Left and right of each of the four LEP interaction points superconducting low-beta quadrupole magnets are installed to squeeze the vertical beam size at the interaction points. These magnets are the dominant source of vertical closed orbit drifts at LEP because of their strength, the large vertical beta function and their support. Hydrostatic Levelling Systems and resistor-based position sensors were installed to measure the vertical movements of these magnets continuously. The correlation between the mechanical movements and closed orbit variations has been studied. The analysis has shown that the orbit can be kept stable by acting on one correction dipole per low-beta quadrupole pair. This has led to a feed-back system which uses the mechanical measurements to correct the closed orbit and to prevent large orbit variations

Coosemans, Williame

Marin, A

Rybaltchenko, K

Tecker, F A

Wenninger, J

CERN Document Server

CLOSED ORBIT FEED-BACK FROM LOW-BETA QUADRUPOLEMOVEMENTS AT LEPF. Tecker, RWTH Aachen, Germany,W. Coosemans, A. Marin, K. Rybaltchenko, J. Wenninger, CERN, Geneva, SwitzerlandAbstractLeft and right of each of the four LEP interaction points su-perconducting low-beta quadrupole magnets are installedto squeeze the vertical beam size at the interaction points.These magnets are the dominant source of vertical closedorbit drifts at LEP because of their strength, the large ver-tical beta function and their support. Hydrostatic LevellingSystems and resistor-based position sensors were installedto measure the vertical movements of these magnets con-tinuously. The correlation between the mechanical move-ments and closed orbit variations has been studied. Theanalysis has shown that the orbit can be kept stable by act-ing on one correction dipole per low-beta quadrupole pair.This has led to a feed-back system which uses the mechan-ical measurements to correct the closed orbit and to preventlarge orbit variations.1 INTRODUCTIONSince the beginning of LEP operation the vertical closedorbit has been found to be drifting with time. This re-quired frequent orbit corrections by the operation crew.The distribution of corrector magnets used for the correc-tions showed that the correctors near the low-beta quadru-poles (QS0) or with a phase advance of n nearby wereused for the majority of corrections.The superconducting low-beta quadrupoles are verticallyfocusing and have a quadrupole strength (k= 0:16 m 2)10 times larger than other magnets in LEP to obtain thesmall beam size at the interaction point. Their strength andthe large vertical beta function at this location make theclosed orbit very sensitive to any vertical mechanical dis-placement of these magnets. In addition, the mounting ofthe magnets makes it likely that they can actually move.Hydrostatic levelling systems (HLS) and potentiometer-based position sensors were installed to measure the move-ments of the magnets continuously. A first analysis of theobserved movements and their correlation to orbit drifts canbe found in [1]. This analysis was extended and led to afeed-back system for the vertical orbit.2 MECHANICAL INSTALLATIONSThe schematic layout of one of the four experimental inter-action point (IP) is shown in Fig. 1. The low-beta quadru-poles are mounted on a cantilever support structure whichextends into the experimental detectors. Since it is notsupported on the inner side, it is possible that the mag-nets move with the support. Hydrostatic Levelling Sys-tems [2, 3] are installed to monitor the movements. Theyconsist of vessels with capacitance-based hydrostatic sen-sors and temperature sensors connected by communicatingtubes. These vessels are placed at different locations on themagnets and supports. One system with horizontal tubes isinstalled on each support for the QS0 and the next closestmagnets (QS1). Since the tubes are horizontal, local tem-perature differences do not introduce a measurement errorand the precision is better than 1m. A different systemof communicating vessels is connecting the supports andthe ground on both sides of a detector. It was not possibleto install the tubes in a horizontal plane and the precisionis limited to O(10)m. The HLS at one interaction point(IP 8) which was installed as a pilot system has also non-horizontal tubes and hence a lower precision.In addition potentiometer-based systems measure the po-sition of the magnets with respect to reference points in theexperimental detectors with precision of a few m.3 OBSERVED MOVEMENTSFig. 