In August 1999, strong pressure increases were observed in the SPS in the presence of the new LHC-type beams. This paper reports on observations of the electron cloud phenomenon and the related pressure increase as a function of parameters such as the number of protons per bunch, the number of bunches per batch, the shape of the vacuum chamber and the electron current collected on pick-ups. Results of the observed clean-up, "beam scrubbing" will be presented as well as the consequences of the e-cloud phenomenon on the SPS operation with the LHC nominal beam intensity

Arduini, Gianluigi

Cornelis, Karel

Gröbner, Oswald

Hilleret, Noël

Höfle, Wolfgang

Jiménez, J M

Laurent, Jean Michel

Moulard, G

Pivi, M

Weiss, K P

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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsELECTRON CLOUD: OBSERVATIONS WITH LHC-TYPE BEAMS IN THE SPSG. Arduini, K. Cornelis, O. Gröbner, N. Hilleret, W. Hofle, J.M. Jimenez, J.M. Laurent,G. Moulard, M. Pivi and K. WeissIn August 1999, strong pressure increases were observed in the SPS in the presence of the newLHC-type beams. This paper reports on observations of the electron cloud phenomenon and the relatedpressure increase as a function of parameters such as the number of protons per bunch, the number ofbunches per batch, the shape of the vacuum chamber and the electron current collected on pick-ups.Results of the observed clean-up, "beam scrubbing" will be presented as well as the consequences of thee-cloud phenomenon on the SPS operation with the LHC nominal beam intensity.LHC and SL DivisionsPresented at the Seventh European Particle Accelerator Conference  (EPAC 2000)26-30 June 2000, Vienna, AustriaGeneva, Large Hadron Collider ProjectAdministrative SecretariatLHC DivisionCERNCH - 1211 Geneva 23SwitzerlandLHC Project Report 434Abstract23 September 20001ELECTRON CLOUD: OBSERVATIONS WITH LHC-TYPE BEAMS IN THESPSG. Arduini, K. Cornelis, O. Gröbner, N. Hilleret, W. Hofle, J.M. Jimenez, J.M. Laurent,G. Moulard, M. Pivi, K. WeissCERN, SwitzerlandAbstractIn August 1999, strong pressure increases wereobserved in the SPS in the presence of the new LHC-type beams. This paper reports on observations of theelectron cloud phenomenon and the related pressureincrease as a function of parameters such as the numberof protons per bunch, the number of bunches per batch,the shape of the vacuum chamber and the electroncurrent collected on pick-ups. Results of the observedclean-up, "beam scrubbing" will be presented as well asthe consequences of the e-cloud phenomenon on the SPSoperation with the LHC nominal beam intensity.1  INTRODUCTIONPressure rises in presence of LHC-type beams wereobserved for the first time during an MD dedicated toLHC in August 1999. Fig.1 shows that the pressureincreases occurs only in presence of the LHC-typebeams. Before and after the MD session, with the normalSPS proton beams, the pressures recover with a timeconstant consistent with the vacuum system. Thepressure increase which occurs all around the machineranges between a factor of 3 to 60 depending on the sizeof the vacuum chamber and on the beam characteristics[1]. The pressure rise can be attributed to gas desorptiondue to beam induced multipacting [2]. To study thevacuum behaviour in more detail the pressures of 42gauges placed in similar positions around the SPS ringwere recorded with an acquisition rate of onemeasurement per second.2  PROTONS PER BUNCH The measurements shown in Fig.2 were made during adedicated MD session with an LHC-beam duty cycleclose to 45% (6.4s out of a total 14.4s SPS cycle) andwith a maximum intensity of 7.4x1010p/bunch. It can beseen that the pressure remains constant and low until athreshold is reached and from then onwards it increasesapproximately linearly with the number of protons. Thesame behaviour with a similar relative pressure increaseis observed in many other positions around the SPS ring.These observations are consistent with the resultspresented at the Chamonix Workshop [1] with a smallerduty cycle of 4%. In this last figure, it appears that thethreshold is slightly higher and more uniform for gaugesfar away from any magnetic field.1.0E-091.0E-081.0E-071.0E-060.25 0.45 0.65 0.85 1.05 1.25 1.45 1.65 1.85Time (hours)Pressures mbar0.0E+005.0E+121.0E+131.5E+132.0E+132.5E+13ProtonsSPS type beams(5 ns)LHC type beams (25 ns)MBA dipole/GAUGE/MBB dipoleFigure 1: Pressure versus