The LEP magnetic bending field and therefore the beam energy is changed by a current flow over the vacuum chamber. The current is created by trains travelling between the Geneva main station and destinations in France. Some of the rail current leaks into earth and returns to the power station via the LEP tunnel, where the vacuum chamber is one of the conductors. Train leakage currents penetrate LEP at the injection lines from the SPS close to IP1 and between IP5 and IP7, thereby interacting with the magnetic dipole field. The observed changes in B field cause beam energy increases of several MeV

Brun, G

Dehning, Bernd

Drees, A

Galbraith, Peter

Geitz, M A

Henrichsen, K N

Koratzinos, M

Placidi, Massimo

Puzo, P

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A newly observed effect affects the LEP beam energyG. Brun, B. Dehning, P. Galbraith, K. Henrichsen, M. Koratzinos, M. Placidi, P. PuzoCERN, Geneva, SwitzerlandA. Drees, BUGH, Wuppertal, Germany, M. Geitz, RWTH, Aachen, GermanyAbstractThe LEP magnetic bending field and therefore the beam en-ergy is changed by a current flow over the vacuum cham-ber. The current is created by trains travelling between theGeneva main station and destinations in France. Some ofthe rail current leaks into earth and returns to the power sta-tion via the LEP tunnel where the vacuum chamber is oneof the conductors. Train leakage currents penetrate LEP atthe injection lines from the SPS close to IP1 and betweenIP5 and IP7, thereby interacting with the magnetic dipolefield. The observed changes in B field cause beam energyincreases of several MeV.1 INTRODUCTIONThe LEP energy was determined with high precision in 1995to improve the errors on the Z mass and width and to con-firm the results of the 1993 measurements [1]. For thefirst time since LEP was commissioned the bending fieldhas been monitored with probes installed inside the tunneldipole magnets.2 MAGNETIC FIELD MEASUREMENTSThe magnetic field was measured simultaneously with 4NMR probes in almost all fills. Figure 1 gives an overviewabout the position of all probes with respect to LEP. TwoNMR probes are located in the reference magnet and the twoother NMR probes in the tunnel dipole magnets close to IP4(NMROCT4) and to IP8 (NMROCT8).The probes in the tunnel magnets are installed in a slit be-tween the pole faces of the magnet and the vacuum chamberwall (fig. 2). At this off centre position the field homogene-ity is ensured by an additional field plate. Induction wiresare installed in the field gap of the bending magnets to mea-sure magnetic flux changes. The induction wires form a oneturn loop (flux loop). The total LEP ring is divided into eightparts such that one flux loop is extending over an octant.The magnetic field for a long LEP fill (fig. 3) shows frommidnight to around 4:00 only a slow drift. The two otherperiods show fast field variations. The field increase dur-ing the 14 hour measurement corresponds to a beam energyrise of 12 MeV. The steps occur each time after a strongfluctuations of the B field. The B field shows a tendencyof saturation at the end. The difference of the tunnel andreference NMR readings is displayed in figure 4. The tun-nel NMR probes expose strong field variation during day-NMRREFNMRVACIP 1OPALSUISSESPSAIRPORTPSLEPCERN PREVESSINCERN MEYRINNMROCT4NMROCT8DELPHIL3ALEPHFRANCEIP 5IP 2IP 3IP 4 IP 6IP 7IP 8Figure 1: LEP ring with its experiments and the positionsofthe 4 NMR probes. Two probes, NMRREF and NMRVAC,are installed in the reference magnet in a surface buildingclose to IP2 (L3) while NMROCT4 and NMROCT8 aremounted underground in two LEP bending magnets nearIP4 and IP8.Excitation bars NMR-probeField plateBeam pipeFlux loopIron yokeFigure 2: LEP tunnel bending magnet with the NMR probeoff centre in the slit between field plate and the vacuumchamber wall. The two flux loop wires are situated under-neath the vacuum chamber.time, whereas the reference show almost no time depen-dance. This characteristic calm period occurs every night.The short term variations (t  5 min) of the tunnelNMR probes are strongly anti-correlated. If NMROCT8measures a magnetic field increase, NMROCT4 measuresa decrease and vice versa. Anti-correlation is presented infigure 5.5 MeVStepNoisy period Calm periodStepbehaviourDriftbehaviourLevel change16:00 18:00 20:00 22:00 00:00 02:00 04:00 06:0046474464784648246486464904649446498time of day [h]beam energy [MeV]bending field [G]509509.1509.2Figure 3: Magnetic field in one tunnel magnet close to IP8as function of the time in the fill. The B field is expressedin corresponding units of the beam energy.bending field [Gauss]NMROCT8beam energy [MeV]NMROCT4NMRREFtimeNMRVAC20:00 00:00 04:00 08:00509.15509.25464804649046500509.15509.25464804649046500849.50849.60464804649046500880.75880.85464804649046500Figure 4: Bending field in Gauss of all NMR probes duringone fill. The dotted lines denote the start and the end of thecalm period between midnight and four o’clock .The flux loop and the NMR readings are correlated. Sinceflux loops measure the time derivative of the magnetic field,changes of the field level should be accompanied by peaksof the flux loop signal. Figure 6 shows the time dependenceof both signals.The characteristic calm period is also recorded by the fluxloop (fig. 7, octant 8). The flux loops in the two ring oc-tants which have NMRs record a short term anti-correlation(fig. 8). The measured correlation coefficient is -2.0. Theadjacent flux loops show almost the same characteristic sig-nals. These obervations and the fact that flux variations in-side the gap of the magnet are likely to be caused by currentson the vaccum chamber point to global current flow.