Note: Descriptions are shown in the official language in which they were submitted.
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SUPERCONDUCTING COIL ASSEMBLY
CROSS REFERENCE TO RELATED APPLICATION
This application claims the benefit of U.S. Provisional Application No.
60/143,666 filed 14 July 1999.
BACKGROUND OF THE INVENTION
1. Field of The Invention
The present invention is related to the field of superconducting devices
energized by an electrical current source and, in particular, superconducting
coils
and superconducting magnets generating very high magnetic forces. Such magnets
have a variety of uses such as separation devices and are useful in the field
of
refining ores, in particular, titanium dioxide ores in the form of a slurry.
2. Description of the Background Art
Superconductivity is a phenomenon whereby certain metals, when cooled to
very low temperatures, become perfect conductors of electricity. In practical
application, superconductive materials may be used to construct powerful
electromagnets capable of generating very high magnetic fields with relatively
little
power consumption. Superconductive materials must be kept at temperatures as
low
as a few degrees Kelvin in order to maintain their superconductive properties.
Superconducting magnets are manufactured by winding turns of
superconducting material, in the form of layers of conductor, insulator, and a
support
material, on a mandrel to form pancake coils that are stacked on one another
to form
a completed magnet winding. The superconducting material is typically formed
into
a flat band of material that has limited flexibility and low structural
strength. To
efficiently wind the flat superconductor and get the end leads located on the
outside
of the coils for electrical termination, the coils are often wound in pairs
from the
inside to the outside which requires a winding transition portion at the
inside that
connects to each coil in the pair. This forms what is called a "double
pancake"
assembly. One double pancake assembly is electrically connected to an adjacent
double pancake assembly with a splice. The splice is often located on the
outside of
the coils at a splice transition portion. Where the coils cross over the
winding
transition portion, as well as where the splice transition portion passes over
underlying coils, there is a high stress placed on the superconductor that may
become a source of electrical failure. During operation, large forces are
developed
on the structure resulting from the high magnetic field exceeding one tesla.
The
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superconducting materials in the pancakes must be cooled to a temperature of
about
-250 to -270°C (23 to 3 °K) to maintain their superconducting
properties. The
materials are placed in a vacuum container to thermally insulate the
materials. In
use, heat is developed in the pancakes that also must be removed from the
assembly.
It is common to achieve the low temperatures in the structure and remove
operating
heat by submerging the entire assembly in a cryogenic fluid, such as liquid
helium,
to achieve temperatures of 2-4 °Kelvin for low temperature
superconductors (LTS)
or of 20-25 °K for high temperature superconductors (HTS). This is
referred to as
an open refrigerant system since cryogenic fluid connections are required to
the
structure that are disconnected and reconnected in use. This requires a
complex
casing design and system for circulating fluid throughout the structure. When
the
magnet is started up and shut down for maintenance, the cryogenic fluid is
removed
and replaced, which subjects the assembly, including the transition portions,
to
thermal stress.
Another cooling system is a closed refrigerant system that is connected to the
magnet coil assembly using conductive connections; the fluid is always
contained in
a separate system terminating in a "cold head" that is never opened when
disconnected and reconnected to the magnet assembly. This is referred to as a
conductive cooling system and it has been used successfully only on small
scale
magnet systems where the conductive distances are small (on the order of
several
inches). A conductive cooling system simplifies the construction, which is
desirable, but in a large coil assembly it introduces challenging problems
making
many high conductivity mechanical connections across many pancake assemblies.
U.S. Patent No. 5,861,788 to Ohkura, et al describes problems with heat
generation
in superconducting magnets using cryogenic refrigerators and cooling heads.
In the field of refining ores, in particular, titanium dioxide ores in the
form of
a slurry, it would be desirable to provide magnetic separation equipment at
remote
locations in order to avoid shipping large amounts of ore to a location where
suitable
magnetic separation equipment exists. The operation of complicated open
cryogenic
cooling systems at remote locations is problematic and powerful
superconductive
magnets generating heat loads that can be managed by closed cryogenic cooling
systems have not been developed to date. There is a need for a system for
removing
heat from a large magnet assembly employing many pancakes without requiring
submersion of the pancake structure in cryogenic fluids. There is also a need
for low
stress transition portions in a large double pancake design that produces high
magnetic forces.
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SUMMARY OF THE INVENTION
The invention provides a portable, high power superconducting coil and
magnet having low stress and minimal temperature rise across the device. The
superconductive coil assembly for generating a magnetic field according to the
invention comprises an end piece, at least one pancake assembly adjacent to
the end
piece, the pancake assembly comprising superconductive material and a radial
heat
transfer element, wherein the superconductive material is disposed between the
radial heat transfer element and the end piece; and a thermally conductive
element
having a thermally conductive connection with the end piece and the radial
heat
transfer element.
The invention also provides a mandrel for a superconductive double pancake
coil assembly having a recessed channel provided therein, said recessed
channel
having a width and depth for placement of a superconductive material.
The invention also relates to a process of removing heat from a stack of
pancakes in a superconducting magnet by providing thermal conductive layers
between the pancakes with a portion extending beyond the pancakes for radial
heat
transfer, and providing thermal conductive elements passing through the
conductive
layer portions extending beyond the pancakes and passing into thermal
conductive
rings at the ends of the stack of pancakes for providing axial heat transfer.
A
thermal conductive path is established between the end rings and an external
heat
transfer device that employs a cryogenic fluid.
The invention is also a low stress transition connector for supporting and
guiding the superconducting material in a low stress path between two pancakes
in a
double pancake structure. The invention is also a splice connector for
supporting
and guiding the superconducting material in a low stress path between one
double
pancake structure and another.
BRIEF DESCRIPTION OF THE FIGURES
Figure 1 is a diagrammatic view of a magnetic separator for a slurry fluid.
Figure 2A is a perspective view of a superconducting coil assembly.
Figure 2B is an enlarged view of a portion of Fig. 2A showing a connection
between a tubular cooling rod and a plurality of cooling plates.
Figure 3 is a plan view of a double pancake coil.
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Figure 4A is a section view 4A-4A from Fig. 3 showing a winding transition
channel in a double pancake mandrel and a splice transition channel in a
splice block
connecting the double pancake coils.
