U.S. patent application number 13/354495 was filed with the patent office on 2012-06-14 for method of manufacturing a solenoidal magnet, and a solenoidal magnet structure.
This patent application is currently assigned to Siemens Plc.. Invention is credited to Graham HUTTON, Jonathan Noys, Adrian Mark Thomas.
Application Number | 20120149580 13/354495 |
Document ID | / |
Family ID | 41058055 |
Filed Date | 2012-06-14 |
United States Patent
Application |
20120149580 |
Kind Code |
A1 |
HUTTON; Graham ; et
al. |
June 14, 2012 |
Method of Manufacturing a Solenoidal Magnet, and a Solenoidal
Magnet Structure
Abstract
A method of manufacturing a solenoidal magnet structure,
comprising the steps of providing a collapsible mold in which to
wind coils; winding wire into defined positions (88) in the mold to
form coils (34); placing a preformed tubular mechanical support
structure (102, 120) over the coils (34) so wound; 10 impregnating
the coils and bonding them to the mechanical support structure by
applying a thermosetting resin and allowing the thermosetting resin
to harden; and collapsing the mold and removing the resultant
solenoidal magnet structure comprising the resin impregnated coils
and the mechanical support structure from the mold as a single
solid piece.
Inventors: |
HUTTON; Graham;
(Oxfordshire, GB) ; Noys; Jonathan; (Oxfordshire,
GB) ; Thomas; Adrian Mark; (Oxfordshire, GB) |
Assignee: |
Siemens Plc.
Frimley
GB
|
Family ID: |
41058055 |
Appl. No.: |
13/354495 |
Filed: |
January 20, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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12832675 |
Jul 8, 2010 |
|
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13354495 |
|
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Current U.S.
Class: |
505/211 ;
335/216; 335/296 |
Current CPC
Class: |
G01R 33/381 20130101;
Y10T 29/49071 20150115; Y10T 29/49014 20150115; H01F 41/048
20130101; Y10T 29/49155 20150115; H01F 41/127 20130101; Y10T
29/49073 20150115; H01F 6/06 20130101; G01R 33/3802 20130101; Y10T
29/49075 20150115 |
Class at
Publication: |
505/211 ;
335/296; 335/216 |
International
Class: |
H01F 6/06 20060101
H01F006/06; H01F 5/00 20060101 H01F005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 16, 2009 |
GB |
0912367.0 |
Claims
1. A solenoidal magnet structure, comprising: wire wound into
coils; a preformed tubular mechanical support structure located
over the coils, the whole structure being monolithically
impregnated with a thermosetting resin.
2. A structure according to claim 1, wherein the support structure
is preformed by assembly of a plurality of sections, each of
arcuate radial cross-section.
3. A structure according to claim 1, wherein the preformed tubular
mechanical support structure carries an axial split.
4. A structure according to claim 3, wherein the tubular mechanical
support structure grips the outer diameter of the coils.
5. A solenoidal magnet structure, according to claim 1, wherein
further electrical leads are retained within the thermosetting
resin.
6. A solenoidal magnet structure, according to claim 1, wherein the
wire is a superconducting wire.
7. A solenoidal magnet structure, according to claim 1, wherein
some cons are of different outer diameter from others, and
differences in the outer diameters of the coils are compensated by
use of a filler layer.
8. A solenoidal magnet structure, according to claim 7, wherein the
filler layer comprises a layer of resin-impregnated cloth.
9. A solenoidal magnet structure according to claim 1, wherein
axially inner coils are of different outer diameter from axially
outer coils, and separate mechanical support structures are
provided for the axially inner coils and the axially outer
coils.
10. A adenoidal magnet structure according to claim 9, wherein an
axially inner mechanical support structure supports axially inner
coils, and axially outer mechanical support structures support the
axially outer coils.
11. A solenoidal magnet structure according to claim 9, wherein a
first mechanical support structure, of a first internal diameter,
supports axially inner coils, and a second mechanical support
structure of a second internal diameter greater than the first
internal diameter, supports the axially outer coils.
Description
[0001] The present invention relates to a method of manufacture of
solenoidal magnet coils, and to solenoidal magnet coils themselves.
In particular, it relates to such coils for generating high
strength magnetic fields, which may be applied in systems such as
nuclear magnetic resonance (NMR) or magnetic resonance imaging
(MRI).
