The LEP storage ring has been operated at energies above 90 GeV for more than 1000 hours during 1997. Because of the rapid increase with the beam energy of the power radiated as synchrotron light, the vacuum system has been submitted to very stringent conditions as far as power evacuation and photon stimulated gas desorption are concerned. The operational experience acquired under these, up to now unexplored, conditions will be reviewed together with an outline of the limitations which were experienced at these high levels of radiation in the use of the available vacuum instrumentation. Based o n the available data detailed predictions concerning the beam lifetime, gas desorption and beam cleaning of the vacuum system under the impact of photons with a critical energy approaching 1 MeV will be formulated

Billy, J C

Bojon, J P

Hilleret, Noël

Jiménez, M

Laugier, I

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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHLaboratory for Particle PhysicsDivisional Report CERN LHC/98-4 (VAC)Experience with the LEP Vacuum System at Energies Above 90 GeVand Future ExpectationsJ.C. Billy, J.-P. Bojon, N. Hilleret, M. Jimenez, I. LaugierAbstractThe LEP storage ring has been operated at energies above 90 GeV for more than 1000 hours during1997. Because of the rapid increase with the beam energy of the power radiated as synchrotron light,the vacuum system has been submitted to very stringent conditions as far as power evacuation andphoton stimulated gas desorption are concerned. The operational experience acquired under these, upto now unexplored, conditions will be reviewed together with an outline of the limitations which wereexperienced at these high levels of radiation in the use of the available vacuum instrumentation. Basedon the available data detailed predictions concerning the beam lifetime, gas desorption and beamcleaning of the vacuum system under the impact of  photons with a critical energy approaching 1 MeVwill be formulated.EPAC’98 Conference, 22-26 June 1998, Stockholm, SwedenAdministrative Secretariat Geneva, SwitzerlandLHC Division 14 July 1998CERNCH - 1211 Geneva 232EXPERIENCE WITH THE LEP VACUUM SYSTEM AT ENERGIES ABOVE90 GEV AND FUTURE EXPECTATIONSJ.C. BILLY, J.P. BOJON, N. HILLERET, M. JIMENEZ, I. LAUGIERLHC-VAC, LHC Division, CERN, Geneva, SwitzerlandAbstractThe LEP storage ring has been operated atenergies above 90 GeV for more than 1000 hours during1997. Because of the rapid increase with the beam energyof the power radiated as synchrotron light, the vacuumsystem has been submitted to very stringent conditions asfar as power evacuation and photon stimulated gasdesorption are concerned. The operational experienceacquired under these, up to now unexplored, conditionswill be reviewed together with an outline of thelimitations which were experienced at these high levels ofradiation in the use of the available vacuuminstrumentation. Based on the available data detailedpredictions concerning the beam lifetime, gas desorptionand beam cleaning of the vacuum system under theimpact of  photons with a critical energy approaching 1MeV will be formulated.1  INTRODUCTIONThe LEP accelerator is now in operation since1989 . After a first period of physics at 45 GeV, it is nowrunning routinely at energies higher than 90 GeV. Areview of the main characteristics and of the pastperformance of the LEP vacuum system can be found inreferences 1, .2. At energies exceeding 90 GeV, theamount of power radiated by the electron and positronbeams exceeds 500 W/m for a total current of 4 mA. Thislarge power, deposited mainly in the vacuum chamberstimulate the desorption of gas and  can lead in someplaces to the creation of leaks. A brief summary of theoperational experience gathered during 1997 will bepresented. Furthermore, in the evaluation of thepossibility to run LEP at higher energies, the achievablebeam gas lifetime at 100 GeV is a very importantconsideration. An attempt to extrapolate pressuremeasurements made above 80 GeV for the prediction ofthe beam-gas lifetime at 100 GeV will be given.2.  