Meddahi, Malika

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Performance and running scenarios in the futureM. Meddahi, CERN, Geneva, SwitzerlandAbstractThe modifications to the cryogenic system of LEP and theRF systems allow to establish new limits on the possibleintensity and therefore on the luminosity performance. To-gether with the experience with the low emittance opticsin 1998 and the expected beam-beam tune shifts, estimatescan be made for the performance in 1999. Further changesto the horizontal and vertical beta function are discussedbased on past experience and running scenarios are pro-posed for optimum performance within the boundary con-ditions. A few speculative remarks are made on runningLEP in the year 2000.1 BOUNDARY CONDITIONS1.1 RF voltage and gradientsCavities Gradient (MV/m) MV272 Nb-Cu 6 277716 Nb 5 136Cu-system - 1201 klystron off - 81.7Total 2951Total available 2833(96 % efficiency)Table 1: RF voltages with SC gradient at 6 MV/m102/90 102/90Jx = 1.0 Jx = 1.5Avail. Voltage 2833 MV 2833 MVEmax (GeV) 97.0 96.5(τq = 24 h)Table 2: Maximum energy for phase 1The strategy for increasing the energy in 1999 and 2000is described in [1]. During a first stage the superconduct-ing accelerating gradient will be fixed to 6 MV/m, giving atotal available voltage of 2833 MV (Tab.1). From this volt-age we calculate the corresponding maximum energy forthe 102/90 optics and two different values of the hori-zontal damping partition number Jx (Tab.2). As a result,96 GeV is proposed as the energy for this first stage. At afixed energy, the gradient will then be slowly increased inCavities Gradient (MV/m) MV272 Nb-Cu 6.5 300916 Nb 5 136Cu-system - 1201 klystron off - 88.5Total 3177Total available 3050(96 % efficiency)Table 3: RF voltages with SC gradient at 6.5 MV/m102/90 102/90Jx = 1.0 Jx = 1.5Avail. Voltage 3050 MV 3050 MVEmax (GeV) 98.8 98.3(τq = 24 h)Table 4: Maximum energy for phase 2order to prepare the next stage.During this second stage the gradient in the SC cavities willbe fixed to 6.5 MV/m. From Tab.3 and Tab.4, 98 GeV is theproposed energy for the second stage.The third stage will depend on the available SC accelerat-ing gradient and it is proposed to aim for 7 MV/m. Thecorresponding available voltage (Tab.5) and the possibleenergy (Tab.6) lead to suggest an energy of 100 GeV.1.2 Intensity limitationsThe various intensity limitations were presented in detailsin 1998 [2, 3]. Only the modifications affecting theoperation in 1999 are discussed here. More details can befound in [4].Cavities Gradient (MV/m) MV272 Nb-Cu 7 324016 Nb 5 136Cu-system - 1201 klystron off - 95.3Total 3401Total available 3265(96 % efficiency)Table 5: RF voltages with SC gradient at 7 MV/mChamonix IX278102/90 102/90Jx = 1.0 Jx = 1.5Avail. Voltage 3265 MV 3265 MVEmax (GeV) 100.5 100.05(τq = 24 h)Table 6: Maximum energy for phase 3Units Power (MW)36 klystrons 361 klystron off - 1Cu-system 0.5Total available 34.1(96 % efficiency)Table 7: RF powerThe total intensity limit (2I0) from the RF power (Prf ) isgiven by Prf = (2I0)U0 where U0 is the energy loss perturn. The total available RF power is 34.1 MW (Tab.7) andthe corresponding maximum intensity is summarized inTab.8. This limit is above 10 mA.The intensity limit by cryogenic cooling power wasaddressed during the workshop [5]. The cooling power at4.5 K per module is given by :Pcm =4(V (Ea))2(R/Q)Q(Ea)+Rm(σs)(2Itot)22kbwhere V (Ea) is the total voltage from one cavity (func-tion of the accelerating gradient Ea), R/Q is the normalisedshunt impedance, Q the quality factor of the cavity, Rmthe loss impedance (function of the bunch length, σs) andkb the number of bunches per beam. The first term de-scribes the power required to cool the RF system (RF dis-sipation) and the second term describes the beam induceddynamic load. This limit has been modified from experi-ence/measurements obtained during the 1998 run.Recent measurements on SC cavities have shown that thelinear dependence of the quality factor as a function of theaccelerating