U.S. patent number 3,708,606 [Application Number 05/036,741] was granted by the patent office on 1973-01-02 for cryogenic system including variations of hollow superconducting wire.
This patent grant is currently assigned to Air Reduction Company, Incorporated. Invention is credited to Bradley S. Kirk, William G. Marancik, Walter J. Shattes.
United States Patent |
3,708,606 |
Shattes , et al. |
January 2, 1973 |
CRYOGENIC SYSTEM INCLUDING VARIATIONS OF HOLLOW SUPERCONDUCTING
WIRE
Abstract
A core element comprising superconducting strands in a normally
conducting matrix is initially extruded in a desired
cross-sectional shape or configuration; and is then interposed in a
prefabricated or postfabricated tube, and coreduced to a final
product of desired dimensions, thus providing a matrix having a
plurality of longitudinal internal channels. The core elements may
assume a variety of cross-sectional shapes, such as ribbon, square,
cross, triangle, star, annulus, etc., or a composite of these. In
an alternative process, a cable is formed by twisting or braiding
together a plurality of superconducting matrix wires, prior to
coreduction in a prefabricated or postfabricated tube. In another
form, a superconducting matrix strip is welded to form a tube. In a
variation of this, superconductor wires are interposed in
longitudinal slots in a billet of normally conducting material
which is rolled into a ribbon, alternatively formed into a tube.
The product of any of these techniques is finally coreduced to a
wire which is formed into a coil which may be cooled internally by
a forced cooling system including fluid or superfluid helium.
Inventors: |
Shattes; Walter J. (Bloomfield,
NJ), Marancik; William G. (Basking Ridge, NJ), Kirk;
Bradley S. (Warren, NJ) |
Assignee: |
Air Reduction Company,
Incorporated (New York, NY)
|
Family
ID: |
21890367 |
Appl.
No.: |
05/036,741 |
Filed: |
May 13, 1970 |
Current U.S.
Class: |
174/15.5; 29/599;
174/125.1; 335/216; 174/27; 174/128.1 |
Current CPC
Class: |
H01B
12/10 (20130101); H01L 39/2403 (20130101); Y02E
40/644 (20130101); Y10T 29/49014 (20150115); Y02E
40/60 (20130101) |
Current International
Class: |
H01B
12/10 (20060101); H01L 39/24 (20060101); H01b
007/34 (); H01v 011/00 () |
Field of
Search: |
;174/15C,DIG.6,126CP,128,27,15R,16R,117F,28 ;335/216 ;29/599
;333/995 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
441,457 |
|
Jan 1968 |
|
CH |
|
6,701,316 |
|
Jul 1967 |
|
NL |
|
Primary Examiner: Gilheany; Bernard A.
Assistant Examiner: Grimley; A. T.
Claims
We claim:
1. As an article of manufacture, superconducting wire comprising in
combination:
a preformed, prereduced matrix of normally conducting material
containing a large number of strands of superconducting material
each initially not exceeding about 10 mils in cross-section
extended along the length of said wire in metallurgically bonded
relation to said matrix, a composite comprising said matrix having
a shape including one or more channels extending internally along
the length of said wire,
said wire including said channelled composite having been reduced
in cross-section through at least one post-reduction step in
addition to said prereduction step.
2. An article of manufacture in accordance with claim 1 wherein
said preformed, prereduced matrix has been twisted prior to
installation as an element of said wire.
3. The combination in accordance with claim 2 wherein said
superconducting material consists essentially of an alloy of
niobium and titanium.
4. The combination in accordance with claim 3 wherein said normally
conducting material is copper.
5. The combination in accordance with claim 2 wherein said
composite comprises an outer tube of conducting material, and
wherein said preformed, prereduced matrix comprises a core element
axially disposed in said outer tube, said outer tube having a
substantially larger initial cross-sectional dimension in at least
one direction than the cross-sectional dimension of said core
element,
said outer tube having been at least reduced to the external
diameter of said core element to form a composite with said core
element.
