Colvin, J C

Huson, F R

MacKay, W W

McIntyre, P M

Pissanetzky, S

Rocha, R

Schmidt, W M

Shotzman, G

Zeigler, J C

CERN Document Server

Particle Accelerators, 1990, Vol. 28, pp. 213-218Reprints available directly from the publisherPhotocopying permitted by license only© 1990 Gordon and Breach, Science Publishers, Inc.Printed in the United States of AmericaTHE HIGH FIELD SUPERFERRIC MAGNET IIF. RUSSELL HUSON, JOHN C. COLVIN, WILLIAM W. MACKAY,PETER M. MCINTYRE, SERGIO PISSANETZKY, RICHARD ROCHA,WILLIAM M. SCHMIDT, GARRY SHOTZMAN, AND JOHN C. ZEIGLERTexas Accelerator Center~802 Research Forest Drive, Bldg. 2,The Woodlands, Texas 77380Abstract A high-field superferric accelerator dipole hasbeen built and tested. The magnet was designed to operateat 7.6 Tesla. It achieved 6.5 Tesla with no training. Thereduced performance is due to inadequate preload of themagnet during cooldown, and can be remedied by modificationof the outer lining of the magnet.INTRODUCTIONThe Texas Accelerator Center (TAC) has conducted a systematicdevelopment of superferric accelerator dipole magnets over thepast five years. The superferric design 1 is characterized by aclose coupling of the superconducting coil to the iron fluxreturn of the magnet. The coil is arranged in a rectangularpackage rather than the cos e geometry which is required when theiron flux return is decoupled from the coil region. The benefitsof the superferric design areo the rectangular coil package and flux return are easyto manufacture and maintain dimensional control;o the Lorentz forces on the coil are reduced in mag-nitude and push the conductor directly against theside wall of the steel flux return, providing forstraightforward preloading of the coil to preventcoil motion and training;o differential contraction of a stainless-steel outerlining can be used to deliver preload only while themagnet is cold, eliminating creep of coil insulationwhile at room temperature;[691]/213214/[692] F. R. HUSON ET AL.o the iron flux return significantly reduces the mag-netic reluctance, and hence reduces the amount ofsuperconductor required to achieve a given field.A year ago TAC successfully tested its first two~modesuperferric magnet. 2 In the two-mode magnet, the rectangularcoil is segmented into two current circuits which are separatelyenergized. The outer circuit is a simple rectangular coil whichfunctions as a classic Panofsky window~frame magnet at fields upto iron saturation (-2 Tesla). The inner circuit, also composedof rectangular coil segments, is arranged to approximate thecos e distribution required as ~ ~ 1 at high field. By currentprogramming the two circuits an excellent dipole field qualitycan be achieved at all excitations over a 20:1 dynamic range. 3The first two-mode superferric magnet was designed for6 Tesla operation (90% of short-sample limit). It achieved5.85 Tesla, exhibited no training, and had excellent fieldhomogeneity. During the past year TAC has designed and built asecond two-mode superferric magnet, designed for 7.6 Teslaoperation. This paper presents the design, the test results, andan analysis of their significance.t-------------51 eM -------------iFIGURE Cross-section of two-mode superferric magnet.THE HIGH FIELD SUPERFERRIC MAGNET IIDESIGNThe magnet is shown in cross-section in Figure 1. It consists ofa two-circuit rectangular coil package, a beam tube/coil mandrelassembly, and a 4-component flux return. Figure 2 shows thecurrent programming of the two circuits which is calculated toproduce homogeneous dipole field over the operating range offield strength.The magnet flux return is designed to accommodate thedelivery of a horizontal preload to the coil which can counterthe Lorentz force on the coil in operation and thereby preventcoil motion. This is achieved by segmenting the flux return into4 rectangular blocks as shown in Figure 1, so that the two side"plungers" can slide under preload wi thin the two large blocks.The blocks and plungers are each fabricated as welded stacks orlaminations in the usual fashion. These stacks must then slideupon one another in the plane of the laminations.We experimentally verified that this indeed occurs andmeasured the coefficient of friction under relevant transverseloading at cryogenic temperatures. Figure 3 shows the coeffi~cient of static friction as a function of loading force.~A horizontal preload F is applied to the coil throughpdifferential contraction of the outer lining. The stainlesssteel lining shrinks more during cool-down than the iron fluxreturn, resulting in a tension T:3500//3000'e If/ PERFECT.20/ IRON~I,\// / DIPOLEj2500 (..u =00),," ~I/ .15~2000 // // )Js00( /. /H1500# / • to#"/'-.'c1000 / .05500,,/[693]/21523456788fTESLA)---. 