Superconducting magnet having a support structure for ring-shaped superconductive coils

A superconducting magnet includes a coil support structure comprising a body formed of a generally-cylindrical wall. The body includes at least a pair of surfaces facing inwardly of said body, each being of generally-cylindrical shape and each being adapted to abut the radially-outer surface of a respective one of a pair of ring-shaped, superconductive coils. The support structure comprises a material such as aluminum which is adapted to shrink around the coils as the body and coils are cooled to a cryogenic temperature, so as to provide interference fits between the body and the coils. To prevent movement of the coils towards each other due to magnetic attraction forces therebetween, the body includes a pair of shoulders extending inwardly from respective adjacent edges of the pair of inwardly-facing surfaces. The shoulders are adapted to abut confronting surfaces of the pair of coils to prevent the coils from moving towards each other.

BACKGROUND OF THE INVENTION 
My invention relates to superconducting magnets, and more particularly to 
support structures for ring-shaped superconductive coils. 
A plurality of ring-shaped superconductive coils having a common 
longitudinal axis are useful in establishing a large magnetic field, 
aligned with such longitudinal axis, in the vicinity of the axis. Such a 
magnetic field is required, for example, in nuclear magnetic resonance 
(NMR) imaging apparatus. In such apparatus, the subject of inquiry, such 
as an entire human body, is placed in the region of high magnetic field 
and exposed to selected radio-frequency electromagnetic waves. The atoms 
of the human body re-emit radio-frequency electromagnetic waves at 
different frequencies depending on the identity of the atoms. Detection of 
such different frequencies reveals the identity of the atoms, and this 
information is used to generate images of internal body structures. 
Superconductive coils must be maintained below a critical, extremely low 
temperature, typically about 4.degree. Kelvin (i.e., 451.degree. Farenheit 
below zero) in order to be superconducting. When superconducting, these 
coils are capable of carrying electric current at an extremely high 
density, permitting the attainment of extremely high magnetic fields. Any 
heating of a superconducting coil that raises the temperature of even a 
single, localized part of the coil above the critical temperature results 
in the entire coil losing superconductivity due to rapidly spreading 
I.sup.2 R or resistive heating of the coil. 
A ring-shaped superconductive coil typically comprises a winding of 
superconductive wire impregnated with epoxy so as to form a monolithic 
structure. The epoxy-impregnation in large measure prevents relative 
movement between the individual wires of the winding that would give rise 
to undesirable frictional heat generation. 
Heat generation in a superconductive coil can also result from interaction 
of the coil and its support structure which maintains the coil in a 
desired position. A first component of such heat generation is the 
frictional heat generated from relative sliding movement between the coil 
and the support structure. A second component of such heat generation 
results from relative movement of internal coil strands, typically 
transitory and of minute magnitude, which is induced from high stress in 
the winding, either compressional or tensile in nature. These transient 
movements give rise to a phenomenon known in the art as "training" of the 
coil. A coil in the process of training experiences interior movement 
during coil energization, even though it is epoxy-impregnated. Such 
movement causes the coil to become frictionally-heated above the critical 
temperature and lose superconductivity at a lower than rated current 
density, and also requires cessation of current therethrough to avoid 
excessive resistive heating of the coil. Upon cooling of the coil below 
the critical temperature and reenergization thereof, it typically 
experiences further interior coil movement, although usually less 
pronounced, again resulting in frictional heating of the coil above the 
critical temperature; however, this occurs at a higher current density if 
the interior coil movement is less. Cycles of cooling and reenergization 
of the coil continue, with the coil attaining higher current densities as 
interior coil movement subsides. In this way, coils are "trained" to 
withstand the high stresses imposed upon them, although such training is 
not always permanent. 
Training of a coil is not without considerable costs arising, for example, 
from the required cycles of coil cooldown and reenergization. 
Additionally, the container in which the cooling medium for the coil 
(i.e., the cryostat) is located must be designed to withstand repeated 
heating and pressure cycles, so as to prevent loss of cooling medium. 
