Support structure for high field magnet coils

A support structure for a superconducting solenoid magnet has a set of five symmetrically and coaxially arranged nonferrous support rings. The central support ring mounts two magnet coils and the other four rings each mount one magnet coil so as to restrain the magnet coils from collapsing toward the center along the magnet bore axis and also to resist radially and circumferentially acting hoop stresses to which the coils are subjected. Tubular struts are symmetrically arranged about the magnet bore axis between adjacent support rings and are seated at their ends in counterbores in the support rings to separate and space apart the support rings. Threaded rods extend through the tubular struts and adjacent support rings to fasten the adjacent rings together.

BACKGROUND OF THE INVENTION 
The field of the invention is support structures for supporting 
electromagnetic coils which are subjected to high axial and radial forces. 
The use of high field magnets, in applications such as . magnetic resonance 
imaging in medical diagnosis, is well known. Such high field magnets are 
typically made with several discrete electromagnetic coils which are held 
coaxially with respect to each other by a common frame. The coils are 
typically superconducting, that is, they are immersed in a cryostat which 
contains liquid helium to cool the magnet coils to approximately 
4.2.degree. Kelvin. At that temperature, the coils have zero resistance 
and are capable of conducting a very high current density and producing 
magnetic field strengths of 1.5 Tesla or more. At such field strengths, 
considerable stress is placed on the magnet coil support structure. 
Adjacent coils have mutual axial attractive forces of 100,000 to 200,000 
lbs. Self repulsion within a coil produces radial or "hoop stresses" of 
200,000-500,000 lbs. 
The magnet support structure must resist these magnetic field related 
stresses, and provide the rigidity necessary to ensure that the magnetic 
field remains uniform over time. This latter requirement was thought most 
recently to require that the magnet support structure be relatively 
massive and inflexible. 
Prior support structure designs have been of a solid walled. barrel shape. 
Such structures have been difficult and expensive to manufacture because 
of the size and weight of the structures which have been used to make up 
the barrel and because of the difficulty of machining such large elements. 
Also, the weight of such designs increased the cost of ancillary magnet 
support structure and limited the use of such magnets to areas where the 
floor would support high loads. The closed construction of such designs 
also made assembly more difficult and obstructed the placement of 
components inside the cryostat. 
Moreover, the rigidity of such magnet support structures may have 
contributed to slippage of the magnet coils relative to the support 
structure when the axial force to which the coils were subjected changed, 
for example, when the magnet was energized, or when there were minor 
misalignments between the coils. 
Also, the construction of the barrel shaped support 10 structure resulted 
in a high mutual inductance between the coils and the support structure by 
providing a path for eddy currents around the support structure's 
circumference and along its length. With such a high mutual inductance, 
changes in the position of the coil with respect to the support structure 
may have resulted in surges in the coil current. Such surges can trigger a 
"quench" of the superconducting magnet in which the magnet reverts to a 
non-persistent or non-superconducting state. 
SUMMARY OF THE INVENTION 
The present invention relates to a support structure for high field 
strength electromagnets and consists of two or more non-ferrous rings 
disposed around and along the magnet bore axis for holding electromagnetic 
coils in compression. Each such non-ferrous ring is separated from the 
adjacent ring by a number of connecting struts oriented parallel to the 
magnet bore axis and spaced around the circumference of the rings. 
It is one object of the invention to provide a readily manufactured, 
lightweight means of supporting coils of a high field magnet. 
A further object of the invention is to reduce mutual inductance between 
the support structure and the magnet coils. The use of struts and the 
orientation of the struts parallel to the magnet axis reduces mutual 
inductance between the magnet coils and the supporting structure. Such 
mutual inductance is believed to interfere with smooth ramping of the 
magnet. 
It is yet another object of the invention to provide a more flexible magnet 
support structure. Such structure is believed to improve magnet ramping by 
accommodating minor alignment errors or sudden changes in axial force. The 
use of low mass struts rather than a more massive support structure 
results in more axial flexibility in the magnet. This reduces the tendency 
of the coils to slip with respect to the support structure, such as may 
occur when the coils are subjected to momentary increases in current and 
hence increases in axial force, or when the coils are subjected to an 
uneven axial force as a result of minor alignment errors during assembly. 
Any such slipping of the coil with respect to its supports during ramping 
increases the possibility that the coil will quench. 
It is a further object of the invention to permit the use of readily 
manufactured laminations to be used for the coil support rings. The use of 
connecting struts to position the rings with respect to each other 
facilitates the construction of the rings from a plurality of laminations, 
such laminations incorporating the required holes and indentations to 
receive the connecting struts and being appropriately sized to define a 
shoulder against which the magnet coil may seat. 