2 4 6 8 10 12PRELOAD (103 LB/IN )FIGURE 2 Calculated current programming FIGURE 3 Measured coefficient of staticof the two coil segments. friction for lamination stacks.216/[694] F. R. HUSON ET AL.T Y t I:i~/~ (1)where Y 30 Mpsi is the Young's modulust 0.63 em is the lining thicknessI:i~/~ 10~3 is the cumulative differential contraction.The stress in the outer lining produces equal horizontal andvertical preloads within each quadrant F T/l2on the plungerspand blocks respectively.The various forces acting on the coil assembly and fluxreturn are shown schematically in Figure 4. The preload wasdesigned to exceed the horizontal Lorentz force F at maximumexdesign field and thereby prevent coil motion at all operatingfield strengths. This consideration neglects several otherforces, however, as will be discussed later.The coil assembly was fabricated in the same fashion as inall previous superferric magnets. The conductor is 18 strandRutherford cable (.15 x .62 cm 2 ), with 2 half-wraps of 25 ~mKapton insulation. The superconductor strand is composed of 2000NbTi filaments, 9 ~m diameter, imbedded in a copper matrix to aratio 1.8:1. The measured short-sample current is 2896 A at5 Tesla.MAGNET TESTSThe magnet attained a field of 6.50 Tesla. On successivequenches, the peak field was 6.48, 6.49, 6.50, 6.50 Tesla: therewas no training. From short-sample measurements we had predicteda peak field of 7.6 Tesla.The measured and calculated multipoles are given in TableI. For the two lowest excitations, the design configurationcalled for I = O. We were unable to deliver less than 10 Acusing the power supplies in the test configuration. Thisproduced the large higher multipoles observed in the three lowestexcitations. The significant and relatively constant value of a1indicates an off-center displacement of the coil package withinthe flux return assembly.[695]/217THE HIGH FIELD SUPERFERRIC MAGNET IIhorizontal, vertical preloadhorizontal Lorentz force on coil assemblyvertical Lorentz force on coil assembly-+- -+-Fsand Fccorrespond to the observed quenchThere are two other forces acting on the3700 lb/in1630 Ib/inF F = 5300 lb/inpy p-165 lb/in horizontal Lorentz force on steel plunger2600 lb/in vertical Lorentz force on steel blockFsxFsyFcxFcyThe values given forfield of 6.5 Tesla.LORENTZ FORCES, FRICTION, AND COIL MOTIONTo understand the disappointing reduction in magnet quench per-formance below the short sample limit, we must return to theanalysis of the forces shown in Figure 4. For one quadrant,.... Fpxassembly: static friction Ff at the interface between the blockand plunger, and compression F of the coil mandrel.mThe initial preload is delivered with no magnetic excita-tion (F = F = 0).s cEquilibrium requires that0;F + F + Fpx f mFf + 1.1 FpF (1-1.1) Fm pAt full excitation, the net force on the steel plunger is2150 lb/inreversed and is acting to move the coil outwards. Coil motionwill occur if the net force exceeds the static friction:F + F + F + F > ~Fcx sx px m fF = -1.1(F + F )f P syF > 21.1 F + 1.1 F - Fcx P sy sxr------~x.FIGURE 4 Diagram of component forces acting on the coilassembly and flux return.218/[696] F. R. HUSON ET AL.Coil motion will therefore result even below the 6.5 Tesla wherequench is observed, and quench finally occurs when the coilmotion results in a superconductor element exceeding criticalcurrent in the presence of trapped flux. Our choice of preloadwas governed by the erroneous consideration that F > F wouldP cxprevent coil motion.This undesirable state of affairs can be remedied bydoubling the thickness of the outer lining, hence doubling Fp •No sliding will then occur up to a field of 7.7 Tesla, beyondshort sample limit. The test magnet will be reassembled with asuitable outer lining and retested. It is interesting to notethat, even when a superferric magnet is erroneously assembledwith inadequate preload, the quench field is well-defined anddoes not train.TABLE I Measured (and calculated) harmonics (cm-n)b ..100200400833150020002500280010(0)10(0)10(0)12350750120014500.250.49o. 96( 1.00)1. 98( 2.01)3.44(3.45)4.46(4.50)5.48(5.51)6.05(6.13)-0.4-2.0-2.0-3.0-3.8-2.9-2.6-3.32.0-1.5-2.1-3.4-4.3-5.2-4.7-4.7-78(-0.4)-37-19(-0.4)-9.5(-0.9)-2.20.4(-0.3)0.5 (0.7)2.5(-0.8)-0.6-0.50.4-0.5-0.6-1.0-1.3-0.83.7(0.4)2.21.9(-0.1 )1 .8( 0.1)1.7( 0.5)1.9( 0.1)1.2(-0.3)0.2(-0.5)REFERENCES1. F.R. Huson, ~ ~., "Superferric Magnet Option for the SSC,"1985 Particle Accelerator Conference, (invited paper),Vancouver, B.C., May 1985.2. J .C. Colvin, ~ al., "The High-Field Superferric Magnet,"Nucl. lnst. and Meth. in Phys. Res. A270, 207-211 (1988).3. w. Xie, ~ al., "A High-Field Two-Mode Superferric Magnet,"1986 Applied Superconductivity Conference, Baltimore,Maryland, September 1986.

The High Field Superferric Magnet. 2

The High Field Superferric Magnet. 2

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