6. The combination in accordance with claim 5 wherein the preformed
shape of said core element is alternatively a ribbon, cross,
polygon, or annulus.
7. The combination in accordance with claim 5 wherein said core
element interposed in said outer conducting tube is twisted.
8. The combination in accordance with claim 5 wherein said core
element comprises a braid of twisted strands including said
preformed matrix of normal and superconducting material.
9. The combination in accordance with claim 8 wherein said core
element comprises a central tube about which is twisted a plurality
of strands comprising said preformed matrix of normal and
superconducting material.
10. The combination in accordance with claim 5 wherein said outer
tube consists essentially of copper.
11. A system comprising in combination a coil of superconducting
wire in accordance with claim 1, connected in circuit relation with
a source of electrical power, a source of fluid helium connected to
said coil, and means for inducing the flow of fluid helium in said
coil.
12. As an article of manufacture, superconducting wire comprising
in combination:
an outer tube of conducting material,
a composite core axially disposed in said outer tube,
said composite core consisting essentially of a ribbon of a
preformed, prereduced matrix of normally conducting material
containing a plurality of superconducting strands extended along
the length of said wire, said ribbon clamped between two
semiannular intermediate tube halves comprising normally conducting
material to form said composite core having a pair of longitudinal
channels,
said outer tube having a substantially larger initial
cross-sectional dimension in at least one direction than the
cross-sectional dimension of said composite core, and having been
at least reduced to the external diameter of said composite core to
form a further composite with said core.
13. The combination in accordance with claim 12 wherein said
intermediate tube halves comprise a matrix of normal and
superconducting material.
14. As an article of manufacture, superconducting wire comprising
in combination a composite including one or more channels extending
internally along the length of said wire, said composite
comprising:
an outer tube which is a laminate comprising a peripheral layer of
metal of relatively higher tensile strength and an inner layer
comprising copper,
a core element axially disposed in said outer tube, said core
element comprising a preformed, prereduced matrix of normally
conducting material containing a plurality of superconducting
strands extended along the length of said wire and twisted about
its long axis prior to installation in said tube,
said laminated outer tube having a substantially larger initial
cross-sectional dimension in at least one direction than the
cross-sectional dimension of said core element, and said laminated
outer tube having been at least reduced to the external diameter of
said core element to form a composite with said core element.
15. The combination in accordance with claim 14 wherein said outer
tube is a laminate comprising a peripheral layer of metal of
relatively higher tensile strength and an inner layer of said
prereduced matrix of normally conducting material containing a
plurality of strands of superconducting material extended along the
length of said wire.
16. The combination in accordance with claim 15 wherein said
peripheral layer is stainless steel and said inner layer of
prereduced matrix material has been twisted prior to rolling for
formation into said tube.
17. A system comprising in combination a coil of superconducting
wire in accordance with claim 14, connected in circuit relation
with a source of electrical power, a source of fluid helium
connected to said coil, and means for inducing the flow of fluid
helium in said coil.
18. The combination in accordance with claim 14 wherein said metal
of relatively high tensile strength consists essentially of
stainless steel.
19. A superconducting wire comprising a laminated tube having an
inner tubular layer comprising a prereduced matrix of normally
conducting material containing a plurality of strands of
superconducting material, said inner tubular layer extended along
the length of said wire, and an outer metal layer of different
composition for strengthening said tubular wire.
20. As an article of manufacture, superconducting wire comprising
in combination a composite including one or more channels extending
internally along the length of said wire, said composite
comprising:
an outer tube,
a core element in said outer tube, said core element comprising a
preformed, prereduced matrix of normally conducting material
containing a plurality of superconducting strands extended along
the length of said wire, said core element twisted about its long
axis prior to installation in said tube,
said outer tube formed from an elongated slotted slab of normally
conducting material wherein wire elements of said preformed,
prereduced matrix material have been interposed into the slots, and
said slab having been rolled to reduce its cross-section and formed
into a tube by welding the longitudinal edges of said rolled slab,
wherein said tube including said core element has been further
reduced in cross-section to form a channelled superconducting
wire,
said outer tube having a substantially larger initial
cross-sectional dimension in at least one dimension than the
cross-sectional dimension of said core element, and said outer tube
having been at least reduced to the external diameter of said core
element to form a composite with said core element.
