Proceedings of ITC/ISHW2007

High-temperature superconducting coil option for the fusion reactor
FFHR and development of indirectly-cooled HTS conductors
Gourab Bansal, Nagato Yanagia, Tsutomu Hemmib, Kazuya Takahataa, Toshiyuki Mitoa, Akio Sagaraa
The Graduate University for Advanced Studies, Toki, Gifu 509-5292, Japana

National Institute for Fusion Science, Toki, Gifu 509-5292, Japanb

Japan Atomic Energy Agency, 801-1, Mukoyama, Naka 311-0193, Japan
Large current-capacity high-temperature superconducting (HTS) conductors using YBCO wires are being
considered as an option for the fusion energy reactor FFHR [1, 2]. Typical required current for such conductors in
FFHR is 100 kA at about 13 T magnetic field. The operating temperature for HTS magnets in FFHR is about 20 Kwith conduction cooling using an indirect cooling method. Since the specific heats of the materials at 20 K aresignificantly higher as compared with 4.2 K, which is the typical operating temperature of low temperature
superconducting (LTS) magnets, the HTS magnets are operated much more stably and reliably against any
mechanical or electromagnetic disturbances, which is one of the biggest advantages over LTS magnets. Anotheradvantage is the reduction in required refrigeration power as the HTS magnets are operated at the elevatedtemperatures. Or, using HTS conductors, it is also considered to be viable to assemble the continuous helical coils
in segments with a number of joints for conductors, as we can allow additional heat generation utilizing the surplus
refrigeration power [3]. Two options of aluminum-alloy and SUS jacketed HTS conductors have been considered.
The maximum strain in the winding has been estimated to be less than 0.3%, which is less than the critical strain of~0.5% for YBCO conductors. As the first step towards the HTS conductor developments, a 10 kA-class conductor
has been fabricated using Bi-2223/Ag tapes, by simply stacking and soldering them inside a copper sheath. The
critical currents of the conductor have been measured at 4.2 K and at elevated temperatures up to 30 K. The
stability margin experiments have also been done. The HTS conductor was found to be highly stable. In near future,
the YBCO coated-conductors are planned to be used for next HTS conductor sample, and the stability and quench
characteristics will be compared with those obtained for the present one.
Keywords: LHD, FFHR, fusion reactor, HTS, YBCO, BSCCO, superconductor, stability.

1. Introduction
Force free helical reactor (FFHR) is a LHD-type
fusion energy reactor, which is being designed at National
Institute for Fusion Science (NIFS) in the framework ofinter-university collaborative research. Several designs of
FFHR-series reactors have been proposed [4]. Amongthem, a state-of-the-art design FFHR2m1 has plasma major
radius of 14 m, toroidal field of 6.2T, maximum field at theconductor 13 T, and fusion power of 1.9 GW. FFHR2m1
consists of one pair of helical coils and two pairs ofpoloidal field coils. Already well-developed low
temperature superconductors (LTS) are being considered
for the helical and poloidal coils of FFHR2m1. However,recently, the high temperature superconductor (HTS)
technology has improved significantly and has shown goodprospects for future applications. Considering this fact, theHTS has emerged as a competitive candidate for the helicalcoils of FFHR. The present study is focused on the HTS
conductor design for the helical coils of FFHR. Hightemperature superconductors are being considered forauthor’s e-mail: bansal@nifs.ac.jp

high field magnets in fusion reactors due to their better
performances in high magnetic field and elevatedtemperature operations [5] – [15]. In a fusion reactor, theHTS magnets can be operated at ~20 K or higher andtherefore reduces the operational cost compared toconventional LTS magnets, which are used at ~4 K.
Secondly, due to the increased specific heat of the HTS
conductors at elevated temperatures, they become lessprone to quench and therefore more safer operations of afusion reactor are possible, which is perhaps the mostdesirable requirement from the magnets. The HTS
magnets can be cooled by conduction cooling methods
and therefore can avoid the complicated networks ofpipings, generally necessary for force flow cooled LTS
conductor magnets. The indirect cooling method has been
proposed for aluminum-alloy jacketed Nb3Sn conductor
for FFHR helical coils [16]. The same indirect coolingtechnique can be easily adopted for HTS conductor coils
as well. The increased thermal conductivity of the metals
at elevated temperatures helps in quickly removing theProceedings of ITC/ISHW2007
heat generated in the conductor due to the AC losses,
mechanical disturbances, nuclear heating and othersources.

