Knowledge of AC loss and dynamic magnetic field distortion in the main LHC dipoles is both important for the assessment of the accelerator performance and providing insight into the properties of assembled magnets. We measured the loss due to the current cycling in a few 1-meter long model dipoles, 15-meter long dipole prototypes and pre-series magnets. As expected the loss depends linearly on the rate of the current change. From the slope of this dependence, the contact resistance between the strands of the opposite layers of the cable, Rc, was evaluated for the inner winding of the dipole. We discuss the method to estimate the Rc value in the outer winding. The Rc value has been also derived independently from measurements of the magnetic field. For this, the ramp rate dependent component of the main field as well as of the harmonics has been measured. The main magnetic field measurements were performed using both stationary coils and Hall probes. Rotating coils were used to perform the harmonic measurements

Akhmedov, A A

Amet, S

Balaazi, I

Bottura, L

Gateau, M

Pugnat, P

Walckiers, L

CERN Document Server

IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 12, NO. 1, MARCH 2002 31Loss and Dynamic Magnetic Field Measurements inLHC DipolesA. Akhmetov, S. Amet, I. Balaazi, L. Bottura, M. Gateau, P. Pugnat, and L. WalckiersAbstract—Knowledge of AC loss and dynamic magnetic fielddistortion in the main LHC dipoles is both important for the as-sessment of the accelerator performance and providing insight intothe properties of assembled magnets. We measured the loss due tothe current cycling in a few 1-meter long model dipoles, 15-meterlong dipole prototypes and pre-series magnets. As expected the lossdepends linearly on the rate of the current change. From the slopeof this dependence, the contact resistance between the strands ofthe opposite layers of the cable, , was evaluated for the innerwinding of the dipole. We discuss the method to estimate thevalue in the outer winding. The value has been also derived in-dependently from measurements of the magnetic field. For this, theramp rate dependent component of the main field as well as of theharmonics has been measured. The main magnetic field measure-ments were performed using both stationary coils and Hall probes.Rotating coils were used to perform the harmonic measurements.Index Terms—AC loss, coupling currents, dipole magnet, mag-netic field quality, superconducting cables.I. INTRODUCTIONSUPERCONDUCTING strands and cables subjected to achanging magnetic field exhibit effects related to persistentcurrents in superconducting filaments and normal currentsflowing between both the filaments of each strand and thestrands of the cable. These currents disturb the backgroundmagnetic field and dissipate the energy provided by themagnetic field source. In accelerator dipole magnets woundwith Rutherford-type superconducting cables, the magneticfield distortion is the main issue that should be addressed, forinstance, by proper synchronization of the power converters.The present paper is based on measurements conducted in1999–2001. We measured the loss and dynamic magnetic fielddistortion in a few 1-meter long model dipoles, 15-m long dipoleprototypes and pre-series dipoles. All these magnets are of thesix-block design shown in Fig. 1. The results are used for theevaluation of the contact electrical resistance between thecrossing strands of the two layers of the cable. The last valuegoverns the current distribution in the flat cable subjected to atime-dependent magnetic field [1], [2]. Due to the wide varietyof the magnets and a gradual development of experimental tech-niques, we do not intend here to characterize every dipole mea-Manuscript received September 24, 2001.A. Akhmetov is with the CERN, European Organization for Nuclear Re-search, CH-1211 Geneva 23, Switzerland (LHC division, MTA group) and waswith the Institute for High Temperatures, Ac. of Science of Russia, Izhorskaya13/19, Moscow, 127412, Russia.S. Amet, I. Balaazi, L. Bottura, M. Gateau, P. Pugnat, and L. Walckiers arewith the CERN, European Organization for Nuclear Research, CH-1211 Geneva23, Switzerland (LHC division, MTA group).Publisher Item Identifier S 1051-8223(02)03424-3.Fig. 1. Upper right quarter of the winding for one aperture of the dipole. Theblocks are numbered.sured. We will rather discuss the possibilities and restrictionsof different kinds of measurements for determination of anddipole properties in general.II. MEASUREMENTS TECHNIQUESAll measurements are conducted during a trapezoidal