Figure 4B is a section view 4B-4B from Fig. 3 showing a detail of a cooling
plate engaged in a groove in a double pancake mandrel.
Figure SA is a perspective view of a mandrel with a connector and Figure SB
is an enlarged plan view of the connector.
Figures 6A and 6B are an elevation view and a plan view, respectively, of a
winding transition channel seen in view 6A-6A of the mandrel of Fig. 5A.
Figures 7A and 7B are an elevation view and a plan view, respectively, of a
splice transition channel seen in view 7A-7A of the splice block in Fig. 4.
Figure 8 is a diagrammatic side view of a splice between two double pancake
assemblies.
Figures 9A, 9B, 9C, and 9D illustrate an assembly procedure for making a
double pancake with the mandrel and cooling plate of the invention.
Figure 10 shows the heat flow in a section of the top of a superconducting
coil assembly according to the invention.
Figure 11 shows a 2-dimensional analysis of a portion of a magnet.
Figure 12 shows hoop strain in a portion of a magnet assembly.
Figure 13 shows radial strain in a portion of a magnet assembly.
Figure 14 shows hoop strain in a portion of an energized magnet assembly.
Figure 15 shows axial strain in a portion of an energized magnet assembly.
Figure 16 shows a two dimensional analysis thermal fringe plot.
Figure 17 is a three dimensional analysis thermal fringe plot.
Figure 18 shows a portion of a magnet, in partial cross section, with an
internal thermal conductive element.
DETAILED DESCRIPTION OF THE INVENTION
The superconducting coil according to the invention may be used in a high
temperature superconducting magnet (HTS) operating at from about 5 degrees
Kelvin to about 50 degrees Kelvin, preferably from about 15 degrees Kelvin to
about
25 degrees Kelvin, more preferably at about 20 degrees Kelvin, depending on
the
design criteria. The design central field may range from about 0.1 Tesla to
about 5
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Tesla, more preferably from about 1.0 Tesla to 3.0 Tesla. Stress/strain levels
achieved are less than 0.2% mechanical strain, preferably less than 0.12%
mechanical strain, more preferably less than 0.1% mechanical strain.
Temperature
rise or gradients in the superconducting coil assembly are on the order of
less than 3
degrees Kelvin, and preferably temperature rise or gradients in the
superconducting
coil of less than 1 degree Kelvin, or more preferably less than 0.5 degree
Kelvin, are
obtained by the invention.
A diagram of a magnetic separator 20 is shown in Figure 1 comprising a
metal yoke 22, a metal pole piece 24, a filter chamber 26 containing a metal
mesh
filter 28, a cryogenic coil cavity 30 containing a superconducting coil 32,
and an
inlet conduit 34 and outlet conduit 36. The inlet conduit 34 contains a
shutoff valve
38, and the outlet conduit 36 contains a shutoff valve 40. In operation, a
slurry of
material, such as titanium dioxide and impurities, passes through the open
valve 38
through inlet conduit 34, through filter mesh 28 and through open shutoff
valve 40
in outlet conduit 36. The superconducting magnet coil is energized with a DC
current to generate a strong magnetic field that acts to attract the
impurities in the
slurry stream to the filter material. The outlet stream flowing from outlet
conduit 36
is purified titanium dioxide slurry. After a period of operation, the filter
28 becomes
clogged with impurities and the flow of slurry is stopped and the valves 38
and 40
are closed and the current to the magnet turned off. The filter can be flushed
with a
cleaning fluid, such as water, by connecting the conduits to a water source
and
flushing out the filter 28. Alternatively, the pole piece 24 containing the
filter 28
can be removed from the yoke and the clogged filter replaced with a clean one.
When the magnet is turned off and on, the collapsing and expanding magnet
field
produces eddy currents and heating in the metal elements in the
superconducting coil
32. This heat must be removed to avoid heating the superconducting material.
Figure 2A shows a superconducting coil assembly 42 that is useful in the
magnetic separator 20 of Fig. 1. The coil assembly 42 is annular and has an
open
bore side 44 and an outer side 46. The coil assembly 42 comprises a plurality
of
pancake coils, which are preferably fabricated in pairs known as double
pancake
assemblies 48. Between each pancake coil is a cooling plate 50 made of a
thermally
conductive material, preferably aluminum, that conducts heat primarily in a
radial
direction between the coils, although it may also conduct heat
circumferentially and
axially (i.e. through the plate thickness). At the top of the coil assembly 42
is an end
ring 52 made of a thermally conductive material, preferably aluminum, and at
the
bottom of the assembly is an end ring 54 made of a thermally conductive
material,
preferably aluminum, which conduct heat circumferentially and radially. The
end
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ring 52 has a gap 53 to interrupt eddy currents in the ring during operation.
A bridge
plate 55, made of an electrical insulator, bridges the gap 53 to strengthen
the ring.
End ring 54 has a similar gap and bridge plate. The end rings, plates and
coils are
held together by tie bolts 56 arranged around the bore side and outer side of
the coil.
The tie bolts have spring washers 58 and nuts 60 on each end to accommodate
thermal expansion and contraction of the coil assembly 42. To avoid completing
an
electrical circuit for eddy currents between the end rings, plates, and tie
bolts, the tie
bolts are covered with a tubular insulator and there is an insulator washer
(not
shown) separating the spring washers from the end rings. Arranged around the
outer
side of the assembly are a plurality of thermally conductive elements, cooling
rods
62 that are shown passing through holes in the end ring 52 and the plates 50
in the
top half of the coil assembly. The cooling rods 62 conduct heat in an axial
direction
indicated by arrow 64 and are preferably copper or copper alloy. Also arranged
around the outer side of the assembly are a plurality of thermally conductive
elements, cooling rods 63 that are shown passing through holes in the end ring
54
and the plates 50 in the bottom half of the coil assembly. The cooling rods 63
conduct heat in an axial direction indicated by arrow 65 and are preferably
copper or
copper alloy. The top ends of the rods 62 are connected by thermally
conductive
straps 66 and 68. The bottom ends of the rods 63 are connected by thermally
conductive straps 70 and 72 (not shown). Straps 66, 68, 70 and 72 are
preferably
copper or copper alloy. It is believed that the copper straps provide better
thermal
conduction circumferentially around the coil assembly 42 compared to relying
solely
on the thermally conductive end rings. The end rings also provide structural
strength
to the assembly.
