We give the present status of the field quality in the main LHC dipoles. Special emphasis is given to the collared coil data: a few tens of coils have been built, allowing a first analysis of the variability between producers, and estimates of the random part. Effects of the corrective actions implemented to fine tune the systematics components of low-order multipoles are presented. Correlations of collared coil data to the magnetic field measurements in operational conditions are discussed. Comparison to specifications imposed by beam dynamics allows to pin out the most critical requirements that will have to be met during the LHC dipole production

Bottura, L

Pauletta, S

Remondino, Vittorio

Sanfilippo, S

Scandale, Walter

Todesco, Ezio

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsSTATUS REPORT ON FIELD QUALITY IN THE MAIN LHC DIPOLESE. Todesco, L. Bottura, S. Pauletta, V. Remondino, S. Sanfilippo and W. ScandaleWe give the present status of the field quality in the main LHC dipoles. Special emphasis is given to thecollared coil data: a few tens of coils have been built, allowing a first analysis of the variability betweenproducers, and estimates of the random part. Effects of the corrective actions implemented to fine tune thesystematics components of low-order multipoles are presented. Correlations of collared coil data to themagnetic field measurements in operational conditions are discussed. Comparison to specificationsimposed by beam dynamics allows to pin out the most critical requirements that will have to be met duringthe LHC dipole production.CERN, Geneva, SwitzerlandPresented at the Eighth European Particle Accelerator Conference (EPAC)3-7 June 2002 - La Villette, Paris, FranceGeneva, Large Hadron Collider ProjectAdministrative SecretariatLHC DivisionCERNCH - 1211 Geneva 23SwitzerlandLHC Project Report 579Abstract2 July 2002STATUS REPORT ON FIELD QUALITY IN THE MAIN LHC DIPOLESE. Todesco, L. Bottura, S. Pauletta, V. Remondino, S. Sanfilippo, W. Scandale, CERN, Geneva, SwitzerlandAbstractWe give the present status of the field quality in the mainLHC dipoles. Special emphasis is given to the collaredcoil data: a few tens of coils have been built, allowing afirst analysis of the variability between producers, and es-timates of the random part. Effects of the corrective ac-tions implemented to fine tune the systematics componentsof low-order multipoles are presented. Correlations of col-lared coil data to the magnetic field measurements in op-erational conditions are discussed. Comparison to specifi-cations imposed by beam dynamics allows to pin out themost critical requirements that will have to be met duringthe LHC dipole production.1 INTRODUCTIONThe magnetic field of the main LHC dipoles is measuredin three conditions. Measurements of the superconductingcoils in the collars (collared coil) at 300 K provide a firstindication of the field quality, and a powerful instrumentto detect assembly errors or faulty components. Then, themagnetic field of the cold mass (collared coils plus the ironyoke and the shrinking cylinder) is measured at 300 K.Both measurements are carried out at the manufacturer’spremises. The cryomagnet is finally tested at CERN at 1.9K and under the nominal current cycle. This provides thefinal assessement of field quality and of correlation with theprevious measurements [1].Here, we present data relative to 34 collared coil, 10 coldmasses and 7 cryomagnets. In the early phase of the pro-duction, a wide range of thickness of the spacers betweencollars and coil poles (shims) has been used to compensateout of tolerance in the coil geometry, aiming at a nominalpre-stress. The successive magnets have been built witha narrower range of shim thickness, and during the pro-duction we aim at using nominal shims. Therefore, mea-surements data are reduced to nominal shims using the ap-proach defined in Ref. [2]. This helps to define strategiesto steer the magnet production. At a later stage, unmodi-fied data will be used to optimize the magnet installationfor beam operation.2 SYSTEMATICSMagnetic measurements of prototypes and first pre-series magnets showed that the systematic part of   and were out of tolerance of about +3 units and +1 unitsrespectively [3]. The origin of this discrepancy is ratherwell understood, i.e. some changes in the magnet structureafter the definition of the coil cross-section, the influenceof coil deformations [4], and some variations in the tar-gets [5]. To recover nominal values for these multipoles,a cross-section correction has been implemented [6]. Cop-per wedges of the internal layer have been modified by atmost 0.4 mm, keeping the same coil shape to avoid costlychanges in the toolings and in the collars. The correctionhas been based on measurements of 9 collared coils andof 4 cryomagnets. Two collared coils with the new cross-section (magnets 29 and 31 in Figs. 1-3) have been built.The obtained shift in field harmonics is shown in Table 1.Measurements are given with a two sigma error. The multi-polar