U.S. patent application number 11/441129 was filed with the patent office on 2006-12-21 for electromagnet.
This patent application is currently assigned to Siemens Magnet Technology Ltd.. Invention is credited to Andrew Farquhar Atkins, Graham Gilgrass, Andrew James Gray.
Application Number | 20060284711 11/441129 |
Document ID | / |
Family ID | 34834652 |
Filed Date | 2006-12-21 |
United States Patent
Application |
20060284711 |
Kind Code |
A1 |
Atkins; Andrew Farquhar ; et
al. |
December 21, 2006 |
Electromagnet
Abstract
An electromagnet comprising a plurality of coils of
superconductive material, monolithically embedded in an embedding
material, which is solid at the temperature of operation of the
superconductive electromagnet, and a method for producing an
electromagnet comprising a plurality of coils of superconductive
material, comprising the steps of winding coils of superconductive
material, retaining the coils at predetermined relative positions,
and monolithically embedding the plurality of superconducting coils
in an embedding material, which is solid at the temperature of
operation of the superconductive electromagnet.
Inventors: |
Atkins; Andrew Farquhar; (Nr
Banbury, GB) ; Gilgrass; Graham; (Wallingford,
GB) ; Gray; Andrew James; (Bicester, GB) |
Correspondence
Address: |
CROWELL & MORING LLP;INTELLECTUAL PROPERTY GROUP
P.O. BOX 14300
WASHINGTON
DC
20044-4300
US
|
Assignee: |
Siemens Magnet Technology
Ltd.
Witney
GB
|
Family ID: |
34834652 |
Appl. No.: |
11/441129 |
Filed: |
May 26, 2006 |
Current U.S.
Class: |
335/216 |
Current CPC
Class: |
H01F 27/322 20130101;
H01F 6/06 20130101; H01F 27/327 20130101; H01F 5/02 20130101; G01R
33/3815 20130101; H01F 6/04 20130101 |
Class at
Publication: |
335/216 |
International
Class: |
H01F 6/00 20060101
H01F006/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 26, 2005 |
GB |
0510716.4 |
Claims
1. An electromagnet comprising a plurality of coils of
superconductive material, arranged for cryogenic cooling,
characterised in that the plurality of coils of superconductive
material is monolithically embedded in a thermoplastic material
which is solid at the temperature of operation of the
superconductive electromagnet.
2. An electromagnet comprising a plurality of coils of
superconductive material, arranged for cryogenic cooling,
characterised in that the plurality of coils of superconductive
material is monolithically embedded in an embedding material, which
is solid at the temperature of operation of the superconductive
electromagnet; at least one of the coils is wound into a preformed
journal; and the preformed journal comprises at least one integral
cooling channel for circulation of a liquid cryogen
therethrough.
3. An electromagnet according to claim 1 wherein the coils are held
apart at appropriate relative positions by spacers mechanically
associated with the coils.
4. An electromagnet according to claim 3 wherein the spacers are
integrally formed with at least one performed journal.
5. An electromagnet according to claim 4, wherein the performed
journal is at least partially formed of a porous material and the
embedding material permeates the porous material.
6. An electromagnet according to claim 5, wherein the porous
material is a metal foam.
7. An electromagnet according to claim 1, wherein coil terminations
and cable runs are also embedded within the thermosetting or
thermoplastic material.
8. An electromagnet according to claim 1, wherein the embedding
material is a thermosetting material or a thermoplastic
material.
9. An electromagnet according to claim 8 wherein the embedding
material comprises one of: water, nitrogen, paraffin wax.
10. An electromagnet according to claim 8 wherein the embedding
material comprises an organic resin.
11. A method for producing an electromagnet comprising a plurality
of coils of superconductive material, said method comprising the
steps of: winding coils of superconductive material and retaining
the coils at predetermined relative positions, characterised in
that the method further comprises the step of monolithically
embedding the plurality of superconducting coils in an embedding
material, which is solid at the temperature of operation of the
superconductive electromagnet, wherein at least one of the coils is
wound into a preformed journal; and the preformed journal comprises
at least one integral cooling channel, for circulation of a liquid
cryogen therethrough.
12. A method according to claim 11 wherein the step of retaining
the coils at predetermined relative positions comprises use of
spacers mechanically associated with the coils.
13. A method according to claim 12 wherein the spacers are
integrally formed with at least one preformed journal.
14. A method according to claim 11, wherein the preformed journal
is at least partially formed of a porous material, and the
embedding material permeates the porous material.
15. A method according to claim 14, wherein the porous material is
a metal foam.
16. A method according to claim 11, wherein the step of embedding
the superconducting coils in a thermosetting or thermoplastic
material further comprises embedding coil terminations and cable
runs within the thermosetting or thermoplastic material.
17. A method according to claim 11, wherein the embedding material
is a thermosetting material or a thermoplastic material.