21. The article of manufacture in accordance with claim 20 wherein
said wire interposed in said slotted slab is twisted.
22. The article in accordance with claim 20 wherein said wire
interposed in said slotted slab has been coated with an inner
coating of copper, an intermediate coating of cupro-nickel, and an
outer coating of copper.
23. A system comprising in combination a coil of superconducting
wire in accordance with claim 20, connected in circuit relation
with a source of electrical power, a source of fluid helium
connected to said coil, and means for inducing the flow of fluid
helium in said coil.
24. As an article of manufacture, superconducting wire comprising
in combination
a preformed, prereduced matrix consisting essentially of an alloy
of cupro-nickel in which are embedded a plurality of copper coated
strands of niobium titanium extended along the length of said
wire,
a composite comprising said matrix having a shape including one or
more channels extending internally along the length of said
wire,
said wire including said channelled composite having been reduced
in cross-section through at least one post-reduction step in
addition to said prereduction step.
Description
BACKGROUND OF THE INVENTION
In order to maintain superconducting materials in the temperature
environment necessary for operation, they are usually submersed in
a bath of boiling liquid helium. Liquid helium at normal pressure
has been found to be a poor medium for cryostatic systems requiring
forced circulation of the coolant. However, pressurized normal (or
supercritical) liquid, or superfluid, helium have been found to be
an excellent coolant in forced circulation, or forced convection
systems.
A most advantageous cooling system is one in which the
superconductor wires are constructed to include lengthwise pores or
channels through which fluid or superfluid helium can be pumped in
a forced circulation system to bring it into direct contact with
the superconductor.
Although prior systems have been developed utilizing supercritical
helium and various conductor configurations, construction of
suitable wires of this form, having optimum superconductive
characteristics, requires expensive and cumbersome processes.
Moreover, it has been found that certain of the superconductive
configurations employed in the prior art have less than optimum
current carrying characteristics, and have poor directional
characteristics, exhibiting a substantial degree of anisotropy.
Accordingly, it is a primary object of the present invention to
provide for the construction with greater facility of higher
quality superconductor wires, particularly of a type including
channels for low temperature fluid or superfluid helium. Another
object of the invention is to provide channeled superconductive
wires of a configuration of improved electrical characteristics in
which anisotropy is reduced. Another object is to provide
superconductor configurations having better heat transfer
characteristics. A still further object of the invention is to
provide for the manufacture of longer lengths of superconductive
wire with greater facility, and of a type which tolerates twisting
without fracture.
BRIEF DESCRIPTION OF THE INVENTION
These and other objects are attained in accordance with the present
invention by a unique type of wire construction for use in the
superconducting coil of a forced convection helium cooling system.
This wire may comprise a core element including a matrix of
normally conducting material containing fine superconducting
strands, the core being preformed to a pre-selected cross-sectional
shape, such as, for example, a ribbon, square, triangle, star,
annulus, etc. or combination of the same; and which is ultimately
interposed in a prefabricated or postfabricated tube, and coreduced
or sink drawn to a final reduced cross-sectional wire product which
contains lengthwise channels or pores.
In one alternative form, the core comprises a cable formed of a
plurality of superconductive matrix wires braided or twisted
together, with each other or with normally conducting strands,
before being placed in a tube for further processing. In another
alternative form, a superconductive matrix strip is welded at the
long edges to form a tube, or placed inside an external tube
comprising normally conducting material. A tube may be formed from
ribbon made from a billet of normally conducting material which has
been slotted to accommodate a wire of superconductive matrix
material.
The product wire, after final cold or hot working and heat
treatment, is used to form a coil, through the channeled interior
of which helium is pumped in a forced convection cooling
operation.