gradient was too pessimistic. A new model(logarithmic and according to RF specifications) is as-sumed, yielding larger Q values at high gradients (Tab.9).As a consequence the cryogenics requirements are reducedand it allows to operate at a slightly higher energy and/orwith an increased total current.Loss impedance measurements performed on the SC cav-ities equipped with new field probe cables showed thatthe new cavities contributed much less to the impedance.E(GeV) 96 98 100U0 (GeV) 2.48 2.70 2.922I0 (mA) 13.7 12.6 11.7Table 8: Maximum intensity from RF powerValues of 1998 New values for 1999Q (6 MV/m)[109] 3.2 3.2Q (7 MV/m)[109] 2.3 2.85Table 9: Quality factor10 MΩ were measured instead of the 16 MΩ (for σs=9 mm)measured in 1997 with the cavities equipped with old fieldprobe cables. The corresponding maximum possible cur-rents tolerated by the cryogenics budget are above 10 mA(Tab.10).As a conclusion, up to 100 GeV, the total intensity limit forE(GeV) 96 98 1002I (mA) 16.1 14.5 12.2(Jx = 1)2I (mA) 15.8 14.1 11.5(Jx = 1.5)Table 10: Maximum intensity from the cryogenics budget1999 is above 10 mA and is also limited to1.0 mA/bunchfrom TMCI. Consequently, the proposed intensities forrunning in 1999 are 6 and 8 mA (with 4 bunches per beam)and 10 mA with 6 bunches per beam.1.3 Beam-beam limitFig.1 represents the maximum vertical beam-beam param-eter (ξy) recorded for many 1998 fills as a function of thecorresponding total intensity (intensity for which ξy wasmaximum). In Fig.2, ξy is averaged in bins of 0.1 mA.The two figures do not show any sign of beam-beam pa-rameter saturation with increasing current.This is also illustrated with the evolution of ξy dur-Figure 1: ξy as a function of the total currenting fill 5259 (Fig.3). As a result, it was proposed to use279Chamonix IXFigure 2: Average ξy in bins of 0.1mA vs currentFigure 3: ξy for Fill 5259 vs currenta maximum beam-beam parameter of 0.08 for the perfor-mance estimate calculation. Such beam-beam parameterlimit would be for example achieved with intensities de-scribed in Tab.11 for various energies (assuming 4 bunchesper beam, an emittance ratio (κ) of 1.2 % and Jx = 1.5).Above these currents, beam blow up is expected and a wayout would be to use more bunches per beam (i.e 6+6).E(GeV) 96 98 1002I0 (mA) 8.1 8.6 9.1Table 11: Maximum intensity at beam-beam parameterlimit1.4 Horizontal and vertical squeezeDuring 1998, three different physics periods can be distin-guished. During the first period (up to fill 5140), βx = 1.5 mand βy = 5 cm were used and both ξy and the peak lumi-nosity were increasing, as the total current was increased(Fig.4, Fig.5).For the second period (fills 5140-5262), βx was squeezedto 1.25 m while βy remained unchanged. Both ξy and thepeak luminosity were increasing compared to the first pe-riod, without increasing further the total current.Finally, from fill 5263 onwards, during the last physicsperiod, βy was squeezed to 4 cm while βx remained un-changed. The result was a decrease of ξy and an increaseof the luminosity compared to the second period, while thetotal current remained unchanged.Figure 4: Beam-beam parameter as a function of fill num-berFigure 5: Luminosity as a function of fill numberChamonix IX280In order to quantify these changes and to study whetherthe effects of the horizontal and vertical squeezes were asexpected, one fill in each of the three physics periods is pre-sented (Tab.12).For the horizontal squeeze, for the same bunch intensity,βx βy ξy ξx Lum. I/bunch frfm cm 1031 µA Hz1.50 5 0.062 0.042 7.3 750 1001.25 5 0.076 0.042 9.0 750 1201.25 4 0.061 0.042 9.5 750 120Table 12: Three fills during the different running periodssqueezing the horizontal beta value from 1m5 to 1m25 andincreasing the RF frequency shift from 100 Hz to 120 Hz,is expected to yield 20 % increase in both the ξy and peakluminosity. Tab.13 shows that the expectations were effec-tively met.