18. A method according to claim 17 wherein the embedding material
comprises one of: water, nitrogen, or paraffin wax.
19. A method according to claim 17 wherein the embedding material
comprises an organic resin.
20. An electromagnet comprising a plurality of coils of
superconductive material, characterised in that the plurality of
coils of superconductive material is monolithically embedded in a
embedding material, which is solid at the temperature of operation
of the superconductive electromagnet.
21. An electromagnet according to claim 20, arranged for cryogenic
cooling.
22. An electromagnet according to claim 21, which is arranged for
cryogenic cooling by at least partial immersion in a bath of liquid
cryogen.
23. An electromagnet according to claim 22 wherein a number of
cooling channels are provided in the embedding material, for
circulation of a liquid cryogen therethrough.
24. An electromagnet according to claim 20, wherein at least one of
the coils is wound into a preformed journal.
25. An electromagnet according to, claim 24, wherein the preformed
journal comprises at least one integral cooling channel for
circulation of a liquid cryogen therethrough.
26. An electromagnet according to claim 20, wherein the coils are
held apart at appropriate relative positions by spacers
mechanically associated with the coils.
27. An electromagnet according to claim 26, wherein the spacers are
integrally formed with at least one performed journal.
28. An electromagnet according to claim 24, wherein the preformed
journal is at least partially formed of a porous material and the
embedding material permeates the porous material.
29. An electromagnet according to claim 20, wherein coil
terminations and cable runs are also embedded within the embedding
material.
30. An electromagnet according to claim 20, wherein the embedding
material is a thermosetting or thermoplastic material.
31. An electromagnet according to claim 30 wherein the embedding
material comprises one of: water, nitrogen, paraffin wax, an
organic thermoplastic resin or an organic thermosetting resin.
32. A method for producing an electromagnet comprising a plurality
of coils of superconductive material, said method comprising the
steps of: winding coils of superconductive material and retaining
the coils at predetermined relative positions, characterised in
that the method further comprises the step of monolithically
embedding the plurality of superconducting coils in an embedding
material, which is solid at the temperature of operation of the
superconductive electromagnet.
33. A method according to claim 32, further comprising the step of
at least partially immersing the embedded coils on in a bath of
liquid cryogen.
34. A method according to claim 32, further comprising the steps of
providing a number of cooling channels in the embedding material,
for circulation of a liquid cryogen therethrough.
35. A method according to claim 32, wherein at least one of the
coils is wound into a preformed journal.
36. A method claim 35, wherein the preformed journal comprises at
least one integral cooling channel, for circulation of a liquid
cryogen therethrough.
37. A method according to claim 32, wherein the step of retaining
the coils at predetermined relative positions comprises use of
spacers mechanically associated with the coils.
38. A method according to claim 37, wherein the spacers are
integrally formed with at least one preformed journal.
39. A method according to claim 35, wherein the preformed journal
is at least partially formed of a porous material, and the
embedding material permeates the porous material.
40. A method according to claim 32, wherein the step of embedding
the superconducting coils in an embedding material further
comprises embedding coil terminations and cable runs within the
embedding material.
41. A method according to claim 32, wherein the embedding material
is a thermosetting or thermoplastic material.
42. A method according to claim 41 wherein the embedding material
comprises one of: water, nitrogen, paraffin wax, an organic
thermoplastic resin or an organic thermosetting resin.
Description
[0001] The present invention relates to an electromagnet assembly.
More particularly it relates to a particularly advantageous
mounting and retaining element for coils and other conductors of an
electromagnet. More particularly still, it relates to a
cryogenically cooled superconducting electromagnet.
[0002] MRI or NMR imaging systems are used for medical diagnosis. A
requirement of such a system is a stable, homogeneous, magnetic
field. Typically, cryogenically cooled superconducting
electromagnets are employed. In order to achieve stability it is
common to use a superconducting magnet system which operates at
very low temperature. The temperature is maintained by cooling the
superconductor, typically by immersion in a low temperature
cryogenic fluid such as liquid helium. Cryogenic fluids, and
particularly helium, are expensive and it is desirable that the
magnet system should be designed and operated in a manner to reduce
to a minimum the amount of cryogenic fluid used.
[0003] The present invention relates to an inexpensive yet
effective way of retaining superconductive coils and other
conductors in place during ramping and operation of an
electromagnet.
[0004] FIG. 4 illustrates a hollow cylindrical cryostat 1 suitable
for housing a solenoidal superconducting magnet for an MRI system,
according to the prior art. The superconducting magnet system 2
typically comprises a set of superconductor windings 12 for
producing a magnetic field, wound on a former 10. A cryogenic fluid
vessel 14 contains the superconductor windings and holds the low
temperature cryogenic fluid, when in operation. One or more thermal
shields 18 substantially surround the cryogenic fluid vessel 14,
and an outer vacuum chamber 16 completely encloses the thermal
shield(s) and the cryogenic fluid vessel 14.