2 shows a typical vertical movement of a low-beta qua-drupole measured by the hydrostatic levelling system. Ingeneral, the pattern of the movement in time is the same forall QS0 magnets. The movement is correlated with temper-ature changes of the support. When the temperature rises,the magnets start moving downwards. The temperaturechanges are mainly caused by the operation cycle of LEP.The temperature starts rising after LEP has been ramped tohigher energy. This can be understood by the higher cur-rent heating up the current feed-throughs which traverse thesupport. The time constant of the thermally driven move-ments is of the order of a few hours and a short refillingtime keeps the amplitude of the movements smaller.4 ORBIT ANALYSISThe vertical orbit drifts were studied in detail to determinethe contribution from the movements of the low-beta QS0magnets. A movement yQof a quadrupole at location s0creates an angular kick y0(s0) = klyQwhich changesthe closed orbit at any location s by y(s) according toy(s)=p(s)(s0) cos(j(s) (s0)j   Q)2 sin(Q)y0(s0):(1),  and Q are the betatron function, phase advance andtune, respectively. Since both k and (s0) are large forthe QS0, the movement yQof a single QS0 generatesa RMS change of the vertical closed orbit y40 timeslarger than the mechanical movement. This factor is only 2for a regular lattice quadrupole.location of potentiometer probessupport structureexperimental detectorIPhydrostatic position sensorsQS2 QS  1 QS2QS  1 QS0 QS0orbit correction dipoleHLS sensors linked across IPtunnel groundFigure 1: Schematic view of the low-beta insertion around an experimental detector. The QS0 are the superconductinglow-beta quadrupole magnets. The layout of one IP is slightly more complicated since the inner magnets (QS0,QS1) andthe inner parts of the detector are mounted in a support tube (not shown).time (h)y QS0.R4 (µm)time (h)temp (C)time (h)energy (GeV)1001201401601802002202400 10 20 30 40 50 60 7026.5270 10 20 30 40 50 60 7002550751000 10 20 30 40 50 60 70Figure 2: Movement of a low-beta quadrupole (QS0.R4)for a period of three days (top), the temperature measured atthe support of this magnet (middle) and LEP beam energy(bottom). The vertical dashed lines indicate the end of theramp to collision energy.The beta function is symmetric around the interactionpoint and the vertical betatron phase advance between thetwo magnets is nearly . Eq. 1 shows that the effect onthe orbit nearly cancels if both magnets move by the sameamount in the same direction. A common movement ofyQ=10m for both QS0s results in a RMS orbit changeof only y= 7m. Fortunately, the pairs of QS0s at thedifferent interaction points tend to move in a similar way.Eq. 1 also implies that the differential movement of the QS0pair can be corrected with only one orbit correction dipole(see Fig. 1) at each IP.The differential movement can be seen in the closed or-bit. Orbits logged during physics data taking were used tocalculate the ‘bare orbit’1 from the measured orbit ymeas1The closed orbit if all correction magnets were switched off.daytime (h)σ∆y (mm)without correctionwith 4 QS0 correction00.511.522.5317:00 18:00 19:00 20:00 21:00Figure 3: RMS of the difference of bare orbits during aphysics fill relative to the first physics orbit before and af-ter orbit correction with only 4 QS0 correctors. A typicalvertical orbit has a residual RMS of about 500m.byybare= ymeas  (2)pbpm2 sin(Q)Xcorpcorcos (jcor  bpmj   Q)corwhere the sum is taken over all vertical correctors. coris the kick angle for the corrector, corand corare betafunction and phase at the corrector, bpmand bpmat thebeam position monitors, respectively. This procedure re-moves the effects of all corrector magnets. The differ-ence of two bare orbits shows the orbit changes from othersources than corrector magnets and can be used to localisethe source of the changes.The bare orbit of the first orbit acquired after the physicsexperiments start taking data is used as a reference for eachfill. For all subsequent orbits the bare orbit is calculatedand the RMS of the difference from the reference yisevaluated. This represents the movements of the orbit if noorbit corrections had been made.An example of the movements during a fill is given inFig. 