2. HTS Conductor Design
Second generation coated conductors, such as
YBCO and GdBCO, are the promising candidates for
future demo fusion reactors as they sustain high criticalcurrents at high magnetic fields. A cross-section of theproposed 100 kA-class HTS conductor for FFHR2m1
helical coils is shown in Fig. 1. This conductor uses
YBCO or GdBCO tapes along with copper tapes inside athick jacket of aluminum-alloy or SUS. The dimensionsof the conductor have been chosen the same with its LTS
counterpart proposed in [16]. The critical current of theHTS tape is considered as 100 A/mm-width at 13 T and
25 K, which is expected to be developed soon. The
copper to superconductor ratio in HTS conductor is 7.0
and the critical current of the conductor is 128 kA at 13 T,
25 K.

Fig.1

Cross-section of proposed HTS conductor forthe helical coils of FFHR2m1.

Fig. 2 shows the rectangular cross-section of thehelical coil (1.8 m wide and 0.9 m high). The helical coilconsists of 12 layers with 36 turns in each layer. Inside thewinding, four 75 mm thick cooling panels with embedded
cooling channels are installed. The winding is cooled byconduction due to the flowing coolant through the coolingchannels of the panels. The expected steady state nuclear
heat load on the superconducting coil in FFHR is 100
W/m3 [16,17], which should be removed by the coolant
effectively. The temperature increase of the conductor,ΔTmax, can be estimated by one dimensional heat
conduction equation.

ΔTmax =Ql 22λe(1)

where Q is the heat load, l is the distance, λe is theeffective thermal conductivity. In Fig. 2, the maximum
distance between heated zone and coolant, l, is 0.1 m. If theconductor temperature is allowed to be increased by 1 K
due to the nuclear heating of 100 W/m3, the required
effective thermal conductivity is 0.5 W/m-K.

Fig.2

Cross-section of helical coil of FFHR2m1.

Fig.3

Effective thermal conductivity in the
cross-sectional direction of the winding as afunction of the jacket and HTS bundle
thermal conductivities.

Fig. 3 shows the calculated effective thermal
conductivity considering different materials over a lengthof 0.1 m in winding cross-section. The insulation thickness
is taken as 1 mm and its thermal conductivity is 0.1
W/m-K at 20 K. The effective thermal conductivity is
calculated by varying the jacket and HTS and copper tapes
bundle thermal conductivities. In the worst case, when
HTS and copper tapes bundle thermal conductivity is
considered to be 1 W/m-K and HTS conductor jacket isSUS, the effective thermal conductivity comes out to be~0.85 W/m-K at 20 K. This value is still higher thanrequired effective thermal conductivity of 0.5 W/m-K,
which suggests that SUS can also be used in HTS
conductors, which was not possible for LTS counterpart
proposed in [16]. Aluminum-alloy jacket provides higher

Proceedings of ITC/ISHW2007
effective thermal conductivity of ~1 W/m-K at 20 K and
therefore is a better option as far as the heat removal isconsidered.

3. Stress and strain in the coil
The helical coils of FFHR will experience large
electromagnetic forces and therefore the stresses and strain
will be developed in the coil. Stresses and strain in the coilhave been estimated by considering the coil as an infinite
solenoid and only the radial forces are taken in account[16,18]. The average radius of curvature of helical coil is
5.5 m and therefore the same radius of curvature isconsidered for infinite solenoid model. The cross-sectionof the solenoid model is also the same as that of helical coilshown in Fig. 2. The calculated stresses and strain are
shown in Fig. 4 for both the options of aluminum-alloy
jacketed and SUS jacketed HTS conductors. The radialstress at inner radius and outer radius of the winding aretaken to be zero as the boundary conditions. The maximum
hoop stress in the SUS cooling panel and aluminum-alloy
jacket are 510 MPa and 210 MPa respectively. Themaximum hoop stress in SUS structure is 370 MPa when
HTS conductor jacket is SUS. The stresses are always lessthan the yield strengths of the materials at 20 K and
therefore safe under large electromagnetic forces in helicalcoils of FFHR. The strain is always less than 0.3%, whichis less than the critical strain of ~0.5% for YBCO. Fig. 4
suggests that SUS jacketed HTS conductor is a better
choice from stress and strain point of view.

