An energy spectrometer has been installed in the LEP accelerator to determine the beam energy with a relative accuracy of 10-4. A precisely calibrated bending magnet is flanked by 6 beam position monitors (BPM). The beam energy is determined by measuring the deflection angle of the LEP beams and the integrated bending field. An accuracy of less than 10-6 m on the beam position is necessary to reach the desired accuracy on the LEP beam energy. Capacitive wire positioning sensors are used to determine the relative mounting stability of each BPM and to calibrate the beam position monitors. Two-dimensional sensors are attached to each side of every BPM support and provide a position measurement with respect to two stretched wires mounted on either side of the LEP beam pipe. The fixing points of each wire are monitored by additional reference sensors. The position information is digitised via a multiplexed high accuracy digital voltmeter and read out continuously during LEP operations. Wire position sensor accuracy was tested in the laboratory with a laser interferometer, while relative accuracy tests are performed in the LEP environment. Systematic effects of synchrotron radiation on the wire position sensor performance were studied

Coosemans, Williame

Dehning, Bernd

Hildreth, M D

Marin, A

Mugnai, G

Prochnow, J

Roncarolo, F

Torrence, E

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN    SL DIVISIONGeneva, SwitzerlandJuly 2000CERN-SL-2000-029 BIDETERMINATION OF THE ACCURACY OF WIREPOSITION SENSORSCoosemans W., Dehning B., Hildreth M., Marin A., Mugnai G.,Torrence E.CERN – Geneva – CHProchnow J.RWTH – Aachen – DRoncarolo F.Polytechnic – Milano - IAbstractAn energy spectrometer has been installed in the LEP accelerator to determine the beam energywith a relative accuracy of 10-4. A precisely calibrated bending magnet is flanked by 6 beam positionmonitors (BPM). The beam energy is determined by measuring the deflection angle of the LEP beamsand the integrated bending field. An accuracy of less than 10-6 m on the beam position is necessary toreach the desired accuracy on the LEP beam energy. Capacitive wire positioning sensors are used todetermine the relative mounting stability of each BPM and to calibrate the beam position monitors.Two-dimensional sensors are attached to each side of every BPM support and provide a positionmeasurement with respect to two stretched wires mounted on either side of the LEP beam pipe. Thefixing points of each wire are monitored by additional reference sensors.The position information is digitised via a multiplexed high accuracy digital voltmeter and read outcontinuously during LEP operations. Wire position sensor accuracy was tested in the laboratory with alaser interferometer, while relative accuracy tests are performed in the LEP environment. Systematiceffects of synchrotron radiation on the wire position sensor performance were studied.Presented at EPAC 2000 – Vienna – Austria26 – 30 June 2000Determination of the Accuracy of Wire Position SensorsJ. Prochnow∗, W. Coosemans, B. Dehning, M. Hildreth, A. Marin, J. Matheson, G. Mugnai, F. Roncarolo+,E. Torrence, ∗ RWTH Aachen, Germany / CERN, Geneva, Switzerland / + Polytechnic Milano, ItalyAbstractAn energy spectrometer has been installed in the LEP ac-celerator to determine the beam energy with a relative ac-curacy of 10−4. A precisely calibrated bending magnet isflanked by 6 beam position monitors (BPM). The beam en-ergy is determined by measuring the deflection angle of theLEP beams and the integrated bending field. An accuracyof less than 1µm on the beam position is necessary to reachthe desired accuracy on the LEP beam energy. Capacitivewire positioning sensors are used to determine the relativemounting stability of each BPM and to calibrate the beamposition monitors. Two-dimensional sensors are attachedto each side of every BPM support and provide a positionmeasurement with respect to two stretched wires mountedon either side of the LEP beam pipe. The fixing points ofeach wire are monitored by additional reference sensors.The position information is digitised via a multiplexed highaccuracy digital voltmeter and read out continuously duringLEP operations. Wire position sensor accuracy was testedin the laboratory with a laser interferometer, while relativeaccuracy tests are performed in the LEP environment. Sys-tematic effects of synchrotron radiation on the wire posi-tion sensor performance were studied.1 SETUP OF THE SPECTROMETERIn Fig. 1 the setup of the spectrometer is sketched as it wasinstalled for the 1999 LEP run. The dipole magnet lies inthe centre of the two arms and is equipped with six beamposition