1252 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 15, NO. 2, JUNE 2005

System Testing and Installation of theNHMFL/NSCL Sweeper Magnet

Mark D. Bird, Steven J. Kenney, Jack Toth, Hubertus W. Weijers, Jon C. DeKamp, Mike Thoennessen, and Al F. Zeller

Abstract—A superconducting dipole, designed for use as asweeper magnet in nuclear physics experiments, has been de-signed and built by the National High Magnetic Field Laboratoryfor operation at the National Superconducting Cyclotron Labo-ratory. The magnet operates at a peak field of 3.8 T in a 140 mmgap. A secondary beam enters the magnet from the upstream sidebefore striking a target. The neutrons continue straight throughto a neutron detector. The charged particles are swept 40 degreeson a one-meter radius into a particle spectrometer. To allow spacefor the exit of the downstream neutron beam, the magnet iron andcoil structure are built in a modified “C” configuration. Thereare two coils of “D” shape, one above and one below the beam.This configuration keeps the magnet compact and removes theneed for a negative curvature side. The peak field in the windingis 6.5 T. The net force on the curved leg of a single “D” is 1.6 MN.Results of system testing including cool-down, quench history, andintegration with the cyclotron are presented.

Index Terms—Large gap dipoles, superferric dipoles, sweepermagnet.

I. INTRODUCTION

THE National High Magnetic Field Laboratory (NHMFL)at Florida State University has completed the design, fabri-

cation, and testing of a large-gap, super-ferric dipole magnet foruse in radioactive beam experiments at the National Supercon-ducting Cyclotron Laboratory (NSCL) at Michigan State Uni-versity (MSU) (see Figs. 1 and 2). The magnet was successfullytested at the NHMFL reaching the design current on March 19,2004 with only a single training quench. The magnet was de-livered to the NSCL in April and was installed and in-serviceperforming experiments along the nuclear drip-lines using thecoupled-cyclotron in June.

II. SYSTEM DESCRIPTION

The physical constraints, optimization, and analysis of thesweeper system have already been discussed in [1]–[7]. Thesweeper magnet consists of four main subsystems: the magnetcryostat, the satellite cryostat, the magnet iron and the powersupply as shown in Figs. 1 and 2.

Manuscript received October 4, 2004. This work was supported in part by theU.S. National Science Foundation under Grant PHY9871462.

M. D. Bird, S. J. Kenney, J. Toth, and H. W. Weijers are with the Na-tional High Magnetic Field Laboratory, Tallahassee, FL 32306 USA (e-mail:bird@magnet.fsu.edu).

J. C. DeKamp, M. Thoennessen, and A. F. Zeller are with the National Su-perconducting Cyclotron Laboratory, East Lansing, MI 48824 USA (e-mail:zeller@nscl.msu.edu).

Digital Object Identifier 10.1109/TASC.2005.849553

Fig. 1. CAD and electromagnetic models of NHMFL/NSCL sweeper magnet.

Fig. 2. Photo of finished NHMFL/NSCL sweeper magnet.

III. MAGNET TESTING AT NHMFL

Welding, leak checking and assembly of the magnet werecompleted in Feb. 2004. An instrumentation system including

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BIRD et al.: SYSTEM TESTING AND INSTALLATION OF THE NHMFL/NSCL SWEEPER MAGNET 1253

TABLE IPRE-TENSION OF LINKS

strain gages on the 13 warm-to-cold links was developed. Thecryogen circuits were purged, the warm-to-cold links were loos-ened, a hi-potential test was performed. The helium circuit wasslowly cooled with cold nitrogen gas over a period of about twodays. The N2 circuit was then slowly cooled starting with coldN2 gas over a period of about one day. The helium circuit wasthen repeatedly evacuated and backfilled with He gas. It wasslowly cooled to LHe temperature over about one day. A coldhi-pot test was performed. The warm-to-cold links were tight-ened. The power supply was connected and the magnet ener-gized at 4 volts (7 A/s) to 150 Amps. The magnet was de-ener-gized and the warm-to-cold links were adjusted to provide moreuniform loading during energization. The magnet was energizeda second time at 4 volts and quenched at 343.7 Amps. Themagnet was energized a third time at 4 volts to 250 Amps andthen at lower voltage to 350 Amps. The warm-to-cold links werethen adjusted a second time. On March 19, 2004 the magnet wasramped at 5 volts to 250 Amps and at lower voltage to the de-sign current of 365 Amps. A couple days later the magnet wasramped from 0 to 375 Amps at 5 volts without quenching. Themagnet displayed a hold time of 3 hours without refilling cryo-gens, which is acceptable for this application.

IV. LINK TENSIONING

The Lorentz forces within and between the superconductingcoils are reacted by the bobbin. However, the net force betweenthe cold mass and the iron yoke as well as the weight must bereacted by warm-to-cold links. One wants to make these linkslong and slender to minimize the conduction heat load, however,they also must have a mechanical stiffness higher than the “mag-netic stiffness” of the magnet system. For this system we choseto make the links from Ti-6Al-4V (ELI) as it is much stiffer thanfiberglass and the combination of thermal conductivity, strength,and stiffness is better than in steel. There are a total of thirteenlinks on the Sweeper, one on the back, four on the front, fouron the top and four on the bottom. In Fig. 1 all but the back linkcan be seen as it is actually enclosed in the iron yoke.