Particular advantages of the processes and products of the present
invention are that the wire can be conveniently fabricated in
longer lengths than in prior art processes. The various
configurations provide for better heat transfer, and in most cases,
for reducing anisotropy. Moreover, the composite strands of
superconductor and matrix material can be readily twisted, prior to
installation as core elements or prior to fabrication as enclosing
tube elements, providing improved electrical characteristics.
These and other objects, features, and advantages will be apparent
to those skilled in the art from a study of the detailed
specification hereinafter with reference to the attached drawings,
in which:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a matrix of a normally conducting material, including
strands of superconducting material, which have been coreduced in
an initial process;
FIG. 2 shows, in cross-section, a combination, before reduction, of
a superconducting matrix element of the form indicated in FIG. 1,
interposed in a tube of normally conducting material;
FIGS. 3A and 3B shows variations of the combination of FIG. 2,
before reduction, in which the superconducting core element
interposed in the tube respectively takes the form of a cross or a
polygon;
FIG. 4 is a further variation of the elements of FIGS. 2 and 3, in
which the core element interposed in the tube takes the form of two
annular halves of normally conducting material, which close
together to hold in place a diametrically-disposed ribbon of
superconducting material;
FIGS. 5A and 5B show the general cross-sectional appearance of the
final form to which the configurations of FIGS. 2 and 3
respectively may be reduced;
FIGS. 6A and 6B show a single strand twist, and multiple strand
twist which can be assumed by the core elements of FIGS. 2, 3 and
4;
FIG. 7A is a further alternative form of the invention in which the
central superconductive matrix comprises a plurality of wires of
the type shown in FIGS; 6A or 6B, braided together.
FIG. 7B is a modification of the above in which the central element
is tubular;
FIG. 8A shows a round wire element of superconductive matrix
material enclosed in a thin copper coating;
FIG. 8B shows a wire element of superconductive matrix material
having a plurality of coatings, including a coating of cupro-nickel
sandwiched between two thin coatings of copper;
FIG. 8C shows a wire of the form of FIGS. 8A or 8B, which has been
twisted;
FIG. 9 shows a slab or normally conducting material, including
multiple longitudinal slots into which are interposed rods of the
form of FIGS. 8A, 8B, or 8C;
FIG. 10A shows, before reduction, a tube made from a slotted slab
of the form of FIG. 9 by standard tube processes, surrounding a
composite cruciform central element;
FIG. 10B shows the configuration of FIG. 10A after reduction;
and
FIG. 11 shows in schematic a forced cooling system employing a coil
of superconductive wire formed in accordance with the present
invention.
DETAILED DESCRIPTION OF THE DRAWINGS
Referring in detail to FIG. 1 of the drawings, there is shown in
cross-sectional view, a matrix 1 of normally conducting material,
including a large number of strands 2 of superconducting material.
In the present illustration, the matrix 1 may comprise a ribbon of
the order of 50 mils thick, 125 mils wide, and of indeterminate
length, containing of the order of 100 strands of superconducting
material, each strand having a cross-sectional dimension of, say 4
to 5 mils.
This product may be fabricated by any one of several different
processes well-known in the art in which, for example, a plurality
of superconducting rods inserted in normal conducting tubes are
packed together in a preselected configuration inside of a
cylindrical shell of normally conducting material several inches in
diameter. This is then evacuated and sealed. The evacuated, sealed
billet is then processed by a combination of hot and cold working
steps to a product of desired cross-sectional dimension and
electrical characteristics. Such a process is described in detail
on page 46 of a book entitled Manufacture and Properties of Steel
Wires by Anton Pomp, published by The Wire Industry, Ltd., London
(1954).