The cooling rods may comprise solid rods or bars that are attached to the
straps, end rings, and plates by soldering, either after passing through holes
in the
straps, rings, and plates, or attached to the peripheral edge of them. The
cooling rods
may comprise a plurality of tubular spacers that are placed between the
straps, end
rings, and plates aligned with holes placed in them. A bolt may be passed
through
the spacers and holes that may be threaded to a particular one of the plates,
or may
be provided with a nut on the threaded end. The cooling rods may comprise a
threaded rod passing through holes in the straps, rings and plates with nuts
on either
side of the holes to firmly clamp the straps, rings and plates between the
nuts. The
cooling rods may comprise tubular members that are placed in holes in the end
rings
and plates and are then swaged outward by conventional mechanical or hydraulic
means to expand and press firmly against the bores of the holes to make a firm
connection with low thermal resistance. To further reduce the thermal
resistance,
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the cooling rod embodiments employing mechanical connections may be plated
with
gold or other low thermal resistance coatings, or thermal conductive grease
may be
applied to points of contact between the cooling rods and other heat transfer
elements, i.e. straps, end rings and plates.
Figure 2B illustrates a preferred embodiment of a cooling rod 62a that is a
tube swaged outward to engage the straps, end rings, and plates. As an
illustration,
the tubular rods 62a are passed through holes 71 in plates SO and are then
expanded
by a conventional hydraulic expansion device placed in the bore 73. The device
temporarily closes off one end, such as end 75 of the bore 73 and applies
hydraulic
pressure to the enclosed bore. This presses the tube firmly against the
perimeter of
the holes 71 and bulges the tube slightly between plates in an outward
direction
relative to the central axis of bore 73, such as at 77. If the closing off
element of the
device prevents expansion at the end 75, this end can be expanded with a
conventional mechanical expansion device inserted in the bore 73. The
placement
and swaging of the tubular cooling rods can be done as a last step in putting
together
the coil assembly 42.
Figure 18 shows an alternate embodiment of the invention where, in addition
to cooling rods, such as 62 (not shown for the top half of the assembly 42)
and 63 (at
the bottom half of the assembly) around the outer side 46 of the assembly,
there are
cooling rods, such as cooling rod 63a and 62a, (not shown in the top half)
around the
inner bore side 44 of the coil assembly 42. Fig. 18 has cooling rod 63 passing
through end ring 54 and cooling plates, such as SOa, SOb, and SOd that are
arranged
within and between double pancake assemblies 48a and 48b as described below.
Cooling plate SOa is arranged within double pancake assembly 48a and is
connected
to cooling rod 63, and cooling plate SOb is arranged within double pancake
assembly
48a and is connected to cooling rod 63. Cooling plate SOd is arranged between
pancake assemblies 48a and 48b and is connected to cooling rod 63 at the outer
side
46 and is connected to cooling rod 63a at the inner bore side 44 spaced from
the bore
centerline 43 (not to scale). Cooling rod 63a also passes through end ring 54.
A
similar arrangement may also be present for the top half of the assembly 42.
In embodiments of the invention, cooling rods may be placed at the inner
bore side 44, cooling rods may be placed at the outer side 46 or they may be
placed
both at the inner bore side 44 and the outer side 46, depending on the cooling
needs
of the superconducting coil assembly 42.
Attached to the end ring 52 is a thermally conductive block 74 that has
connected thereto a flexible thermally conductive assembly 76 that is also
connected
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to another thermally conductive block 78 which is attached in a conventional
manner
to a cold head 80 of a cryogenic refrigerator for cooling the top half of the
coil
assembly. The flexible assembly 76 preferably consists of a plurality of
braided
thermally conductive material, preferably copper strips that are soldered to
blocks 74
and 78. The ends of conductive straps 66 and 68 are also physically and
thermally
connected to thermally conductive block 74. This arrangement of thermal
blocks,
flexible thermal assembly and conductive strap ends is repeated at end ring 54
for
connection to a cold head 82 of another cryogenic refrigerator for cooling the
bottom
half of the coil assembly. The coil assembly 42 is shown with an axial slot 81
that
passes through the top and bottom end rings 52 and 54, respectively, and
plates 50,
that permits access from the top to the bottom of the coil assembly 42 after
it is
assembled in the superconducting coil 32 which includes additional
conventional
elements such as a infra-red radiation shield and a vacuum chamber.
Figure 3 shows a plan view of a double pancake assembly 48a oriented with
the cutout for axial slot 81 shown at the top of the figure. The assembly 48a
comprises a mandrel 84 around which a superconducting coil 86a is wound on the
top side of annular plate SOa; another superconducting coil 86b (not seen in
this
view) is wound on the bottom side of plate SOa. Coil 86a is separated from
plate SOa
by an insulating washer 88a. In preferred embodiments, insulating washer 88a
is
made from G-l OCR material, which is a glass fiber reinforced phenolic
available
from Industrial Laminates/Noplex, Inc. of Postville, Iowa. Plate SOa may be
provided with a plurality of radial slots 90 to interrupt any eddy currents
that might
form in the plate in use and that can be used to aid in removing plate SOa
from the
assembly. In case the double pancake assembly needs to be repaired and the
superconducting material salvaged, cuts can be made from the outer periphery
of
plate SOa to the slots 90 to separate plate SOa into small segments that can
be
removed. After placement of double pancake assembly 48a in coil assembly 42,
one
radial slot is cut through to the periphery to stop the circulation of eddy
currents
around plate SOa while in use. Large diameter holes 89 in plate SOa are
provided for
rods 62 (Fig. 2A); small diameter holes 91 are provided for tie bolts 56 (Fig.
2A).
At the inner diameter of the coil 86a there is a segment 85 where a winding
transition portion is located in the mandrel 84. At the outer diameter of coil
86a is a
segment 83 where a splice transition is located between double pancake coils.
The
two segments 83 and 85 are shown in Figure 3 as overlapping, but they may be
displaced from one another so as to be non-overlapping if desired.