Thus, it would be desirable to provide a coil support structure that 
interacts with a coil in such a way as to reduce stresses in the coil that 
would require training of the coil. 
A commercially-available superconducting magnet including ring-shaped 
superconductive coils utilizes a support structure comprising an aluminum 
body of generally-cylindrical shape and which has outwardly-opening 
recesses extending around the circumference thereof. Ring-shaped 
superconductive coils are manufactured in the recesses by winding 
superconductive wire directly into the recesses and then impregnating the 
resulting winding with epoxy. These coils, however, are undesirably 
subject to two different sources of heating, both arising from the fact 
that a ring-shaped superconductive coil tends to expand in diameter upon 
energization. One potential source of heating constitutes relative sliding 
movement between the axial sides of the coil and the adjacent walls of the 
recess in which the coil is formed. Another potential source of heating 
results from the high tensile stresses that arise in the expanded coil and 
undesirably may require that the coil be trained. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of my invention to provide a superconducting 
magnet including a support structure for ring-shaped superconductive coils 
wherein relative sliding motion between the coils and the support 
structure is greatly reduced. 
Another object of my invention is to provide a superconducting magnet 
including a support structure for ring-shaped superconductive coils 
wherein stresses in the coil are reduced to such a degree that the need 
for training of the coils is substantially eliminated. 
In accordance with a preferred embodiment of my invention, I provide a 
superconducting magnet in which support structure for ring-shaped 
superconductive coils comprises a body formed of a generally-cylindrical 
wall. The body includes at least first and second inwardly-facing surfaces 
of generally-cylindrical shape, which are adapted to abut the 
radially-outer surfaces of first and second ring-shaped supercondutive 
coils, respectively. The support structure body comprises a material such 
as aluminum which is adapted to shrink around the first and the second 
coils as the body and coils are being cooled to a cryogenic temperature. 
Consequently, interference fits between the body and the coils are 
provided. To prevent the first and second coils from moving towards each 
other due to electromagnetic attraction forces between these coils, the 
body includes first and second shoulders extending inwardly from the 
adjacent edges of the first and second surfaces, respectively. These 
shoulders are adapted to abut the adjacent surfaces of the first and 
second coils so as to restrain movement of the coils towards each other. 
In practice I have found that there is very little movement, if any at all, 
between the coils and the support structure, inasmuch as the coils have 
not lost superconductivity despite repeated energization. Additionally, 
the coils do not require training to reach their rated current. This 
apparently is because the compressional forces exerted on the coils by the 
support structure substantially counterbalance the tensile forces that 
would otherwise tend to arise in the coils upon their being energized.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates a preferred support structure in accordance with my 
invention. The support structure comprises a body 10 formed of a 
generally-cylindrical wall 11, a longitudinal axis 12 being located 
inwardly of body 10. Also illustrated in FIG. 1, in schematic form, are 
the following ring-shaped superconductive magnet coils: axially-inner 
coils 14C and 16C, and axially-outer coils 18C and 20C. Body 10 includes 
inwardly-facing surfaces 14S, 16S, 18S and 20S, each of a 
generally-cylindrical shape, preferably as close to being exactly 
cylindrical as possible, which surfaces abut the radially-outer surfaces 
of coils 14C, 16C, 18C and 20C, respectively. Body 10 is comprised of a 
nonmagnetic material selected to shrink as it is cooled to a cryogenic 
temperature, which depends upon the properties of the coils (typically 
about 4.degree. Kelvin for niobium-titanium superconductors), so as to 
provide interference fits between body 10 and the coils. 
Due to the interference fits between body 10 and coils 14C, 16C, 18C and 
20C, I have found that, during operation, the hoop stress in each coil is 
markedly reduced from what it would be if the coils were not restrained by 
interference fits with body 10 such as describes the prior art structure 
discussed above. This can be quantitatively appreciated as follows. In the 
absence of coil energization, for a superconducting magnet with a magnetic 
field intensity of 0.8 Tesla and a current density through the aggregate 
cross-sections of the coils of 30,000 amps/cm.sup.2, (hereinafter, "0.8 
Tesla magnet"), the compressional stress on the coils, resulting from the 
interference fit of body 10, is approximately 12,000 psi at 4.degree. 