3. If the QS0 magnets are the source of the drifts thenit should be possible to correct most of the effect with onlyone orbit corrector magnet per IP. This was done with theCOCU package [4] using the MICADO algorithm [5]. Theresult showed that this correction is very effective, takingaway about 70–90% of the orbit movements (Fig. 3). Thisdaytime∆ y (µm)daytimecalculated ∆y‘ (µrad)∆ y (µm)calculated ∆y‘ (µrad)2400241024202430244024502460247016:00 18:00 20:0002.557.51012.51517.52016:00 18:00 20:0002.557.51012.51517.5202400 2410 2420 2430 2440 2450 2460 2470Figure 4: Example of the mechanical movement (top left),the calculated orbit correction kick (top right) and their cor-relation (bottom) at one IP during a fill with large quadru-pole movements.proves that the superconducting low-beta magnets are in-deed the major source of the drifts.5 CORRELATION AND FEED-BACKThe calculated orbit correction kick y0 should be propor-tional to the differential movements of a QS0 pair. Thiswas examined for the different interaction points. Only theHLS with horizontal connecting tubes on the supports areused for the analysis. Due to the lower precision, includ-ing the HLS system connecting the supports on both sidesof the IP does not improve the correlation. An example isshown in Fig. 4.A proportionality constant between the correction kickand the differential movement was derived for each QS0pair from this correlation. A software feed-back was devel-oped which reads the mechanical position of the magnetsand calculates the necessary correction kick from the dif-ferential movement. Whenever the correction kick exceedsa certain threshold, it is sent to the orbit corrector. The HLSis used except for IP 8 where the potentiometers are moreprecise than the pilot HLS.The feed-back has been running very successfully at theend of the 1996 operation period of LEP. Fig. 5 shows anexample. The feed-back was compensating well for themovements of the orbit. The potentiometer system used atIP 8 is less precise than the HLS but still capable of track-ing the movements. The feed-back significantly reducedthe number of orbit corrections that had to be made by theoperator.daytime (h)kick IP2 (µrad)daytime (h)kick IP4 (µrad)daytime (h)kick IP6 (µrad)daytime (h)kick IP8 (µrad)from orbit analysisQS0 Feed-back trim0510156:00 8:00 10:000510156:00 8:00 10:000510156:00 8:00 10:000510156:00 8:00 10:00Figure 5: Example of the feed-back during one fill. Thecorrection kicks sent by the feed-back are compared to thecorrections calculated from the off-line orbit analysis.Another application of the QS0 measurements is thereload of a previously stored set of corrector excitations ofan orbit which gave high luminosity and good backgroundconditions. The positions of the QS0 magnets of storageand reload time are compared and the necessary correc-tion is incorporated. An experiment was performed andit demonstrated that the orbit with the incorporated correc-tion had a significantly lower RMS excursion. This reducesthe risk of accidental beam loss when an old set of correctorsettings is reloaded.6 CONCLUSIONSThe superconducting low-beta quadrupoles are the majorsource of vertical orbit drifts at LEP. Hydrostatic Lev-elling Systems with horizontal connecting tubes measurethe vertical movements of these magnets very precisely.A feed-back system based on the measurements has beenimplemented and prevents large orbit variations during thephysics data taking. The HLS at IP 8 will be upgraded thisyear and the more precise HLS measurements will be usedfor the feed-back at all IPs.7 REFERENCES[1] F. Tecker. Low-beta quadrupole movements as source of ver-tical orbit drifts at LEP, 1996. CERN-SL/96-40 (BI).[2] Fogale Nanotech, 190, Parc Georges Besse, 30000 Nimes,France. Sensors and Actuators for alignment of large ma-chine components.[3] LEP hydrostatic levelling systems. http://www.cern.ch/-CERN/Divisions/SL/groups/bi/hls/WelcomeHLS.html.[4] D. Brandt, W. Herr, J. Miles, and R. Schmidt. A newclosed orbit correction procedure for the CERN SPS andLEP. Nucl.Instr.Meth., A(293):305–307, 1990.[5] B. Autin and Y. Marti. Closed orbit correction of A.G. ma-chines using a limited number of magnets, 1973. CERN ISRMA/73-17.

Closed orbit feed-back from low- $\beta$ quadrupole movements at LEP

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Closed orbit feed-back from low-β\betaβ quadrupole movements at LEP

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Closed orbit feed-back from low- $\beta$ quadrupole movements at LEP