In the embodiment shown in Fig. 18, where cooling rods are placed at the
inner side of the coil assembly, it may be desireable to allow segments of
plate SOa
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to pass through the mandrel in the region remote from segment 85 (the segment
where the superconductive material is undergoing a winding transition from one
side
of plate SOa to the other). In the region where plate SOa can pass through the
mandrel, it can be engaged by the cooling rods 63a in Fig. 18 as shown by the
long
and short dashed lines at 248. This technique can be applied to the other
plates
within each double pancake coil assembly so all plates, such as plates SOa,
SOb, and
SOc, can be engaged by the cooling rods 63a as shown in Fig. 18, if such
measures
are required to achieve the desired cooling of the coil assembly.
Figure 4A is a section through three double pancake assemblies 48a, 48b,
and 48c taken at the position of section 4A-4A in Fig. 3. The double pancake
48a is
shown at the bottom of the coil assembly 42 (Fig. 2A) placed on top of end
ring 54
separated from it by two slip sheets 92 and 94 of electrically insulating
material,
such as G-l OCR. During thermal movement of the double pancake coils the slip
sheets reduce friction with the end rings to prevent damage to the rings and
coils.
Similar slip sheets are located at the top end ring as well. Each coil 86a and
86b
making up double pancake 48a is made up of a laminate of superconductive
material
strips and insulating material strips, collectively shown as 95 in coil 86b,
and a
support material strip shown as 98, the laminate of 95 and 98 collectively
shown as
87in coil 86b. The laminate 87 is helically wound around the mandrel 84a of
double
pancake 48a. Coil 86a is helically wound in a first circumferential direction
around
mandrel 84a and coil 86b is wound in second circumferential direction around
mandrel 84a, the second circumferential direction being opposite the first
circumferential direction. For example, if viewed from above (i.e., the
perspective
of Figure 3), if the first circumferential winding direction would be
clockwise, then
the second circumferential winding direction would be counter-clockwise. The
support material 98 typically comprises a band that is slightly higher than
the
remaining laminations to take the vertical load in the coil to prevent damage
to the
superconducting material and preferably is made from stainless steel. The
support
material 98 also takes the tensile loads during winding, and hoop stresses
during
operation, to protect the superconducting material. Plate SOa is placed
between coils
86a and 86b. Located between plate SOa and coil 86a is insulating washer 88a
and
located between plate SOa and coil 86b is insulating washer 88b. Plate SOb is
similarly arranged in double pancake 48b and plate SOc is similarly arranged
in
double pancake 48c.
Plate SOd is placed between double pancake 48a and double pancake 48b.
An insulating washer 88c is placed between plate SOd and coil 86a, and
insulating
washer 88d is placed between plate SOd and coil 86c in double pancake 48b.
Plate
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SOe is similarly arranged with insulating washers between double pancake 48b
and
double pancake 48c. To improve thermal conductivity between the coils,
insulating
washers, and plates, it is desirable to apply a conductive grease to the
interfaces. An
indium grease can be used or a grease of silicon oil solvent containing
ceramic
grains of Zn0 or other conductive greases or oils.
Coil 86b is connected to coil 86a by a connecting portion 100 of
superconducting laminate material that fits in a recessed channel 102 in
mandrel 84a.
At the location of the section 4A-4A, the connecting portion 100 is shown
halfway
between coil 86a and 86b although it forms a continuous serpentine transition
between the two coils. This connecting portion is typical for all the double
pancakes. Coil 86a of double pancake 48a is connected to coil 86c of double
pancake 48b by a splice portion 104 where a splice is formed between the end
of
superconducting laminate on coil 86a and the end of superconducting laminate
on
coil 86c. The splice portion is contained in a recessed channel 106 of a
splice block
108. The splice block 108 is arranged to rest on the outer turns of the
superconducting material of coils 86a and 86c. The splice block 108 has a
groove
110 that engages the plate SOd. At the location of the section 4A-4A, the
splice
portion 104 is shown halfway between coil 86a and 86c although it forms a
continuous serpentine transition between the two coils. This splice portion is
typical
for connecting all double pancakes in the coil assembly. The end 112 of
superconducting material on coil 86b is electrically connected to a source of
DC
current in operation. The current will pass through the turns of coil 86b,
through the
connecting portion 100, through the turns of coil 86a, through splice portion
104,
through the turns of coil 86c and continue in this fashion through all the
pancake
coils to the top coil in assembly 42 (Fig. 2A) where the superconducting
material
terminates and is electrically connected to the opposite end of the DC source.
Figure 4B is a partial view of the cross-section 4B-4B from Fig. 3 that shows
the relationship of a mandrel, such as mandrel 84a, and a cooling plate, such
as plate
SOa, at a location remote from the winding transition segment 85 (Fig. 3). The
mandrel 48a has a groove 132 that accepts the inner edge 109 of plate SOa as
well as
the inner edge 111 of insulating washer 88a and the inner edge 113 of
insulating
washer 88b. This serves to axially and radially position the washers and plate
with
the mandrel to facilitate fabrication of the double pancake. In certain
embodiments
of the invention, such as the embodiment of Fig. 18, the groove 132 may go
completely through the mandrel 84a to allow segments of plate SOa to pass
through
the mandrel to engage cooling tubes on the inner side of the coil assembly.