Fig.4

Hoop stress, radial stress, and strain in thehelical coil of FFHR2m1. Both options ofaluminum-alloy jacketed and SUS jacketed HTS
conductor are shown.

4. Quench detection and protection
Due to the increased specific heat of the materials at
elevated temperatures, the thermal diffusivity becomes
smaller and therefore the quench propagation also becomesslower. Hence, the voltage development in HTS conductors
at elevated temperatures is very slow and therefore thequench detection becomes difficult. This is one of thebiggest problems in HTS conductors. Fig. 5 shows thevoltage across the conductor as a function of conductor
length at different temperatures and 100 kA current. At
45K, the conductor length is about 6 m to observe avoltage of 100 mV whereas it is about 2.5 m at 50 K. The
required length further reduces with increased temperature.
Fig. 6 shows the final hot-spot temperature with different
jacket materials in adiabatic condition. The coil current is
100 kA at 25 K and the stored magnetic energy is dumped
into an external resistor with a time constant of 20 s after
the quench detection.

Fig.5 Voltage development as a function ofconductor length at different temperaturesand 100 kA current.

Fig.6

Final hot-spot temperature as a function of
initial hot-spot temperature (just before thedumping) and jacket materials.

Fig. 6 suggests that SUS jacket for HTS conductor
allows higher initial hot-spot temperature (for a conditionof final hot-spot temperature less than 150 K) beforedumping. This means that less conductor length is required
to develop larger voltage as shown in Fig. 5, and therefore
quench can be detected rather quickly and easily with SUS
jacketed HTS conductor.

Proceedings of ITC/ISHW2007

5. Proposal of segmented helical coils
It may not be easy to realize a continuous winding of
the huge helical coils in FFHR, therefore, the segmented
helical coils might be a viable choice to wind the helicalcoils with a number of joints between the segments. Thisidea was first proposed by Hashijume et al. [3]. Due to theelevated temperature operation of HTS coils, the surplus
refrigeration power can be used to take away the heatgenerated by the joints between the helical coil segments.
Since, the HTS conductor has large temperature margin,
the temperature rise of few Kelvin due to the joints may
not be a big concern for the stability of the coils. Fig. 7
shows the maximum temperature rise of the conductor as afunction of heating density calculated by equation 1. Boththe options of SUS jacketed and aluminum-alloy jacketed
conductor have been considered. If a temperature rise of5K of the conductor is acceptable, the heat density of about
990 W/m3 on the winding can be allowed. This means ajoint resistance of about 3 nΩ is acceptable as the numberof joints between the conductors in one helical coil, madeby 10 segments, is 4320.

simple soldered lap joint. A soldered joint configuration
with few numbers of HTS tapes is shown in Fig. 8. The
HTS tapes are cut in step like structure and thenoverlapped and joined YBCO side with YBCO side.

6. Development of 10 kA-class HTS conductor
As the first step towards the development of large
current capacity HTS conductor, we have developed a 10kA-class HTS conductor using Bi2223/Ag HTS tapes. The
critical current of one Bi2223/Ag tape is 140 A at 77K andself field. The HTS tapes were soldered inside a copper
sheath of outer dimension of 12 mm × 7.5 mm. The HTS
tapes were simply stacked and soldered. One HTS
conductor consists of 34 HTS tapes and no copper tape.
Thin stainless steel heaters were attached on the surface ofFig.9 (a) Cross-section of 10 kA-class HTS conductor,(b)
GFRPinsulated
HTSconductor
configuration (to realize conduction cooling) inexperiments.

Fig.7

Maximum temperature rise of the conductor
as a function of continuous heating density
on the helical coil winding.

Fig.8 (a) HTS tapes cut in step like structure (b)overlap joint between two HTS conductors.
The joint between the conductors might be themechanical joint proposed by Hashijume et al. in [3] orthe conductor to elevate the conductor temperature up to
30 K for the critical current and stability measurements atelevated temperatures. Then the conductor was insulatedusing epoxy and GFRP sheets to isolate it with liquid

Fig.10Measured critical currents of HTS conductor atdifferent temperatures.

helium in the cryostat. During the experiments, theconductor was cooled by conduction through the ends. A
cross-section of the HTS conductor is shown in Fig. 9. The