SYNCHROTRON POWERDEPOSITIONWith the constant increase in beam energy, theenergy spectrum of the LEP synchrotron radiation hasconsiderably changed. On figure 1, the power radiated permeter of chamber, per mA of stored beams and per keV ofphoton energy interval is given as a function of thephoton energy for different beam energies. On the samefigure, the mass attenuation coefficients for thephotoelectric effect and for  the compton effect are alsodisplayed as a function of the photon energy.The critical energy of these photons varies between 7.6and 720 keV when the beam energy changes from 22 GeVto 100 GeV.3.  RADIATION EFFECTS ONTHE VACUUM SYSTEMMost of the vacuum problems which occurred inLEP during the 1997 running period were due to anexcessive unhomogeneous heating of Conflat® vacuumflanges adjacent to cross section changes. These flangeswere exposed to a higher heat load deposited very locally,thus mechanically distorting the flange-gasket assembly.In one occasion, the repeated thermal cycles due to theLEP operation (acceleration and coast) have led to thefailure of a poorly penetrated weld. The design of thesetransitions have been improved to avoid these failures.The main improvements consisted in:-Improving the cooling of the zone where the poweris deposited-Avoiding the excessive heating of the flanges byprotecting them from the impact of the hardsynchrotron radiation using external copperabsorbers.As a matter of fact the very hard part of thesynchrotron radiation spectrum can escape through1.E-061.E-051.E-041.E-031.E-021.E-011.E+000.01 0.1 1 1 0Photon ENERGY (MeV)POWER (W/mA/m/keV)0.0010.010.111 0100Mass Attenuation ( cm2/g)100 GeV80 GeV45 GeV22 GeVPhotoelec.absComptonAbsFigure 1: Synchrotron radiation spectrum atvarious beam energies3the vacuum chamber and represents a powersufficiently large to overheat massive elemnts likeflanges assembly.More details on damages created by the synchrotronradiation on LEP equipment can be foundin reference 34.  DYNAMIC PRESSUREThe total pressure in an operating electronstorage ring is the sum of 2 main contributions:-The static pressure, measured in the absence ofbeam, which is due to the thermal degassing of thevacuum chambers.-The dynamic pressure (D.P.), measured in thepresence of beams, which results from the desorptionof various gases, mainly H2  and CO, under thephoton impact. This D.P. is proportional to thebeam current and is expressed in Torr/mA.The D.P. was measured during the first part of the 1998LEP running period and typical results obtained in an arcsector (hence exposed to the full radiation of the mainbending magnets) are given on figure 2. Before operationthe static pressure is closed to 10 -11 Torr. During the firstperiod of operation at 45 GeV, the initial D.P. was 10-10Torr/mA decreasing very quickly ( beam dose~ 200 mA.h)to 10-11 Torr/mA. The increase of the beam energy from45 to 94.5 GeV resulted in a D.P. increase by a factorclose to 165.   BEAM ENERGY DEPENDENCEOF THE D.P. DURING 1998In order to study its dependence on the beamenergy, the evolution of the D.P. in the arcs of LEP wasmeasured as a function of the beam energy (E). Thepressure was measured on Bayard-Alpert gauges at thefollowing beam energies: 22, 45, 65, 80, 86, 91.5 GeVand the corresponding D.P. calculated. The results aregiven figure 3 after normalisation to 1 at 45 GeV. Thesame graph shows also the total power radiated by thebeam, normalized to 1 at 45 GeV. The decrease of thenormalized D.P.between 91.5 and 45 GeV exhibits thesame E4 dependence as the total emitted power. Below 45GeV this E4 dependence is no more valid for the D.P.which decreases only slightly (a factor 3 instead of theexpected 15 if the E4 law was followed).0.010 .111 01 0 02 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 00          BEAM ENERGY (GeV)NORMALISED  ENERGY                    ABSORBED PRESSURE INCREASENORM P DYN2NORM P DYN1TOTAL RADIATED POWERP.DYN.Figure 3: Comparison between the totalemitted power and the pressure increaseThis effect can be explained by considering the strongdecrease in the photon attenuation (see figure 1) due to theimportant loss of efficiency of the photoelectric effectwhen the photon energy increases from 7.6 keV ( thecritical energy at 22 GeV) to 65 keV (the critical energycorresponding to a 45 GeV beam). This very strongreduction of the photon absorption cross section partlycompensates, between 22 and 45 GeV the increase ofradiated power. On the contrary, for beam energiesbetween 45 and 100 GeV, the compton scatteringbecomes the dominant attenuation process and theattenuation in the vacuum chamber of the emittedphotons is only slowly varying with the photon energy.Under these circumstances, the energy deposition rate isunchanged and the amount of absorbed energy variesproportionally to the incident energy flux. Hence thedesorption rate, directly related to the energy depositionrate follows the same E4 law as the radiated power.6.   