Kelvin. Upon coil energization, however, the coils tend to expand, giving 
rise to a tensile hoop stress in the coils, which, to a high degree, 
counteracts the compressional stress due to the interference fit. The net 
stress on the coils comprises a compressive hoop stress of only about 
3,000 psi. 
Aluminum is particularly preferred for body 10, because the coils can then 
be readily inserted into body 10 of aluminum at room temperature, and then 
cooled to a cryogenic temperature to provide interference fits with the 
coils. Nonmagnetic materials other than aluminum can be utilized, however, 
such as copper, brass, bronze or fiberglass. The use of any of these 
alternative materials for body 10, however, typically requires that the 
coils be inserted into body 10 at a temperature considerably higher than 
room temperature, inasmuch as these materials shrink less than does 
aluminum for the same drop in temperature. 
In order to achieve a high homogeneity of magnetic field in the vicinity of 
longitudinal axis 12, it is desirable that support structure body 10 be 
symmetrical about a plane of symmetry 22 located orthogonal to 
longitudinal axis 12. Thus, for example, axially-inner coil 14C is 
symmetrically positioned with respect to axially-inner coil 16C, and 
axially-outer coil 18C is symmetrically positioned with respect to 
axially-outer coil 20C. While support structure body 10, as illustrated, 
is designated to accommodate four coils (i.e. coils 14C, 16C, 18C and 20C) 
it could accommodate other symmetrically arranged numbers of coils. By way 
of example, a further coil could be positioned directly on plane of 
symmetry 22, or a pair of additonal coils could be provided which are 
spaced symmetrically about plane of symmetry 22. Other variations will be 
apparent to those skilled in the art. 
Body 10 comprises mid-section 24 and end-sections 26 and 28, which are 
tightly joined to mid-section 24, as with rabbet joints 30 and 32, 
respectively. As can best be seen in the detail view of FIG. 2, rabbet 
joint 30 comprises the left axial end of mid-section 24 inserted into an 
axially-opening groove or rabbet 32 in end-section 26 and secured in 
rabbet 32, as with, preferably a plurality of bolts 34, extending from 
flange 38 of end-section 26 into the left axial end of mid-section 24, 
and, further, preferably a plurality of alignment pins 36 similarly 
extending from flange 28 into mid-section 24. In addition to providing 
rabbet 32 for implementing rabbet joint 30, outwardly-extending flange 38 
also provides added structural rigidity to end section 26 which is useful 
in maintaining the cylindrical shape of end-section 26, whereby high 
magnetic homogeneity is maintained in the vicinity of longitudinal axis 
12. 
To prevent inner coils 14 and 16, shown in FIG. 1, from movement towards 
each other due to electromagnetic attraction forces therebetween, which 
typically exceed 10,000 pounds for a 0.8 Tesla magnet, shoulders 40 and 42 
of body mid-section 24 extend inwardly from adjacent edges of first and 
second surfaces 14S and 16S, respectively. As best illustrated in the 
detail view of FIG. 3, shoulder 40 thereby overlaps coil 14C for a short 
distance, preventing movement to the right of coil 14C. With coil 14C 
overlapping such a short projection 40, it may be thought that coil 14C 
would rotate to the position shown in phantom at 43, due to the 
electromagnetic attraction forces pulling coil 14C to the right. However, 
in a 0.8 Tesla magnet, I have found this not to be the case, apparently 
due to the tight interference fit between body surface 14S and coil 14C. 
To ensure that coil 14C tightly abuts shoulder 40, a series of wedges 44 
are disposed within inwardly-opening recess 46 of body mid-section 24, 
spaced along the circumference thereof. Wedge 44 is retractable into 
recess 46, for example, by means of a bolt 48 threaded into wedge 44 from 
the outer surface of mid-section 24. Wedge 44 can be retracted into recess 
46 by turning bolt 48 in an appropriate direction. This causes wedge 44 to 
press against coil 14C and push it tightly against shoulder 40. A 
belleville or spring-like washer 50 is preferably provided around bolt 48 
to allow wedge 44 freedom of movement during any transient movement of 
coil 14C while it is being cooled down to a cryogenic temperature. 