variation due to tolerance on the copper wedges ( 	microns) is associated to model estimates. One finds agree-ment between simulations and experimental data.Table 1: Differential effect of cross-section correction inthe collared coil on low-order allowed multipoles   Model       Measurements ﬀ   ﬁ ﬁ  ﬂThe measured values of the allowed low-order multi-poles in the straight part of 34 collared coils are shownin Figs. 1-3. Different markers are used to single out thethree manufacturers, and the two apertures are plotted foreach magnet. The best estimate for the running systematicis shown as a solid line. Since each firm will produce onethird of the dipoles, the systematic is defined as the aver-age of the three manufacturers averages. Comparisons aregiven to the allowed range for the systematic (dashed line),deduced by extrapolating the beam dynamics targets in op-erational conditions to the straight part of the collared coilthrough correlations discussed in Section 4.Figure 1: Average in the straight part of the collaredcoil versus magnet number: values of the three manufactur-ers (one marker per aperture) and running systematic (solidline) versus allowed range for the systematic (dashed line)The graph of  (see Fig. 1) shows an upward trend frommagnet 1 to magnet 15 of up to 7 units. This trend, commonto all manufacturers, is relevant since the allowed range forthe systematic is 6 units. The change of the cross-sectionhas been carried out when the average had already driftedby 3 units, and therefore the new magnets feature a system-atic  in the upper part of the allowed range. No systematicdifferences between manufacturers are observed.Figure 2: Average in the straight part of the collaredcoil versus magnet number: values of the three manufactur-ers (one marker per aperture) and running systematic (solidline) versus allowed range for the average (dashed line)The control of  (see Fig. 2) is critical since the vari-ability (1.5 units peak-to-peak) is large compared to theallowed range for the systematic (0.6 units). No drift isobserved in the production, but there are differences be-tween the manufacturers after the first 10 magnets. Man-ufacturer 1 has an average of 1.8 units, whilst firms 2 and3 are around 1 unit. If the same difference will be pre-served in new cross-section magnets, the systematic willbe around -0.1, i.e. in the upper part of the allowed range.Figure 3: Average in the straight part of the collaredcoil versus magnet number: values of the three manufactur-ers (one marker per aperture) and running systematic (solidline) versus allowed range for the average (dashed line)The control of systematic is also rather critical, sincethe spread of this multipole is close to the allowed range forthe systematic (see Fig. 3). Also in this case, a differencebetween manufacturers is observed after the first 10 mag-nets: manufacturer 3 has an average of about 0.4 units,whilst in firms 1 and 2 are around 0.7 - 0.8 units. Afterthe correction, some changes in the correlation to field inoperational conditions and in beam dynamics targets haveshifted downward and reduced the allowed range. The bestestimate for the systematic  is now out of the allowedrange by a small amount (+0.1 units), if the difference be-tween firm 2 and firm 1,3 are preserved (see Fig. 3).Data relative to the new cross-section show that the mea-surements are now within or close to the allowed range forlow-order multipoles (see Figs. 1-3). The effect on the in-tegrated main field is shown to be negligible, as designed.More statistics is needed to determine the average of thenew production, and if additional corrections are necessary.LHC dipoles have a two-in-one collar structure thatbreaks the left-right symmetry, giving rise to systematiceven normal multipoles. These components are analysedin Ref. [7]: data show that the modification of the ironlaminations implemented in the pre-series dipoles has beensuccessful and that both  ﬃ and   are within targets.3 RANDOMSThe standard deviation of the measured main field andharmonics in the collared coil is shown in Fig. 4 (mark-ers) for each manufacturer. The solid line represents theallowed budget for the random component. In section 6 wewill show that the main source of the randoms is already inthe collared coil, and therefore the comparison of collaredcoil standard deviations to beam dynamics budget is sig-nificant. Measured values are close or within the boundsalready in this early stage of the production. The only ex-ception is , due to the drift that has been observed in thefirst 15 magnets. The random   of the successive magnetsis within the target of 1.5 units. In the same figure we alsoplotted the standard deviation of the multipoles of the dis-tribution of all magnets. Also in this case, this quantity isclose or within targets with the exception of . This meansthat the allowed budget for the random part could be com-patible with an installation scenario where magnets of allmanufacturers are mixed in the arcs.Figure 4: Standard deviation