Superconducting cable conductor

In order to provide a flexible oxide superconducting cable conductor which is reduced in ac loss, tape-shaped multifilamentary superconducting wires covered with a stabilizing metal are spirally wound on a flexible former. Each of the multifilamentary superconducting wires has a plurality of filaments. The filament contains an oxide superconductor. The superconducting wires are preferably wound on the former at a bending strain of not more than 0.3 %. In winding on the former, a prescribed number of tape-shaped multifilamentary superconducting wires are wound on a core member in a side-by-side manner, to form a first layer. Then, an insulating layer is provided on the first layer. This insulating layer can be formed by an insulating tape. A prescribed number of tape-shaped superconducting multifilamentary wires are wound on the insulating layer in a side-by-side manner, to form a second layer. The insulating layer is adapted to reduce ac loss of the conductors. When the former is made of a metal, it is more preferable to provide an insulating layer between the former and the multifilamentary superconducting wires.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a superconducting cable conductor 
employing an oxide superconductor, and more particularly, it relates to a 
flexible conductor which is applicable to a superconducting cable. 
2. Description of the Background Art 
An oxide superconductor which enters a superconducting state at the liquid 
nitrogen temperature is expected for application to a superconducting 
cable with a cooling medium of liquid nitrogen. When such application is 
implemented, it will be possible to simultaneously attain simplification 
of a thermal protection system and reduction of a cooling cost in relation 
to a metal superconducting cable which requires high-priced liquid helium 
at present. 
A superconducting cable must be capable of transmitting a heavy current 
with low energy loss in a compact conductor. Power transmission is 
generally made through an alternating current, and a superconductor 
employed under an alternating current is inevitably accompanied by energy 
loss which is generically called ac loss. The ac loss such as hysteresis 
loss, coupling loss or eddy current loss depends on the critical current 
density (Jc) of the superconductor, sizes of filaments, the structure of 
the conductor, and the like. 
Various types of superconducting cables have heretofore been manufactured 
through metal superconductors, with study of structures for reducing ac 
loss. For example, Japanese Patent Publication No. 6-36329 (1994) 
discloses a superconducting conductor which comprises a normal conductor, 
and composite multifilamentary superconductors which are spirally wound 
along the outer periphery of the normal conductor. The conductor disclosed 
in this gazette is formed by clockwisely and anticlockwisely wound layers 
of the composite multifilamentary superconductors, which are alternately 
superposed with each other. The directions for winding the conductors are 
varied every layer for reducing magnetic fields generated in the 
conductors, thereby reducing impedance and increasing current carrying 
capacity thereof. This gazette also proposes provision of a 
high-resistance or insulating layer between the layers, in order to reduce 
ac loss. 
When a cable conductor is formed by an oxide superconductor, a technique 
employed in a metal superconductor cannot be applied as such. An oxide 
superconductor, i.e., a ceramics superconductor, is fragile and weak 
against mechanical strain as compared with a metal superconductor. For 
example, Japanese Patent Publication No. 6-36329 (1994) discloses a 
technique of spirally winding the superconductors around the normal 
conductor so that the winding pitch is equal to the diameter of each 
superconductor. However, when a recently developed superconducting wire 
comprising an oxide superconductor which is covered with a silver sheath 
is wound at such a short pitch, for example, there is a high possibility 
that the oxide superconductor is broken to disable current feeding. When 
an oxide superconducting wire is extremely bent, its critical current may 
be remarkably reduced. In manufacturing of a cable conductor, therefore, 
an important subject is how to arrange the oxide superconductor. 
Further, the cable conductor must be flexible to some extent, for 
facilitating handling. It is also an important subject how to manufacture 
a flexible cable conductor through a hard and fragile oxide 
superconductor. 
In addition, ac loss inevitably follows a superconductor which is employed 
under an alternating current, as described above. Thus, it also remains as 
an important subject how to reduce ac loss in manufacturing of a cable 
conductor through an oxide superconducting wire. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a superconducting cable 
conductor having flexibility and exhibiting excellent superconductivity, 
particularly a high critical current and a high critical current density, 
through an oxide superconductor. 
Another object of the present invention is to provide a superconducting 
cable conductor which is further reduced in ac loss through an oxide 
superconducting wire. 
According to the present invention, provided is a superconducting cable 
conductor employing an oxide superconductor, which comprises a long 
flexible core member, a plurality of tape-shaped multifilamentary oxide 
superconducting wires which are spirally wound on the core member, and an 
electric insulating layer. In the inventive conductor, each of the 
tape-shaped multifilamentary oxide superconducting wires includes a 
plurality of filaments consisting essentially of an oxide superconductor, 
and a stabilizing metal covering the same. The plurality of tape-shaped 
superconducting wires wound on the core member form a plurality of layers, 
each of which is formed by winding a plurality of tape-shaped 
superconducting wires in a side-by-side manner. The plurality of layers 
are successively stacked on the core member. The electric insulating layer 
is at least provided between the plurality of layers. This core member 
provides the inventive superconducting cable conductor with flexibility. 