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Figure SA shows mandrel 84, which is preferably fabricated from stainless
steel, and has a gap 114 closed by an electrically insulating connector 116 to
interrupt eddy currents in the mandrel during use. As described above, one
acceptable insulating material is G-IOCR. The connector is shown in greater
detail
in Figure SB where the connector 116 has a segment 118 that forms part of the
outer
surface 120 of the mandrel and a segment 122 that forms part of the inner
surface
124 of the mandrel 84. Fasteners, shown in this embodiment as screw 126, hold
the
connector 116 to the mandrel 84. When the screws are removed, the connector
can
be removed and the diameter of the mandrel can be reduced by moving free end
128
toward free end 130 which is useful during assembly of the mandrel with the
plates
50, such as plates SOa, SOb, and SOc in Fig. 4. The mandrel has groove 132 in
a
portion of the outer surface 120 that accepts the inner edge of annular plate
50 (50a,
SOb, SOc). In embodiments of the invention, a portion 134 of the outer surface
120
of the mandrel contains features associated with the recessed channel 102
(Fig. 4A)
where groove 132 is not necessary and it may be missing or is not used. The
angular
length of portion 134 depends on the flexibility of the superconductive
material
used, the geometry of the coil assembly, and the thermal cycling the coil
assembly is
expected to experience in use. Small diameter coils and/or less flexible
superconductive materials and/or more frequent thermal cycling may require a
large
angular length of portion 134, and large diameter coils and/or more flexible
superconductive materials and/or infrequent thermal cycling may only need a
small
angular length of portion 134. In preferred embodiments of the invention
portion
134 is about 90 degrees. Although some portion of a groove 132 is useful in
alignment and assembly of the associated parts of the double pancake assembly,
it
may be only a small portion, so the angular length of recessed channel 102 may
be
from about 60 degrees to about 360 degrees. Preferably it is 60 degrees or
more to
less than 360 degrees and more preferably it is greater than or equal to 60
degrees
and less than or equal to 180 degrees.
Figures 6A and 6B show details of channel 102 in mandrel 84. Figure 6A is
an elevation view 6A-6A from Fig. 5A that shows outer surface 120 that has
groove
132 interrupted in the vicinity of channel 102. In Fig. 6B, the
superconducting
laminate 87 is shown placed in channel 102 and a portion of a plate 50 is
shown in
an alternating long and short dashed line. The bottom surface 133 of recessed
channel 102 forms a serpentine path for the superconducting laminate and lies
below
the surface 120 starting at position 136 where it is at the same level as
surface 120
and above a top side 138 of groove 132; and ending at position 140 where it is
at the
same level as surface 120 and below a bottom side 142 of groove 132. At a
point
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144 intermediate points 138 and 140, surface 133 is below surface 120 at a
distance
139 about equal to the width 141 of the superconducting laminate 87 (Fig. 6B)
so
the laminate is completely contained within the channel 102. Since groove 132
accepts plate 50 as discussed above, the channel passes from a position on one
side
of plate 50 at point 136 to a position on the opposite side of plate 50 at
point 140. In
Fig. 6A, plate 50 has been omitted for drawing clarity. In embodiments of the
invention channel 102 is helical, relative to the central axis of the magnet
assembly.
The height 146 of channel 102 is about equal to the height of the
superconducting
material laminate 87 (Figs. 4A and 6B).
Referring to Fig. 6B, to the left of point 140, the laminate 87 rests in
contact
with outer surface 120 of mandrel 84. Moving to the right from point 140, the
laminate 87 contacts the bottom surface 133 of channel 102 and gradually moves
below surface 120 until at position 144 the laminate is completely below
surface 120
or the surface of the laminate 87 and the surface 120 are about coincident.
This
allows the laminate to move past the plate 50 that is also relieved on its
inner edge
109a slightly in the region 148 between the ends of groove 132. The mandrel
with a
recessed channel having a serpentine-path, bottom surface that continuously
supports the superconducting laminate and eliminates abrupt direction changes
radially and circumferentially as the turns of superconducting laminate
overlap one
another, is believed to minimize the chance of damaging the semiconducting
material in the laminate. The plate 50 (such as 50a, 50b, and 50c in Fig. 4)
is also
stabilized in position in the double pancake by engagement of groove 132 with
plate
50 which facilitates installation and eliminates shifting in the axial
direction of the
coil assembly 42 (Fig. 2A).
Figure 7A shows an elevation view 7A-7A from Fig. 4, but showing only the
splice block 108 made from an insulating material such as G-IOCR. Block 108
has
an outer surface 150 that has recessed channel 106. In Fig. 7B, the block 108
is
shown placed against the last full turns, 87cd of the superconducting laminate
in
coils 86a and 86c. Block 108 has slots 152 and 154 aligned with groove 110 on
an
inner surface 158, which slots and groove are arranged to engage plate 50d
shown in
long and short dashed lines in Fig. 7B. The bottom surface 160 of recessed
channel
106 forms a serpentine path for the superconducting laminate and lies below
the
surface 150 throughout its length and tapers to a sharp edge at the ends162
and 163
where it is at the same level as inner surface 158. Since groove 110 accepts
plate
50d as discussed above and in Fig. 4, the channel passes from a position on
one side
of plate 50d at position 164 to a position on the opposite side of plate 50d
at position
166. Plate 50d has been omitted from Fig. 7A for drawing clarity. The height
168
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of channel 106 is more than the height 170 of the superconducting laminate 87
placed against the bottom surface 160 of channel 106 so the spliced
superconducting
laminate 87 is completely contained within the channel 106. Superconducting
laminate end 87a at the right side of Fig. 7B is coming from coil 86a of
double
pancake 48a in Fig. 4 and end 87b at the left side of Fig. 7B is coming from
coil 86c
of double pancake 48b in Fig. 4. The two ends meet in channel 106 over region
172
to form a splice, which is slightly higher than height 170 of the
superconducting
laminate. The splice block, supported by the last turns 87cd of the coils in a
double
pancake coil assembly, and with a recessed channel having a serpentine-path,
bottom
surface that continuously supports the superconducting laminate and eliminates
abrupt direction changes radially and axially, is believed to minimize the
chance of
damaging the semiconducting material in the laminate. The splice block is also
stabilized in position by engagement of groove 110 with plate SOd which
facilitates
installation and eliminates shifting in the axial direction of the coil
assembly 42 (Fig.
2A). Since the splice block is made from an electrically insulating material
it
prevents accidental contact of the superconducting material in the splice with
other
conductive elements in the coil assembly 42. If the splice block is made of an
electrically conductive material, then additional insulation provisions will
be
necessary. In embodiments of the invention channel 106 is helical, relative to
the
central axis of the magnet assembly.