LIFETIME CONSIDERATIONSThe observed lifetime in LEP is the combinationof various effects leading to a decrease with time of thebeam intensity. In the absence of collisions, the lifetime4is mainly given by:1 1 1τ τ τb C V≈ +where :  t b is the measured beam lifetimet c is the Compton lifetime.t v is the beam - gas lifetime1E-121E-111E-101E-092 / 0 5 7 / 0 5 12 /05 17 /05 22 /05 27 /05 1 / 0 6DATEDYN. PRESSURE (Torr/mA)45 GeV 94.5 GeVFigure 2: D.P. evolution in a LEP arc4The beam-gas lifetime is of course depending on the beamintensity (I) and can be more easily compared from fill tofill by using the product : I * t v. which is a constant for agiven D.P., i.e. at a given level of cleaning and pumpingspeed.0 .100.200.300.400.500.600.700.800.900.1000.1100.1200.1300.1400.1500.3 1 / 7 2 0 / 8 9 / 9 2 9 / 9 19 /10 8 / 1 1 28 /11DATELIFETIME * BEAM INTENSITY   (mA*h)f i tMEASUREMENTSFigure 4: Evolution of I * t v during 1997This product has been calculated during the 97 runningperiod by measuring the decay of the total beamsintensity before they were brought into collisions. Theresults are given on figure 4. The scatter between pointsis relatively large and can be explained by several reasons:- t v is long with respect to t c (~50 hours) hence asmall error on t b has a great influence on t v- These measurements were done parasitically andsimultaneous adjustment on the beam could haveinfluenced t b in an uncontrolled wayA fitted line shows an increase in the product I * t v thatcan be attributed to a beam cleaning effect. The I * t vproduct calculated from these data will be usedsubsequently for the estimation of the vacuum behavior athigher energies.7.  BEAM-GAS LIFETIME ATHIGHER ENERGIESThe beam gas lifetime is directly proportional tothe pressure  and more specifically to the D.P. in the arcsas it is in our case, the dominant contribution to theintegrated pressure. Hence a prediction of I * t v at higherenergies can be based on the dependence of the D.P. onthe beam energy. Considering now the high energy partof the preceding measurements (above 80 GeV),normalized to 80 GeV, the variation of the D.P. can beextrapolated for higher energies. The results are given onfigure 5. it is possible to give a forecast of the product I *t v at higher energy. For 1998 this product should decreaseby a factor 1.9 with respect to the situation at 80 GeV,i.e. a product I * t v  close to 700 mA.h. At 100 GeV, itshould decrease by a factor 1.5 with respect to the actualsituation, giving a product I * t v still greater than 500mA.h.11.21 .41 .61 .822 .22 .42 .680.00 90.00 100.00BEAM ENERGY (GeV)DYNAMIC PRESSURE RATIO1998 : 1.872000 :  2.481997 : 1.68Figure 5: Variation of the D.P. at energiesabove 80 GeV8.   CONCLUSIONSThe large energy increase of the synchrotronration power irradiating the LEP vacuum system createdsome difficulties, mainly located at cross section changesand due to a strong inhomogeneous heating of flanges.Nevertheless the beam gas lifetime and the backgrounddue to the scattering of particles on the residual gas weresufficiently low to ensure good operating conditions forthe high energy physics experiments. Using themeasurements of the D.P. variation with the beamenergy, it was shown that the desorption induced by thesynchrotron light stays proportionnal to the total radiatedpower for beam energies above 45 GeV. This dependenceallows to calculate the beam gas lifetime for higherenergies corresponding to the exploitation of LEP in thenear future. These predictions were confirmed by themeasurements of the product I * t v carried out at 94.5GeV in 1998.ACKNOWLEDGMENTThe authors are indebted to Dr. O. Gröbner and P.M.Strubin for many fruitful discussions.                                                REFERENCES1 C. Benvenuti, Nucl. Inst. Meth 205, 391-401, 19832 J.P. Bojon, O Gröbner,J.M. Laurent, P.M. StrubinProc. 3 rd. EPAC, pp 1564-1566 Berlin 19923 R. Bailey, B. Bahlan and al. WEP01H this conference4 H. Burkhardt, SL/Note 93-73 (OP) 1993

Experience with the LEP vacuum system at energies above 90 GeV and future expectations

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