For the purposes of the present invention, the superconducting
material may comprise, for example, an alloy ranging in composition
from 60 percent niobium, 40 percent titanium to 40 percent niobium,
60 percent titanium. In the present illustrative embodiment the
superconductor material is niobium titanium in an alloy consisting
of essentially 55 weight percent niobium and 45 weight percent
titanium. This is formed from what is known in the art as electron
beam niobium and crystal bar titanium, the total alloy containing
oxygen to the amount of about 200 - 1,000 parts per million, the
remaining impurities, not including oxygen, being under about 0.11
percent by weight. It will be understood that in the fabrication of
alternative embodiments, other alloys can be employed in different
proportions of niobium and titanium, such as, for example, an alloy
consisting essentially of 44 weight percent niobium and 56 weight
percent titanium, having an oxygen content of about 600 parts per
million and having impurities, not including oxygen, of less than
about 0.15 percent by weight. It is contemplated that any material
known as Class II or a "hard" superconductor, may be used for the
purposes of the present invention.
In the present example, it is contemplated that the normally
conducting material may consist of what is known in the art as
"certified oxygen free high conductivity copper", known by the
trademark OFHC Brand Copper, and described in detail in a technical
survey entitled "OFHC Brand Copper High Conductivity Oxygen Free
Copper," copyright 1957, available from The American Metal Company
Ltd., 61 Broadway, New York, New York. Alternatively, it is
contemplated that other normally conducting materials may be sued
for this purpose such as, for example, aluminum, gold, or
silver.
In accordance with another alternative, the core element may
comprise strands of superconductive material in a matrix comprising
high resistance normally conducting material, such as alloys of
cupro nickel or German silver. In a convenient form, strands of
niobium titanium encased in copper sleeves are embedded in a matrix
of cupro-nickel alloy, constructed in the manner disclosed in
application Ser. No. 36,740, filed by W. Shattes and W. Marancik,
at even date herewith.
Assuming that the material prepared in the manner previously
described has been reduced to the cross-sectional dimensions
indicated with reference to FIG. 1, such an element, which may be
in the form of a ribbon as much as 1,200 feet long, may be then
interposed in a tube, 3 as shown in FIG. 2, which may be of, say,
OFHC Brand Copper which in the present example is 30 mils in wall
thickness and 5/8 inches in outer diameter. Alternatively, the
outer tube 3 can comprise aluminum, stainless steel, or in fact any
metal having the requisite strength and relative conductivity. In
addition to being a single integral tube, say 30 mils thick, it can
alternatively comprise a laminate of a plurality of tubes of lesser
thickness fitted together coaxially. For example, an outer tube of
stainless steel can be welded to an inner tube of, say, copper to
provide additional strength.
The structure, as indicated, may be reduced in cross-section about
20 percent by what is known in the art as "sink drawing," in which
cold working is applied to uniformly reduce the diameter of the
outer tube to crimp it about the inner superconducting matrix
element. The principal object of this step is to fasten the core
element to the inner wall of the enclosing tube in such a manner as
to form good mechanical, thermal and electrical contact. The
composite is then cold worked through one or more dies until the
resulting product is of the order of 50 to 80 mils in cross-section
and may be shaped either to have a rectangular or circular
cross-section, depending on the type of die through which it is
drawn in the final processing steps. The inner or core element 1
need not necessarily be a single piece, but can comprise a laminate
of layers from 2 to 25 mils thick, and each 125 mils wide. These
may be superposed to form a single integral element or may be
separated.
In accordance with another alternative, instead of the central
element 1 being of the form of a ribbon, it may be processed by
metal working techniques well-known in the art including drawing it
through dies which will give it a final cross-sectional
configuration in the form of a cross, such as, for example, shown
as element 5 in FIG. 3A. The latter, in a manner similar to that
indicated in FIG. 2, is interposed into a tubular shell 6 of
normally conducting material which may be of a thickness and form
similar to that indicated with reference to FIG. 2. Like the
element previously shown, this may also be sink drawn to reduce it
to the desired cross-section. Another illustrative variation of the
performed core element is shown in FIG. 3B. It will be apparent
that the core cross-section may assume many additional variations,
such as rectangles, triangles, and other polygons, and stars of
various shapes.