Figure 8 shows a diagrammatic view of a typical splice between two ends of
superconducting laminate, 87a and 87b. An inner end of insulating film 174a
from
end 87a is overlapped with an inner end of insulating film 174b from end 87b
at the
top of the figure. Below that in the figure are the ends of six layers of
superconducting material, 176a,b,c,d,e,f from end 87a and 176g,h,i,j,k,1 from
end
87b. The ends are abutted, such as end 176a with end 176g, and all abutted
ends are
staggered apart from one another as shown. A solder 178 is applied at each
pair of
abutted ends to laminate all the ends together and to the superconducting
material
adjacent each pair of abutted ends. An outer end of insulating film 180a from
end
87a is overlapped with an outer end of insulating film 180b from end 87b.
Finally, a
band 98a from end 87a is overlapped with a band 98b from end 87b and a solder
182
is applied therebetween. Bands 98a and 98b are preferably made from stainless
steel. Care should be taken that the solder in the splice does not create an
electrical
path between the superconducting material and the stainless steel band. This
type of
staggered, abutted splice limits the height buildup in the splice compared to
a splice
where the superconducting material is overlapped.
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Figures 9A, 9B, 9C, and 9D illustrate an assembly procedure for making a
double pancake assembly in accordance with the invention using the mandrel and
cooling plate of the invention.
Refernng to Fig. 9A:
1. Align the insulating washer 88a, cooling plate 50 and washer 88b in the
order illustrated and rotationally align circumferential features as at 184.
Referring to Fig. 9B:
2. Pass roll 186 through the inner diameters of assembled parts from step 1.
Roll 186 represents the superconducting laminate 87e required for one coil of
a
double pancake, and roll 188 represents the superconducting laminate 87f
required
for the other coil of a double pancake. Rolls 186 and 188 are a continuous
piece of a
typical superconducting laminate comprising one insulating ply, six high
temperature superconducting (HTS) plies, one insulating ply and one stainless
steel
support ply.
3. Wrap four turns of a piece of insulating ply material around the six HTS
plies for a length slightly greater than what will be required in the winding
transition
segment and centered on the laminate at 190 between rolls 186 and 188.
Referring to Fig. 9C:
4. Align and center the winding transition channel 102 of mandrel 84 so it
aligns with the wrapped HTS plies (step 3.) at position 190 and rotationally
align
circumferential features of the mandrel with the assembled parts from step 1.
5. Carefully place the step 2 ply lengths and the step 3 wrapped HTS plies
into the winding transition channel 102.
Referring to Fig. 9D:
6. Remove the connector 116 in mandrel 84.
7. Carefully close the mandrel ends together and fit the step 1 insulating
washers and cooling plate into the mandrel groove 132 (also in Figs. 5A, 6A &
6B).
The plate cutout (148 in Fig. 6B) must be centered on the mandrel winding
transition
channel at 190.
8. Move the mandrel ends apart and reinstall the connector 116 making sure
the insulating washers and cooling plate are fitted into the mandrel groove
132.
9. Begin winding the laminate from roll 186 around the mandrel, contacting
washer 88a on top of cooling plate 50 in a clockwise direction. When the first
turn
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of the laminate approaches the winding transition channel, wind four turns of
a piece
of insulator ply material around the HTS plies for the length of the channel.
10. Complete winding the first side of the double pancake with roll 186.
Additional turns of laminate or addition of filler may be needed to achieve
the
required pancake outer diameter. The outer turn shall be secured to prevent
loss of
winding tension. Adequate additional laminate shall be provided for the
pancake-to-
pancake splicing or for pancake end termination to the current source.
11. Repeat steps 9 and 10 with roll 188 winding in a counterclockwise
direction on the opposite side of plate 50 contacting washer 88b to complete
the
second side of the double pancake.
Figure 10 shows a partial sectional view of the top of superconducting coil
assembly 200 (similar to assembly 42 in Fig. 2A) for generating a magnetic
field in
accordance with the invention and in particular illustrates the heat flow
through the
superconducting coil assembly 200 when it is in operation. Although single
pancake
configurations may be used, in preferred embodiments double pancake assemblies
(shown in brackets and similar to assemblies 48a,b,c of Fig. 4A) 210a, 210b
and
210c (partially shown) are used and have separating plates 50x and 50y located
between them. The structure of these double pancake assemblies is described in
detail above. Separating plates 50x and 50y are similar in structure and
function to
plates 50k, 50m, 50n and are, in preferred embodiments, provided with openings
89
to allow for installation of a thermally conductive element, shown here as
tube 62.
Tube 62 may be hollow and also may be swaged by mechanical or hydraulic means
to provide a thermally conductive connection with each plate 50k, 50x, 50m,
50y
and 50n. Tube 62 is also thermally connected to end ring 52, high heat flux
strap 66,
and flexible thermally conductive assembly 76. With selection of proper
materials
having different coefficients of thermal expansion/reduction for the materials
of end
ring 52, each plate 50k, 50x, 50m, 50y and 50n, and rod 62, the thermal
connections
between each improve with a reduction of temperature because the plates shrink
to a
greater extent than the rod. For example, thermally conductive material,
preferably
aluminum or aluminum alloy may be used for of end ring 52, each plate 50k,
50x,
50m, 50y and 50n and thermally conductive material, preferably copper or
copper
alloy may used for rod 62. These principles apply to the construction of the
lower
half of the superconducting coil assembly of the invention as well.
In Figure 10, the heat flows are indicated by the arrows and when the source
of heat is considered as the superconducting material in the pancake, the heat
flows
both axially and radially, albeit to different extents as it will follow the
path of least
CA 02374326 2001-12-19
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resistance. The heat in the superconducting material flows to and through the
vertically oriented metal support material 98 in the pancake and then through
layers
88e and 88f and into, for example, plates 50k & 50x for superconducting
material
positioned between them: Plates 50k and 50x transfers the heat energy and the
heat
flows radially outward to rod 62. Rod 62 transfers the heat energy and heat
flows
axially to end ring 52 and flexible thermally conductive assembly 76. The heat
flows predominantly in the radial directions, such as 212, 214, 216, 217, 218
and
219, and in the axial directions, such as 220, 222, 224, 226, 228, 230 and
232. The
heat energy flowing through flexible assembly 76 may then be removed by the
cryogenic refrigeration unit 78. The bottom half of the coil assembly 42
operates in
much the same manner. Thus, in operation, the superconducting coil in
accordance
with the present invention may be maintained at nearly constant temperatures
throughout, with variations of less than 1 degree Kelvin, preferably less than
0.5
degree Kelvin.