The foregoing and other objects and advantages of the invention will appear 
from the following description. In the description, reference is made to 
the accompanying drawings which form a part hereof, and in which there is 
shown by way of illustration a preferred embodiment of the invention. Such 
embodiment does not necessarily represent the full scope of the invention, 
however, and reference is made therefore to the claims herein for 
interpreting the scope of the invention.

DETAILED DESCRIPTION OF THE INVENTION 
Referring particularly to FIGS. 1 and 3, a magnet support structure 
according to the teachings of the present invention includes a set of 
magnet coil support rings 1-5 which are spaced apart along a central 
solenoid magnet bore axis 6 and are aligned concentrically therewith. The 
support rings 1-5 are separated apart and fastened together by sets of 
supporting struts 7-10. The struts in each set 7-10 are disposed around 
the central magnet bore axis 6 and extend in a direction parallel to the 
axis 6 to connect with the associated supporting rings 1-5 at their ends. 
The support structure is preferably made of non-magnetic materials. For 
example, the rings 1-5 and the struts 7-10 in the preferred embodiment are 
made of aluminum. 
The magnet of the preferred embodiment is symmetric about the bore axis 6, 
and is also symmetric along the axis 6 about the central support ring 3. 
As a consequence, the support structure is also symmetric about the 
central support ring 3, with the struts 8, intermediate support ring 2, 
struts 7 and end support ring 1 being a mirror image of the respective 
struts 9, intermediate support ring 4, struts 10 and end support ring 5. 
The construction of rings 1-3 and their associated strut sets 7 and 8 will 
now be described in detail, however, the same description also applies to 
the symmetrical rings 3-5 and associated struts 9 and 10. 
Referring to FIG. 4 there is shown a cross section taken through end ring 1 
and one of the struts 7. A strut 7 is tubular and a threaded non-magnetic 
(e.g. brass) rod 11 extends through the strut 7. End ring 1 has a 
longitudinal throughbore 15 into and beyond which the rod 11 extends. A 
counterbore 17 is provided at the axially outer end of the through-bore 15 
and a counterbore 16 is provided at the axially inner end of the 
through-bore 15. The end of strut 7 is received in a sliding fit in the 
counterbore 16 and a nut 18, which is screwed onto the rod 11, is housed 
in the counterbore 17. Prior to insertion, the end of strut 7 is coated 
with a low temperature epoxy which bonds the strut 7 to the end ring 1 to 
prevent loosening of the nut 18. 
An electromagnetic coil 20 is received on the radially inside surface 19 of 
end ring 1. The diameter of the radial inside surface 19 is sized so that 
the magnet coil 20 may be readily inserted into end ring 1 when end ring 1 
and magnet coil 20 are at room temperature. When magnet coil 20 and end 
ring 1 are cooled to superconducting temperatures, the high coefficient of 
expansion of the aluminum of end ring 1 compared to the coefficient of 
expansion of the epoxy and wire of magnet coil 20 causes magnet coil 20 to 
be held tightly in compression by end ring 1. The compressional stress 
generated by end ring 1 helps the coil 20 to resist high hoop stresses 
resulting from the internal self-repulsion of magnet coil 20 when 
energized. The compression of magnet coil 20 by end ring 1 also provides a 
tight friction fit to hold magnet coil 20 against axial movement as a 
result of magnetic forces between magnet ring 20 and adjacent coils in the 
magnet support structure. Further restraint against axial movement by 
magnet coil 20 is provided by the clamping means described below. 
Radially inside surface 21 of end ring 1 forms a lip 22 (FIGS. 4 and 7) 
which extends radially inward around the entire inside circumference of 
ring 1 to define a shoulder or land 23 which faces axially outward. A set 
of clamps 24 are fastened to the axially outer face of the ring 1 by bolts 
25 which are bolted to the ring 1. The clamps 24 are disposed around the 
circumference of the ring 1 to bear on and secure a magnet coil 20 against 
the land 23 when the clamps 24 are tightened against the magnet coil 20. 
The magnet coil 20 is thus securely fastened to the inner surface of the 
ring 1 and restrained further from moving in the axial direction. It is 
noted that the clamps 24 are primarily for holding the magnet coils when 
the coils are deenergized and also to position the coils before shrinking 
the rings around the coils. When the coils are energized, magnetic forces 
on the coils pull them in the axial direction toward one another and 
toward the radial central plane of the assembly so as to shorten the 
assembly or collapse it toward the middle. The position of the clamps 
described above corresponds to co-currents in the magnet coils, e.g., 
where the current in adjacent coils flows in the same direction, clockwise 
or counterclockwise around the bore. If the currents in adjacent coils may 
be counter to one another, as in the case of an actively shielded coil, 
the clamp and lip would have to be on the other face of the magnet. 