The superconducting cable conductor according to the present invention can 
maintain a superconducting state at the liquid nitrogen temperature. 
The conductor according to the present invention is applicable to an ac 
conductor which is reduced in ac loss. 
The foregoing and other objects, features, aspects and advantages of the 
present invention will become more apparent from the following detailed 
description of the present invention when taken in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
According to the present invention, each tape-shaped multifilamentary oxide 
superconducting wire generally has such a structure that a number of 
filaments consisting essentially of an oxide superconductor are buried in 
a stabilizing material of silver or a silver alloy. The oxide 
superconductor may be prepared from yttrium, bismuth or thallium based 
oxide superconductor such as Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-X 
(0.ltoreq.X&lt;1), (Bi,Pb).sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10-Y 
(0.ltoreq.Y&lt;1) or Tl.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10-Z 
(0.ltoreq.Z&lt;1), for example. A bismuth based ceramics superconductor is 
preferable in consideration of a high critical temperature, a high current 
density, low toxicity and easiness in formation of a wire. A tape-shaped 
superconducting wire is generally manufactured through steps of preparing 
raw material powder for an oxide superconductor, charging the powder in a 
stabilizing material sheath, performing plastic working, and performing 
sintering. In the step of preparing raw material powder, powder materials 
of oxides or carbonates of elements for forming a superconductor are mixed 
with each other at prescribed blending ratios and sintered, so that the 
sintered mixture is thereafter crushed to obtain raw material powder. The 
powder is charged in a sheath which consists essentially of silver or a 
silver alloy, for example. The plastic working step is carried out through 
drawing and rolling, for example. After the rolling, the wire which is 
shaped into a tape is sintered at a temperature of about 800.degree. C. to 
about 900.degree. C., preferably at about 840.degree. C. to 850.degree. 
C., so that the superconductor provided in the sheath attains high 
orientation and a high critical current density. In order to prepare a 
multifilamentary wire, a plurality of wires obtained after drawing are 
assembled with each other and subjected to plastic working and sintering. 
In the aforementioned process, it is possible to form a substantially 
single superconducting phase having high orientation through combination 
of the plastic working and the sintering. Filaments of the tape-shaped 
superconducting wire prepared through the aforementioned process have 
substantially homogeneous superconducting phases along the longitudinal 
direction of the tape wire, while c-axes of the superconducting phases are 
oriented substantially in parallel with the direction of thickness of the 
tape wire. The filaments are formed by crystal grains in the form of 
flakes extending along the longitudinal direction of the tape wire, which 
are strongly bonded with each other. The flaky crystal grains are stacked 
along the direction of thickness of the tape wire. The tape-shaped 
superconducting wire as employed is not particularly restricted in size, 
but the same has a width of 1.0 mm to 10 mm, preferably 2 mm to 6 mm, and 
a thickness of 0.05 mm to 1 mm, preferably 0.1 mm to 0.4 mm, for example. 
In such sizes, the tape wire having filaments of the aforementioned 
structure can maintain a critical current density of 4.times.10.sup.3 to 
3.0.times.10.sup.4 A/cm.sup.2, for example. The tape wire having the 
filaments of the aforementioned structure is relatively resistant against 
bending, and maintains a high critical current density also when the same 
is held at a bending strain of a specific range, as described later. The 
tape-shaped multifilamentary superconducting wire can be provided with 7 
to 10,000, preferably 37 to 1,000 filaments. 
In the inventive conductor, the core member, which is generally called a 
former, is adapted to hold the tape-shaped superconducting wires at a 
bending strain of the prescribed range. This former has a length which is 
required for the superconducting cable conductor, and is provided at the 
center of the superconducting cable conductor. The former is in a 
substantially cylindrical or spiral shape so that the tape wires are wound 
thereon, and generally has a substantially constant diameter along its 
overall length. The former can consist essentially of at least one 
material selected from the group consisting of stainless steel, copper, 
aluminum and FRP (fiber-reinforced plastic), for example. 
According to the present invention, the former is preferably in the form of 
a tubular member having flexibility. It is possible to preferably employ a 
pipe having a spiral groove (hereinafter referred to as a spiral tube) as 
a former having sufficient strength and flexibility, as shown in FIG. 1, 
for example. A bellows tube having a bellows is also preferably employed 
as a former as shown in FIG. 2. Referring to FIGS. 1 and 2, symbols 
R.sub.1 and R.sub.2 denote outer diameters, symbols T.sub.1 and T.sub.2 
denote thicknesses, symbols P.sub.1 and P.sub.2 denote pitches, and 
symbols D.sub.1 and D.sub.2 denote gaps respectively. Further, the former 
can also be prepared from a spirally wound material such as that called a 
spiral steel strip shown in FIG. 3, for example. Each of these shapes is 
adapted to provide the former with sufficient flexibility. The spiral tube 
or the bellows tube can also be prepared from stainless steel, copper, 
aluminum or FRP. The flexible former provides the inventive conductor with 
flexibility. The flexible conductor can be taken up on a drum. 
