The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

DEVELOPMENT OF A MAGNET WITH HIGH CRITICALTEMPERATURE SUPERCONDUCTING WIRES*K. Hatanaka#, S. Ninomiya, Y. Sakemi, RCNP, Osaka University, Osaka, Japan
T. Kawaguchi, N. Takahashi, KT Science Ltd, Kakogawa, Hyogo 675-0012, Japan
Abstract
A solenoid magnet using high Tc superconducting wire
was designed and fabricated. After the successful coolingtests, performance was studied by operation with both DCand AC currents. We found the magnet can be operated
with 100 A at frequencies higher than 1 Hz. The present
results indicate many possible applications. The magnet
will be used to generate a mirror field of the ECR ion
source at 2.45 GHz.

INTRODUCTION
Since
thediscoveryofhigh-temperature
superconductor (HTS) materials in 1986, many efforts
have been performed to apply them to various apparatuses
as well as to develop new materials [1]. Recently it has
become possible to supply long high-Tc wires in greaterthan 100 m lengths [2]. Although many prototype devises
using HTS wires have been fabricated and evaluated,
there are only a few applications in the accelerator fields[3].
We designed and fabricated two solenoid coils with
HTS wires and made some preliminary tests to investigate
the performance and perspectives of the HTS magnets for
future applications. These coils will be used to generate amirror field of an Electron Cyclotron Resonance (ECR)
ion source operating at 2.45 GHz. In this paper, we report
the design and performance of the coil system.

Table1: Parameters of the HTS-magnet
Superconductor
Bi 2223/ Ag tape
Total length 360 mNo. of turns
292 x 2 coils
Winding construction 4 pancakes / coil
Rated current
90 A
Max. magnetic field 5.2 kG in parallel toin the coiltape
3.6 kG in normal totape
Cryostat Cooling method
Conduction cooling by a
G-M refrigerator
Thermal insulation Vacuum isolation, 80K
shieldandsuperinsulation
Cooling power of the 9 W at 20 K, and
G-M refrigerator
18 W at 80K

HTScoils

G-M refrigerator

80K first stage
CRYOSTAT

20K second stage
Thermal shield

DesignSpecifications of the present solenoid coils are
summarized in Table 1. Cross sectional view of themagnet is shown in Fig. 1. The ECR ion source consistsof two HTS coils, 6 permanent magnet (NEOMAX-44H)
bars to generate a sextupole field, a plasma chamber and,a micro-wave guide. A two-stage G-M refrigerator is used
to cool the coils. The cooling power is 18 W and 9 W at
the first (80K) and the second stage (20 K), respectively.
Current leads are made of electric copper and thermal
anchors are installed in the middle to connect current
leads to the first stage of the refrigerator. In order toreduce power losses HTS tapes are soldered on the leads
between the thermal anchors and the HTS coils. Powerlosses at current leads are estimated to be 10 W and 2 W
at the high and low temperature region, respectively. Total
heat load is estimated to be 18 W and 3 W at the first andsecond stage of the refrigerator.
_________________*Supported in part by Ministry of Education, Culture, Sports, Scienceand Technology of Japan.
#hatanaka@rcnp.osaka-u.ac.jp

Magnetic shield

1170 mm

HTS COILSHTS-COIL

Coil support

Thermal shield
640 mmFigure1: Cross sectional view of the HTS solenoid
magnet for an ECR ion source.
The HTS wire is a flexible composite consisting offilaments of nominal composition (Bi,Pb)2Sr2Ca2Cu3Ox ina silver matrix, which also provides mechanicalrobustness and transient thermal stability. The tape is 4.2

