LHC model magnets have dynamic field imperfections of various nature. Two effects of particular importance are field component decay during injection and ''snapback'' during the first few seconds of acceleration, which happens over typically 15 to 20 mT. The dynamic behaviour of the model magnets was measured as a function of several parameters in the operation cycle and powering history. We demonstrate how the systematic variation of only one single operation cycle parameter can affect the behaviour of the sextupole component

Bottura, L

Haverkamp, M

Schneider, M

Bottura, L.

Haverkamp, M.

Schneider, M.

University of Twente Research Information

Studies of decay and snapback effects on LHC dipole magnets

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EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsSTUDIES OF DECAY AND SNAPBACK EFFECTS ON LHC DIPOLE MAGNETSM Haverkamp1,2, L Bottura1, M Schneider1 LHC model magnets have dynamic field imperfections of various nature. Two effects of particularimportance are field component decay during injection and ''snapback'' during the first few seconds ofacceleration, which happens over typically 15 to 20 mT. The dynamic behaviour of the model magnets wasmeasured as a function of several parameters in the operation cycle and powering history. We demonstratehow the systematic variation of only one single operation cycle parameter can affect the behaviour of thesextupole component.1) CERN, Division LHC, CH-1211 Geneva 232) University of Twente LT / TN, P.O. Box 217, NL-7500 AE EnschedePresented at the 4th European Conference on Applied Superconductivity (EUCAS'99)14-17 September 1999, Sitges, Barcelona, SpainGeneva, Large Hadron Collider ProjectAdministrative SecretariatLHC DivisionCERNCH - 1211 Geneva 23SwitzerlandLHC Project Report 336Abstract1 December 1999Studies of Decay and Snapback Effects on LHC DipoleMagnetsM Haverkamp1,2, L Bottura1, M Schneider11) CERN, Division LHC, CH-1211 Geneva 232) University of Twente LT / TN, P.O. Box 217, NL-7500 AE EnschedeABSTRACT: LHC model magnets have dynamic field imperfections of various nature. Twoeffects of particular importance are field component decay during injection and ''snapback''during the first few seconds of acceleration, which happens over typically 15 to 20 mT. Thedynamic behaviour of the model magnets was measured as a function of several parametersin the operation cycle and powering history. We demonstrate how the systematic variation ofonly one single operation cycle parameter can affect the behaviour of the sextupolecomponent.1. INTRODUCTIONThe field in a plane perpendicular to a magnet’s axis can be expanded in a Fourierseries, where bn and an are the normal and skew multipole coefficients, which are expressed inrelative ”units”. B1 is the main field and Rref the reference radius of 17 mm:( ) ( ) .10,1114-¥=- å ÷÷øöççèæ ++=+=nn refnnxy RiyxiabBiBByxB (1)Field errors in superconducting dipole magnets have several origins. Geometric field errorsresult from the deviation of the real current distribution in the cables from the ideal currentdistribution of the desired magnetic field. At high fields a further deviation is caused by thesaturation of the iron yoke. Both contributions are reproducible, can be predicted accuratelyand may be inferred from warm measurements. Additional effects are caused by thediamagnetic properties of the superconducting filaments in the cable, which are due topersistent currents. This contribution can be important at the low injection field level. Duringramps, eddy currents are induced in the Rutherford cable and contribute to its diamagneticbehaviour. Persistent and eddy current effects are reproducible, but require measurements inthe superconducting state. Finally, the LHC will have to deal with a new class of effects(Bottura 1997). During long periods of constant current excitation, especially during injection,all field components show a decay behaviour. As soon as, after the end of injection, the currentis ramped up again, the field components rapidly recover from the decay and follow the courseof the original hysteresis curve (snapback). In the LHC magnets the typical sextupole changeduring snapback is in the range of 10-4 (1 unit). On the other hand, a sextupole change of 1 unitcauses a chromaticity change of about 50 units, one order of magnitude larger than the requiredtolerance. The