currentcycle from to and back lasting a total time . Thecurrent ramps up and down are executed with constant ramprates and correspondingly. The totalloss was obtained by recording the current and the totalvoltage applied to the magnet as(1)In addition to the total voltage we measured voltage drops inportions of the magnet. In this case (1) provides an apparentloss in the portion of the magnet measured.The magnetic field harmonics , where stands for themain field, have been measured during the up and down ramps.As the result we obtained the values(2)where . Measurements of were doneby the stationary coils [3], [4] and the Hall probe setup describedin [5]. Rotating coils [6] were used for the evaluation of thehigher order harmonics at the reference radius 17 mm. The ex-perimental techniques are described in more detail in [3]–[7].1051-8223/02$17.00 © 2002 IEEE32 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 12, NO. 1, MARCH 2002Fig. 2. Loss in the pre-series dipole MBPSO01.III. LOSS MEASUREMENTSA. Total Loss and Its ScalingFig. 2 shows the results of loss measurements conducted onthe Alstom-made pre-series dipole MBPSO01 forA and A. Each point in this figure corresponds tothe average of five measurements, the error bars show standarddeviations. In spite of the fact that the same conditions were usedfor the measurements, the points exhibit substantial scatter. Thisis probably due to the deviation of the performed current cyclefrom the ideal one. Nevertheless, it is seen that the dependencecan be fitted by the linear expression:(3)where J and Js/A. The correspondingstandard deviations are J and Js/A.The ramp rate independent component of the loss, is thehysteresis loss in the superconducting filaments and, to a muchlesser extent, in the iron yoke of the dipole. The coefficientdefines the loss in the metallic elements of the construction, andamong them mainly in the inter-strand contacts and in the matrixof the strands.Experiments conducted for different and indicatedthat within the measurement accuracy the coefficient can beexpressed via another coefficient as(4)where . This relation can be used to comparemeasurements performed in different current intervals.To evaluate the interstrand contact resistance , the co-efficient is corrected for the strand matrix (interfilament)contribution and the contribution from the metallic parts ofthe magnet, mostly copper wedges inserted between the cableblocks (see Fig. 1). The coefficient obtained after such acorrection, , is compared to the interstrand loss coefficientcalculated for the reference value by means of computercodes [8], [9]. This allows us to evaluate the contact resistanceof the magnet. Calculations show that the main interstrandcontribution to the total loss comes from the 3rd, 4th, and the5th blocks of the inner winding. Hence, the determinationbased on the loss measurements provides the value averagedover these three blocks.Fig. 3. Apparent loss in the inner ( ) and outer ( ) windings of the dipole;(a) the first aperture of MBP2O1 prototype, (b) the 1-m long one apertureMBSMS21 dipole. Contrary to direct calculations based on [8] and [9], theapparent loss is higher in the outer winding. The negative slope of the lossdependence for the inner winding reflects small contact resistance in the outerwinding.B. Apparent LossLoss measurements can be also conducted on different por-tions of the dipole winding thus providing the partial values ofthe loss. For instance, and shown in Fig. 2 characterizethe loss in the first and the second aperture of the MBPSO01dipole. Also, for a few dipoles we measured and ,which are the losses in the upper and lower windings of one ofthe apertures. In the same way, and are the mea-sured losses in the inner and outer windings of the same aper-ture. The corresponding partial loss coefficients can be definedin accordance with (3) and (4) and denoted as and .The ratio (see Fig. 3) found in theexperiments with the 1-m long dipoles and dipole prototypesis in a gross contradiction with the ratio of calculated values[9]. In some cases, e.g., for the one-aper-ture dipole MBSMS21/V2, the coefficient is negativeenough to even make the apparent loss negative at highramp rates. A possible explanation of this effect is given in [10].An apparent loss measured in a portion of the magnet does notcoincide with the real energy deposited in it. Rather it dependson both the magnetic flux penetrating the portion in questionand the local contact resistance. In the absence of inter-filamentAKHMETOV et al.: LOSS AND DYNAMIC MAGNETIC FIELD MEASUREMENTS IN LHC