βx Meas. ξy(expected) Meas.Lum. (expected)(m) (1031 cm−2s−1)1.50 0.062 7.51.25 0.076 (0.075) 9.0 (9.0)Table 13: Measured and expected ξy and peak luminositiyFor the same bunch intensity, and a vertical squeeze from5 cm to 4 cm, one expects 11 % decrease in ξy and 11 %increase in luminosity. Tab.14 shows that the expectationwere met in ξy and close in luminosity. The performancegain from the squeeze was also discussed in [6].Possibilities to squeeze further the β have already beenβy Meas. ξy(expected) Meas.Lum. (expected)(cm) (1031 cm−2s−1)5 0.076 9.04 0.068 (0.067) 9.5 (10.0)Table 14: Measured and expected ξy and peak luminosityaddressed in [7, 8].For the horizontal squeeze, background considerations dueto the aperture in the horizontally focusing quads limit themaximum βx through the following expression :βxx>1.2546 10−9For Jx =1, the limit is 1.21 m at 100 GeV (Tab.15) which isvery close to the operational value (1.25 m).If one would like to always fit in the machine an optics witha Jx =1 and keep the same optics all the way up to 100 GeV,1.25 m would be the right choice. Also, it was shown [9]that the background storms observed during physics fillsand optimised by small horizontal tune change, are less se-vere for larger βx, which would bring another argument tonot further reduce this value.Concerning a further reduction of the vertical beta func-Energy (GeV) 96 98 100x(nm) 41 42.8 44.6βx min (m) 1.11 1.16 1.21Table 15: Minimum βx for different energytion, beam-beam effects are imposing a limit on the βy inorder not to affect the luminosity. This is the hour glasseffect which was discussed for LEP in [10].With βy  2σl and for σl = 1cm , the limit on βy is 2 cm,compared to the present value of 4 cm.A further reduction has to be tested in order to study its fea-sibility in operation.However an attempt to match the physics optics at βy =3.5 cm showed that βy values were increasing in QS3-QS4 (200 m at collimator). Also, the necessary sextupolestrength on SD1 severely increased (from KSD1=−.35 at4cm to KSD1=−.38 at 3.5cm). This would limit this 3.5cm optics to 94.4 GeV (max−.036). It has to be remindedthat large value of the sextupole strength enhances exci-tation of non-linear resonances and increases the tune de-pendence with amplitude. Both effects reduce the dynamicaperture [11].With the 3.5 cm optics, the non-linear chromaticity cor-rection showed a reduced aperture in momentum p/p =0.013 (was 0.015) and it was demonstrated in [12] thatfurther reduction of the βy would lead to marginal momen-tum aperture.In summary, it is recommended to use βx of 1.25 m and βyof 4 cm, at the start-up and through 1999.2 PERFORMANCE ESTIMATE IN 1999Tab.16 summarized the parameters used for the 1999 per-formance estimate. As can be observed in Fig.6 and Fig.7,Energy (GeV) 96 98 100Jx 1.5 1.5 1.5VRF (MV) 2833 3050 3265x (nm) 27.4 28.5 29.7σs (mm) 10.8 11 11.2βx (m) 1.25 1.25 1.25βy (m) 0.04 0.04 0.04f (Hz) 90 90 90κ (%) 1.2 1.2 1.2Table 16: Proposed parameters for 1999 with the 102/90the only performance limitations in 1999 will come fromboth the maximum possible current to be accumulated inLEP and from the achievable accelerating gradient.The integrated luminosity Lint is given by Lint = η L0 Twith the global efficiency η = 0.2 or 0.15, L0 the peak lu-minosity and T the total time considered. In Tab.17, three281Chamonix IXenergy cases are treated (96, 98, 100 GeV) for three in-tensities (6, 8, 10 mA). For 6 mA, the expected integratedluminosity is  1.5 pb−1/day, assuming a global efficiencyof 0.20. While increasing the intensity and the energy, theglobal efficiency is expected to be reduced. At 8 mA theluminosity is constant with the energy E ( 2 pb−1/day),due to the fact that in the range of energies considered theratio ξy  E is constant.96 GeV 6 mA 8 mA 10 mAL0 /1030 (cm−2s−1) 92.5 145 170Lint (pb−1)/day, η=.2 1.59 2.50 2.93η=.15 1.19 1.88 2.2098 GeV 6 mA 8 mA 10 mAL0 /1030 (cm−2s−1) 88.7 145 163Lint (pb−1)/day, η=.2 1.53 2.50 2.82η=.15 1.15 1.88 2.11100 GeV 6 mA 8 mA 10 mAL0 /1030 (cm−2s−1) 85.2 145 156Lint (pb−1)/day, η=.2 1.47 2.50 2.70η=.15 1.10 1.88 2.03Table 17: Performance