time, total number ofprotons=4.5x1012 (5.6x1010 p/bunch), duty cycle 62 %.02468100.0E+00 1.0E+12 2.0E+12 3.0E+12 4.0E+12 5.0E+12 6.0E+12 7.0E+12Number of protonsPressures (mbar)VG50660 Dipole magnetic fieldVG51880 No magnetic field Figure 2: Pressure versus number of p/batch betweentwo dipoles i.e. without magnetic field, duty cycle 45 %.3  BUNCHES PER BATCH Apart from the number of protons per bunch animportant parameter has been the number of bunches ina batch. A batch consists of a train of 81 bunchesseparated by 25 ns. These measurements were made witha duty cycle of 4 %. At 4.4x1010p/bunch (full batch 3.6x1012p), no pressureincrease was observed when increasing the number ofbunches in the batch to the nominal value of 81.By increasing the bunch density up to 7.7x1010 (fullbatch 6.25x1012p), the relative pressure increase is twiceas large as with 6.5x1010 p/bunch (Fig.3). This densitycorresponds to two third of the nominal intensity to beinjected into the LHC. The measurements show that thethreshold of the pressure rise decreases from 34 down to24 bunches in a batch when increasing the density from6.5x1010 up to 7.7x1010p/bunch (Fig.4).21.01.52.02.53.03.54.04.55.05.56.017:42 17:49 17:57 18:04 18:11 18:18 18:25 18:33 18:40 18:47 18:54Relative pressure increase0.0E+001.0E+122.0E+123.0E+124.0E+125.0E+126.0E+127.0E+12ProtonsMBB-31540-MBBMBA-30060-MBBMBA-30660-MBBStd dia159mm chamber-31933Number of protons6.25x1012 protons 81 bunches70 b53 b47 b37 b27 b23 b15 b3154031933Duty cycle ~ 3 %Figure 3: Relative pressure increase versus number ofbunches at 7.7x1010p/bunch.01234560 10 20 30 40 50 60 70 80 90Number of protonsPressures (mbar)MBB-31540-MBB 6.4x1012 protons/batchMBB-31540-MBB 5.25x1012 protons/batchFigure 4: Relative pressure increases versus number ofbunches per batch at 6.5x1010 and 7.7x1010p/bunch.4  PICK-UPS AND DAMPER A pick-up collector, mounted on top of a dipolechamber was used to monitor the electron currentproduced by the e-cloud. The pick-up electrode wasshielded from the beam by a metallic grid and could bebiased (+45 V) with respect to ground in order tosuppress secondary electrons emitted from the collectoritself. At 3.6x1010p/bunch (2.9x1012p/batch), below thethreshold, no pressure rise was observed and no signalcould be seen on the pick-up. However, by increasingthe number of protons above the threshold of the e-cloud,the pick-up current showed distinct peaks separated by23 µs which corresponds to the revolution time in theSPS. Fig.5 shows a picture of the fast scope for a protonbunch density of 7.5x1010 (6.1x1012p/batch). Special beam pick-ups are used in the SPS by thetransverse feedback system (“damper”) which dampsinjection oscillations and stabilises the beam againsttransverse coupled bunch instabilities. In presence ofLHC beams, this damper was found to be stronglyperturbed [4]. The electrostatic pick-ups used areequipped with high-impedance FET amplifiers. Theseamplifiers have been found to be sensitive to the beaminduced position signal as well as to deposited charges.The performance of the damper system has sufferedseverely due to the electron cloud saturating completelythe beam position signal. Fig.6 shows a vertical positionsignal for an LHC proton beam batch with a drift of thesignal starting about half way through the 2 µs longbatch. This drift is due to electrons hitting the pick-upelectrode. The threshold intensity of this phenomenonhas been found to vary between 2.5x1012 and3.5x1012p/batch. A solenoid magnet field of ~100 Gausscould suppress the effect up to about 5-6x1012p/batch ascan be seen in Fig.7, a clear indication of low energyelectrons being the cause of the effect [4].Figure 5: Structure of the current collected by the biased(+45 V) pick-up with a proton bunch density of 7.5x1010Figure 6: Vertical damper pick-up signal perturbed bythe e-cloud effect (5.5x1012p/batch).Figure 7: Vertical damper signal with a solenoidmagnetic field of 100 Gauss to suppress the e-cloud(5.5x1012p/batch).3 5  CLEANING OR SCRUBBING A specific MD session was dedicated to the study of apossible cleaning effect by the beam. During approx. 