As shown in FIG. 5, the construction of the intermediate magnet coil 
support ring 2 is virtually the same as that of ring described above. The 
struts 7 are bonded in counterbores in the axial outer face of ring 2 and 
are fastened with nuts 18 along the axially inner face of ring 2 (shown in 
FIG. 2). Similarly, tubular struts 8 are bonded in counterbores 26 formed 
around the circumference of the axial inside face 27 of the ring 2, and 
are attached to the ring 2 by threaded rods 12 which extend through the 
struts 8 and the ring 2 and are fastened with nuts 18 at the axially outer 
face of the ring 2. The nuts 18 are not recessed in counterbores because 
the nuts on ring 2 do not add to the length of the magnet which is 
desirably as short as possible. The ring 2 is cold shrunk around the coil 
30 and the radially inside periphery 27 of ring 2 defines a lip 28 having 
an axially outward facing land 29 against which a magnet coil 30 is 
secured by clamps 31 in the manner previously described. 
As shown in FIG. 6, the construction of ring 3 is substantially different 
from the rings 1 and 2, since it holds two magnet coils 34 and 35. The 
close proximity of these two coils 34 and 35 obviates the need for 
connecting struts and instead ring 3 is elongated in the axial direction 
to hold both coils 34 and 35. Struts 8 are affixed to ring 3 by means of a 
tapped, blind hole 36 with a counterbore 37. The end of strut 8 to be 
connected to ring 3 is painted with epoxy and fitted into counter bore 37 
so that the epoxy bonds the strut to the rings. The corresponding end of 
the threaded rod 12 is threaded into tapped hole 36. Coil 34 is secured to 
the support ring 3 by a shrink fit as previously described and a radially 
inwardly extending lip 38 which defines an axially outwardly facing land 
39. Clamps 40 also are used which bear on the outboard faces of the coil 
34. 
Each support ring 1-5, in the preferred embodiment, is formed and fitted by 
a shrink fitting process to the coils so as to constrain movement of the 
coils during operation of the magnet and to aid the coils in resisting 
hoop stresses at their rated currents as previously described and as is 
described in U.S. Pat. No. 4,467,303 which is hereby incorporated by 
reference. 
In one embodiment described above, each ring 1-5 is machined from a solid 
casting or forging. In an alternate embodiment each such ring may be 
composed of a number of laminations having the correct sequence of holes 
and then fastened together by means of auxiliary bolts and nuts and the 
connecting struts and threaded rods. FIG. 8 shows this technique as 
applied to ring 1. Laminations 45-47 and 50-53 include oversized holes 55 
which form the receiving counterbores for nuts 18 and struts 7. 
Laminations 48 and 49 have smaller holes 56 slightly larger than the 
outside dimension of threaded rod 11. End laminations 52 and 53 have a 
somewhat smaller inner radius to form lip 22 and land 23. 
The struts 7-10 are spaced uniformly around the periphery of each ring 1-5 
so as to share equally the compressive force resulting from the mutual 
attraction of each contained magnet coil. The struts are staggered to aid 
in assembly of the structure. Ten such struts separate each ring 1-5 in 
the preferred embodiment, however, the number of struts 7-10 may be varied 
if the individual strut strength is correspondingly varied. For a 1.5 
Tesla magnet with a bore of approximately 50", the outermost rings 1 and 8 
are separated by struts of approximate length 14" which must be 2.0" in 
diameter to accommodate a centerbored hole of 1.0" and provide adequate 
strength. Rings 2 and 4 are separated from ring 3 by struts of approximate 
length 9" but which maintain the same diameter despite their 
proportionally higher bending strength for reasons of manufacturing 
convenience. The particular dimensions of the magnet bore and the 
separation of the rings and the rings' diameter may be varied according to 
the desired characteristics of the magnetic field to be created. For any 
given magnet configuration the following is a description of the method 
for determining the acceptable strut strength, dimensions and number for 
the weakest strut set in accordance with the present invention. 
The minimum size of the struts is governed by the elastic buckling limit 
for a set of columns represented by these struts. This buckling limit may 
be determined as follows. Referring to FIG. 9 there is shown a schematic 
view of a three-coil structure as seen along a line perpendicular to the 
magnet bore axis, z. In general, a multi-ring magnet may be decomposed 
into such three ring elements. In practice, the following equations are 
used to determine the required strength of the outermost struts 7 
connecting rings 1, 2 and struts 10 connecting rings 4 and 5. These struts 
7 and 10 are most prone to buckling both because of their high slenderness 
ratio and the high total compressive force between rings 1, 2 and rings 4, 
5. The same number of struts and strut diameter may be used for struts 8, 
9 between rings 2, 3 and rings 3, 4. The lower slenderness ratio of struts 
8 and 9 insures that these struts have ample resistance to buckling. 
