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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 15, NO. 2, JUNE 2005
System Testing and Installation of the
NHMFL/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 a
sweeper magnet in nuclear physics experiments, has been designed and built by the National High Magnetic Field Laboratory
for operation at the National Superconducting Cyclotron Laboratory. The magnet operates at a peak field of 3.8 T in a 140 mm
gap. A secondary beam enters the magnet from the upstream side
before striking a target. The neutrons continue straight through
to a neutron detector. The charged particles are swept 40 degrees
on a one-meter radius into a particle spectrometer. To allow space
for the exit of the downstream neutron beam, the magnet iron and
coil structure are built in a modified “C” configuration. There
are two coils of “D” shape, one above and one below the beam.
This configuration keeps the magnet compact and removes the
need for a negative curvature side. The peak field in the winding
is 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, and
integration with the cyclotron are presented.
Index Terms—Large gap dipoles, superferric dipoles, sweeper
magnet.
Fig. 1.
CAD and electromagnetic models of NHMFL/NSCL sweeper magnet.
Fig. 2.
Photo of finished NHMFL/NSCL sweeper magnet.
I. INTRODUCTION
T
HE National High Magnetic Field Laboratory (NHMFL)
at Florida State University has completed the design, fabrication, and testing of a large-gap, super-ferric dipole magnet for
use in radioactive beam experiments at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University (MSU) (see Figs. 1 and 2). The magnet was successfully
tested at the NHMFL reaching the design current on March 19,
2004 with only a single training quench. The magnet was delivered to the NSCL in April and was installed and in-service
performing experiments along the nuclear drip-lines using the
coupled-cyclotron in June.
II. SYSTEM DESCRIPTION
The physical constraints, optimization, and analysis of the
sweeper system have already been discussed in [1]–[7]. The
sweeper magnet consists of four main subsystems: the magnet
cryostat, the satellite cryostat, the magnet iron and the power
supply as shown in Figs. 1 and 2.
Manuscript received October 4, 2004. This work was supported in part by the
U.S. National Science Foundation under Grant PHY9871462.
M. D. Bird, S. J. Kenney, J. Toth, and H. W. Weijers are with the National 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 Superconducting Cyclotron Laboratory, East Lansing, MI 48824 USA (e-mail:
zeller@nscl.msu.edu).
Digital Object Identifier 10.1109/TASC.2005.849553
III. MAGNET TESTING AT NHMFL
Welding, leak checking and assembly of the magnet were
completed in Feb. 2004. An instrumentation system including
1051-8223/$20.00 © 2005 IEEE
BIRD et al.: SYSTEM TESTING AND INSTALLATION OF THE NHMFL/NSCL SWEEPER MAGNET
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TABLE I
PRE-TENSION OF LINKS
strain gages on the 13 warm-to-cold links was developed. The
cryogen circuits were purged, the warm-to-cold links were loosened, a hi-potential test was performed. The helium circuit was
slowly cooled with cold nitrogen gas over a period of about two
days. The N2 circuit was then slowly cooled starting with cold
N2 gas over a period of about one day. The helium circuit was
then repeatedly evacuated and backfilled with He gas. It was
slowly cooled to LHe temperature over about one day. A cold
hi-pot test was performed. The warm-to-cold links were tightened. The power supply was connected and the magnet energized at 4 volts (7 A/s) to 150 Amps. The magnet was de-energized and the warm-to-cold links were adjusted to provide more
uniform loading during energization. The magnet was energized
a second time at 4 volts and quenched at 343.7 Amps. The
magnet was energized a third time at 4 volts to 250 Amps and
then at lower voltage to 350 Amps. The warm-to-cold links were
then adjusted a second time. On March 19, 2004 the magnet was
ramped at 5 volts to 250 Amps and at lower voltage to the design current of 365 Amps. A couple days later the magnet was
ramped from 0 to 375 Amps at 5 volts without quenching. The
magnet displayed a hold time of 3 hours without refilling cryogens, which is acceptable for this application.
IV. LINK TENSIONING
The Lorentz forces within and between the superconducting
coils are reacted by the bobbin. However, the net force between
the cold mass and the iron yoke as well as the weight must be
reacted by warm-to-cold links. One wants to make these links
long and slender to minimize the conduction heat load, however,
they also must have a mechanical stiffness higher than the “magnetic stiffness” of the magnet system. For this system we chose
to make the links from Ti-6Al-4V (ELI) as it is much stiffer than
fiberglass and the combination of thermal conductivity, strength,
and stiffness is better than in steel. There are a total of thirteen
links on the Sweeper, one on the back, four on the front, four
on the top and four on the bottom. In Fig. 1 all but the back link
can 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 strain
gages mounted on the outside of the link cans. As the magnet is
energized, the coils tend to become slightly rounder and there is
a net force on the coils backward (toward the iron) of about 30
kN. Vertically, the two superconducting coils are only connected
on the backside. The net force on each coil is toward the nearby
iron rather than the more remote other coil and has a magnitude
of 150 kN. Thus, the bobbin “opens up” slightly and the top and
bottom links toward the front of the magnet go slack.