monitors (BPM).Fig. 2 shows the setup of one BPM station with the BPMSteelDipoleNMR ProbesBPM PickupsWire Position SensorsSynchrotronAbsorbersQuadQuad0m 10mFigure 1: Layout of the Spectrometerin the centre flanked by the two copper synchrotron lightabsorbers. The wire, its stretching weight and the posi-tion sensors attached to the BPM can be seen. The BPMsare mounted on individual limestone blocks to reduce vi-brations. For avoiding unnoticed movements of the wholeBPM blocks with respect to each other a system of wireswith wire position sensors (WPS) attached to both sides ofthe BPMs was installed. The sensors detect movements ofJura Limestone BlockStretched WirePosition SensorsAbsorbersBPMFigure 2: One of the BPM stations with its wire system.the BPMs with respect to the wire and thus with respectto each other. The position of the wire mounting is mea-sured by WPS which are attached to the supporting lime-stone block by a temperature regulated copper tube. Thewire position system uses three wires, which consist ofcarbon surrounded by kevlar. The wires have a diameterof 0.4 mm. The first extends over the whole spectrometerand is intended to observe movements of the arms of thespectrometer with respect to each other. Two further wiresare installed, one on each arm for redundancy and also todistinguish between shifts and expansions of the BPM sup-ports.2 PRINCIPLE OF OPERATIONThe principle of the position measurement is based on sens-ing the capacitance between opposite planar electrodes ofthe sensor and the wire. This is achieved by measuringthe current from the electrodes while an alternating voltageis applied to them. The voltage has an amplitude of 10 Vpeak to peak relative to the grounded conducting wire anda frequency of 4 kHz. The capacitance is, to the first order,inversely proportional to the distance in this arrangement.To obtain the wire position, the analogue signals from op-posite plates are subtracted. To ensure that the electrodesmeasure only the capacitance with respect to the wire, theyare surrounded by guard rings, which are supplied by an os-cillator in phase with the potential on the electrodes. Sincethe potential difference between guard ring and electrodeis zero, the field lines do not bend towards the groundedframe of the sensor head.3 MEASUREMENTSA basic test to check the behaviour of the sensors is tochange the set point of the water temperature regulationsystem for the BPMs. A temperature increase ∆T leadsto an expansion of the BPMs ∆l, which is measured bythe wire position system. This test was performed on sev-eral occasions for all BPMs. The expansion of the BPMsdetermined from the WPS signals can be compared withthe expansion coefficient of aluminium α, from which theBPMs are made :∆l = l ∆T α . (1)Fig. 3 shows the results of such an experiment; the top plotshows the introduced temperature change ∆T = 10oC,the lower the resulting expansion. Taking into account thedistance between the wires ∆l = 30 cm and the expansioncoefficient α = 2.4 × 10−5 1/K this should result in anexpansion of 69 µm, which agrees with the measurement.Fig. 4 shows the movements of one BPM during an energy3035400 1 2 3 4 5 6 7time [h]Temperature [oC]time [h]Expansion [µm]0204060800 1 2 3 4 5 6 7Figure 3: Top: Temperature of the BPMs against time, Bot-tom: Expansion of the BPM measured by the WPS againsttime.calibration measured by the WPS. The setup enables us toaccess four modes of motion: horizontal and vertical shifts,expansion and tilt of the BPMs.4 RESOLUTION AND STABILITY OFTHE WIRE POSITION SENSORSAn upper limit of the resolution of the wire position sys-tem can be determined by the differences between succes-sive readings of the WPS. If the wire does not move in sucha 12 s period, this difference provides the resolution. Thedistribution of the difference between successive readingswas found to be Gaussian (see Fig. 5, left); such Gaussianswere plotted for all 18 WPS and the mean value of theirRMSs was found to be (153 ± 11) nm (see Fig. 5, right).The resolution of all sensors is better than the accuracy re-quired. To determine the absolute accuracy of one WPS,-2-10120 5-2-10120 5-2-10120 5time [h]horiz. shift [µm]time [h]horiz. expansion [µm]time [h]vert. shift [µm]time [h]tilt [µrad]-6-4-202460 5Figure 4: Horizontal shift (left top), horizontal expansion(right top), vertical shift (left bottom) and tilt (right bottom)of the BPM measured by the wire system.051015202530-200 0 200  81.65    /    84Constant   19.32Mean   .1026Sigma   91.55resolution [nm]IDEntriesMeanRMS          12345             18  152.9  44.97RMS of the distributions [nm]00.250.50.7511.251.51.752100 200 300Figure 