According to the present invention, it is possible to wind several 10 to 
1,000 tape-shaped multifilamentary superconducting wires on the former. 
The tape wires are wound in at least two or more layers while directing 
principle surfaces thereof to the former. Each layer may be formed by an 
arbitrary number of the tape wires. When several 10 tape wires are wound 
on the former in parallel with each other so that the surface of the 
former is filled up with the tape wires, additional several 10 tape wires 
are further wound thereon. When a sufficient number of tape wires are 
wound on the first layer of the tape wires as a second layer, a third 
layer of tape wires are then wound thereon. The insulating layer is 
provided between each adjacent pair of layers. 
According to the present invention, each tape-shaped multifilamentary oxide 
superconducting wire is wound on a former as shown in FIG. 4, for example. 
Referring to FIG. 4, a tape wire 11 is wound on a former 10 having a 
prescribed diameter at a bending strain or a curvature of a prescribed 
range and a pitch (P) of a prescribed range. At this time, a principal 
surface 11a of the tape wire 11 is directed to the former 10. Thus, 
relatively loose bending is applied to the tape wire 11 along its 
longitudinal direction. When the bending strain is defined as follows, the 
tape wire 11 which is wound on the former 10 is bent at a bending strain 
of not more than 0.5 %, preferably not more than 0.3%. Superconductivity 
of the tape wire 11 is hardly deteriorated upon bending at a bending 
strain of such a range, as compared with that in a linear state. 
bending strain (%)={thickness of tape-shaped superconducting wire/(bending 
diameter+thickness of the same)}.times.100. 
When a tape-shaped superconducting wire having a thickness t is spirally 
wound on a former having a diameter D at a pitch P, a bending strain 
.epsilon. is obtained in the following equation. According to the present 
invention, it is preferable to adjust the pitch P and the diameter D of 
the former so that the bending strain .epsilon. is not more than 0.3%. 
EQU .epsilon.=t.times.100/(D.sub.1 +t) 
EQU D.sub.1 =(P.sup.2 +(.pi.D).sup.2).sup.1/2 /.pi. 
According to the present invention, each tape-shaped multifilamentary oxide 
superconducting wire is preferably wound on the former with tension of not 
more than 2 kgf/mm.sup.2 in a range of 0.5 to 2 kgf/mm.sup.2, for example. 
The core member (former) can be formed by either an electric insulating 
material or an electric conductor. The electric insulating material is 
preferable in consideration of reduction in ac loss, while a metal which 
is a conductor is preferable in consideration of strength. A metal pipe 
having a spiral groove or a metal bellows tube is a particularly 
preferable core member for providing the conductor with flexibility while 
maintaining constant strength. A metal core member can also be employed 
for feeding an abnormal current upon an accident. In this case, it is 
possible to set optimum resistivity of the core member in consideration of 
ac loss of the conductor and a burden of the core member for the abnormal 
current. 
When the metal pipe having a spiral groove or the metal bellows tube is 
employed as the core member, the inventive conductor can further comprise 
a metal tape which is spirally wound on the core member, and an insulating 
tape which is spirally wound on a smooth surface formed by the metal tape. 
The metal tape can form a smooth surface for covering the grooves of the 
core member so that the superconducting tapes are not buckled by these 
grooves and for receiving the superconducting tapes. It is possible to 
cover the grooves while maintaining flexibility of the core member by 
winding the metal tape. The insulating tape which is wound on the metal 
tape cuts off electrical connection between the core member and the metal 
tape and the superconducting tapes. The insulating tape may be replaced by 
an electric insulating layer consisting essentially of another material. 
When the core member is prepared from a metal, this core member is 
preferably electrically insulated from the superconducting tapes. 
According to the present invention, the electric insulating layer is at 
least provided between the layers, each of which is formed by a plurality 
of superconducting tapes. Further, it is possible to further insert an 
electric insulating layer between a plurality of side-by-side tape-shaped 
superconducting wires at least in any of the plurality of layers. When the 
core member consists essentially of a metal as described above, an 
electric insulating layer is preferably further inserted between the core 
member and the tape-shaped superconducting wires. Alternatively, a 
plurality of tape-shaped multifilamentary oxide superconducting wires 
which are previously covered with insulating layers may be wound on the 
core member. According to the present invention, the insulating layer 
suppresses electric connection between the superconducting tapes, thereby 
reducing ac loss of the conductor. 
FIGS. 5 and 6 typically illustrate insulation between layers and that 
between superconducting tapes in the layers. Insulating materials 50 are 
provided between superconducting strands 51 respectively. An interlayer 
insulating layer 60 is provided between first and second layers 61 and 62 
which are formed by prescribed numbers of superconducting tapes, while the 
second layer 61 is further covered with an insulating layer 65. The 
insulating materials 50 are formed by cord-shaped or tape-shaped bodies 
which are spirally wound on a core member 55. The interlayer insulating 
layer 60 and the insulating layer 65 can be formed by wide tape-shaped or 
strip-shaped bodies. These materials are spirally wound on the strands 51. 