Fig. 3. The sweeper in place in the N4 vault at the NSCL. The quadrupole
triplet is to the right and the focal plane detector is to the left. MoNA is not
shown.
During the first energization of the magnet, the links on the
top developed too high a strain level. The magnet was de-energized and the links adjusted such that the cold mass moved up a
few millimeters. During the third energization the bottom links
developed too high a strain and the cold mass was moved down
again slightly. During the fourth run the link tension were as intended with the appropriate symmetries and magnitudes.
V. SYSTEM TESTING AT NSCL
In April the Sweeper arrived at the NSCL where it was installed on the beamline in the N4 vault along with the Modular
Neutron Array (MoNA) and the focal plane detectors. The installation is shown in Fig. 3 where the Sweeper has the NHMFL
logo, a red focusing quadrupole is shown to the right and the
focal plane detectors to the left. MoNA is not shown.
The first run consisted of observing neutrons detected by
MoNA in coincidence with charged particles bent into the focal
from
plane detectors by the Sweeper. A secondary beam of
the Coupled Cyclotron Facility bombarded a beryllium target
in front of the sweeper magnet. The single proton stripping
which is unbound
reaction populates the ground state of
. The goal of
and decays immediately into a neutron and
the test run was to observe this decay and extract the ground
.
state energy of
Neutrons emitted near zero degrees through the large gap of
the Sweeper were detected in MoNA located about 13 m behind
the Sweeper. The Sweeper operated at 300 A corresponding to a
rigidity required to bend the charged fragments into the detector
system of the focal plane box located at 40
The upper left panel of Fig. 4 shows the
-E plot recorded
by detectors in the focal plane box following the sweeper. Three
, and
and
redistinctive groups identified as scattered
action products (from top to bottom) can be seen. The other three
spectra are time-of-flight spectra of neutrons recorded by MoNA
relative to the fragments detected in the sweeper detector setup.
(bottom left),
(bottom
The three spectra are gated on
(top right). The
gated spectrum shows only
right) and
random coincidences from cosmic ray background as expected.
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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 15, NO. 2, JUNE 2005
ACKNOWLEDGMENT
This paper is dedicated to the memory of Jack E. Crow, the
founding director of the NHMFL, a man of tremendous vision
and energy without whose enthusiasm this project would not
have been undertaken.
The authors would also like to express their tremendous
appreciation to the numerous people at the NHMFL and at
the NSCL who contributed to the successful delivery of this
magnet system. Principal among them are: Soren Prestemon
(FEA); Scott Gundlach (CAD); Denis Markiewicz (superconducting magnet technology); Iain Dixon, George Miller, Bianca
Trociewicz, (system testing), J. Bierwagen, and D. Sanderson
(installation), plus numerous technicians, machinists and
welders at the NHMFL an the FSU physics departments.
REFERENCES
Fig. 4. On-line spectra from the first test run of the sweeper magnet in
coincidence with the Modular Neutron Array MoNA. The top left figure
shows the particle identification of fragments in the detector box following the
sweeper. Bands of lithium, helium and hydrogen (from top to bottom) can be
clearly identified. The other three spectra are time of flight spectra of MoNA
relative to the fragments in the sweeper.
The
spectrum exhibits two distinct peaks corresponding to
forward and backward emitted neutrons from the ground state
. The spectrum in coincidence with
shows only one
of
does not have a sharp unbound resobroad peak because
nance. The spectra shown in Fig. 4 were recorded online and
are not calibrated. The fact that we were able to extract these
distinct features already online shows that the Sweeper and all
the detectors of MoNA and the Sweeper focal plane box worked
great in this first test run.
[1] M. D. Bird et al., “Bucket testing of a compact sweeper magnet for
nuclear 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 sweeper
magnet 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 nuclear
physics,” IEEE Trans. Appl. Supercond., vol. 12, no. 1, pp. 341–344.
[5] J. Toth and M. D. Bird, “Convergence studies of D-shaped coil/bobbin
interactions in a sweeper magnet system,” IEEE Trans. Appl. Supercond., vol. 13, no. 2, pp. 1400–1403, Jun. 2003.
[6] J. Miller, G. Miller, and D. Richardson et al., “A novel concept for a
modular helium-vapor-cooled lead pair,” in Advances in Cryogenic Engineering: 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 the
NHMFL/NSCL sweeper magnet,” IEEE Trans. Appl. Supercond., vol.
14, no. 4.