Although the end piece has been depicted in the Figures as a ring, the radial
heat transfer element a plate and the thermally conductive element as a rod or
tube,
different geometries for each piece may be used depending upon the particular
application of the magnet.
Experimental Model
The thermal and stress/strain analysis of a 0.8 m diameter cryocooled High
Temperature Superconducting (HTS) magnet is provided by computer modeling.
The design central field of the magnet is ~ 2 T. The HTS magnet is cryocooled
with
a target operating temperature of 20 K. A non-linear stress/strain analysis is
performed to determine the stress/strain values on the HTS conductor and its
corresponding support structure at four critical stages of manufacture and
operation:
1) tension winding, 2) axial pre-compression, 3) cool down to 20 K, and 4)
magnet
energization. Results indicate the HTS conductor and its support structure
remain
below acceptable stress/strain levels (<0.2 % mechanical conductor strain)
during
the fabrication and operation of the device. In addition, a steady state
thermal
analysis is performed in order to demonstrate adequate heat removal within the
HTS
conductor windings. Results indicate a maximum temperature rise of <3 K for a
12
W input, which represents the worst case operational scenario.
HTS coil performance specifications developed for the magnetic separator
are listed below in Table 1 and are based upon existing commercial LTS
magnetic
separators presently used by the kaolin industry.
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Table 1. Major HTS Coil Performance Requirements
Parameter Requirements
Magnetic Field 2 T
Coil diameter 0.8 m
Coil height 50 cm
Refrigeration Cryocooled
Thermal cycles 25
Magnetic cycles 200,000
Cost equivalent LTS unit
Ramp time < 2 minutes
Useful life 5 years
A central magnetic field of 2 T is chosen to represent what is commercially
available in batch type separators. Dimensional requirements for the HTS coil
are
determined by those presently used in the kaolin and Ti02 industries. Other
requirements in terms of useful life, magnetic and thermal cycles, ramp time,
etc.,
are representative of those used in commercial LTS magnetic separators, with
the
exception of the refrigeration system. Presently, the LTS coils used in
commercial
magnetic separators of this diameter have been limited to using liquid helium
as a
refrigerant.
Based upon the HTS coil performance requirements, a detailed HTS coil
conceptual design is generated. Key elements of the design are summarized in
Table
2.
Table 2. HTS Coil Conceptual Design.
Parameter Design
Operating current 400 A
HTS conductor Bi-2223 (PIT)
Winding type Double pancake
No. of pancakes 102
No. of turns per pancake2g
Intercept temperature60 K
Operating temperature20 K
Iron yoke Yes
Refrigerator Two 2 stage G-M
Current leads HTS - binary type Ag - 10%
Au PIT
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The HTS conductor for the coil is several strands of silver sheathed
Bi2Sr2CazCu30X (Bi-2223) powder-in-tube mufti-filamentary tape. Each Bi-
2223 tape is approximately 4 mm wide and 0.25 mm thick and contains over
61 HTS filaments. The HTS tapes have a normal metal to superconductor
ratio of ~ 2.5 to 1. The conductor is tension wound into double pancake coils
on a pre-formed winding mandrel. A stainless steel support strip is used to
support the hoop tension in the coil. Each double pancake coil is stacked and
spliced together to form a single continuous HTS coil. The stacked HTS coil
assembly is axially pre-compressed via tension rods to minimize conductor
movement during cool-down and energization. Kapton~ and G-10 is used
for the turn-to-turn and pancake-to-pancake insulation, respectively. The
current leads are of a binary-type HTS current lead with an intercept
temperature of about 60 K. The lower stage (HTS portion) of the current
leads are of silver - 10 % gold alloyed Bi-2223 powder-in-tube tape. A 6061-
T6 aluminum shield operating at about 60 K is used to intercept the room
temperature radiation heat load. The HTS coil assembly is mounted in a
donut shaped vacuum cryostat. The cryostat is mounted inside an iron yoke
structure. The entire magnet assembly (including the iron yoke) is designed
for transportability.
Stress/Strain Analysis
A non-linear finite element analysis (FEA) is used to determine the
stress/strain
with the HTS conductor and its corresponding support structure at four
critical states
of HTS coil fabrication through operation: 1) tension winding, 2) axial pre-
compression at room temperature, 3) cool-down to 20 K, and 4) magnet
excitation.
For the magnet excitation stress/strain analysis, two dimensional (2D) and
three
dimensional (3D) electromagnetic analysis using a commercial Boundary Element
Method code determined the magnetic forces.
ABAQUS Version 5.8 FEA code is used to complete the structural analysis.
Figure 11 shows the 2D axisymmetric analysis of'/2 of a double pancake cross
section with the various material parts shaded. The analysis elements include
the
insulation layers 250 and metallic layers 252 along with half of each pancake
coil
above and below the layers. By using axisymmetric elements, the analysis
approximates the spiral wound coil as a series of concentric rings around the
mandrel 254. The first 3 inner diameter (ID) coil plies in the ID region 256
and last 3
outer diameter (OD) coil plies in the OD region 258 included separate HTS
plies 260
and stainless steel (SS) support plies 262 to better simulate the actual
conditions of
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these extremes. The central plies are simulated with average properties to
model the
basic behavior. Contact surfaces are defined between each part of the
analysis.
The lower cut plane through the pancake coil winding is constrained against
axial movement. The upper cut plane is constrained to move together axially.
These
boundary conditions simulate the central portion of the full magnet.
The analysis of the pancake winding process involves defining an initial hoop
stress in each HTS and SS ply pair. These stressed ply pairs are released one
pair at a
time to mimic the actual winding process. The initial stress is adjusted such
that the
hoop stress in the HTS and SS plies, after release, matches the winding
tension. As
each new pair is released, the previously released ID plies are compressed and
eventually lose their initial tension.
Figure 12 shows the resulting hoop strain in the wound pancake analysis for
only the enlarged ID region 256 and OD regions 258 that have separate HTS and
SS
plies. The mandrel and ID HTS plies are all in compression. The ID SS plies
are still
in tension although considerably less than the winding tensions. The OD SS
plies are
all in tension at about the winding tension level. Friction is not included
between the
contact surfaces during the winding simulation.