According to the present invention, the materials excluding the core member 
are preferably formed by tape-shaped or cord-shaped bodies, in order to 
maintain flexibility of the core member and to provide the superconducting 
cable conductor with desired flexibility. Therefore, the electrical 
insulating layer is also preferably formed by a tape-shaped or cord-shaped 
body. In this case, it is possible to form the insulating layer by 
spirally winding an insulating tape or an insulating cord along the 
longitudinal direction of the core member. The insulating tape or cord can 
be wound with tension of 0.5 to 2 kgf/mm.sup.2, for example. 
In order to provide an electric insulating layer between a plurality of 
side-by-side superconducting tapes in each layer of the superconducting 
tapes, it is possible to employ superconducting tapes which are previously 
insulation-coated as a whole, for example. However, it is relatively 
difficult to sufficiently insulation-coat surfaces of flat tapes, 
particularly edge portions, and a high cost is required in this case. When 
superconducting tapes which are not coated in edge portions are lined up, 
electrical connection takes place therebetween. When an electric 
insulating layer is provided between side-by-side superconducting tapes in 
a layer, therefore, it is preferable to arrange an insulating tape as 
shown in FIG. 7. Referring to FIG. 7, each insulating tape 70 is held 
between adjacent superconducting tapes 71 and 71'. This insulating tape 70 
is spirally wound along the superconducting tapes 71 and 71' to cover one 
major surface 71a of the superconducting tape 71 as well as one major 
surface 71'b of the superconducting tape 71'. In other words, the 
insulating tape 70 covers the major surface 71a of the superconducting 
tape 71 which is closer to the core member and the major surface 71'b of 
the other superconducting tape 71' which is opposite to the core member in 
pairs of opposite major surfaces (71a and 71b of the tape 71 and 71'a and 
71'b of the tape 71') of the superconducting tapes 71 and 71' 
respectively. Due to this arrangement, one of the superconducting tapes is 
completely insulated from the other one, thereby solving the 
aforementioned problem on the edge portions. Further, it is also possible 
to insulate layers from each other by winding the insulating tape in the 
aforementioned manner. 
The insulating layer can be prepared from an insulating material such as 
Kapton (polyimide based material), a polypropylene laminate paper (PPLP), 
polyethylene (PE) or a kraft paper, for example, while the insulating 
material preferably causes no deterioration such as cracking in liquid 
nitrogen. The insulating material is employed for forming an insulating 
layer in the form of a paper, a sheet, a film, fabric or a tape. The 
insulating layer is preferably not more than 0.1 mm in thickness, to allow 
compaction of the conductor. On the other hand, the insulating layer which 
is previously formed on each superconducting tape preferably consists 
essentially of enamel, for example. 
According to the present invention, it is possible to employ tape-shaped 
multifilamentary wires each having twisted filaments. FIG. 8 typically 
shows such a superconducting wire. Referring to FIG. 8, filaments 2 
forming a superconducting multifilamentary tape 1 are twisted at a 
prescribed pitch L, for example. Due to such twisting of the filaments 2, 
an induction current flowing between a stabilizing metal 3 and the 
filaments 2 is parted every twisting pitch L into small loops, and hence 
the value of the current is limited. Thus, generation of Joule heat is 
suppressed in the stabilizing metal 3 and ac loss is reduced as compared 
with a superconducting wire having untwisted filaments. It is possible to 
prepare a multifilamentary wire having such twisted filaments in the 
following manner, for example: First, a plurality of single-filamentary 
wires each having a filament of an oxide superconductor are engaged in a 
metal pipe, and this metal pipe is drawn into a wire. Then, the wire is 
twisted in a state of a round wire, for forming twisted filaments. Then, 
the wire is again drawn and thereafter rolled and heat treated. Through 
such steps, the filaments are changed in diameter and thickness by 
drawing, rolling etc. while maintaining twisted shapes. In the twisting, 
drawing and rolling steps, the twisting pitch is preferably set at a level 
of more than five times or preferably ten times as large as the diameter 
of the wire to be twisted, so that the wire is not parted. 
The superconducting cable conductor according to the present invention has 
such flexibility that its superconductivity is substantially not 
deteriorated also when the same is bent up to 1.5 m, preferably up to 2.6 
m, in bending diameter. This conductor can be wound on a drum, to be 
stored and carried. 
According to the present invention, it is possible to provide a long oxide 
superconducting cable conductor having flexibility as well as excellent 
superconductivity. In the present invention, an eddy current or a coupling 
current transferred between and flowing across the superconducting tapes 
is suppressed by the insulating layer which is effectively provided 
according to the present invention. It is possible to reduce ac loss of 
the conductor by at least one digit, due to the insulating layer. The 
present invention provides a further practical ac superconducting cable 
conductor. 
The present invention is now further concretely described. 
