserved.Experiments have been carried out at the CERN-SPS tostudy the dependence of the vertical electron cloud insta-bility on the energy and on the beam size. The reductionof the physical transverse emittance as a function of energyis considered in fact to be the main reason for the unusualdependence of this instability on energy.INTRODUCTIONStudies for the Large Hadron Collider (LHC) perfor-mance upgrade include the improvement of the existingLHC injectors and the design of new rings in the injectorchain [1]. Several scenarios, aimed at overcoming theexisting bottlenecks, are presently being taken into con-sideration. The most promising option requires raisingthe injection momentum into the existing SPS from thepresent 26 GeV/c to 50 GeV/c. This is believed to improvethe machine performance in many regards (e.g., less spacecharge and intra beam scattering, more rigid beams againstcoupled bunch instabilities, no transition crossing, lowerinjection and capture losses) [2]. Furthermore, it wouldallow for an upgrade of the SPS to a 1 TeV extractionenergy ring, with the related advantages for injection intothe LHC. This scenario is based on the replacement of thepresent SPS injector, the Proton Synchrotron (PS) ring, bythe PS2 [3].However, the steps for the realization of this upgradeplan crucially depend on the effect of a higher injectionenergy on the collective phenomena that are presentlyconsidered as the main limitations in the SPS performance.In particular, the vertical single bunch ECI [4] limited for along time the number of batches that could be injected intothe SPS and it is currently overcome by operating the ringwith large vertical chromaticity after a scrubbing run [5].An intuitive semi-analytical approach to the ECI us-ing a broad-band resonator with beam dependent param-eters [6] shows that its scaling may become alarmingly un-favourable at high energies far from transition, under thefurther assumptions of constant bunch length and normal-ized transverse emittances.However, a comprehensive study of the effect of higher in-jection energy on the ECI has been carried out via simula-tions and experiments in the CERN-SPS. Table 1 shows alist of the essential parameters used for the numerical study(typical LHC-type bunch in the SPS). The main assump-tions of our model (reflecting the real situation) are:• The longitudinal emittance and the bunch length arekept constant. The momentum spread Δp/p0 and thematched voltage are re-adjusted with the energy. Thematched voltage scales like |η|/γ.• The normalised transverse emittances are constant.Consequently the transverse beam sizes are scaled as√1/γ.Table 1: Parameters used in our studyParameter Symbol ValueMomentum p0 (GeV/c) 14–450Norm. transv. emitt. x,y (μm) 2.8Long. emitt. z (eVs) 0.35Bunch length σz (m) 0.3Bunch population N 1.1× 1011Vertical tune Qy 26.13Momentum comp. α 0.00192SIMULATION STUDY WITH HEADTAILThe dependence of the ECI threshold on energy was sim-ulated with the HEADTAIL code [7]. The action of the elec-tron cloud on the bunch is lumped in one or more pointsalong the ring. The Nsl slices of which the bunch is made,successively interact with the electron cloud. The elec-trons are modeled as Ne macro-particles, whose distribu-tion in the pipe cross-section comes from the build up codeECLOUD [8]. Each slice sees the electron cloud as de-formed by the interaction with the preceding slices. Thedistortion of the cloud distribution induced by the bunchpassing through is the mechanism that couples the motionof subsequent slices and can give rise to instability. To gainan insight into the physical mechanism that determines thetype of dependence of the instability threshold on energy,we have identified the thresholds at different momenta. Theresult of a scan over a few points extending to 270 GeV/cis displayed in Fig. 1 for a maximum SEY (δmax) of 1.4.The threshold for instability decreases with increasing en-ergy and levels off to the threshold for electron cloud buildup for momenta higher than 100 GeV/c. Our explanationfor this anomalous behaviour is that, although the bunchbecomes more rigid at higher energies, and therefore lesssensitive to collective effects, it also shrinks transversely,which enhances the effect of the electron cloud pinch. Inaddition, the matched voltage scales like |η|/γ, causing adecrease of the synchrotron tune far above transition. Thistranslates into a slower longitudinal motion and thereforelarger time scales for damping.Figure 1: Simulated ECI thresholds at different momenta, studydone with quasi-self-consistent e-cloud distribution.To prove that the main cause of the destabilization ofthe beam at higher energies lies in the smaller transversesizes, we simulated the ECI at 26 GeV/c for beams withdifferent emittances in both transverse planes. It was foundthat emittance values about a factor 1.5 larger than nom-inal are sufficient to stabilize the SPS bunch against ECI(see Fig. 2). This feature is also inherent to ECI and candistinguish it from other types of collective instabilities,which do not depend on x,y to the first order (except spacecharge, which is weak at the energies under considerationin this paper).EXPERIMENTAL VERIFICATION ATTHE CERN-SPSAn experimental study to prove the scaling law found bysimulations was carried out at the CERN-SPS during the2007 run. An LHC-type beam made of 4 batches with 72bunches each, was injected into the SPS at 26 GeV/c dur-ing a flat bottom of about 11 s, then accelerated to an in-termediate plateau of 55 GeV/c (about 6 s) and eventuallytaken to 270 GeV/c and dumped. The 