The inductance of the pancakes was measured at room
temperature and was 6.0 – 6.4 mH which is consistent
with the design value of 6.2 mH. The critical current (Ic)of the pancake was measured in liquid nitrogen. Results
are 48 – 52 A (1 µV/cm, self-field) for all pancakes. ThisIc value is same as that of the tape itself. This shows thewire was not damaged by winding procedure. Afterstacking two pancakes to make a coil, Ic was again
measured in liquid nitrogen to be 36 - 39 A. Same results
were obtained after three heat cycles. The achieved
performance is better than anticipated and we can expect
higher operating current than 90 A at 30 K.
Two coils are mounted on a bobbin made of SUS316.
They are connected electrically in series and assembled ina cryostat. Thermal shields are installed and connected tothe first stage of the refrigerator. Twenty layers superinsulations are put between the thermal shields and room
temperature wall of the cryostat. Figure 2 shows aphotograph of the system with thermal shields. Thebobbin is supported by four rods of titanium alloy whichis 4 mm in diameter. The cryostat is evacuated by a turbomolecular pump and cooled by the refrigerator. Figure 3shows the temperature and the ohmic resistance. The HTS
coils transited in superconducting state at 105 K with 25
hours cooling time. Coils are cooled down to 13 K, while
the equilibrium temperature of thermal shields is about100 K. Coils are excited with 100 A current without any

20250

16200

12150

8100

45000-4061218243036

Total resistance of two coils (Ohm)

Performance

30042Time (hrs)
Figure 3: Temperatures and coil resistance on the cooling
down with a G-M refrigerator. Circles and squares showtemperatures of the coils and the second stage of therefrigerator. Pluses crosses are temperatures of a thermal
shield and the first stage of the refrigerator. Solid curveshows the total ohmic resistance of two coils
2500

2000

Bz (Gauss)

mm wide and 0.21 mm thick. The wire (Bi2223) was
supplied by Sumitomo Electric Industries, Ltd. Each coilconsists of two double-pancakes. The inner and outer
diameter of the pancake is 156 and 225 mm, respectively.
Each pancake has 146 (73x2) turns and is 9 mm in height.
The operating temperature of HTS coils is designed to be
30 K and the maximum current is 90 A.

Temperature (K)The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

1500

1000

5000- 40- 30- 20- 10010203040

Distance along the axis (cm)Figure 4: Longitudinal magnetic field Bz distribution
along the central axis. The coil current is 100 A.

Figure 2: Photograph of the magnet assembled in thecryostat. We can see thermal shields and supporting rods

noticeable rise in temperature. Although we can expect
the coil critical current higher than 200A, the operating
current was limited up to 100 A because of the thermal
capability of the current leads in vacuum. Longitudinalmagnetic field distributions are measured along thecentral axis (z) at 100 A and are shown in Fig.4 together
with calculated fields with the program code TOSCA. We
can see good agreements between them.
Owing to the good performance above described, wecan expect a large thermal margin for the fabricated coils.
This makes it possible to operate the magnet in an ACmode with high frequencies and to keepsuperconductivity. Figure 5 shows the coil currents

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

measured by a DCCT. The magnet is excited by a homopole DC current source at 0.25 Hz. There is a heating load
due to the AC loss of superconducting wire. The
temperature rise was 2 K after 5 minutes AC operation.
At this temperature, the critical current is higher than 100A. Present results show a large applicability of HTS wires
to rapid cycling AC magnets such as scanning magnets
for the cancer therapy, high duty synchrotrons, etc.
70

A solenoid magnet was fabricated with the HTS wire
Bi2223. A two-stage G-M refrigerator could cool thesystem to 13 K and the critical current of the wire ishigher than 100 A at 20 K for the present configuration.
The magnet can be excited by AC currents at frequencies
higher than 0.5 Hz. This shows a large applicability ofHTS wires to rapid cycling magnets.

60

REFERENCES

50[1] K. Sato et al., Proc. of MT-15, Beijing (1997) 24-29.
[2] LJ. Masur et al., Proc. of MT-17, Geneva (2001) 1-5.
[3] D.M. Pooke et al., Proc. of CIMTEC’98, Italy (1998).

40Current (A)SUMMARY

3020100-1 0200

202

204

206

208

210

212

214216Time (s)Figure 5: AC excitation pattern with a homo-pole DCcurrent. The frequency is 0.25 Hz. Current is measured atthe current lead by a DCCT.