14hours, an LHC beam with a bunch density slightly abovethe threshold of the e-cloud (5.6x1010p/bunch) wasmaintained in the machine with a duty cycle of 3.4 %.The results [1] indicate that only three out of 20 gaugesshow a measurable pressure decrease, between 7% and12%. Nevertheless, a clear correlation could be foundbetween the intensity of the e-cloud, as measured by thepressure increase, and the observed cleaning effect i.e.the larger the local pressure increase, the larger thecleaning. Since the cleaning of the vacuum chamber with beamwill be an important process for the future operation ofthe LHC, it was decided to repeat this measurement witha higher duty cycle of ~45%. The experiment was madewith bunch densities ranging from 5.0x1010 up to8.0x1010p. In total 42 pressure gauges around the SPSring were recorded. Fig.8 shows the evolution of the relative pressureincrease versus beam time for bunch intensities largerthan the e-cloud threshold value of 5.0x1010. This figure shows clear evidence of a cleaning effectof the vacuum chamber. No pressure increase could beseen after 56 hours of LHC beam time indicating that themultipacting threshold had increased beyond theoperating conditions.y = 32.8e-1.5 x1101000:00 10:00 20:00 30:00 40:00 50:00 60:00LHC beam time (hh:mm)Relative Pressures decreaseFigure 8: Relative pressure increase versus effectivebeam time. The effective time is the total time spentwith LHC beams and bunch intensity above 5x1010.6 DISCUSSION - CONCLUSIONS The experimental results show that the pressureincreases only in presence of the LHC-type beam. Thepick-ups measure electron current signals only inpresence of LHC-type beams and the behaviour of thepressures versus bunch densities are consistent withobservations in other machines such as PEP2 and KEKB.These observations clearly favour the e-cloudphenomenon as the origin of the pressure rises observedin the SPS. Even though this phenomenon was not anticipated tooccur in the SPS more recent calculations are in goodagreement with the observed threshold [3]. In deed thevertical dimension of the vacuum chambers in thedipoles of the SPS is very similar to the height of theLHC beam screen. The lack of synchrotron radiation andof the resulting photoelectrons in the SPS is not asufficient guarantee against the electron cloud build-upsince the avalanche may start, as in the ISR [2], withelectrons created by residual gas ionisation as well as bymany other processes. The simple threshold conditionfor wall to wall multipacting [2] depends only on thebunch intensity, the bunch distance and the vacuumchamber dimension (beam pipe radius). For the LHC-type beams, and for the SPS vacuum chamber, thisthreshold bunch intensity is 4.5x1010p/bunch, inreasonable agreement with observations. The observed increase of the SPS beam emittanceabove the electron cloud threshold is so far not fullyunderstood. To explain this beam blow-up by residualgas scattering, the average pressure in the SPS wouldhave to exceed by far the pressures measured duringthese experiments. A more likely explanation could bethe emittance growth due to the direct interaction of theprotons with the electron cloud.Conditioning of the vacuum chamber with the LHCbeam at intensities above the threshold for the onset ofthe beam-induced e-cloud was performed. This showedan exponential reduction of the dynamic pressure rise inthe presence of the LHC beam so that no significantpressure increase was visible after about 60 h ofconditioning with beam. For the time being neitherdirect evidence of a reduction of the electron density isavailable nor of a reduction of the Secondary EmissionYield. Finally, the installation of the pumping port shieldingelements which are required to reduce the impedance ofthe so far unshielded SPS vacuum system will slightlyreduce the pumping speed of the ion pumps and willreduce by a significant amount (~25 %) the efficiency ofthe titanium sublimation pumps. As a consequence of thereduced pumping speed from 70 l/s to 55 l/s, acorresponding increase of the average pressure must beexpected. REFERENCES [1] G. Arduini and al., Proc. Workshop on LEP-SPSPerformance-Chamonix X, 119-122 (2000) [2] O. Gröbner, 10th Int. Conf. on High Energy Accel.Protvino, July 1977, p. 277 and PAC 1997 [3] F. Zimmermann, Chamonix Workshop, 2000 [4] W. Hofle, Proc. Workshop on LEP-SPSPerformance-Chamonix X, 112-118 (2000)ACKNOWLEDGEMENTSMany thanks to the operation team of both SPS and PSmachines for their helpful collaboration and toF.Ruggiero and F.Zimmerman for helpful discussions.

Electron Cloud: Observations with LHC-Type Beams in the SPS

Electron Cloud: Observations with LHC-Type Beams in the SPS

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