DIPOLES 33Fig. 4. The difference B in the 2nd aperture of the MBPSA01 dipolemeasured with stationary coils.coupling within a strand and neglecting both hysteresis lossand contribution from the metallic parts, it can be shown that(5)where and are the contact resistances in the outerand inner windings correspondingly. It follows from (5) thatif . More detailed consid-erations including inter-filament and hysteresis loss are givenin [10].C. Symmetry ConsiderationsThe magnetic flux linking the apertures of the dipole is negli-gible in comparison with that between the elements of one aper-ture. Due to this, the values and represent with good ap-proximation the real loss in the corresponding apertures. Hence,the results shown in Fig. 2 allow us to evaluate the average ineach aperture. If the loss measurement is done simultaneously intwo apertures, although the absolute error is large for each aper-ture, the relative accuracy is still excellent. The reason is that thedeviation from the ideal trapezoidal current cycle, which is themain cause for the scattering of the results, is the same for bothapertures. A single measurement can be sufficient to monitor a2% difference between and . Based on the above argu-ment, a much larger effort is needed to distinguish the origin ofan asymmetry, i.e., whether it comes from the difference in thecoupling currents or in the hysteresis loss.A magnetic interaction between upper and lower halves of thedipole is very strong and the effect of the difference in on theapparent loss cannot be estimated by means of [8], [9]. How-ever, it is clear that in an ideal dipole . Hence, asingle ramp can be used to verify the up-down symmetry of themagnet.IV. MAGNETIC FIELD MEASUREMENTSA. Measurements With Stationary CoilsThe field measurement using stationary coils has the advan-tage of providing integral values along the magnet, important toassess the accelerator performance. Unluckily, it is affected by asubstantial dispersion. Fig. 4 shows the results obtained for theFig. 5. The coefficient S for the MBPSO01 dipole obtained in theexperiment ( ), calculated for the interfilament contribution ( ) and estimatedfor the interstrand contribution ( ).2nd aperture of the Ansaldo-made pre-series dipole MBPSA01.Magnetic field and loss measurements were conducted simulta-neously for A and A. Each point inthis figure corresponds to the average of five measurements, theerror bars show standard deviations. The linear fit(6)results in a negative offset mT,mT consistent with the magnetic field generated by supercon-ducting filaments. The coefficient T s/A,T s/A reflects mostly inter-strand coupling [8]. The cur-rent cycles were optimized for the loss measurements. Properoptimization for the magnetic field measurements could lead toa decrease in the standard deviations.The 3rd, the 4th and the 5th blocks of the inner winding are themain contributors to the difference in between the ramps upand down. Hence, for evaluation of the above data providesinformation equivalent to that from the loss measurements.B. Measurements with Hall ProbesReproducibility of measurements using Hall probes ismuch higher than with stationary coils. However, the resultsobtained with the probes located even a few centimeters apartalong the magnet differ within a few percent. The presence of aperiodic magnetic field pattern [11] in the dipole is mainly re-sponsible. Another drawback of the measurements using Hallprobes is that the integral magnetic field cannot be obtained di-rectly, especially in the presence of a substantial variation ofalong the length of the magnet.C. Measurements of and With Rotating CoilsIn case of allowed harmonics, i.e., for (sextupole) and(decapole) the value was found to depend on theramp rate as(7)Experiments conducted on the pre-series dipoles showthat contrary to , the coefficient in (7) increases withcurrent. As an example, Fig. 5 shows the dependence of34 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 12, NO. 1, MARCH 2002on current for the MBPSO01 dipole before and after acorrection for the interfilament contribution done for a strandtime constant s [12]. It is seen that the correctionmakes this dependence less pronounced but does not eliminateit entirely.V. EXAMPLESLet us take pre-series magnets measured so far as examples.In case of the MBPSO01 dipole, the loss (see Fig. 2) andmagnetic field measurements conducted with both Hall probesand stationary coils provide averaged for the firstaperture and for the second one. Corrections for theinterfilament contributions have been done for the strand timeconstant