Hilleret, Noël

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LEP VACUUM IN 1998: OVERVIEW AND FUTURE PERSPECTIVEJ.-C. Billy, J.-P. Bojon, N. Hilleret, M. Jimenez, I. Laugier, P. StrubinCERN, Geneva, SwitzerlandAbstractThe pressure evolution in the LEP vacuum systemduring 1998 will be summarized as well as the mainfailures encountered. More details will be givenconcerning the evolution of the beam-gas lifetime, the gascomposition and the temperature of critical componentsduring 1998. Based on these considerations, an estimate ofthe future current limit as a function of the beam energywill be given.1 INTRODUCTIONFollowing a difficult year in 1997, modifications wereintroduced at various levels in the design, the control andthe operation of the LEP vacuum system in order toimprove its reliability. After reviewing thesemodifications, the operation of the system during 1998will be considered from the point of view of pressure,temperature, gas composition and lifetime. These data,combined with those concerning the evolution of thepressure with the beam energy, may allow conclusionsbeing drawn concerning the expected performance athigher energy during the 1999 running period. They willbe highlighted in the final paragraph, together with adescription of the on-going modifications to the LEPvacuum.2 MODIFICATIONS INTRODUCEDFOR THE 1998 RUNNING PERIODAfter the 1997 running period, when the down time dueto vacuum incidents was unusually high, several weakspots were identified and actions were undertaken in orderto:- Eliminate known weak points by removing obsoleteequipment (e.g. pretzel separators) and improving thecooling at cross-section changes of the vacuumchamber exposed to high heat load whenever possible.- Improve our monitoring systems both for thetemperature and the pressure in order to obtain an earlydetection of potentially critical points.In addition, efforts were made to rejuvenate and reinforcethe team in charge of emergency interventions on theLEP and SPS vacuum systems.2.1  Component modificationsMost failures experienced in 1997 were due to theoverheating of cross-section changes. During the 1997-98shutdown, these failures were modified or eliminatedwhere possible1. The main modification consisted ineliminating all welds located at cross-section changes andin cooling these regions uniformly. External absorberswere also mounted to protect the flanges from the localheating induced by the synchrotron radiation escapingfrom the transition chambers.2.2  The temperature and pressure loggingsystemsAs stated above, the heat load intercepted by the LEPvacuum system has become critical in several places andan extensive program of temperature monitoring andlogging has been launched. More than 500 thermocoupleswere installed in LEP during the 1997-98 shutdown andare now available on the LEP vacuum man-machine-interface (the so-called “Russian view”). All of thesetemperature measurements are also stored for one weekand can be retrieved for further studies. The temperatureprofile along a LEP arc can also be displayed as well asthe temperatures exceeding a given threshold. Thesetemperature measurements were very helpful to findoptimum operation conditions to protect the vacuumsystem, i.e. by adjusting a collimator position to reducethe heat load on a ZL transition or by following anothertransition temperature as the wigglers were powered at ahigher energy. The monitoring of pressures is also ofgreat importance for the survey of the LEP vacuumsystem. A very helpful new program was introduced in1998, which allows the study of the long-term evolutionof pressures in LEP over a period of several years. This isespecially useful for the detection of leaks which evolveslowly and hence very difficult to detect on a day-to-daysurvey 2.3  S.O.S. VIDEIn order to be able to react efficiently in emergencysituations in the LEP and SPS vacuum systems, a newsystem of stand-by was introduced within the LHCVacuum Group. Young technicians working mainly forLHC also participate in this new system (common toboth machines LEP and SPS). This has two mainadvantages: Firstly, the intervention team is rejuvenatedand this was mandatory to ensure the future interventions(before, the average age exceeded 50). Secondly, theseyoung technicians acquire a direct experience of theexploitation of large accelerator vacuum systems. Thisteam was called 10 times for intervention on LEP and 9times for an intervention on the SPS outside workinghours.Chamonix IX178Figure 1: Leak on the window of the wire scanner(H.C. 