[0002] FIGS. 1A-1B illustrate cross-sectional and axial sectional
views, respectively, of a conventional solenoidal magnet
arrangement for a nuclear magnetic resonance (NMR) or magnetic
resonance imaging (MRI) system. A number of coils 34 of
superconducting wire are wound onto a former 1. The resulting
assembly is housed inside a cryogen vessel 2 which is at least
partly filled with a liquid cryogen 2a at its boiling point. The
coils are thereby held at a temperature below their critical
temperature.
[0003] The former 1 is typically constructed of aluminum, which is
machined to ensure accurate dimensions of the former, in turn
ensuring accurate size and position of the coils which are wound
onto the former. Such accuracy is essential in ensuring the
homogeneity and reliability of the resultant magnetic field.
Superconducting magnets may quench due to even a small amount of
movement of even one turn of the coil. The formers must therefore
be very rigid. These requirements combine to render the production
of formers very expensive.
[0004] Also illustrated in FIGS. 1A-1B are an outer vacuum
container 4 and thermal shields 3. As is well known, these serve to
thermally isolate the cryogen tank from the surrounding atmosphere.
Insulation 5 may be placed inside the space between the outer
vacuum container and the thermal shield. However, as can be seen in
FIGS. 1A-1B, these elements also reduce the available inside
diameter 4a of the solenoidal magnet. Since the available inside
diameter 4a of the solenoidal magnet is required to be of a certain
minimum dimension to allow patient access, the presence of the
outer vacuum container 4 and the thermal shields 3 effectively
increases the diameter of the magnet coils and the former 1, adding
to the cost of the overall arrangement.
[0005] The cost of producing a former 1 such as illustrated in
FIGS. 1A-1B and described above is accounted for approximately
equally by labor costs and material costs. Among other objectives,
the present invention seeks to reduce the labor costs involved in
producing a solenoidal magnet structure.
[0006] U.S. Pat. No. 5,917,393 describes a superconducting magnet
arrangement wherein superconducting wire is mounted on an inner or
outer surface of a thermally conductive cylinder, whereby cooling
may be applied through the material of the former. The
superconducting wire is thermally connected to, but electrically
isolated from, the material of the cylinder.
[0007] The present invention aims to alleviate at least some of the
problems of the prior art, and provides a relatively inexpensive
and lightweight solenoidal magnet structure which is capable of
withstanding the forces applied to it in use, and provides accurate
and stable positioning of the coils of the solenoidal magnet.
[0008] Accordingly, the present invention provides apparatus and
methods as defined in the appended claims.
[0009] The above, and further, objects, characteristics and
advantages of the present invention will become more apparent from
consideration of the following description of certain embodiments
thereof, given by way of examples only, in conjunction with the
accompanying drawings, wherein:
[0010] FIGS. 1A-1B show a solenoidal magnet structure housed within
a cryostat, according to the prior art;
[0011] FIG. 2 shows an overall perspective view of an assembly of
coils and a mechanical supporting structure according to an
embodiment of the present invention;
[0012] FIG. 2A shows an overall perspective view of an assembly of
coils and a mechanical supporting structure according to another
embodiment of the present invention;
[0013] FIG. 3 shows an axial half-cross-section through an assembly
of coils and a mechanical supporting structure according to an
embodiment of the present invention; and
[0014] FIGS. 4A-4D show steps in a method for producing an assembly
of coils and a mechanical supporting structure according to an
embodiment of the present invention.
[0015] The present invention particularly provides a method of
manufacturing a solenoidal magnet structure, An embodiment of the
present invention will now be discussed with reference to FIGS.
2-4D.
[0016] FIG. 2 shows an overall perspective view of an assembly of
coils 34 and a mechanical supporting structure 102 according to an
embodiment of the present invention. The invention provides that
the mechanical supporting structure 102 is a preformed cylinder,
such as of a metal or a composite material, bonded to the radially
outer surface of the coils. Typical example materials include
aluminum, and fiberglass reinforced epoxy resin. Coils 34 are
formed within a mold, the mechanical supporting structure 102 is
slid over the outside of the coils, and the whole structure is then
resin impregnated to bond the coils onto the support structure and
produce a single solid piece which comprises the coils 34 and the
mechanical support structure 102.