By tightly pressing coil 14C against shoulder 40, wedge 44 (FIG. 3) 
prevents undesirable movement between surface 14S and coil 14C, and also 
prevents the buildup of high shear stress therebetween. Advantageously, 
bolt 48 can be turned so as to retract wedge 44 into recess 46 after 
support structure body 10 is assembled. 
Considering again FIG. 1, support structure end-sections 26 and 28 include 
inwardly-extending shoulders 52 and 54, repectively, for restraining the 
respective coils 18C and 20C from movement towards each other due to 
electromagnetic attraction forces therebetween. Shoulders 52 and 54 are 
essentially similar in construction to shoulders 40 and 42, discussed 
above. To hold coil 18C tightly against shoulder 52, a plurality of 
L-shaped backets 56 are provided, as best illustrated in the detail view 
of FIG. 4. As shown in FIG. 4, brackets 56 are secured to the inner 
surface of body end-section 26, such as with bolts 60. 
Referring back to FIG. 1, the wall of support structure body 10 in the 
vicinity of surfaces 14S, 16S, 18S and 20S should be varied in thickness 
along longitudinal axis 12 so as to maintain substantially unchanged the 
configuration of these surfaces during cooldown of body 10 to a cryogenic 
temperature. This is necessary to prevent distortion of the coils and 
resultant high stresses in the coils. Distortion of the axially-outer 
coils, for example coil 18C, can be substantially eliminated by including 
flange 61 in support structure end-section 26, which can be better 
appreciated by considering the detail view of FIG. 5, illustrating in 
enlarged form flange 61 and coil 18C. Flange 61 advantageously prevents 
end portion 26 from contracting during cooldown in such a way that it 
would tend to become distorted in shape as shown in phantom at 64, with 
coil 18C likewise tending to become distorted in shape as shown in phantom 
at 66, because coil 18C would be forced to assume the shape of surface 68. 
The inclusion of flange 61 in end-section 26 markedly reduces such 
distortion of coil 18C from occurring, because the large effective mass of 
flange 61 results in the illustrated portion of end-section 26 (FIG. 5) 
contracting at the same rate at its left side as at its right side. 
As mentioned in the foregoing discussion, the provision of flange 61 in 
end-section 26 (FIG. 1) markedly reduces distortion of coil 18C. However, 
as illustrated in the enlarged, detail view of FIG. 6, end-section 26 
still tends to contract upon cooldown in such a way as to tend to become 
distorted in shape to the position shown in phantom at 70, since the 
portion of wall 11 in the vicinity of surface 18S is not rigid enough to 
fully prevent such distortion. Accompanying this distortion, the 
configuration of surface 18S tends to change to that shown in phantom at 
72. This results in compressional stresses on coil 18C that are maximum at 
its upper right portion and at its upper left portion, as viewed in FIG. 
6. To hold this unfavorable result to within innocuous limits, the 
thickness of wall 11 of end-section 26 should be varied along longitudinal 
axis 12 (FIG. 1) so as to minimize the disparity between distorted surface 
72 and surface 18S. Specifically, the thickess of the wall of end-section 
26 should, in general, increase from centerline 74 of end-section 26 (FIG. 
6) to the left, as viewed in FIG. 6. From viewing end-section 26 as 
depicted in FIG. 6, it can be appreciated that flange 61 constitutes a 
portion of wall 11 of end-section 26 that is considerably greater in 
thickness than the illustrated portions of end-section 26 to the right of 
flange 61. In particular the portion of wall 11 of end-section 26 that 
extends radially-outwardly from surface 18S is thicker than the portion of 
wall 11 at the centerline 74 of end-portion 26. 