[0005] The shield(s) reduce(s) the incidence of radiated heat from
the outer vacuum chamber 16 which may reach the cryogen vessel 14.
A service neck 20 is typically included. In operation, this neck
may house a recondensing refrigerator 21.
[0006] It is common practice to use a refrigerator 21 to cool the
thermal shields 18 to a low temperature in order to reduce the heat
load onto the cryogenic fluid vessel 14, and thus the loss of
cryogen, for example liquid helium (not shown in FIG. 4), by
boil-off. It is also known to use a refrigerator 21 to directly
refrigerate the cryogen vessel 14, thereby reducing or eliminating
cryogen fluid consumption.
[0007] An object of the present invention is to reduce the
likelihood of unintentional quench of a superconducting
electromagnet during operation. A quench occurs when a
superconductor, such as used in superconducting magnets, reverts to
a resistive state. This may be caused, for example, by localized
heating in one part of the superconductor, either due to local
movement or friction. For any of these reasons, a small part of the
superconductor ceases to be superconductive, and enters a resistive
state. Any current flowing through the resistive part will cause
local Joule heating. This will cause the adjacent parts of the
superconductor to quench, resulting in a larger resistive part, in
turn causing further Joule heating. Very rapidly, a large part of
the superconductor enters a resistive state, with a potentially
very large current still flowing. When this happens, a large part
of the stored field energy, which may be in the order of several
mega joules, will be dissipated as heat. If this process is not
adequately managed, this heat may be dissipated in confined areas,
resulting in local temperature rises which can damage the coil
areas at or near the part where the quench was initiated.
[0008] As mentioned above, one possible cause for a quench in a
superconducting magnet is localised heating due to movement of a
conductor. Since electromagnets such as used in imaging systems
include conductors carrying very high currents in a very strong
magnetic field, appreciable forces are experienced by the
conductors. Any freedom of a conductor to move, even slightly, may
lead to sufficient localised heating to cause quench of the magnet
when in operation.
[0009] At least two different mechanisms can be identified for
movement of superconducting wires. For convenience, these are
commonly referred to as `ramp shift` and `stick shift`. When a
magnet is being brought into operation, the coils and the former on
which they are wound are cooled to operating temperature. Different
thermal expansion of the coils, the former and other materials used
may mean that the wires of the coils become free to move slightly.
The magnitude of current flowing in the magnet coils is ramped. The
magnetic field may act on the current-carrying wires of the coils
to displace them from their former position. This is known as ramp
shifting. The displaced wires cause a variation on the homogeneity
of the magnetic field produced, and a re-shimming process will be
required to compensate for this variation. Instead of a gentle
movement, the wires may initially stick in one place, and when the
force on them has built up to a certain level, the wire may
suddenly move. This sudden movement may be sufficient to cause a
quench. One object of the present invention is to prevent any such
movement, either `ramp shift` or `stick shift`, thereby avoiding
the problems of quench, or the need for reshimming caused by such
movement.
[0010] Conventional superconducting electromagnets are cooled by
immersion in a bath of liquid cryogen, for example liquid helium.
The temperature of the bath is maintained by boiling off the liquid
cryogen. It is necessary to provide a recondensing refrigerator to
recondense boiled-off helium back to a liquid state, or the boiled
off cryogen must be vented to the atmosphere.
[0011] A difficulty arises when the system is in transit, awaiting
installation. Typically, the system is transported filled with
liquid cryogen, but the recondensing refrigerator 16 is unable to
operate due to the absence of a suitable power source. During the
transit time, the cryogen is allowed to boil, keeping the coils 12
at the required temperature. The cryogen thus acts as a thermal
battery. Service neck 20 provides an escape path for boiled off
cryogen to leave the cryogen vessel 14. The boiled off cryogen is
allowed to vent to atmosphere. The system may be required to be
capable of remaining in this boiling thermal battery state for a
duration of up to about 30 days. When a cryogen such as helium is
used, the cost of the cryogen lost by boiling may become
significant.
[0012] It is required to keep the coils at a low temperature, since
otherwise the commissioning of the system on installation becomes
difficult and time consuming. If the system has heated up to
ambient temperature, which will happen if the liquid cryogen boils
dry, the system must be cooled and refilled with liquid cryogen
before being commissioned. In some regions of the world, it is very
difficult to obtain the large supplies of the liquid cryogen
required for such an operation if not planned for in advance. Such
a re-cooling and refilling is also time consuming, and costly both
in terms of the time a field engineer must spend on site installing
the system, and the material cost of the cryogen used.