[0017] FIG. 2 shows an example of a cylindrical mechanical support
structure 102 as used in the present invention. As shown, the
structure 102 may have an axial split 103 along its length,
bordered with retaining flanges 104. When assembling a magnet
structure of the present invention, the axial split 103 may be
opened, to increase the internal diameter of the support structure
102. The support structure may then be slid easily over the coils
34. Once in position, the retaining flanges 104 may be pulled
together, for example using bolts, clamps, screws or any other
suitable arrangement. This reduces the internal diameter of the
support structure, and may cause the internal surface of the
support structure to bear onto coils 34, retaining them in
position.
[0018] In an alternative arrangement illustrated in FIG. 2A, the
cylindrical mechanical support structure 102 is a complete
cylinder. The internal diameter of the mechanical support is
somewhat larger than the outer diameter of the coils 34. To
install, the support structure 102 is slid over the cons 34,
leaving a clearance between the outer surface of the coils and the
inner surface of the mechanical support. This clearance is then
filled with a filler, for example a granular filler such as sand or
glass beads, before the impregnation step. During the impregnation
step, resin fills the clearance, around the filler, impregnates the
coil windings and causes the coils to adhere to the inner surface
of the cylindrical mechanical support.
[0019] In both examples, as shown in FIG. 2 and FIG. 2A, mechanical
support structures such as shown at 101 may be provided on the
external surface of the support structure, for use in mounting the
magnet structure--for example within a cryogen vessel or vacuum
vessel.
[0020] FIG. 3 illustrates an axial half-cross-section through an
assembly of coils and a mechanical supporting. structure according
to an embodiment of the present invention. An advantage of the
arrangement of the present invention is that the frictional
interface between the radially inner surface of the coils and
former is eliminated. This frictional interface may provoke quench
in prior art arrangements, since movement of a coil in frictional
contact with a former may cause sufficient localized heating to
bring about a quench. The radially inner surface of a
superconducting coil is the part most susceptible to quench, so it
is particularly useful to eliminate frictional heating in that
region. Since there are no such frictional interfaces in the
arrangement of the present invention, such risks are not present.
The coils are separated from the mechanical support structure by
filer layers 40, which also serve as thermal diffusion barriers, as
will be explained below. The body force, that is the axial
electromagnetic force acting on the coils, is restrained by the
shear bond strength between the coils 34 and the mechanical support
structure 120. The interfacial shear strength required between the
coils 34 and the mechanical support structure 102 to maintain their
position when subjected to the electromagnetic loads is within the
capability of existing technology of bonding methods described
herein.
[0021] In some conventional arrangements, slip planes were
provided. These are interface surfaces along which the coils can
move relatively easily over a former. Rather than trying to prevent
movement, their aim is to make any movement as frictionless as
possible to reduce the frictional heat produced by any such
movement. In the present invention, the coils are firmly attached
to the mechanical support structure, and there is no need to
provide slip planes.
[0022] As illustrated in FIG. 3, a thermal diffusion barrier 40 may
be provided between the coils and the mechanical support structure.
This thermal diffusion barrier 40 should be electrically
insulating, magnetically inert and thermally conductive. The
thermal diffusion barrier may be a layer of wire wound over the
superconducting coil. The thermal diffusion barrier serves to
spread heat, to prevent heat from the former reaching the coil, or
at least to spread any heat from the former over a larger surface
of the coil, to prevent hot spots. The thermal diffusion barrier
may also serve to bring the outer diameter of certain coils 34 to
nearer the inner diameter of the mechanical support structure. The
thermal diffusion barrier may, as described further below, include
a composite material such as fiberglass reinforced epoxy resin.
[0023] In the arrangement of FIG. 3, the three axially inner coils
have a smaller outer diameter than the axially outer coils. This is
believed to be typical of current superconducting magnets for MRI
imaging systems. Evidently, the support structure 102 cannot be
slid over the axially outer coils, as it has an internal diameter
less than the external diameter of the axially outer coils. The
support structure may be split axially, as shown in FIG. 2, the cut
103 opened until the support 102 can pass over the radially outer
coils, then be clamped back together once aligned with the coils
34. One alternative method for constructing such an arrangement,
according to the present invention, is illustrated in FIG. 3A. The
magnet assembly is constructed in three parts. A first, central,
mechanical support structure 102a is assembled to the axially inner
coils of reduced internal diameter. Second and third, outer,
mechanical supports 102b, 102c are then assembled to respective
axially outer coils. The three parts of the mechanical support
structure may then be assembled together, and the whole assembly
impregnated. Radial flanges 105 may be used to join the parts
together. Flanges 105 may be joined using bolts, clamps, screws or
any other suitable arrangement. In an alternate method, illustrated
in FIG. 3B, a first, smaller diameter, mechanical support structure
102d is assembled to the axially inner coils of reduced internal
diameter. A second, larger diameter, mechanical support structure
102e is then slid over the first mechanical support 102d and the
axially outer coils. The resulting assembly is then impregnated. It
may be preferred that the second, larger diameter, mechanical
support structure is axially split, as shown in FIG. 2. This will
allow an increase in the internal diameter of the mechanical
support structure to allow it to slide easily over the first,
smaller diameter, mechanical support structure. The split 103 may
then be closed, as discussed with reference to FIG. 2.