∆BNMROCT4 [G]∆BNMROCT8 [G]-1.7-0.1-0.0500.050.1-0.1 -0.05 0 0.05 0.1Figure 5: Anti-correlation of NMROCT4 and NMROCT8.The differences of subsequent magnetic field measurementsB of NMROCT4 and NMROCT8 are correlated with acoefficient of -1.7.time of fill [h]BNMROCT8 [G]time of fill [h]∆B/∆t*3sec [G]489.75489.8489.85489.922:59 23:01 23:03-0.100.122:59 23:01 23:03Figure 6: Flux loop readings in octant 8 (bottom) comparedto NMROCT8 measurements.The path of the currents can be determined by measure-ments at different locations. For technical reasons a currentis measured on a earth cabel which is connected in parallelto the vacuum chamber every 40 m. Current clamps wereused to measure the parasitic flow. Figure 9 shows the cor-relation factor of two simultaneous measurements taken atdifferent places along the LEP tunnel. The lower part showsthe correlation factor as a function of the position in the stor-age ring. The upper part of figure 9 shows the correlationplot projected on the LEP ring. One device acts as a ref-erence and stays at the magnet which is equipped with theNMROCT8 probe, whereas the other is displaced. A cor-time of fill [h]∆B/∆t*3sec [G]16:00 20:00 00:00 04:00-0.100.1Figure 7: The flux loop readings in octant 8 note the calmperiod between midnight and four o’clock as well as theNMR probes.∆Bfluxloop 4 [G]∆Bfluxloop 8 [G]-2.0-0.200.2-0.2 0 0.2Figure 8: Anti-correlation of the flux loop readings. Thedifference of consecutive flux loop measurements in octant4 and 8 displayed one against the other.LEPSPSCERNairportBellegardeLausanneLa VersoixLac deGeneveMeyrinZimeysaCornavinIP1IP2IP3IP4IP5IP6IP7IP81 kmCorrelation versus IP1 2 3 4 5 6 7 8-1-0.500.51Figure 9: Correlation of earth cable currents within LEP.Bottom: The correlation factor of two simultaneous earthcurrent measurements near at position of NMROCT8 andthe position marked on the horizontal axis is shown. Top:Map of the Geneva area with LEP, the river La Versoix,the Geneva lake and the railways to Lausanne, Bellegarde(Lyon, Paris) and the airport.relation denotes a current flow in the same direction of thering, an anti-correlation an opposite flow. The correlation ischanging close to IP1 and IP6. At IP1 the inversion of thecorrelation occurs around the electron and positron injectionline (fig. 9 at abscissa value 0.8 and 1.2). Between IP6 andIP7 a more gradual decrease of the correlation is recorded.The flux loop measurements and the earth cable currentmeasurements pointing to a current which is flowing be-tween two electrical poles in the Geneva area. Therefore thetime dependence of the potential differences measured be-tween rails and ground, between the LEP vacuum chamberand a reference ground as well as the B field was recorded(fig. 10). The voltage between the rails and ground has beenVoltage on rails [V]RAILTGVGeneveMeyrinZimeysa 17.11.1995 Voltage on beampipe [V]LEP beam pipeTimeBending B field [Gauss]LEP NMR-70-0.024-0.02-0.016-0.012746.28746.30746.32746.34746.3616:50 16:55Figure 10: Time dependence of three signals recorded atdifferent locations. Top: Voltage between the rails and areference ground at the Zimeysa railway station. Middle:Voltage between the LEP vacuum chamber and a referenceground near IP8. Bottom: The magnetic field changes mea-sured by NMROCT8.measured at the Zimeysa train station (fig. 9) several kilome-tres away from the voltage and B field measurements nearIP8. The measured potential difference between the railsand ground is proportional to the current leaking into groundand penetrating LEP. The shape of the three signals is almostidentical and caused by a train leaving the main station ofGeneva as indicated in figure 9.3 CONCLUSIONThe trains operating between the Geneva main station Cor-navin and destinations in France (e.g. TGV, Paris - Geneva)cause a current flow on the LEP vacuum chamber. The cur-rents enter and leave the LEP tunnel near IP1 and IP6. Themeasured B field variations cause beam energy increases ofup to 12 MeV [2, 3].4 REFERENCES[1] R. Assmann et al, (The working group on the LEP energy)“The energy calibration of LEP in the 1993 scan”, Zeitschriftfu¨r Physik C 66, 567-582 (1995).[2] M. Geitz, “Investigation of the Environmental Effects onthe LEP Beam Energy”, diploma thesis, RWTH Aachen, PI-THA Nr.96/14, also to be published, CERN SL/96-xx (BI).[3] “The Train Effect”, CERN, to be published.

A newly observed Effect affects the LEP Beam Energy

A newly observed Effect affects the LEP Beam Energy

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