Study on Bending Strain of Superconducting Tape wound on Former 
Oxides or carbonates were mixed with each other to contain Bi, Pb, Sr, Ca 
and Cu in composition ratios of 1.84:0.36:1.99:2.18:3.00. This mixed 
powder was heat treated to obtain powder containing 85% of a 2212 phase 
and 15% of a 2223 phase as superconducting phases while mainly containing 
(Ca,Sr).sub.2 PbO.sub.4 and Ca.sub.2 CuO.sub.3 as non-superconducting 
phases. The powder as treated was charged in a silver pipe of 12 mm in 
outer diameter and 9 mm in inner diameter, and the silver pipe was drawn 
into 1.3 mm in diameter. A prescribed number of strands obtained in the 
aforementioned manner were charged in a silver pipe of prescribed sizes, 
and the silver pipe was drawn into 1.0 mm in diameter. The wire as 
obtained was rolled into 0.3 mm in thickness. This wire was heat treated 
at 845.degree. C. for 55 hours, and thereafter rolled at a draft of 15%. 
The tape wire as obtained was heat treated at 838.degree. C. for 48 hours. 
Through the aforementioned process, six types of tape-shaped 
superconducting wires were obtained as shown in Table 1. Critical current 
densities (Jc) of these tape wires were measured under liquid nitrogen 
along proper lengths. The critical current densities Jc were measured when 
the tape wires were in linear states and bent in prescribed diameters 
respectively. Table 2 shows results of the critical current densities Jc 
which were measured with respect to five bending strains. As understood 
from Tables 1 and 2, reduction in critical current density Jc of the wire 
to which bending strain is applied is reduced as the thickness percentage 
of the superconductor with respect to the thickness of the wire is 
reduced. The percentage is preferably not more than 10%. On the other 
hand, each wire preferably has at least 37 filaments. In a superconducting 
tape having at least 61 filaments, the critical current density Jc is not 
much reduced by bending 0.5%. It is understood possible to hold 
multifilamentary superconducting wires prepared along the aforementioned 
process at a bending strain of not more than 0.5%, preferably not more 
than 0.3% in practice. 
TABLE 1 
__________________________________________________________________________ 
##STR1## 
1 #STR2## 
2 #STR3## 
__________________________________________________________________________ 
1 169 26 19.5 4.0 
2 91 19 14.3 5.4 
3 61 15.6 11.7 6.6 
4 37 12 9.0 8.6 
5 19 8.7 6.5 12 
6 7 5.2 3.9 20 
__________________________________________________________________________ 
TABLE 2 
______________________________________ 
255 mm .phi. 
128 mm .phi. 
85 mm .phi. 
64 mm .phi. 
25 mm .phi. 
A B A B A B A B A B 
______________________________________ 
1 0.1 1.50 0.2 1.50 0.3 1.50 0.4 1.50 0.5 1.50 
(100) (100) (100) (100) (100) 
2 0.1 1.70 0.2 1.70 0.3 1.70 0.4 1.70 0.5 1.70 
(100) (100) (100) (100) (100) 
3 0.1 1.90 0.2 1.90 0.3 1.84 0.4 1.82 0.5 1.79 
(100) (100) (97) (96) (94) 
4 0.1 2.0 0.2 1.92 0.3 1.84 0.4 1.80 0.5 1.76 
(100) (96) (92) (90) (88) 
5 0.1 2.1 0.2 1.95 0.3 1.83 0.4 1.74 0.5 1.63 
(100) (93) (87) (83) (78) 
6 0.1 2.2 0.2 2.00 0.3 1.83 0.4 1.72 0.5 1.63 
(100) (93) (83) (78) (74) 
______________________________________ 
A: bending strain (%) 
B: Jc (.times. 10.sup.4 A/m.sup.2) 
*Parenthesized numerals indicate percentages of critical current densitie 
Jc after bending with respect to those before bending. 
Measurement of ac Loss 
Bi.sub.2 O.sub.3, PbO, SrCO.sub.3, CaCO.sub.3 and CuO were blended with 
each other to contain Bi, Pb, Sr, Ca and Cu in composition ratios of 
1.81:0.40:1.98:2.21:3.03. The blended powder was heat treated a plurality 
of times, with crushing after every heat treatment Powder obtained through 
such heat treatment and crushing was further crushed by a ball mill, to 
obtain submicron powder. This powder was heat treated at 800.degree. C. 
for 2 hours, and charged in a silver pipe of 12 mm in outer diameter and 9 
mm in inner diameter. 
The silver pipe charged with the powder was drawn and cut into a plurality 
of wires, which were thereafter engaged in another silver pipe of 12 mm in 
outer diameter and 9 mm in inner diameter, to prepare a multifilamentary 
wire having 61 filaments. This multifilamentary wire was further drawn, 
rolled into 3.0 mm in width and 0.22 mm in thickness, and then heat 
treated. Thereafter the wire was further rolled into 0.20 mm in thickness 
and heat treated, thereby obtaining a silver-coated bismuth oxide 
superconducting wire having 61 filaments. 