[0024] Another alternative arrangement for forming the mechanical
support structure is schematically illustrated in radial
cross-section in FIG. 3C. A plurality of sections 106, each of
arcuate cross-section, are assembled together to create a
cylindrical support structure. They may be assembled using axial
flanges 107 running along the edges of each section. As the flanges
104 of FIG. 2, the flanges 107 may be joined together using bolts,
clamps, screws or any other suitable arrangement.
[0025] Patient comfort and accessibility of clinicians in MRI
systems may both be improved by reducing the length of the magnet.
Coil arrangements which permit this length reduction, while
maintaining field quality may result in coil body forces which act
in an axial direction away from the centre of the magnet.
Conventional methods of restraining these forces require additional
former material to be positioned on the end of the coil, which
increases the length of the magnet system. By utilizing the shear
strength at the interface between the coils and the mechanical
support structure, such additional material can be avoided and a
shorter magnet system length achieved in embodiments of the present
invention.
[0026] A method of producing a magnet structure according to the
present invention will now be described. Firstly, an accurate mold
is required in which to wind the coils. As no former is to be
provided, the accurate dimensions and relative spacing of the coils
are defined by this mold, which must accordingly be very accurately
made, and of a durable material, to allow a single mold to be used
to produce many magnet structures. The mold is arranged to be
collapsible. Superconducting wire is wound into defined positions
in the mold. Typically, these positions will be recesses in the
surface of the mold. A mechanical support structure 102, such as a
tube of metal such as aluminum or stainless steel; or of a
composite material such as fiberglass reinforced plastic, is placed
over the coils 34 so wound. A layer of fiberglass cloth may be
placed over the outer circumference of the coils before the
mechanical support structure 102 is positioned. The layer of
fiberglass cloth will provide thermal diffusion barrier 40. A
further mold may be placed over the coils 34 and the mechanical
support structure 102 to form an enclosed mold cavity. The coils
and the mechanical support structure within the mold are
monolithically impregnated with a thermosetting resin. This is
allowed to harden and the resin impregnated coils and the
mechanical support structure, now bonded to the coils, is removed
from the mold as a single solid piece. The impregnation step is
preferably performed in a vacuum, to avoid bubbles of air or other
gas which might otherwise be trapped in the coil windings and cause
stresses in the finished piece. The mold may be provided with a
lining, such as of polytetrafluoroethylene PTFE, to aid in
releasing finished articles.
[0027] A particular advantage of this method of forming the
solenoidal coil arrangement is in that the coils are accurately
dimensioned and positioned relative to each other by the shape of
the mold. The mechanical support structure which is bonded onto the
coils does not need to be accurately dimensioned, since the
positioning of the coils is defined by the mold, and the mechanical
support structure merely serves to securely retain the coils in
their relative positions as defined by the mold.
[0028] This is particularly advantageous since the mold, which must
be very accurately dimensioned, and is relatively expensive, may be
re-used several times to produce a number of similar solenoidal
magnet structures. Hitherto, the former has been the accurately
dimensioned, expensive component, and of course can only be used
for one magnet structure. Use of the method of the present
invention, using an accurately machined mold to define the
dimensions and relative positions of the coils, accordingly allows
production of accurate solenoidal magnet structures for a reduced
cost, and in reduced time, as compared to existing methods of
production.
[0029] According to an aspect of the present invention, a
preformed, tubular mechanical support structure 102 is positioned
over the coils 34 in a mold prior to the impregnation step when
thermosetting resin is applied to monolithically embed the coils
and bond them to the mechanical support structure to produce a
single solid article. Typically vacuum impregnation is used. The
coils 34 may be over-wound with cloth 40 to bring their outer
diameters to the same size as the inner diameter of the mechanical
support structure, and to provide a thermal diffusion barrier
layer. The cloth will be impregnated with the thermosetting resin
during the molding step. Again, the moulds may be provided with a
lining, such as of polytetrafluoroethylene PTFE, to aid in
releasing the finished articles.