55 GeV/c flat portionwould serve to show that the beam still suffers from ECI atthis higher energy. In addition, it could be used to provethat artificially enlarging the beam size would be an effi-cient tool to suppress this effect. Actually, the observation 0 2 4 6 8 10 12 14 16 0  2  4  6  8  10  12% of εy growtht (ms)εx,y0=2.8 μmεx,y0=3.1 μmεx,y0=3.5 μmεx,y0=4.0 μmFigure 2: Simulated percents of vertical emittance growth dueto ECI for LHC-type beams in the SPS having different initialtransverse emittances (as labelled).Figure 3: Evolution of the electron flux (nA) at different hori-zontal positions in the beam pipe along the cycle. (a) is withouttransverse damper excitation, (b) is with the damper on.of the beam behaviour at this energy had a two-fold inter-est, because the value is close to the new injection energy.After reaching stable conditions with a 4-batch LHC-typebeam inside the SPS (tunes Qx = 26.13, Qy = 26.185,and vertical chromaticity ξy = 0.15 along the whole cycle),the beam was intentionally driven unstable at 55 GeV/c byreducing the vertical chromaticity to ξy = 0.05 toward theend of the plateau. The evolution of EC in the machine wasmonitored through the signal of an EC detector (see Fig. 3).As Fig. 4 depicts, the ECI manifested itself with the loss ofthe bunches in the tail of the fourth batch.The instability was proven to be independent of the op-eration of the SPS transverse feedback system. The twovertical dampers were switched on and off (alternately andtogether) with no evident effect on the appearance of theinstability. Most of the measurements were taken eventu-ally with both dampers on in order to rule out the onsetof coupled-bunch modes. After proving that the ECI ap-peared at 55 GeV/c, it was attempted to suppress it throughtransverse emittance blow-up. The longitudinal beam pa-rameters were kept unchanged in this exercise, and theywere constantly monitored (4σz=3 ns in the flat parts of theBunch index300 350 400 450 500 550 600t (s)05101520020040060080010001200I vs bunch vs time26 GeV/c55 GeV/c0.150.05ξ  =yξ  =yFigure 4: Top picture: Evolution of the bunch by bunch intensitywith the 4 injections, a small uniform loss at the start of the firstramp, and the final loss at the tail of the fourth batch after loweringthe vertical chromaticity.t (s)0 2 4 6 8 10 12 14 16 18 20)13 (x 10pN11.522.533.5Figure 5: Typical beam intensity as a function of time alongthe cycle. The red trace shows a stable shot when the damperexcitation was on, whereas the black trace shows a loss due to avertical instability occurring with the damper excitation off. Inboth cases, chromaticity is reduced at 17 s.cycle). The transverse blow-up was introduced at the endof the ramp to 55 GeV/c by means of transverse damperexcitation. Testing the method during stable operation withhigh chromaticity, it was measured that the beam size at thewire scanner location was increased by a factor 2 horizon-tally and by a factor 1.4 vertically (which would translate infactors of emittance increase of 4 and 2, respectively). Aspredicted by simulations, the transversely enlarged beamremained stable also when lowering chromaticity to 0.05.Turning on and off the damper excitation several times, wecould observe that the instability would systematically dis-appear for large beam sizes and re-appear for small beamsizes (Fig. 5).CONCLUSIONSIn summary, the 55 GeV/c experiment described aboveshows that one of the bottle-necks for LHC-type beams inthe SPS, i.e. the ECI, is still present at higher energy andcan lead to beam loss, if it is not suppressed by high chro-maticity. Furthermore, it demonstrates the principle thatthe ECI depends critically on the transverse beam size, aswas anticipated by simulations, and can be cured by us-ing beams with larger transverse emittances. This evidencecan be inferred as an indirect way to prove the physicalinterpretation of the counterintuitive decrease of the ECIthreshold with energy.In conclusion, both simulations and experiments showthat the ECI becomes more severe with increasing beamenergy. As a consequence, upgrade plans with higher in-jection energy for proton machines which suffer from ECImust foresee a program of EC suppression. Promising ECcountermeasures exist and their effectiveness is presentlyunder study. They consist mainly of different types of coat-ing, grooved surfaces and clearing electrodes. These tech-niques normally reduce the effective SEY at the inner pipesurfaces.REFERENCES[1] Proceedings of LUMI’05, Arcidosso, Italy, 31 August-3September 2005, edited by F. Ruggiero, W. Scandale andF. Zimmermann, published as CERN Yellow Report CERN-2006-008 (2006); and references therein.[2] G. Rumolo, E. Me´tral, E. Shaposhnikova, in Proc. ofLUMI’06, Valencia, Spain, 16–20 October 2006, edited byW. Scandale, T. Taylor and F. Zimmermann, published asCERN Yellow Report CERN-2007-002 (2007)[3] M. Benedikt, in Proc. of LUMI’06, see above Ref. [2][4] Proceedings of ECLOUD’04, Napa, CA, USA, 19–23 April,2004, edited by M. Furman, S. Henderson, and F. Zimmer-mann, published as CERN Yellow Report CERN-2005-001(2005); and references therein.[5] G. Arduini et al., CERN-SL-2001-003-DI (2001)[6] K. Ohmi, F. Zimmermann, E. Perevedentsev, Phys. Rev. E65, 016502 (2001)[7] G. Rumolo, and F. Zimmermann, Phys. Rev. ST Ac-cel. Beams 5, 121002 (2002)[8] G. Rumolo and F. Zimmermann, CERN-SL-Note-2002-016-AP (2002)

Experimental Study of the Electron Cloud Instability at the CERN SPS

Experimental Study of the Electron Cloud Instability at the CERN SPS

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