Then, a steel tape of 0.33 mm in thickness and 10 mm in width was spirally 
wound into a former having an outer diameter R of 19 mm.phi., a winding 
pitch L of 4 mm and a gap D of 2 mm, as shown in FIG. 9. 
20 tape-shaped multifilamentary superconducting wires obtained in the 
aforementioned manner were spirally wound on the former in a side-by-side 
manner at a winding pitch of 250 mm. FIG. 10(a) is a sectional view 
showing a single-layer conductor as obtained. Referring to FIG. 10(a), 
superconducting multifilamentary wires 11 are wound on a former 10 in a 
side-by-side manner. The single-layer conductor as obtained exhibited a 
critical current value (Ic) of 550 A. 
Then, 22 superconducting multifilamentary wires were spirally wound on the 
single-layer conductor in a side-by-side manner at a winding pitch of 250 
mm, in a direction opposite to the winding direction for the first layer. 
FIG. 10(b) is a sectional view showing a two-layer conductor thus 
obtained. Superconducting multifilamentary wires 11' are further wound on 
the superconducting wires 11 wound on the former 10. The two-layer 
conductor as obtained exhibited a critical current Ic of 850 A. 
In this conductor, ac loss values were measured in states of the 
single-layer conductor having 20 strands and the two-layer conductor 
having 42 strands. FIG. 11 illustrates relations between ac loss values 
per strand and energization currents in the respective cases. FIG. 11 also 
illustrates a relation between ac loss and an energization current in each 
strand having a critical current Ic of 20 A, which was measured before 
preparation of the conductor. Referring to FIG. 11, black circles, white 
circles and black triangles show values related to each strand, the 
single-layer conductor and the two-layer conductor respectively. As shown 
in FIG. 11, the single-layer conductor exhibited ac characteristics which 
were substantially identical to those of the unassembled strand. In the 
two-layer conductor, on the other hand, ac loss per strand was increased 
as compared with that of the independent strand. Through this experiment, 
it has been proved that a single-layer conductor has lower ac loss than a 
multilayer conductor. It has been forecasted that this results from 
generation of an eddy current or a coupling current transferred and 
flowing across the layers, which is not present in the single-layer 
conductor. In order to verify this hypothesis, superconductors provided 
with insulating materials between layers for cutting off electrical 
conductivity were prepared thereby reducing ac loss in multilayer 
conductors. 
EXAMPLE 1 
20 multifilamentary wires obtained in the aforementioned manner were wound 
on a spiral tube former of 19 mm.phi. in outer diameter and 0.3 mm in 
thickness with gaps of 2 mm at a pitch of 4 mm shown in FIG. 1, in a 
side-by-side manner. The wires were wound at a pitch of 250 mm. Then, an 
insulating material prepared from a PPLP paper of 140 .mu.m in thickness 
and 30 mm in width was spirally wound on the multifilamentary wires by one 
layer at a pitch of 40 mm with gaps of 0.5 mm. Then, 22 strands identical 
to the above were spirally wound thereon at a pitch of 250 mm, in a 
direction opposite to that for the first wires. 
FIGS. 12 and 13 show the conductor as obtained. In the two-layer conductor 
as obtained, superconducting multifilamentary wires 11 are wound on a 
former 10 in a side-by-side manner to form a first layer, as shown in 
FIGS. 12 and 13. An insulating layer 20 of a PPLP paper is provided on the 
superconducting multifilamentary wires 11, while superconducting 
multifilamentary wires 11' are wound thereon in a side-by-side manner to 
form a second layer. The conductor as obtained exhibited a critical 
current Ic of 850 A. In this conductor, ac loss was reduced by about one 
digit as compared with that of a two-layer conductor which was 
manufactured with no insulating layer. In terms of ac loss per strand, the 
ac loss of the conductor approached that of an independent unassembled 
strand. Through the aforementioned experiment, it has been proved 
effective to provide an insulating layer between layers of a multilayer 
conductor, for reducing its ac loss. 
While the above Example has been described with reference to a two-layer 
conductor, it is possible to attain the effect of the insulating layer 
similarly to the above also as to superconducting multifilamentary wires 
which are superposed with each other in three or more layers. As shown in 
FIG. 14, for example, it is possible to provide a compact conductor which 
can reduce ac loss and supply a heavy current by a structure obtained by 
successively stacking a first layer of superconducting multifilamentary 
wires 31, an insulating layer 32, a second layer of superconducting 
multifilamentary wires 33, an insulating layer 34, a third layer of 
superconducting multifilamentary wires 35, an insulating layer 36, and a 
fourth layer of superconducting multifilamentary wires 37. 
It is also possible to cover surfaces of superconducting multifilamentary 
wires with insulating layers, for winding the wires on a former. For 
example, a superconducting multifilamentary wire 41 is covered with an 
insulating layer 40, as shown in FIG. 15(a). It is possible to wind a 
plurality of such superconducting multifilamentary wires 41 on a former 
56, as shown in FIG. 15(b). While this structure conceivably requires a 
long time for insulation coating and a higher cost as compared with the 
case of inserting the insulating material between the layers, insulation 
is more reliably performed in this case. 