Another alternative may be, for example, of the form indicated in
FIG. 4 of the drawings, in which the core element is a composite
formed of two doughnut-shaped halves 7a, 7b of copper or other
normally conducting material of the order of, say 5/8 inches in
outer diameter and having an inner diameter opening 8, say, 3/8
inches across. The two halves 7a, 7b accommodate between them a
strip or ribbon 9 running lengthwise and bifurcating the chamber
formed by the inner opening 8. The ribbon 9 comprises a matrix
element of superconducting material of the general form indicated
in FIG. 1. In an alternative embodiment, the donut-shaped halves
7a, 7b of FIG. 4 can also be formed of superconducting material of
the form indicated in FIG. 1. The composite cone comprising the
annular halves 7a, 7b and ribbon separator is interposed, in the
manner indicated with reference to the previous figures, in an
outer tube 11 of normally conducting material, having an outer
diameter of 0.750 inches, and a wall thickness of about 30 mils,
which is processed in the manner previously described by sink
drawing it to conform to the outer diameter of the annular halves
7a, 7b with a reduction of about 20 percent in area. By cold
working, the composite is ultimately reduced to a cross-sectional
dimension of 50 to 80 mils, which may be either circular or
rectangular, in the manner indicated in FIGS. 5A and 5B of the
drawings, depending on well-known metal working techniques.
As indicated in FIGS. 6A and 6B of the drawings, in accordance with
a preferred alternative, the inner core member, comprising the
element 1 of FIG. 2, the element 5 of FIG. 3, and the element 9 of
FIG. 4, may be twisted, as shown in FIG. 6A of the drawings, before
it is rolled or passed through a die for forming in the desired
shape for assembling in the enclosing cylinder, in each case. For
example, it is contemplated that the pitch of twists may vary from
several feet to one-tenth of an inch. In a further alternative
form, shown in FIG. 6B, the core elements 1, 5 and 9 in FIGS. 2, 3
and 6, respectively, may comprise multistrand twists, in which
strands of superconductive matrix wire are twisted together, or
with strands of normal conductor. It has been found that the finer
the superconductor filaments, the more electrically stable is the
configuration.
In a further alternative, the twisted super conductive matrix, as
shown in FIGS. 6A and/or 6B, may be first rolled into a flat sheet,
and joined such as by welding with a metal sheet of other
compositions to form a laminate. The laminate may then be formed
into a tube of laminar construction with the superconductive matrix
forming the innermost layer. The tube formation may be accomplished
by drawing. The outer laminate is desirably a relatively stronger
metal intended to afford desired structural properties and may be
made from a sheet of normally conducting material or preferably, a
stronger metal such as stainless steel. The tube then formed may be
used as a hollow superconductive wire or may be substituted for the
outer tubes disclosed in FIGS. 2, 3, 4, 7 or variations of
these.
An additional modification is shown in FIG. 7A of the drawings. In
FIG. 7A a plurality of wires of composite superconducting matrix
material of the type described with reference to FIG. 1 of the
drawings, each having a cross-sectional diameter of about 0.1 inch,
are braided together in spiral fashion as shown in FIG. 6B to form
a cable 12 about 0.450 inches in diameter. This braid may comprise
only a few strands, or many strands, and may be intermixed with
strands of normally conductive material. The spiral may have a
pitch of from several feet to about one-tenth inch. As with the
other embodiments, this may be interposed into an enclosing outer
tube 13 which is 0.75 inches in diameter and about 30 mils thick,
and sinkdrawn and further processed, as previously described, to
the desired shape and cross-section. In a variation of this
embodiment, the central or core wire 12A of the cable can be
substantially larger than the peripheral wires which may be wound
about it at varying pitches and using various numbers of strands
from as few as one to up to 19. In another variation, indicated in
FIG. 7B core 12a may comprise a tube.