estimate: 102/90 Jx = 1.53 LIMITS AND ESTIMATE FOR 2000In 2000, the maximum SC accelerating gradient will stillremain a challenge and the cryogenic cooling power willstart to limit the performances. This is illustrated in Fig.6where for energies above 100 GeV, the 6 and 8 mA curvescut the cryogenics limit curve at 7.4 and 7.6 MV/m respec-tively.The corresponding maximum energies for the SC gradientare summarized in Tab.18.Tab.19 summarizes the maximum intensity from cryo-Ea 7.2 7.4 7.6E (GeV) 101.6 102.3 102.8(Jx = 1)E (GeV) 101.1 101.8 102.3(Jx = 1.5)Table 18: Maximum energy for different SC gradientsgenic cooling power, and it can be seen that the intensitydecrease is very steep above 100 GeV.A word of caution has to be added regarding the maximumE(GeV) 100 102 1032I (mA) 12.2 8.4 5.0(Jx = 1)2I (mA) 11.5 7.1 1.4(Jx = 1.5)Table 19: Maximum intensity from the cryogenics budgettolerable horizontal emittance in LEP (Tab.20). It remainsto be demonstrated that a horizontal emittance of 47.3 nmat 103 GeV can be accommodated in the machine.E(GeV) 100 103x (nm) 44.6 47.3 !(Jx = 1)x (nm) 29.7 31.5(Jx = 1.5)Table 20: Horizontal emittance for 102/90 optics4 CONCLUSIONSThe performance limitations in 1999 will depend on themaximum possible intensity and on the maximum reliableaccelerating gradient. For 2000, the cryogenics coolingpower will also be a limitation for energies above 100 GeV.For 1999, the following recommendations were made : Operate at 96 GeV, 98 GeV and aim for 100 GeV Use the 102/90 optics with a Jx = 1.5, βx = 1.25 mand βy = 4 cm Start with a total intensity of 6 mA and then 8 mAusing 4 bunches per beam When the total intensity available would exceed 8 mA,6 bunches per beam could be considered.Performance estimates are given for 1999 and with6 mA the integrated luminosity is expected to be about1.5 pb−1/day. With 8 mA, 2 pb−1/day are expected. How-ever it is emphasized that for high intensity and high energythe global efficiency is likely to be reduced.5 REFERENCES[1] G. Geschonke; Strategy for increasing the energy in 1999;These Proceedings.[2] D. Brandt, K. Hu¨bner, S. Myers On LEP Performance Limitsin 1999 and 2000; CERN SL/98-005 (DI).[3] D. Brandt; Performance and running scenarios in the future;Proceedings of the eighth LEP performance workshop, Cha-monix, January 27-30, 1998, and CERN SL/98-006 (DI).Chamonix IX282[4] D. Brandt, S. Myers LEP Performance Limits re-visited;CERN SL/98-074 (DI).[5] P. Gayet; Cryogenics issues for LEP in 1999; These Proceed-ings.[6] M. Lamont; Review of 1998; These Proceedings.[7] A. Verdier; Reduction of β; Proceedings of the seventh LEPperformance workshop, Chamonix, January 14-17, 1997, andCERN SL/97-06 (DI).[8] J. Gareyte; LEP2 Optics : Options and Implications; Pro-ceedings of the eighth LEP performance workshop, Cha-monix, January 27-30, 1998, and CERN SL/98-006 (DI).[9] G. von Holtey; Particle background in LEP experiments andbeam tails; These Proceedings.[10] S. Myers; Simulation of the beam-beam effect for e+ e−storage rings; Nucl. Inst. and Meth. 211 (1983) 263-282,North Holland Publishing Company.[11] D. Brandt et al; A candidate low emittance lattice for LEPat its highest energies; EPAC-98, Stockholm, June 1998 andCERN SL-98-030 (AP).[12] A. Verdier; Optics limits for the highest energy; These Pro-ceedings.283Chamonix IXPerformance Limits (102/90 - Jx = 1.5 - 4 bunches)(Q(Ea) = 10**(9.806 - 5.017e-2 * Ea (MV/m)))02040608010012014016018020096 97 98 99 100 101 102 103Energy (GeV)Lo/10**30 (cm-2s-1)Itot = 8 mAItot = 6 mA6.2 kW12 kW6.2 kW12 kW6 mA (5.9 h)8 mA (4.8 h) TMCI6 MV/m7 MV/m 7.4 MV/m7.6 MV/mFigure 6: Performance limitations for 4 bunchesPerformance Limits (102/90 - Jx = 1.5 - 6 bunches)(Q(Ea) = 10**(9.806 - 5.017e-2 * Ea (MV/m)))02040608010012014016018020096 97 98 99 100 101 102 103Energy (GeV)Lo/10**30 (cm-2s-1)Itot = 8 mAItot = 10 mA6.2 kW12 kW6.2 kW 12 kW8 mA (6.3 h)6 MV/m7 MV/m10 mA (5.3 h)Figure 7: Performance limitations for 6 bunchesChamonix IX284

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