[0030] In some alternative embodiments, a wet lay-up process may be
employed, where the resin is applied as a coating on the coil
conductor, and as part of the mechanical support structure, either
as a coating on the mechanical support structure or as an
impregnated cloth at layer 40 or similar material.
[0031] The mechanical support structure 102 may be manufactured as
an aluminum extrusion, or formed as rolled and welded tubes of
stainless steel; it may be formed as a filament wound tube of
fiberglass reinforced plastic, or may be molded from suitable
materials. It may comprise one or more layers of wire wound into a
cylinder and embedded in a thermoset material.
[0032] Typically in solenoidal magnet structures, some coils will
be of different internal or external diameter from others. In this
case, it may be necessary to bring all coils to a common diameter
to enable them all to be attached to a mechanical support structure
of constant internal diameter. Differences in the relevant
diameters of the coils may be corrected by use of a filler layer 40
of resin-impregnated cloth overwrap. typically employing glass
fiber cloth. This may be added while the coil is in a mold, and may
be added either as resin impregnated cloth or as a dry cloth to be
impregnated in the mold.
[0033] The coils 34 are wound within corresponding parts of the
mold, according to the method described above. In the mold, a
filler material 40 such as resin impregnated glass fiber may be
wound over coils in order to fill the mold to the top. For example,
such filler material may be provided to a depth of 5-10 mm. The
mold may comprise a collapsible mandrel having at least one
removable section, allowing the mold to be disassembled and removed
from the interior of the molded coils 20.
[0034] The use of relatively inaccurate mechanical support
structure 102 for the coils, being a preformed tube, typically of
metal or composite material, and molded resin is rendered possible
by use of accurate tooling. All of the important relative positions
of features of the solenoidal structure are defined by the mold or
other assembly tooling, resulting in a relatively low unit cost of
the solenoidal magnet arrangements produced, while the relatively
expensive mold and tooling may be re-used a number of times to
produce several solenoidal magnet coil assemblies.
[0035] The final structure may be further strengthened to prevent
any significant deformation.
[0036] FIGS. 4A-4D illustrate several views of steps in a method of
manufacturing a solenoidal magnet structure according to the
present invention, in part axial half cross section. In this
example, the mechanical support structure consists of an aluminum
tube 102. FIGS. 4A and 4B show a partial end-view and a partial
axial cross-section, respectively, of a collapsible mold 80, 82, 84
into which coils 34 have been wound in predefined positions 88,
during the manufacture of a solenoidal magnet coil assembly
according to the present invention, with an aluminum tube
mechanical support structure 102. The collapsible mold includes an
inner cylinder member 80 which retains tool segments 82, 84. The
tube 80 may be a single complete cylindrical tube, or may be
divided into segments. The tube 80 may be a collapsible mandrel
having at least one removable section. In use, the cylinder 80 and
tool segments 82, 84 are retained together by detachable mechanical
retaining means, such as the bolts 86 illustrated, to form a
generally cylindrical inner surface of the mold.
[0037] As shown in FIG. 4B, the tool segments 82, 84 have cavities
88 for retaining coils 34 as they are wound onto the mold, and
cavities 92 for retaining electrical leads and other service
components.
[0038] As illustrated in FIG. 4B, over the top of coils 34 and
leads and service components 92, a pre-formed tube 102 of composite
material, or of metal, or of other suitable material, is slid over
the coils from one end of the mold. A mold outer 100 is provided to
surround the tool segments and the pre-formed tube 102, and to
define mold cavity 99 with the tool segments 82, 84 and end pieces
98, if any. While the tool segments 82, 84 must be accurately
formed and accurately positioned, it is not necessary to apply such
a degree of accuracy to the position and shape of the mold outer
100.
[0039] The mold cavity 99 is open in certain locations 108, e.g. at
its ends. Openings may also be provided through the mold outer 100.
An impregnation trough 110 is affixed around the mold structure,
and a thermosetting impregnation resin 122 is forced 124 through
the openings 108 from the impregnation trough into the mold. The
resin 122 monolithically impregnates the coils 34, leads and
service components 92, and any filler material layers 40, 114, and
to bond the coils to the preformed cylindrical mechanical support
structure 102 and produce a single solid article, being the
solenoidal magnet structure comprising coils and the preformed
cylindrical mechanical support structure.