In accordance with a further variation of the invention, wires or
rods of superconductive matrix material of the form indicated in
FIG. 8A, having a solid central element 21 formed in the manner
indicated with reference to the matrix element of FIG. 1, is coated
with a thin coating 22 of normally conducting material, such as a
copper coating 0.001 inches thick. A variation is shown in FIG. 8B,
in which the central superconductive matrix element 24, which may
be 0.050 inches in diameter, includes an inner coating of copper
25, which is 0.001 inches thick, an intermediate coating of
cupro-nickel 26 which is 0.001 inches thick, and an outer coating
of copper 27 which is 0.001 inches thick. Either of the elements
shown in FIGS. 8A or 8B can, in the case of a preferred
alternative, be twisted as shown in FIG. 8C.
Elements 20 of the form of any of those shown in FIGS. 8A, 8B or 8C
are then interposed, as shown in FIG. 9, into a series of
longitudinal slots 28 which are 0.05 inches wide and 0.07 inches
deep, parallel to the long edges of a rectangular slab of normally
conductive material, such as copper, say, 2 inches wide and 1 inch
thick, and of indeterminate length.
This is cold worked and rolled to a thickness of 0.080 inches and a
length of about, say, 1,200 feet. It can then be formed by welding
the edges by tube-making processes well-known in the art, to form a
tube 29, such as indicated in FIG. 10A, having an outer diameter of
0.5 inches and an inner channel 31, say 0.340 inches diameter, and
discrete islands 32 of superconductive matrix material. It is
contemplated that a composite core element 30, having a cruciform
cross-section, and similar to core element 5 of FIG. 3, and
preferably twisted, may be interposed inside of the tubular shell
29, before coreduction to the form shown in FIG. 10B. It will be
understood that the element 30 can also assume any of the forms
shown on FIGS. 2, 3, and 4, or other forms, such as rectangles,
triangles, stars, and other polygons. The form of FIG. 10A may be
reduced by the usual cold working techniques to an element of the
form shown in FIG. 10B, having an outer diameter of 0.400
inches.
It is contemplated that wire formed in accordance with the
specifications set forth hereinbefore may be embodied in the coil
element 36 of a forced cooling convection system employing
supercritical fluid helium, such as shown in FIG. 11 of the
drawings. This comprises a Dewar type vessel 37, properly insulated
in the manner known in the art to maintain the helium at the
desired temperature and pressure. Vessel 37 is partially filled
with a bath of liquid helium 38. Helium gas is initially introduced
into the system from a source 39 through the line 41 and cryogenic
valve 42 to the junction 43, from which it flows through the heat
exchanger 44 interposed in the neck of the vessel, and heat
exchanger 45, submersed in the liquid helium, to coil 36 comprising
hollow superconductive wire of one of the types described with
reference to the earlier figures. The helium circulated through
this circuit by the action of the liquid helium pump 47 is brought
to a temperature of the liquid bath 38 in the heat exchanger 45,
subsequently cooling down the superconductive coil 36. In the
present illustrative embodiment, bath 38 is maintained at about
4.2.degree. Kelvin. The heat exchanger 44 functions to partly
recuperate the enthalpy of helium vapors exhausted through vent 48
in the top of the vessel. Helium in the closed loop including heat
exchangers 44 and 45 and coil 36 is maintained at high pressure,
whereas pump 47 is required to produce only a small pressure drop
for recirculation in the circuit. Until equilibrium is reached,
helium is introduced continuously from the source 39, valve 42
being closed when equilibrium is reached. Details of such a system
are disclosed in an article entitled "Construction of a
Superconducting Test Coil Cooled by Helium Forced Circulation" by
M. Morpurgo of Cern, Geneva, Switzerland, reprinted from N.P.
Division Report CERN 68-17 (1968).
The test coil 36 may, for example, have the form indicated in the
above-named article.
The superconducting strands formed in accordance with the present
invention have been found to have a current carrying capacity of 1
.times. 10.sup.5 amps./cm..sup.2 at a field of 60 kilogauss. In
addition, the forms shown in FIGS. 3A and 7A, 7B are substantially
isotropic in their behavior.
* * * * *