s. The corrected value averaged overthe current interval A, A is approx-imately 1.5 T s/A for the first aperture (see Fig. 5). The lastvalue corresponds to [9]. An apparent contradic-tion between the values is, probably, due to mostly two fac-tors. First, the value depends on the contact resistances inthe blocks of the dipole in a very different way than the loss and. Second, according to preliminary estimations, contribu-tion to the from the wedges can be comparatively high andcapable of screening the cable contribution.The dipole MBPSO02 has not been studied extensively withrespect to the dynamic effects in the loss and . No substan-tial difference between the apertures has been observed either.The rough estimation of the contact resistances isfrom the loss measurements and from themeasurements.Measurements of the MBPSA01 dipole provide a good ex-ample of the symmetry check-up. While for the first aperturethe loss , in the second one there is 15% differ-ence between them. Indeed, the upper and lower poles of theouter winding of the second aperture were wound from the ca-bles supplied by different manufacturers. With high certaintythis results in the different hysteresis loss and, thus, symmetryviolations.In general, all pre-series dipoles have the coefficientT s/A, much less than the upper limit for the accelerator oper-ation. The obtained values of the order of tens of microohmsare in line with the results obtained on single cables [13].VI. CONCLUSIONSOf all methods discussed above none demonstrates clear ad-vantage over the others in characterizing the dipole. The lossmeasurements are prone to high dispersion of the points. At thesame time, even single loss measurements can be enough to as-sess the dipole symmetry. Besides, apparent loss measurementsprovide the only way to evaluate of the outer winding.The magnetic field measurements made with stationarycoils are characterized by low reproducibility but provide an in-formation directly related to the accelerator performance. Per-formed with Hall probes these measurements are much moreaccurate locally but do not give integral values.More information about distribution in the dipole can beobtained by harmonic measurements. Yet additional analysishas to be done to achieve consistent results in this way.ACKNOWLEDGMENTThe authors thank S. Bouchenoua, A. Rijllart, S. Sanfilippoand T. Schreiner for their valuable help in organizing and con-ducting experiments.REFERENCES[1] A. Devred and T. Ogitsu, “Ramp-rate sensitivity of SSC dipole magnetprototypes,” in Frontiers of Accelerator Technology, S. I. Kurokawa,M. Month, and S. Turner, Eds. Singapore: World Scientific, 1996, pp.184–308.[2] A. A. Akhmetov, “Compatibility of two basic models describing a.c. lossand eddy currents in flat superconducting cables,” Cryogenics, vol. 40,pp. 445–457, 2000.[3] J. Buckley, D. Richter, L. Walckiers, and R. Wolf, “Dynamic magneticmeasurements of superconducting magnets for the LHC,” IEEE Trans.Appl. Superconduct., vol. 5, pp. 1024–1027, 1995.[4] Z. Ang, I. Bejar, L. Bottura, D. Richter, M. Sheehan, L. Walckiers, andR. Wolf, “Measurements of AC loss and magnetic field during ramps inthe LHC model dipoles,” IEEE Trans. Appl. Superconduct., vol. 9, pp.1039–1044, 1999.[5] L. Bottura, L. Larsson, S. Schloss, M. Schneider, N. Smirnov, and M.Haverkamp, “A fast sextupole probe for snapback measurement in theLHC dipoles,”, CERN-LHC-Project-Report-362, 1999.[6] Z. Ang, L. Bottura, S. Russenschuck, A. Siemko, D. Tommasini, and L.Walckiers, “Coil size and geometic field quality in short model dipolesfor LHC,” IEEE Trans. Appl. Superconduct., vol. 10, pp. 53–56, 2000.[7] A. P. Verweij, D. Leroy, L. Walckiers, and R. Wolf, “Analysis of theAC loss measurements on the one-meter dipole model magnets for theCERN LHC,” IEEE Trans. Magn., vol. 30, pp. 1758–1761, 1994.[8] L. Bottura, personal communication, CERN, LHC/MTA, Geneva,Switzerland, 2001.[9] A. Verweij, “Print-out of the CCDI2 program,” CERN, LHC/MMS,Geneva, Switzerland, 1997.[10] A. A. Akhmetov, “Apparent energy deposition during the current cyclein the dipole magnets,”, LHC-MTA-IN-2000-127, Sept. 2000.[11] L. Bottura, L. Walckiers, and Z. Ang, “Experimental evidence ofboundary induced coupling currents in LHC prototypes,” IEEE Trans.Appl. Superconduct., vol. 7, pp. 801–804, 1997.[12] R. Wolf, personal communication, CERN, LHC/MMS, Geneva,Switzerland, 2000.[13] D. Richter, personal communication, CERN, LHC/MMS, Geneva,Switzerland, 2001.

Loss and Dynamic Magnetic Field Measurements in LHC Dipoles

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