sextupole changes during snapback therefore need to be corrected accurately.Note that in contrast to older machines, corrections not only have to be applied to thesextupole, but also to the decapole component. This effect was noticed for the first time as atime dependent chromaticity variation during injection tests at the Tevatron, followed by a fastchromaticity change during the initial acceleration phase at the end of particle injection.1Initially flux creep was thought to be the source of the change, due to the logarithmic decaybehaviour. Measurements were performed at HERA on all magnets, which showed deviationsfrom the logarithmic decay. Brück (1989) reported a large variation in the decay rate frommagnet to magnet, which could be grouped into two families, corresponding to themanufacturer. As the initial current cycle showed a strong influence on the decay behaviourand as it is not possible to heat up all HERA magnets before each run, investigations focusedon finding a current cycle, which reduces the decay behaviour. A 30 minute long pre-injectionporch at low current turned out to be quite effective. For further compensation look-up tablesand magnetic measurements from two reference magnets were used. At the SSC Devred (1991)identified two different components of the field component decay. The first and logarithmic oneis caused by flux creep, lasts for about 300 s, is not affected by the excitation history and canbe stopped by decreasing the magnet temperature. The second component, however, exhibits along lasting decay, cannot be stopped, depends on the powering history and only appears whenthe magnet is precycled to a high current. A source of the second decay component can befound in a “current imbalance“ in the Rutherford cable, where different strands take differentfractions of the transport current. During a ramp supercurrents are induced in the magnetheads, where the cable is exposed to a variation in the field sweep rate (Verweij 1995). Duringstable current conditions the amplitude decays due to currents diffusing into the straight sectionof the magnet, which changes the magnetisation in the filaments. A ramp after injectionimmediately “switches on” the original magnetisation (Wolf 1997).2. EXPERIMENTSA series of short models was manufactured at CERN within the frame of the R&Dprogram for LHC dipoles. The magnets were assembled from four superconducting coils in asupport stucture of laminated collars. The assembly was completed by an iron yoke andenclosed into a shrinking cylinder of stainless steel. The magnets considered in our experimentsare the 1m long single aperture models MBSMS1 to MBSMS23. MBSMS1 to MBSMS13 aremade from 5 blocks of cable per quadrant and Al-alloy collars. The following modelsMBSMS15 to MBSMS23 feature the  new optimized 6 block design. Four of them(MBSMS15 to MBSMS18) have Al-alloy collars. The other five (MBSMS19 to MBSMS23),however, contain collars from austenitic steel and represent the closest match to the baselinedesign of the LHC main bending dipoles. Several of the magnets have been reworked indifferent “versions”, changing the collaring and yoking conditions. The measurements of decayand snapback were performed in a vertical test set-up. The magnets were cooled withsuperfluid helium at a temperature of 1.8 K. During our experiments rotating coils were used tomeasure the components of the magnetic field. They were mounted on a glass-fibre shaft androtated inside the bore of the magnet. All coils were read-out simultaneously by a chain ofVME integrators. Thus, the magnetic flux signal was measured as a function of the rotationangle. The major problem in the understanding of dynamic effects in the magnets is related to astrongly non linear dependence on the excitation history. Thus, we modified single parametersof a standardized operation cycle step by step and performed snapback measurements on themodel magnets.2.1 Variation of Operation CyclesThe current cycles used during our experiments are shown in Fig.1. An initial quencherases all persistent currents, the following pre-cycle moves the operating point away from thevergin curve onto a reproducible hysteresis curve. The pre-injection porch reduces the decayand snapback during injection and acceleration. Three parameters were found to be of specialimportance: The pre-cycle flat-top current, the flat-top duration