of magnetic length (  ), mainfield ( ! ), integrated main field !" ), and low-order multi-poles in the collared coil (markers) versus beam dynamicstargets (solid line)4 CORRELATIONSCorrelations between field harmonics measured in thecollared coils and in operational conditions determine thepossibility of steering the production through magneticmeasurements carried out at the manufacturers [8, 9, 10].Experimental data relative to   are shown in Fig. 5. Oneobserves very good correlations with a slope between 0.8and 0.9. This value of the slope is due to the rescalingof multipoles by a factor #ﬂ$&%'#() ﬁ since in the LHCdipoles the main field is enhanced by the iron yoke of 20 %.The deterministic part of the relation between harmonics inthe collared coil and in the cold mass is +*-,.%&$ +*-*.0/21.where 1 . is negligible for multipoles with 354 (see [8]).The effect of cooling down, of persistent currents, of ironsaturation and of Lorentz forces can be approximated atfirst order as additional offsets.Figure 5:   in the straight part of the collared coil versus at injection and at high fieldThe 7 magnets that have been tested cold feature a widerange of that is mainly due to difference in shim thick-ness. In a production phase where only nominal shims willbe possibly used, the expected range of allowed multipoleswill be much smaller. In this case, the above graph could bemisleading, showing a poor correlation. Indeed, the quan-tity relevant to the dependence of field harmonics in oper-ational conditions on the measurements in the collared coilis the standard deviation of the difference  +687.:9 $   *-*. (seeTable 2). Comparison are given with the standard deviationof the harmonics in the collared coil. Data show that thevariability of the collared coil is much larger than the vari-ability due to cooling down, persistent currents, saturationand Lorentz forces, in agreement with previous estimates[11]. To steer field harmonics one has to compare these sig-mas to the allowed ranges for the systematics. For and the range is much larger than the sigma (6 units against0.4, and 0.4 units against 0.03 respectively). The case of is less confortable, since the range is 0.6 units and thecorrelation (one sigma) is 0.2: this quantity should be care-fully monitored during the production. Since collared coildata give only estimates of the geometric component andno indication on persistent currents, correlations to opera-tional conditions must especially rely on a careful controlof the magnetization cable properties.Table 2: Sigma of integrated main field and  ,  ,  in thecollared coil, and of differences between rescaled collaredcoil and operational conditions (see Eq. 4)!;      Collared coil 9.0 1.9 0.45 0.19Injection - $ collared coil 4.3 0.4 0.14 0.03High field - $ collared coil 3.6 0.2 0.15 0.025 CONCLUSIONSWe presented measurements of the magnetic field of themain LHC dipole, and comparison to beam dynamics tar-gets. Systematic values of and   in the first magnetswere out of the allowed ranges. A change in the coil cross-section has been implemented. First data of 2 collared coilswith the new cross-section show that we are close to accep-tance ranges. Comparison between experimental data andallowed ranges for the systematics show that   will be themost difficult component to control, since its random vari-ation is large compared to the acceptance range.The random part of the multipoles is within targets, withthe exception of that has shown a large upward trend.Measurements show that we are close to a situation wherethe standard deviation of all magnets is within the budgetallowed for the random component. Therefore, a mixing ofthe three manufacturers inside the ring could be tolerablefor beam dynamics. This should be confirmed by a widersample of measurements at 1.9 K.We presented the dependence of the magnetic field mea-sured in operational conditions on the collared coil mea-surements. This relation shows a good reproducibility; thecollared coil is shown to be the main source of variablity inthe magnetic field. Comparison to allowed ranges for beamdynamics show that correlations for are the most criticalfor the steering of the production.6 ACKNOWLEDGEMENTSWe would like to thank P. Ferracin and A. Devred forprecious help in the analysis of data, and E. Wildner forcomments and discussions.7 REFERENCES[1] L. Walckiers et al, these proceedings.[2] P. Ferracin, et al, Phys. Rev. STAB, 5 (2002).[3] P. Ferracin, et al, LHC Project Report 467 (2001).[4] P. Ferracin, et al, IEEE Trans. Appl. Supercond. (2002), inpress.[5] S. Fartouk, O. Bruning, LHC Project Report 501 (2001).[6] E. Todesco, private communication.[7] E. Todesco at al, these proceedings.[8] A.V. Tollestrup, Fermilab Report UPC-86 (1979).[9] A. Bonito Oliva et al, Cryogenics 30 589 (1990).[10] R. Gupta et al, MT15 proceedings (1997) 110.[11] L. Walckiers, Field Quality Working Group, private com-munication.

Status Report on Field Quality in the Main LHC Dipoles

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