139)3  INCIDENTSDuring 1998, 6 leaks were detected on the LEP vacuumsystem, causing 4 interruptions to the LEP operation.Three affected  components like, for example, a viewport(HC139, wire scanner, 13/6/98), a pick-up button (HC842,16/6/98) and a coupler window (H.C.640, 14/09/98).The viewport of the wire scanner was submitted toinvestigations2 in order to determine the origin of the leakand Figure 1 shows its location. This print (located on thevacuum side of the viewport) is very similar to those lefton other viewports exposed to electron impact.Three leaks were located on two transitions close toZL20 (H.C. 845/855, 13/06/98) and required thereplacement of a conflat gasket. These leaks were due tothe excessive heat deposited in the vertical plane by thesynchrotron radiation emitted by the superconductingquadrupole and efficiently repaired by an appropriatesetting of the collimators during the energy ramp.Two other leaks were detected by comparing the staticpressure before and after the 1998 running period and werelocated at a gasket ( H.C. 268, AFK) and a pick-up button(H.C. 458). They were sufficiently small to be hiddenduring LEP operation by the beam induced desorption.0510152025300 50 100 150 200 250 300Time (min)020406080100120RF screen of the bellowsFigure 2: Temperature increase on the bellows RFcontacts4 THE TEMPERATUREMEASUREMENTSThe temperatures of critical components weremonitored in order to locate critical points and to avoidoverheating.0102030405060Half cell numberGrey : Standard Physic pBlack : MD e- single beaFigure 3: Temperature distribution along the straightsection in point 6R.F. contacts used in the bellows to avoid cross-sectionchanges are delicate and can be damaged by short bunches.Figure 2 shows the temperature variation measured closeto the rf contacts in presence of a 6 mA beam. Thetemperature does not exceed 80 ºC and these contacts aresafe under the present operating conditions, provided thatthey are not exposed directly to the synchrotron light.179Chamonix IXTEMPERATUREVARIATION H.C.66117/8/98 - 10H11239661261861839639461439TIME110 ºC80 ºC4 mn.Figure 4: Temperature variation at the transitionsarc/straight sectionsThe temperature monitoring system permits thedisplay, as shown in Figure 3,of the temperature alongone straight section (here pt.6).The highest temperature is measured at the transitionbetween elliptical and circular cross-section (40 ºC/mAsingle beam) and is significantly higher on the "incomingelectron" side  (H.C. 661). his dissymmetry betweenelectrons and positrons is visible in almost all straightsections.The heating of this transition is very sensitive tothe beam conditions.Figure 4 shows as function of time, the evolution of thetemperature measured at the transitions between Arc andstraight section for all IP's. The transition 661 shows thehighest temperature (close to 110° C for a 4.7 mA beam).At 10h11 on the 17/08/98 this temperature decreasedsuddenly by 30° C and stabilised around 80 ° C. At thistime, a minor orbit change affected the orbits on the rightof IP6 (incoming electron side) and the control teamreported an RF shift. Although not fully understood, thesetemperature changes (recorded several times) and theasymmetry between electrons and positrons point to thefact that the heating of vacuum components might beaffected by slight differences on the beam trajectories.01020304050Half cell numberGrey : Standard Physic pBlack : MD e- single beaFigure 6: Temperature variation around IP1Finally, it should also be noted that a significant levelof radioactivity has also been measured on thesetransitions in the vertical plane.Other places in LEP have also shown worryingtemperature increases. For example, on a transition in IP7hit by the synchrotron light emitted by the wigglerswhere a steep temperature rise was recorded during theenergy ramp (Figure 5) reaching a maximum close to200° C.The injection regions seem to be more critical andespecially H.C. 124 where the light radiated in theinjection dipole by the electrons illuminates a transition.Figure 6 shows the temperature variation around IP1. Inthese injection regions, the temperatures are again higheron the parts irradiated by electrons than on the partsirradiated by positrons. To summarize, the temperatureincreases recorded this year, with the exception of theinjection region, do not show obvious weak points for theoperation at higher energy. The wiggler region in Point 7has been improved and with the replacement of theantenna cable in the modules, these devices will no