EXAMPLE 2 
Bi.sub.2 O.sub.3, PbO, SrCO.sub.3, CaCO.sub.3 and CuO were blended with 
each other to contain Bi, Pb, Sr, Ca and Cu in composition ratios of 
1.81:0.30:1.92:2.01:3.03. The blended powder was heat treated a plurality 
of times, with crushing after every heat treatment. Powder obtained 
through such heat treatment and crushing was further crushed by a ball 
mill, to obtain submicron powder. This powder was heat treated at 
800.degree. C. for 2 hours, and thereafter charged in a silver pipe of 12 
mm in outer diameter and 9 mm in inner diameter. The silver pipe charged 
with the powder was drawn and cut into a plurality of wires, which were 
thereafter engaged in another silver pipe of 12 mm in outer diameter and 9 
mm in inner diameter, to prepare a multifilamentary wire having 61 
filaments. This wire was further drawn, then rolled into 3.0 mm in width 
and 0.22 mm in thickness, and heat treated. Thereafter the wire was 
further rolled into 0.20 mm in thickness and heat treated, thereby 
obtaining a silver-coated bismuth oxide superconducting wire having 61 
filaments. The wire was sintered and cut into samples of 1 m in length, 
which were subjected to measurement of critical direct currents. It was 
confirmed that the 100 samples exhibited stable critical currents of 
23.+-.1 A. 
The following conductors were manufactured through the wires of 1 m in 
length, and subjected to investigation of ac characteristics. 
Superconducting wires each having 61 filaments were spirally wound on FRP 
formers of 1 m in length and 19 mm.phi. in outer diameter in a 
side-by-side manner, to prepare two types of single-layer conductors A and 
B respectively. The superconducting wires were wound at pitches of 250 mm. 
In the conductor A, 20 wires were densely assembled with each other with 
no insulating material arranged therebetween. In the conductor B, on the 
other hand, 17 superconducting wires were assembled with each other with 
interposition of a cord-shaped insulator of 0.5 mm.phi., which was 
prepared by stranding kraft papers for serving as a spacer, and spirally 
wound. 
As to the conductors, ac loss values were measured in liquid nitrogen of 
about 77 K in temperature by an energization method, and each ac loss 
value was defined by the product of the energization current and a voltage 
component which was in phase with the current. Each of the energization 
current and the loss value was divided by the number of the strands as 
employed, to calculate ac loss per strand. It has been confirmed that ac 
loss of the conductor A was about twice that of the conductor B in such a 
region that a current flowing every strand was not more than 23 Ap. 
Through this experiment, it has been confirmed effective to electrically 
insulate multifilamentary superconducting wires forming the same layer 
from each other, for reducing ac loss. 
EXAMPLE 3 
Wires of 1 m in length, which were identical to those in Example 2, were 
employed to prepare a conductor, for investigating its ac characteristics. 
Multifilamentary superconducting wires were spirally wound on a copper 
former of 1 m in length and 19 mm.phi. in outer diameter in a side-by-side 
manner, to prepare a single-layer conductor C. The wires were wound at a 
pitch of 250 mm. 20 wires were densely assembled with each other, with no 
arrangement of an insulating material therebetween. As to this conductor 
C, ac loss was measured in liquid nitrogen. Ac loss per strand was 
obtained similarly to Example 2, to confirm that the ac loss of the 
conductor C was twice to five times that of the conductor B in such a 
region that an energization current to the strands was not more than 23 
Ap. Through this experiment, it has been confirmed that ac loss of a 
conductor is increased when its core member is prepared from a metal and 
superconducting wires are in contact with the core member. 
EXAMPLE 4 
Wires of 1 m in length, which were identical to those in Example 2, were 
employed to prepare conductors, for investigating ac characteristics. 
Multifilamentary superconducting wires were wound on aluminum spiral pipes, 
having shapes similar to that shown in FIG. 1, of 1 m in length and 28 
mm.phi. in outer diameter, in a side-by-side manner, to prepare two types 
of single-layer conductors D and E respectively. The wires were wound at 
pitches of 250 mm. 20 wires were assembled on each spiral pipe, with 
arrangement of a kraft paper of 1 mm in width and 0.1 mm in thickness 
between the strands. In the conductor D, a copper tape was spirally wound 
on the aluminum pipe so that the multifilamentary superconducting wires 
were spirally wound thereon in one layer. In the other conductor E, a 
copper tape was spirally wound on the aluminum pipe, and a Lumirror tape 
(polyester based tape) of 0.1 mm in thickness was further spirally wound 
thereon for electrical insulation. The multifilamentary superconducting 
wires were spirally wound on this insulating tape in one layer. 