5: Left: Distribution of the difference between suc-cessive readings of one WPS over 17 h. Right: The stan-dard deviation of these distributions for all 18 Sensors.a test stand was constructed to move the sensor head withrespect to a fixed wire employing stepping motors. Themeasurement of the WPS is compared with that of the dis-placement measuring interferometer [3], which determinesdisplacements of the WPS.The resolution of the laser interferometer was estimated bycalculating the difference between successive readings andwas found to be of the order of 20 nm.The gain of the WPS was estimated as follows: The sen-sor was shifted such that the wire was moved in 100 stepswithin the centre (± 0.5 mm) of the aperture of the sensor.The gain results from the slope of the linear fit to the cor-relation between the WPS reading and the position mea-surement of the interferometer. This procedure took ap-proximately 20 min. To test the stability of the responsecharacteristics this gain estimation was repeated 99 times.The results are plotted in Fig. 6. A relative stability of thegains of 5 × 10−4 was found.Entries : 99Mean    : 0.851454RMS     : 0.000454723gain in x for the central region (±0.5mm) of the WPS [V/mm]02468100.8505 0.8508 0.851 0.8512 0.8515 0.8517 0.852 0.8522 0.8525Figure 6: Stability of the WPS response.5 SYNCHROTRON LIGHT AFFECTSTHE MEASUREMENTThe LEP filling pattern is clearly seen in the raw signal ofthe WPS: During a beam dump and the particle accelera-tion, there are fast position changes in the signal (so-called“jumps”) with a height of approximately 4 µm.During particle acceleration the energy deposition from thebeams due to synchrotron light increases from almost 0 toabout 700 W/m for a beam current of 10 mA. The particleacceleration and the beam dump correspond to the appear-ance and disappearance of synchrotron light. The high ra-diation affects the sensors significantly as is shown in [2]and [1]. The effect was suppressed by shielding the sensorheads with about 2 cm of lead. The unpleasant effect dis-appeared, after the sensor heads were shielded with about2 cm of lead.To test the effect of the air in the sensor, another type ofsensor was brought into the LEP tunnel. This sensor mea-sures a capacitance, in a similar way as the wire sensors,but allows evacuation of the sensor head. With this sensorit is possible to measure the capacitance between two fixedplates, with and without air between them. The jumps oc-curring during the beam dump are reduced by a factor of7 when the sensor is put under vacuum, as can be seen inFig. 7, in which the signal of the sensor is plotted versustime. The bars in the top part of the plot indicate whenLEP was operating and the arrows above the plot indicatewhen the sensor was evacuated.The size of the signal while the sensor is under vacuumis similar to the situation where LEP is running and thereis air between the electrodes. Changes in the signal canonly be explained by variations of the dielectric propertiesof the medium between the plates. The synchrotron lightcoming from the beams ionises the air surrounding andwithin the sensor head. This leads to the creation of freetime [h](signal - 10.281) [mV]02468100 2 4 6 8 10 12 14 16 18airLEP LEPLEP LEPLEPvacuumFigure 7: The signal from the sensor (inversely propor-tional to the distance), after offset subtraction, against time.charges. Furthermore the N2 and O2 molecules are split[4]. This results in a decrease of the dielectric constant 	r(= 1 + δ, δ = 5.94 10−4 for dry air), as observed in Fig. 7.While the sensor is under vacuum or LEP is running, deltais zero or close to zero respectively.6 SUMMARY AND CONCLUSIONSThe performance of the wire position system was studied.A resolution of less than 300µm and a relative stability of5× 10−5 was found. The disturbing effect of high radiationon the measurement was inhibited by shielding the sensorheads with lead.7 ACKNOWLEDGEMENTSWe would like to express our gratitude to the people whoaided in either practical work or fruitful discussions, espe-cially Denis Azzedine, Gian Paolo Ferri, Fre´de´ric Ossartfrom Fogale, M. Placidi8 REFERENCES[1] J. Prochnow, The LEP Energy Spectrometer, Diploma Thesis,RWTH Aachen, Germany (May 2000).[2] W. Coosemans, Performance of Wire Position Sensors in aRadiation Environment, IWAA 1999.[3] Hewlett Packard, Laser Measurement System Operator, Op-eration Manual.[4] M. Hoefert et al., Combined influence of humidity and radia-tion in the LEP tunnel, CERN SL-95-08 (DI).

Determination of the Accuracy of Wire Position Sensors

Determination of the Accuracy of Wire Position Sensors

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