longerneeded to be powered at high energy.05010015020010 20 30 40 50 60Time (min)02468EnergyCurrentFigure 5: The temperature variation at a transitionbeside the damping wiggler (H.C. 758)Chamonix IX180STATIC PRESSUREI~4mA  1.E-121.E-111.E-101.E-091.E-080 2 0 0 4 0 0DISTANCE TO IP (m)23-aoû-981 -sep -97PT 2S.C. CAVITIESIP2AFKSTATIC PRESSUREI~4mA  1.E-121.E-111.E-101.E-091.E-080 2 0 0 4 0 0DISTANCE TO IP (m)23-aoû-981 -sep -97PT 4ARCS.C. CAVITIESIP4STATIC PRESSUREI~4mA  1.E-121.E-111.E-101.E-091.E-080 2 0 0 4 0 0DISTANCE TO IP (m)23-aoû-981 -sep -97PT 6ARCS.C. CAVITIESIP6STATIC PRESSUREI~4mA   1.E-121.E-111.E-101.E-091.E-080 2 0 0 4 0 0DISTANCE TO IP (m)23-aoû-981 -sep -97PT 8ARCS.C. CAVITIESIP8Figure 7.: Pressure variation (Torr) around even IP’s inpresence of a 4 mA beam as a function of the distance ( inmeter) to the IP.5 THE PRESSURE MEASUREMENTSThe pressure in presence of a 4 mA beam is given forthe 4 even straight sections in Figure 7. For comparison,the squares indicate the pressure measured with a beam ofequivalent intensity during the 1997 running period.Pressures recorded during 1998 were consistently lowerthan in 1997 especially in the region close to the IP (~80m). In the straight sections with a circulating beam of 4mA, the measured pressures were close to 10-9Torr. In the arcs, the dynamic pressure was measured and theresults are given in Figure 8 for 18 gauges.DYN. PRESSUREEVOLUTION WITH TIME(ARCS)199 81.E-111.E-101.E-091 E-0812/ 05 /9 8 1/ 07/ 9 8 20/ 08 / 98 9/ 10/ 9 8DATEOPENED TO AIRNOT OPENED TO AIRFigure 8: Dynamic pressure (Torr/mA) in the pilot sectorsas a function of timeThe mean dynamic pressure in the arcs is 2.5x10-10Torr/mA.  One gauge (triangles) gives the pressure in asector exposed to air during the 1997-98 shutdown. Theinitial pressure increase at the start of the running periodwas more than two orders of magnitude higher than thatfor the gauges located in a sector not exposed to air. Atthe end of the running period, the difference between thesegauges was a factor of 4, showing the very efficientcleaning effect of the synchrotron light. 6 THE MEASUREMENTS OF THEGAS COMPOSITIONA calibrated residual gas analyzer has been installed in asector adjacent to an arc to monitor the gas compositionduring the operation of LEP. This is displayed as afunction of the beam dose in figure 9.The first spectrum taken at 45 GeV for a small dose(~ 20 mAxh) shows an equivalent pressure increase for themain gases H2, CH4, CO and CO2 close to 2x10-9Torr/mA. The dynamic pressure decreases differently forthe various gases: CH4 cleans very fast followed byCO2. After a dose of 180 mAxh H2 and CO are the mostabundant.181Chamonix IXEVOLUTION OF THE GAS COMPOSITION WITH THE BEAM DOSE (45 GeV)1.0E-121.0E-111.0E-101.0E-091.0E-080.00E+00 5.00E+01 1.00E+02 1.50E+02 2.00E+02 2.50E+02 3.00E+02BEAM DOSE (mA x h)H2 CH4CO CO28/10/9822/05/98Figure 9: Gas composition variation versus the beam doseat the start of the running period (45 GeV)On a longer dose scale, at 94.5 GeV (Figure 10), the gascomposition stabilises with a dynamic pressure increaseclose to 2 x10-10 Torr/mA for CO and H2 , 4 x10-11Torr/mA for CH4 and 2 x 10-11Torr/mA for CO2. Theeffect of the conditioning is visible by comparing, forexample, the pressure increase for CH4 and CO2 beforeand after the conditioning: CH4, not pumped by the getteris unaffected but CO2  decreases by a factor 2. The sameobservation is also valid for H2 andCO.EVOLUTION OF THE GAS COMPOSITION WITH THE BEAM DOSE94.5 GeV1.0E-111.0E-101.0E-091.0E-080.00E+00 2.00E+04 4.00E+04 6.00E+04 8.00E+04DOSE (mA* h)H2 CH4CO CO2CONDITIONNINGCONDITIONNINGFigure 10: Gas composition versus the beam dose at94.5 GeV7 BEAM - GAS LIFETIMELIFETIME 98-  45/94.5 GeV01 0 02 0 03 0 04 0 05 0 06 0 07 0 02 2 / 4 3 1 / 7 8 / 1 1DATE97 PREDICTION98 LIFETIME100 GeV105 GeVelectronspositronsFigure 11: Lifetime versus time during the 98 runningperiodThe lifetime of the electron and positron beams hasbeen obtained by measuring their intensity decay justbefore the beams were brought into collision. Thefollowing formula was used:1 1 1τ τ τb C V≈ +where :  t b is the measured beam lifetimet c is the Compton lifetime.  (~50 hours)t v is the beam - gas lifetimeThe Compton lifetime has been calculated byH. Burkhard3 and is much smaller (~50 hours) than thebeam - gas lifetime ( t v) (~600 hours). This, together withthe measurement uncertainties, leads to the importantscatter visible in Figure 11, which shows the evolution ofthe product t v by the beam intensity(I * t v). This product was 620 mAxh at 94.5 GeV. It wassystematically greater for the positrons than for theelectrons and decreased significantly when a stableoperation period was interrupted by a long period (days)with different beam conditions (e.g. power cut, machinedevelopment, etc..)8 VARIATION OF THE PRESSUREWITH THE BEAM ENERGY:THE FUTUREThe variation of the dynamic pressure in the pilot arcsectors has been studied as a function of the beam energyduring special energy ramps. The results of theseexperiments have been averaged for each gauge andnormalised to 1 at 94.5 GeV. These results are plotted inFigure 12 together with the variation of the emittedsynchrotron light power. A dotted line gives the averageChamonix IX182NORMALIZED PRESSURE INCREASE VERSUS BEAM ENERGYARC 19980.00.20.40.60.81.01.21.41.62 0 4 0 6 0 8 0 100 120BEAM ENERGY (GeV)11.251.5RADIATED POWERMEAN PRESSURE INCREASEFigure 12: Normalized pressure increase as a functionof the beam energyof the different pressure increases. This graph showsclearly that for energies above 70 GeV, the pressureincrease closely follows the evolution of the synchrotronlight power. At lower energies, the 2 curves diverge andthe mean pressure increase stabilises below 45 GeV,showing an enhanced efficiency of the photons for thedesorption in that energy range.This behavior can be understood by comparing inFigure 13 the increase of the power radiated by the beamsand the drastic decrease with the photon energy of thephotoelectric cross-section. Above 70 GeV, thephotoelectric absorption is negligible with respect to theCompton absorption that varies slowly in the photonenergy range corresponding to beam energies up to100 GeV. The consequence of this is that the absorbedpower leading to desorption follows directly the radiatedpower (E4) .Figure 12 shows the extrapolation of the pressure increaseto higher energies: at 100 GeV the expected pressureincrease will be in the arcs 1.25 greater than at 94.5 GeVand the product I * t v will be close to 500 mAxh.To be prepared for this 25% increase in absorbed power,and to the increasing power radiated in the vertical planeby the quadrupoles, new transition chambers have beendeveloped providing a better cooling, both in thehorizontal and the vertical plane. They will equip thetransitions between arcs and straight sections and theseparators close to the IP.9 CONCLUSIONSThe modifications introduced during the 1997-1998shutdown have significantly improved the reliability ofthe LEP vacuum system. The percentage of the downtime linked to vacuum problems has decreased from 23%in 1997 to 10% in 1998.LEP SYNCHROTRON RADIATION1.0E-061.0E-051.0E-041.0E-031.0E-021.0E-011.0E+000.01 0.1 1 10Photon ENERGY (MeV)0.0010.010.1110100Ec 45 GeVEc 80 GeVEc 100 GeVP H O T E L .  A B S .C O M P T O N .  A B S .22 GeV45 GeV80 GeV100 GeVFigure 13: Photon spectrum and absorption coefficientas a function of the photon energyThe new stand-by service, introducing youngtechnicians to emergency interventions on the SPS andLEP vacuum systems worked efficiently for bothmachines. Thanks to the new temperature and pressuremonitoring programs, it was possible to watch accuratelythe evolution of pressure and temperature in critical pointsand to give the necessary feedback to the operation team.Some anomalies were detected in the temperaturedistribution around LEP, e.g. the puzzling differences intemperature increase observed between elements exposedto the radiation of electrons and their symmetric elementsexposed to the radiation of positrons.The observations made in 1998 allow a confidentapproach to the LEP energy increase towards 100 GeV:the vacuum lifetime should not be a problem and thebackground should stay, after a period of initial cleaning,at an acceptable level. Nevertheless, particular attentionmust be paid to the injection regions where thetemperature increase might become critical at 100 GeVand with high currents.ACKNOWLEDGMENTSThe results exposed in this report and more generallythe overall performance of the LEP vacuum system aredue to the efficient work of many colleagues in the LHC-VAC Group as well as a fruitful collaboration with manymembers of the SL and EST Divisions.REFERENCES                                                1 LHC-VAC/SL,   “97 : A Hot Year for the VacuumSystem”,  Proceedings of the 8 Th LEP performanceWorkshop,  Chamonix 1998.2 J.P. Bacher, J. Jesse,R. Matet  Rapport 98/06/333 H. Burkhardt, SL/Note 93-73 (OP)183Chamonix IX

LEP Vacuum in 1998: Overview and Future Perspective

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