As to each of the conductors D and E, ac loss per strand in the conductor 
was measured in liquid nitrogen, to confirm that the ac loss of the 
conductor D was five to ten times that of the conductor E in a region of 
not more than 23 Ap. Through this experiment, it has been confirmed 
possible to suppress increase of ac loss by arranging an insulating 
material on a surface of a core member for assembling superconducting 
wires thereon when the core member is prepared from a metal. 
EXAMPLE 5 
Bi.sub.2 O.sub.3, PbO, SrCO.sub.3, CaCO.sub.3 and CuO were blended with 
each other to contain Bi, Pb, Sr, Ca and Cu in composition ratios of 
1.81:0.30:1.92:2.01:3.03. The blended powder was heat treated a plurality 
of times, with crushing after every heat treatment. Powder obtained 
through such heat treatment and crushing was further crushed by a ball 
mill, to obtain submicron powder. This powder was heat treated at 
800.degree. C. for 2 hours, and charged in a silver pipe of 12 mm in outer 
diameter and 9 mm in inner diameter. After the silver pipe charged with 
the powder was drawn, a plurality of wires were engaged in another silver 
pipe of 12 mm in outer diameter and 9 mm in inner diameter, to prepare a 
multifilamentary wire having 61 filaments. This wire was further rolled 
into 3.0 mm in width and 0.22 mm in thickness, and heat treated. 
Thereafter the wire was further rolled into 0.20 mm in thickness and heat 
treated, thereby obtaining a silver-coated bismuth oxide superconducting 
wire having 61 filaments. The wire was sintered and cut into 200 samples 
of 1 m in length, which were subjected to measurement of critical direct 
currents. It was confirmed that the 200 samples exhibited stable critical 
currents of 23.+-.2 A. 
The wires of 1 m in length as obtained were employed to prepare conductors, 
for investigation of ac characteristics. The multifilamentary 
superconducting wires were spirally wound on FRP formers of 1 m in length 
and 19 mm.phi. in outer diameter in a side-by-side manner in double 
layers, to prepare five types of two-layer conductors F, G, H, I and J. 
The wires were wound at pitches of 500 mm. 
In the first conductor F, no insulating material was arranged between 
adjacent multifilamentary superconducting which were assembled on the 
former, in the first or second layer. Further, no insulating material was 
arranged between the first and second layers either. 40 wires were densely 
wound on the former in two layers. 
In the second conductor G, no insulating material was arranged between the 
wires forming the first or second layer. On the other hand, a PPLP paper 
of 30 mm in width and 0.17 mm in thickness was spirally wound between the 
first and second layers, to insulate these layers from each other. 40 
multifilamentary superconducting wires were wound on the former, to form 
two layers. 
In the third conductor H, Kapton tapes (polyimide based tape) of 0.5 mm in 
width and 0.2 mm in thickness were inserted between the multifilamentary 
superconducting layers as spacers, for forming first and second layers. 
Further, a spirally wound Kapton tape of 30 mm in width and 0.2 mm in 
thickness was arranged between the first and second layers. 
In the fourth conductor I, a Lumirror tape (polyester based tape) of 5 mm 
in width and 0.02 mm in thickness was held between adjacent 
multifilamentary wires in the first or second layer. This Lumirror tape 
was held between the adjacent superconducting wires as shown in FIG. 7. In 
other words, the Lumirror tape was spirally wound along superconducting 
tapes to cover an upper portion of one of the adjacent superconducting 
tapes while covering a lower portion of the other superconducting tape. 
Thus, it was possible to insulate the first and second layers from each 
other with no interposition of an insulating tape therebetween. 
In the fifth conductor J, multifilamentary superconducting wires which were 
previously covered with enamel were assembled with each other, while 
holding Kapton tapes of 3 mm in width and 0.02 mm in thickness between 
adjacent superconducting wires in the first and second layers 
respectively. The insulating tapes were arranged between the 
superconducting wires similarly to those in the conductor I. 
Ac loss was measured as to each of the conductors F to J in liquid nitrogen 
of about 77 K in temperature by an energization method. The ac loss was 
defined by the product of the energization current and a voltage which was 
in phase with the current. Each of the energization current and the loss 
value was divided by the number of the strands as employed, to calculate 
ac loss in terms of that per strand. When currents of 20 Ap per strand 
were fed, the conductors F, G, H, I and J exhibited ac loss values of 7 
mW/m, 1 mW/m, 0.7 mW/m, 0.7 mW/m and 0.7 mW/m respectively. As to regions 
of not more than 25 Ap, the conductors F and G exhibited the maximum and 
next loss values respectively. In the conductors H, I and J, on the other 
hand, the ac loss values remained substantially at the same levels in 
overall regions, thereby implementing reduction of ac loss. Thus, it has 
been confirmed effective to electrically insulate strands from each other 
between layers and in the same layer. In more concrete terms, the 
structures of the conductors H, I and J are further effective. 
Although the present invention has been described and illustrated in 
detail, it is clearly understood that the same is by way of illustration 
and example only and is not to be taken by way of limitation, the spirit 
and scope of the present invention being limited only by the terms of the 
appended claims.