The three rings 60-62 shown in FIG. 9 are separated and supported by struts 
63 and struts 64. The rings are subject to a compressive force P and a 
transverse force Q resulting from a magnetic interaction between the coils 
contained by rings 60-62. L represents the distance between each 
supporting ring and y represents the deflection resulting from the 
compressive force P and the transverse force Q. As a result of this 
deflection, the magnetic field B.sub.o is curved within the bore of the 
magnet. The radius of that curvature is denoted R. 
The buckling limit for the generalized structure shown in FIG. 3 is given 
by the following variation of Euler's elastic buckling equation for 
pivoted struts under normal load: 
##EQU1## 
where: y=transverse displacement of the center ring 
L=distance between rings, 
P=axial buckling force, 
Q=transverse force on the central ring due to unit displacement of the 
central ring in the transverse direction, 
N=number of struts between two rings, 
z=axial distance 
I=moment of inertia for the struts, 
E=modulus of elasticity for the strut material, 
Solving this differential equation yields the following formula which 
provides guidance as to the rigidity and minimum number of struts 
necessary to resist buckling. 
##EQU2## 
The transverse force Q may be estimated as follows: When the central ring 
is displaced by an amount y, the curvature of the field lines is given by: 
##EQU3## 
The gradient of the field is then given by the expression: 
##EQU4## 
Where: 
B=the nominal axial magnetic field strength. The transverse force on the 
center ring due to this gradient is given by: 
EQU F=.mu..gradient.B (5) 
Where: 
##EQU5## 
Where: N.sub.t I=ampere turns of the magnet coil, 
a=average radius of the magnet coil, and 
B=the nominal magnet field strength. Therefore: 
##EQU6## 
The term QyL, in equation 1 and 2 is not present in a typical column under 
a constant load, such as an architectural column, but is the result of the 
magnetic interaction of the coils. As should be noted, this term may 
increase as the deflection y increases, depending on the orientation of 
the magnet coils, creating a potential instability or non-linear "kink" 
mode. 
The above described procedure is used to determine the number, material and 
dimension of strut sets 7 and 10 which are the longest struts and under 
the most compression and hence most susceptible to buckling. As mentioned 
above, the same number, material and diameter of struts is used for strut 
sets 8 and 9, for reasons of manufacturing convenience. Accordingly, 
struts 8 and 9 are insured of being even more resistant to buckling than 
strut sets 7 and 10. This makes the center portion of the support 
structure from ring 2 to ring 4 more rigid than the end portions. Within 
the center portion, the ring 3, being made unitary, is stiffer than the 
portion between the ring 3 and the rings 2 and 4. Thus, the support 
structure becomes progressively more rigid toward the axial center. 
It should be understood the minimum strut strength indicated by Equation 1 
may be increased to provide an acceptable margin of safety. It should be 
noted that equation 1 assumes that the struts are free to pivot while in 
fact they are seated and bonded in counterbores in the supporting rings. 
Moreover, the threaded rods preload each strut affording the struts a 
somewhat higher resistance to transverse bending forces than is assumed 
under these formulas. 
A magnet support frame of the invention has now been fully described. This 
construction can be readily manufactured from common materials. Each ring 
1-5 may be cast separately in a much smaller mold than required if all the 
rings were unitary. Subsequent machining operations are also easier 
because of the relatively convenient size of each ring. Moreover, even 
after the rings are produced, latitude and modifications in the design of 
the magnet is provided for simply by changing the length of the sets of 
struts. Moreover, the open design of the support structure has the 
additional advantage of making it relatively easy to mount ancillary 
devices which may be included with the products, such as switches, 
heaters, resistors and other equipment. The second embodiment disclosed, 
having laminated support rings, lends itself to further manufacturing 
efficiencies (e.g. a stamping process may be employed). 
In ramping and operation of the superconducting magnet, the construction of 
a support structure of the invention also reduces the mutual inductance 
between the magnet coils and the supporting structure. The open 
construction of the support structure reduces the possible eddy current 
paths through the support structure to minimize any interference with 
ramping otherwise caused by such eddy currents and resulting mutual 
inductance. 
The invention also provides a more flexible magnet support structure. 
Magnets made using a support structure of the invention have been found to 
ramp smoothly. It is believed that this is at least partly due to the fact 
that the support structure is somewhat less rigid than prior designs and 
allows for some flexure and small movements to accommodate and correct for 
any misalignments of the magnet coils. Thus, although the frame is rigid, 
some self adjustment is inherent in the construction of a support 
structure of the invention. Many modifications and variations of the 
preferred embodiment which will still be within the spirit and scope of 
the invention will be apparent to those of ordinary skill in the art. For 
example, although the central ring 3 of the preferred embodiment is shown 
as a unitary part, it could be made as two separate rings spaced apart by 
suitable struts. Another example is that a support structure of the 
invention could be adapted to any number of magnet coils. Thus, the 
invention is not limited to the preferred embodiment but is defined by the 
claims which follow.