Referring again to FIG. 1, it is desirable that the portions of wall 11 of 
mid-section 24 in the vicinities of surfaces 14S and 16S, respectively, be 
varied along longitudinal axis 12 so as to maintain substantially 
unchanged the configuration of surfaces 145 and 165 during cooldown of 
mid-section 24 to a cryogenic temperature. Considering surface 14S, for 
instance, it can be seen that the thickness of the portion of wall 11 of 
mid-section 24 immediately radially-outwardly from surface 14S is greater 
in thickness than the portion of wall 11 of mid-section 24 near the center 
thereof (i.e., near plane of symmetry 22). 
In order to enable superconducting coils 14C, 16C, 18C and 20C to be cooled 
to a cryogenic temperature, such as 4.degree. Kelvin, support structure 
body 10 is placed in a cryostat 88 (shown diagrammatically and partially 
broken away), in which the coils, along with body 10, are entirely 
immersed in a coolant such as liquid helium. Body 10 can be suitably 
secured to the housing (not shown) of cryostat 88 as by attaching end 
flanges 60 and 62 thereto. 
Support structure body 10 is especially adapted for use where axially-inner 
coils 14C and 16C are larger in outside diameters than axially-outer coils 
18C and 20C. This is because body end-sections 26 and 28 are separable 
from mid-section 24 in order to permit insertion of inner coils 14C and 
16C into mid-section 24 prior to the attachment of end-sections 26 and 28 
onto mid-section 24. Where, however, it is desired to support 
axially-outer coils larger in outside diameters than axially-inner coils, 
a support structure 110 as shown in FIG. 7, may advantageously be 
utilized. 
Referring now to FIG. 7, it can be seen that body 110 comprises a unitary 
body formed of a generally-cylindrical wall 111, with a longitudinal axis 
112 being located inwardly of body 110. Ring-shaped coils 114C, 116C, 118C 
and 120C are supported in body 110 through interference fits with body 
surfaces 114S, 116S, 118S and 120S, respectively. To obtain the maximum 
homogeneity of magnetic field in the vicinity of longitudinal axis 112, it 
is preferably that body 110 be symmetrical about a plane of symmetry 122 
orthogonal to longitudinal axis 112. 
Body 110 includes shoulders 124 and 126 restraining motion of coils 114C 
and 116C towards each other, and similarly includes shoulders 128 and 130 
restraining motion of coils 118C and 120C towards each other. 
In contrast to the embodiment of FIG. 1, the thickness of wall 111 of body 
110 in the vicinity of surface 114S is not greater than the thickness of 
such wall in the vicinity of line of symmetry 122. This is because wall 
111 of body 110 is considerably thicker than wall 11 of body 10 (FIG. 1), 
resulting in surface 114S remaining substantially unchanged in shape 
during cooldown of body 110 to a cryogenic temperature. With regard to 
surface 118S, on the other hand, the thickness of the wall in the vicinity 
thereof does increase to the left along longitudinal axis 112, as viewed 
in FIG. 7. That is, flange 132 constitutes a portion of wall 111 with 
greater thickness than the portion of wall 111 near plane of symmetry 122. 
Flange 132 is instrumental in maintaining substantially unchanged the 
configuration of surface 118S during cooldown of body 110 to a cryogenic 
temperature. Flange 134 of body 120 similarly maintains the configuration 
of surface 120S substantially unchanged, inasmuch as flange 134 is 
symmetrical to flange 132, at least in the preferred embodiment. 
To cool to a cryogenic temperature coils 114C, 116C, 118C and 120C and also 
body 110, a cryostat 136, shown in diagrammatic form, is utilized. 
The foreging desribes support structures for ring-shaped superconducting 
magnet coils that greatly reduce sliding motion between the coils and 
support structure, and that, further, substantially eliminate high 
stresses in the coils that would give rise to the necessity of training 
the coils. Additionally, the support structures interact with the coils in 
such a way as to minimize distortion of the coils that would give rise to 
inhomogeneities of the magnetic field induced within the ring-shaped 
coils. 
While my invention has been described with respect to specific embodiments, 
many modifications and substitutions thereof will be apparent to those 
skilled in the art. It is, therefore, to be understood that the appended 
claims are intended to cover all such modifications and substitutions as 
fall within the true spirit and scope of the invention.