Applying an axial load to the analysis simulates the clamped assembly of the
stacked pancakes. The resulting axial compression loads to the wound pancake
analysis are not large compared to the energized loads.
The next simulation involves cooling the magnet to 20 K. For this state,
friction
between the contact surfaces is included in the analysis. Due to the
difference in
coefficient of thermal expansion, the HTS, SS, insulator and metallic
materials all
shrink differing amounts. This tends to unload the compressive stress holding
the
HTS/SS plies tightly against the mandrel. The challenge of the modeling task
is to
determine the level of wind tension necessary to cause the ID HTS plies to
maintain
contact with the mandrel without overstraining the HTS material. If
insufficient
winding tension is used, the ID HTS plies will lose contact with adjacent
plies and
jeopardize heat transfer capability.
Figures 13 shows the radial strain in the ID regions 256 and OD regions 258
for
the 20 K cooled condition for the magnet. The intervening plies fall between
these
bounding strain states. The important result is that the HTS plies are all in
radial
compression with the adjacent SS support plies. The only tensile radial
strains are
due to friction between the insulation layers and OD SS plies.
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The final state simulates the excitation loads. This is accomplished by
applying
a radial acceleration to the HTS material in the analysis. The acceleration
varied
linearly in a 2:1 ratio from ID to OD of the coil. The acceleration magnitude
is set to
produce the total radial force generated in the energized magnet. In addition,
an axial
compression load from the magnetic analysis is applied to the top HTS/SS
pancake
cut plane.
Figure 14 shows the HTS hoop strains in the ID regions 256 and OD regions
258 of the energized coil. As shown, the strain levels are well below the 0.2
capability of the HTS material.
The HTS axial strains are shown in Figure 15. These strains are compressive
due to the large axial energized load. Since the SS support plies carry a
large portion
of the axial force, the load carried by the HTS is low.
Table 3 details the strains in the HTS and SS support plies for the first and
last
plies at the three highest load states. Stresses are within the capability of
the HTS
material.
Table 3. Strains in Pancake Analysis
State Wound Cooled Energized
to
20
K
Direction R A H R A H R A H
R - Radial (%) (%) (%) (%) (%) (%) (%) (%) (%)
A - Axial
H-Hoo
Mandrel .0049.0051-.018.0087.0099-.031.0014.0014-.0049
15~ HTS PI .0057.0045-.015-.003-.003-.0038.0137-.051.022
15~ SS Su -.002-.005.007.006 -.010-.006.013-.049.019
ort PI
Last HTS .0013-.003.0003-.0051-.005.015 -.013-.046.038
PI
Last SS Su -.006-.010.023-.0034-.020.015 -.006-.059.037
ort PI
Thermal Analysis
A static linear FEA is used to determine the maximum coil temperature under
normal operating conditions. The AC loss and DC static heat loads used as
inputs to
the thermal analysis (~ 12 W at 20 K) are calculated in a separate analysis. A
temperature intercept of 60 K is assumed for the radiation shield and the HTS
current leads.
Pro/ENGINEER~ and Pro/MECHANICA~ THERMAL Release 19.0 software
running on a HP Visualize C160 Workstation are used to create the models and
perform the analyses of the models, respectively. Two types of steady-state
thermal
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conductive analysis are performed: A 2D Axisymmetric analysis of a %Z cross-
section through the magnet and a 3D analysis. Thermal analysis inputs are
summarized in Table 4.
For the 2D Axisymmetric model, both the number of coil pancakes and turns
per coil (see Table 2) are consolidated ~ 10:1 while maintaining overall
physical
dimensions of the coil. This consolidation yielded an equivalent 10 pancake
structure, in which each pancake is a detailed 3 turn structure (see Figure
16). A thin
outer copper shell 264 simulates the actual axial cooling feature. The 12 W
heat load
is distributed to the HTS elements with a decreasing linear radial gradient.
Figure 16
is the 2D analysis thermal fringe plot for the '/z consolidated structure
cross section.
Line 266 is the centerline of the coil assembly and line 268 is the axial
midplane of
the assembly. For a starting operating temperature of 20 K, the 2D analysis
predicts
a maximum temperature of 20.46 K within the HTS material.
For the 3D model, the pancake coil structure is lumped together to give an
equivalent single pancake of 3 turns. Orthotropic thermal properties,
calculated and
verified with the 2D model, are assigned to the lumped structure. The 3D model
includes the asymmetric features of eddy current breaks, pancake splices,
current
lead connections and the discrete mounting of the two cryocoolers (see Table
4).
Table 4. Thermal Model Inputs
Parameter 2D Axisymmetric3D Solid
Heat Load & Distribution12 W 12 W
Radial (ID ~ OD) Linear DecreaseLinear Decrease
Axial Uniform Uniform
Operating Temperature20 K 20K
Material PropertiesIsotropic Orthotropic &
Isotropic
Thermal Contact 0 0
Resistance
Eddy Current BreaksNone Included
Splice features None Included (per
pancake)
Current Lead featuresNone Included
Cryocooler ConnectionDistributed Discrete at each
at each end end
Outer Conductive Uniform Discrete
Shell
Figure 17 is the 3D thermal analysis fringe plot of the lumped coil structure.
For a starting operating temperature of 20 K, the 3D analysis predicts a
maximum
temperature of 22.19 K within the HTS coil. The addition of a 0.50 K
calculated
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temperature difference across the flexible cryocooler connection (not shown)
yields
a maximum HTS coil temperature to 22.69 K.
In summary, A 2D axisymmetric stress/strain analysis is performed to
simulate the four states of magnet assembly through operation. Results of the
analysis indicate that the mechanical stress is less than 10 MPA at room
temperature
and 32.6 MPA at 20 K and 400 A. These values are well within the operating
limits
of the HTS material. A 2D axisymmetric and 3D thermal analysis is performed to
determine the maximum temperatures of the coil under steady state operating
conditions. For a 12 W heat load, the 2D and 3D analysis results show a
maximum
SOT of ~ 0.5 K and 3K, respectively. These values are well within the safe
operating envelope of the HTS magnet system.
22