and the pre-injection duration.Different pre-injection currents, however, did not significantly affect the snapback (Schneider1998).2Fig. 1: Variation of flat-top current, flat-top duration and pre-injection duration2.2 Variation of Pre-Cycle Flat-Top Current and DurationDuring the pre-cycle experiments nopre-injection porch was used. For flat-topcurrents between 0 and 4000 A no significantinfluence on the snapback amplitude was found.Above 4000 A, however, we observed an almostlinear increase of the snapback amplitude withthe pre-cycle current (Fig.2). Many of the 6block magnets (MBSMS15 to MBSMS23)showed a higher snapback amplitude and astronger dependence on the flat-top current thanthe 5 block ones (MBSMS1 to MBSMS13).Also the influence of the pre-cycle flat-topduration on the snapback amplitude is differentfor the 5 and 6 block design (Fig.3). The 5block magnets show an “exponential” increaseand a saturation for an increasing flat-topduration. A time constant of less than 1 hourcan be found by fitting the data to anexponential model. The snapback amplitude ofmost 6 block magnets, however, increases to amaximum at a flat-top duration of about 10 to20 minutes, decreases significantly above thisthreshold, and possibly saturates for higher flat-top durations. MBSMS9 shows an anomalousbehaviour.2.3 Pre-Injection DurationA pre-injection porch at a current 25 to 75 A lower than the injection plateau can beused to significantly reduce the snapback value (Fig. 4). For the 5-block model dipoleMBSMS6-V1 a reduction of 50 % was found after 30 minutes of pre-injection duration. The 6-blockmagnets rather show a saturation behaviour. MBSMS19-V2, however, only showed a verysmall dependence on the pre-injection porch duration.2.4 StatisticsIn a statistical analysis of the snapback behaviour the whole population of our 1-meterlong model magnets was investigated. The average snapback amplitudes for allowed harmonicsFig. 2: Flat-top current influenceFig. 3: Flat-top duration influence-2.2-1.7-1.2-0.7-0.20 20 40 60 80 100 120Precycle Flat-Top Duration in MinutesAmplitude of Sextupole Snapback in UnitsMbsms17-V3Mbsms19-V2Mbsms22-V1Mbsms23-V3Mbsms6-V1Mbsms12-V1Mbsms17-V1Mbsms15-V4Mbsms9-V2Mbsms5-V1Mbsms18-V1-2.1-1.6-1.1-0.6-0.10 2000 4000 6000 8000 10000 12000 14000Pre-Cycle Flat-Top Current in AmpereAmplitude of Sextupole Snapback in UnitsMbsms17-V3Mbsms17-V1 Mbsms18-V1Mbsms19-V2Mbsms23-V3Mbsms22-V1Mbsms12-V1Mbsms9-V2Mbsms15-V4Mbsms5-V1Mbsms6-V13are distributed systematically either withnegative or positive sign, alternating in signfrom one harmonic order to the other. Thesnapback values of non-allowed harmonics,however, are randomly distributed on thenegative or positive side. Their average valuesare embedded into error bars around zero(Schneider 1998).3. CONCLUSIONFor all magnets we found a strong dependenceof the snapback amplitude on the poweringhistory. Especially the maximum current and the duration of previous cycles have an importantinfluence. A pre-injection porch, however, can be used in order to reduce these dynamic effects.The snapback amplitude depends on the detailed features of each magnet. Magnets of twodifferent geometric designs were used in the experiments. They were wound from the same kindof cable, but especially the interstrand resistances varied through the years with differentcoatings or due to different collaring and yoking conditions. The differences in the measuredsnapback amplitudes are quite large in spite of the apparent similarity of the magnets. Inpractice this implies that all magnets should be measured before they are installed into theLHC.REFERENCESBottura L et al. 1997 IEEE Trans. Appl. Sup. 7, 2, p 602-605Brück H et al. 1989 Proc. 11th Int. Conf. on Magnet Technology, Tsukuba, JapanDevred A et al. 1991 Proc. IEEE Part. Acc. Conf., San Francisco, CA pp 2480-2482Schneider M 1998 PhD thesis, TU WienVerweij A 1995 PhD thesis, University of TwenteWolf R 1997 Proc. 15th Int. Conf. on Magnet Technology, Beijing, China pp 238-241-1.4-1.3-1.2-1.1-1-0.9-0.8-0.7-0.6-0.5-0.40 5 10 15 20 25 30 35Pre-Injection Duration in MinutesAmplitude of Sextupole Snapback in UnitsMbsms22-V1Mbsms19-V2Mbsms6-V1Fig. 4: Influence of pre-injection duration4

Studies of Decay and Snapback Effects on LHC Dipole Magnets

Studies of Decay and Snapback Effects on LHC Dipole Magnets

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