Proceedings of ITC/ISHW2007
epoxy and GFRP insulated conductor configuration (torealized conduction cooling in the experiments) is alsoshown.
Fig. 10 shows the measured critical current of theconductor at 4.2 K and elevated temperatures up to 30 Kunder a bias field of 8 T parallel to ab-plane of the HTS
tapes. The critical currents are measured with 1μV/cmcriterion. During the critical current measurements at 4.2K,
it was not possible to maintain the conductor temperatureexactly at 4.2 K due to the heating by flux flow resistancein the HTS conductor. At 1 μV/cm (criterion for criticalcurrent), the conductor temperature was about 5.5 K,
which is reported in Fig. 10. The similar temperature rise
was observed at elevated temperature measurements at10K, 20K, and 30K as well. Fig. 10 suggests that thecritical current of HTS conductor shows a linear
dependence on temperature.
The stability of the HTS conductor was also measured
using thin film heaters attached on the surface of theconductor. The conductor was found to be very stable andcould not be quenched even with very high energy input of
30 J/cm3 at 20 K and 10 kA conductor current (~90% ofthe critical current) under a bias field of 8 T. With 30 J/cm3
energy input, the temperature of the conductor increased up
to 34 K, which was more than the current sharing
temperature of 31 K. But the conductor temperature goes
down quickly due to the thermal conduction towards theends of the sample and therefore no thermal runaway wasobserved. The typical stability margin for a LTS
cable-in-conduit conductor (CICC) is 300 – 500 mJ/cm3 at~4 K and 30% – 50% of the critical current in theconductor. Hence, one can say that HTS conductors arehighly stable compared to their LTS counterparts andtherefore can provide much safer operation of the fusion
machines. The details of the experimental set-up andresults of the 10 kA-class HTS conductor can be foundelsewhere [19].

7. Summary
The feasibility study of HTS conductor option for
LHD-type helical fusion energy reactor FFHR has started.
The preliminary design of the HTS conductor has been
proposed, which seems to be suitable for FFHR helicalcoils. Quench detection and stress calculations suggest thatSUS should be adopted as a jacket material for theconductor. On the other hand, aluminum-alloy might be abetter choice from winding point of view being a softer
material compared to SUS. Segmented helical coil withmechanical or soldered joints might be a viable choice dueto the large temperature margin of the HTS conductor andavailable surplus refrigeration power, which is a bigadvantage in HTS conductors over their LTS counterparts.
A 10 kA-class HTS conductor with Bi2223/Ag HTS tapes

has been successfully fabricated and tested at 4.2 K andelevated temperatures up to 30 K. More studies e.g. error
fields due to the shielding currents, current distribution in
the conductor, and AC losses are planned to be done on theHTS conductors. The development and testing of a 10kA-class conductor using YBCO tapes is also planned innear future.

References[1][2]A. Sagara et al., Fusion Eng. Design 81, 2703 (2006).
T. Hemmi et al., to be published in NIFS Annual Report(2007).
[3] H. Hashizume et al., J. Plasma Fusion Res. SERIES 5, 532(2001).
[4] A. Sagara et al., Nucl. Fusion 45, 258 (2005).
[5] J. R. Hull, Jour. Nucl. Materials 191-194, 520 (1992).
[6] T. Ando et al., presented at SOFE 2005.
[7] F. Dahlgren et al., Fus. Eng. Design 80, 139 (2006).
[8] T. Isono et al., Fus. Eng. Design 81, 1257 (2006).
[9] G. Janeschitz et al., Fus. Eng. Design 81, 2661 (2006).
[10] P. Komarek, Fus. Eng. Design 81, 2287 (2006).
[11] W.H. Fietz et al., Fus. Eng. Design 75-79, 105 (2005).
[12] L. Bromberg et al., Fus. Eng. Design 54, 167 (2001).
[13] T. Ando et al., IEEE Trans. Appl. Supercond. 14, 1481
(2004).
[14] T. Isono et al., IEEE Trans. Appl. Supercond. 13, 1512
(2003).
[15] W Goldacker et al., J. Phys. Conf. Ser. 43, 901 (2006).
[16] K. Takahata et al., to be published in Fus. Eng. Design(2007), doi: 10.1016/j.fusengdes.2007.04.050.
[17] A. Sagara et al., Fus. Eng. Design 81, 1299 (2006).
[18] M. N. Wilson, Superconducting Magents, (ClarendonPress, Oxford, 1983) p. 41.
[19] G. Bansal et al., submitted to IEEE Trans. Appl.
Supercond., August (2007).