After cool-down all the links were pre-tensioned per Table I.Tension was measured both by torque wrenches and by straingages mounted on the outside of the link cans. As the magnet isenergized, the coils tend to become slightly rounder and there isa net force on the coils backward (toward the iron) of about 30kN. Vertically, the two superconducting coils are only connectedon the backside. The net force on each coil is toward the nearbyiron rather than the more remote other coil and has a magnitudeof 150 kN. Thus, the bobbin “opens up” slightly and the top andbottom links toward the front of the magnet go slack.

Fig. 3. The sweeper in place in the N4 vault at the NSCL. The quadrupoletriplet is to the right and the focal plane detector is to the left. MoNA is notshown.

During the first energization of the magnet, the links on thetop developed too high a strain level. The magnet was de-ener-gized and the links adjusted such that the cold mass moved up afew millimeters. During the third energization the bottom linksdeveloped too high a strain and the cold mass was moved downagain slightly. During the fourth run the link tension were as in-tended with the appropriate symmetries and magnitudes.

V. SYSTEM TESTING AT NSCL

In April the Sweeper arrived at the NSCL where it was in-stalled on the beamline in the N4 vault along with the ModularNeutron Array (MoNA) and the focal plane detectors. The in-stallation is shown in Fig. 3 where the Sweeper has the NHMFLlogo, a red focusing quadrupole is shown to the right and thefocal plane detectors to the left. MoNA is not shown.

The first run consisted of observing neutrons detected byMoNA in coincidence with charged particles bent into the focalplane detectors by the Sweeper. A secondary beam of fromthe Coupled Cyclotron Facility bombarded a beryllium targetin front of the sweeper magnet. The single proton strippingreaction populates the ground state of which is unboundand decays immediately into a neutron and . The goal ofthe test run was to observe this decay and extract the groundstate energy of .

Neutrons emitted near zero degrees through the large gap ofthe Sweeper were detected in MoNA located about 13 m behindthe Sweeper. The Sweeper operated at 300 A corresponding to arigidity required to bend the charged fragments into the detectorsystem of the focal plane box located at 40

The upper left panel of Fig. 4 shows the -E plot recordedby detectors in the focal plane box following the sweeper. Threedistinctive groups identified as scattered , and and re-action products (from top to bottom) can be seen. The other threespectra are time-of-flight spectra of neutrons recorded by MoNArelative to the fragments detected in the sweeper detector setup.The three spectra are gated on (bottom left), (bottomright) and (top right). The gated spectrum shows onlyrandom coincidences from cosmic ray background as expected.

1254 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 15, NO. 2, JUNE 2005

Fig. 4. On-line spectra from the first test run of the sweeper magnet incoincidence with the Modular Neutron Array MoNA. The top left figureshows the particle identification of fragments in the detector box following thesweeper. Bands of lithium, helium and hydrogen (from top to bottom) can beclearly identified. The other three spectra are time of flight spectra of MoNArelative to the fragments in the sweeper.

The spectrum exhibits two distinct peaks corresponding toforward and backward emitted neutrons from the ground stateof . The spectrum in coincidence with shows only onebroad peak because does not have a sharp unbound reso-nance. The spectra shown in Fig. 4 were recorded online andare not calibrated. The fact that we were able to extract thesedistinct features already online shows that the Sweeper and allthe detectors of MoNA and the Sweeper focal plane box workedgreat in this first test run.

ACKNOWLEDGMENT

This paper is dedicated to the memory of Jack E. Crow, thefounding director of the NHMFL, a man of tremendous visionand energy without whose enthusiasm this project would nothave been undertaken.

The authors would also like to express their tremendousappreciation to the numerous people at the NHMFL and atthe NSCL who contributed to the successful delivery of thismagnet system. Principal among them are: Soren Prestemon(FEA); Scott Gundlach (CAD); Denis Markiewicz (supercon-ducting magnet technology); Iain Dixon, George Miller, BiancaTrociewicz, (system testing), J. Bierwagen, and D. Sanderson(installation), plus numerous technicians, machinists andwelders at the NHMFL an the FSU physics departments.

REFERENCES

[1] M. D. Bird et al., “Bucket testing of a compact sweeper magnet fornuclear physics,” IEEE Trans. Appl. Supercond., vol. 13, no. 2, pp.1250–1253, Jun. 2003.

[2] A. F. Zeller et al., “A compact sweeper magnet for nuclear physics,” Adv.Cryogenic Eng., vol. 45.

[3] S. Prestemon et al., “Structural design and analysis of a compact sweepermagnet for nuclear physics,” IEEE Trans. Appl. Supercond., vol. 11, no.1, pp. 123–135, Mar. 2001.

[4] J. Toth et al., “Final design of a compact sweeper magnet for nuclearphysics,” IEEE Trans. Appl. Supercond., vol. 12, no. 1, pp. 341–344.

[5] J. Toth and M. D. Bird, “Convergence studies of D-shaped coil/bobbininteractions in a sweeper magnet system,” IEEE Trans. Appl. Super-cond., vol. 13, no. 2, pp. 1400–1403, Jun. 2003.

[6] J. Miller, G. Miller, and D. Richardson et al., “A novel concept for amodular helium-vapor-cooled lead pair,” in Advances in Cryogenic En-gineering: Proc. Cryogenic Engineering Conf., vol. 47, S. Breon et al.,Eds., 2002, pp. 559–566.

[7] M. D. Bird et al., “Cryostat design and fabrication for theNHMFL/NSCL sweeper magnet,” IEEE Trans. Appl. Supercond., vol.14, no. 4.