[0040] Once the assembly has been fully impregnated and the resin
has set, the various pieces of the mold 82, 84, 98, 100 are moved
away from the resultant single solid article, the molded structure.
Firstly, the impregnation trough 110 and end pieces 98 should be
removed from the mold. The mold outer 100 may also be removed at
this stage, or may be removed later. Typically, and with reference
to FIGS. 4A, 4B, cylinder 80 is detached from the tool segments. If
the cylinder 80 is in a single piece, it can be slid out from the
central bore of the assembly. If the cylinder 80 is split into
segments, these segments may be dismantled and removed from the
bore of the molded structure. The tool segments 82, 84 are then
removed from the molded article. In the example shown in FIG. 4A,
the tool segment 82 is tapered to narrow away from the bore of the
mold. Such segments should be removed first, to provide clearance
for removal of the remaining tool segments 84.
[0041] FIG. 4C shows an example of a solenoidal coil assembly 111
produced according to the method described above. The coils 34 are
impregnated with resin and have dimensions defined by the accurate
surfaces of the tool segments 82, 84. They are bonded by the
impregnated resin to the preformed cylindrical mechanical support
structure 102. The shear strength of the mechanical bond between
coils and the mechanical support structure provided by the resin
impregnant is equally effective in both directions. The coils are
accordingly rigidly held in accurate relative positions by the
mechanical support structure.
[0042] As shown in FIG. 4C, holes may be provided through the
mechanical support structure, for example to allow access to the
coils 34 for electrical cables, to assist with circulation of
coolant, to provide for mechanical retention of the structure
within a cryostat, and so on. The holes may be formed by drilling
or cutting after the impregnation step, or the mold may be provided
with corresponding features to ensure that the holes remain
open.
[0043] FIG. 4D shows a partial cross-section of a shield coil
arrangement according to an embodiment of the present invention. A
shield coil 118 is impregnated with resin, and bonded by that resin
to a mechanical support structure 120, being a pre-formed cylinder
of a composite material such as fiberglass-reinforced plastic, or
of a metal such as aluminum or stainless steel, or other suitable
material. The structure is shown mounted inside a vessel 122, for
example a cryogen vessel, provided with locating means 124.
[0044] United Kingdom patent application GB2437114 describes
methods of manufacturing solenoidal magnets, and solenoidal magnets
so made, which bear some resemblance to certain embodiments of the
present invention. However, significant differences of the present
invention as compared to this prior art include the following. The
preformed cylinders used for the mechanical support structure in
the present invention may be produced in a "parallel" process. For
example, a stock of preformed cylindrical mechanical support
structures may be kept, avoiding the risk that a poorly formed
cylindrical mechanical support structure would result in a scrap or
reworked magnet, as may be the case with the methods of GB2437114.
Any defective cylindrical mechanical support structures would be
rejected before use, and so could not result in a reject solenoidal
magnet structure. Each preformed cylindrical mechanical support
structure may be tested before use to make sure that they meet the
requirements of the design before being bonded to magnetic coils.
The cylindrical mechanical support structures could be bought from
a third party supplier, simplifying the manufacturing process for
the magnet manufacturer. The cylindrical mechanical support
structures themselves need not be particularly accurately formed,
and so may be produced at relatively low cost.
[0045] The use of metal cylindrical mechanical support structures
provides further advantages, for example that they are recyclable
while composite materials are generally not recyclable; they have a
relatively high thermal conductivity, which may allow cooling of
the coils through or around the mechanical support structure; they
have a relatively high electrical conductivity. Such conductive
mechanical support structure will support eddy currents. The eddy
currents are dependent on the rate of change of the magnetic field
during quenching, ramping and imaging in an imaging system such as
a magnetic resonance imaging (MRI) system. Control of the
conductivity of the mechanical support structure may be used during
the design process to alter stray field bursts during quench
events; and can be used to control the rate of temperature increase
during a quench, which would affect the pressure within a cryogen
vessel containing the magnet. The mechanical support structure
should also be stiff and strong. A composite material may be found
to provide greater strength and/or stiffness per unit volume, or
weight, than a metal, when used for the mechanical support
structure of the present invention.
* * * * *