[0013] The distribution and cost of large quantities of helium
required for such cryogen baths is causing difficulties, and may
prevent the installation of equipment in remote locations. It is a
further object of the present invention to reduce the quantity of
liquid cryogen required in the magnet system, and to reduce the
requirement for replenishing the liquid cryogen.
[0014] The present invention accordingly addresses these and other
problems and provides apparatus and methods as defined in the
appended claims.
[0015] The above, and further, objects, advantages and
characteristics of the present invention will become more apparent
from a consideration of the following description of certain
embodiments, given by way of examples only, in conjunction with the
accompanying drawings wherein:
[0016] FIG. 1 represents a perspective view of an electromagnet
according to the present invention;
[0017] FIG. 2 represents an axial cross-section of an electromagnet
according to the present invention, such as that shown in FIG.
1;
[0018] FIG. 3 represents a cross-section and partial perspective
view of a journal, used to accommodate magnetic coils in certain
embodiments of the present invention; and
[0019] FIG. 4 represents a cryostat housing a solenoidal
superconducting magnet according to the prior art.
[0020] FIG. 1 shows a partial cut-away perspective view of an
electromagnet according to an embodiment of the present invention.
A plurality of coils 12 are monolithically embedded within a
thermosetting or thermoplastic material 22 which is solid at the
temperature of operation of the electromagnet. Since the coils are
embedded in a solid material, individual conductors have no freedom
to move, even under significant force caused by elevated currents
flowing through an intense magnetic field. The moulding step should
preferably be carried out in a vacuum to ensure that no voids are
present, particularly in between conductors of a particular coil.
By ensuring that solid material fills all spaces between conductors
of a coil, the possibility of movement of the conductors is
removed.
[0021] Similarly, by monolithically embedding the plurality of
coils 12 within a single artefact of solid material, the
possibility of relative movement between the coils is also
removed.
[0022] Feature 24 is a heat exchanger manifold, whose function will
be briefly described below with reference to certain embodiments of
the invention.
[0023] FIG. 2 shows an axial cross-section of the electromagnet
structure of FIG. 1. As shown, the coils 12 may be of differing
sizes, and spacing. The coils may include field coils, RF coils and
shield coils among others. The function and arrangement of each
type of coil is well known in the relevant art. In order to achieve
and maintain the required level of magnetic field homogeneity, the
spacing between the coils must be accurately set, and must be
maintained despite forces experienced by the coil as a result of
their conducting high levels of current through an intense magnetic
field. The present invention provides this stability by embedding
the coils 12 in a monolithic solid structure.
[0024] In typical known solenoidal electromagnets, a solid former
such as shown at 10 in FIG. 4 is provided. This may be of a
composite material such as fibreglass reinforced resin, or any
material of relatively low thermal conductivity, such as certain
grades of stainless steel. The coils 12 are then wound into
recesses in the former, or are wound onto support structures such
as 13, which are then assembled onto the former itself. An
advantage according to certain embodiments of the present invention
is that a former such as shown at 10 in FIG. 4 is no longer
required. As illustrated in FIGS. 1 and 2, a solenoidal
electromagnet according to the present invention is formed by
embedding a plurality of coils in a monolithic solid structure of
thermoplastic or thermosetting materials. No former is needed,
since the structural strength of the electromagnet is provided by
the thermosetting or thermoplastic material. The absence of a
former in the electromagnet of the present invention may lead to a
reduction in the size and weight of the structure. Of course, if
required, a former may still be employed, in which case the former
is embedded with the coils into the monolithic thermoplastic or
thermosetting material.
[0025] In embodiments where no former is provided, the coils may
need a retaining structure into which they are wound. As
illustrated in FIG. 2, the coils 12 may be wound into preformed
journals 26. These preformed journals define the dimensions of the
coil and hold the conductor in place prior to the coils being
embedded. The size of the journals, and the separation between
them, must be accurately controlled. In certain embodiments,
mechanical spacers may be provided to hold the coils, and the
journals if provided, at the appropriate relative positions.
Preferably, journals 26 are provided having such spacers integrally
formed therewith.
[0026] An example journal 26 is more clearly shown in FIG. 3. As
shown in FIG. 3, the journal 26 may have one or more channels 28
running through it. The purpose of these channels will be explained
later. The preformed journal may appropriately be formed from one
of a large variety of materials. The material must be able to
withstand pressure from the coils and the temperature of operation
of the electromagnet when in operation. For example, certain
plastics, sand-filled resin or open celled foam may be used. Open
celled foams are particularly attractive since the thermoplastic or
thermosetting material will permeate through the open celled
material of the journal 26, allowing it to more easily permeate
through the conductors of the coils. The open celled foam material
effectively becomes part of the thermoplastic or thermosetting
material, adding to the security of the conductors of coils 12
within the material, compared with journals made of a non-permeable
material which will present a continuous interface with the
thermoplastic or thermosetting resin, adding the possibility of
delamination. On the other hand, an epoxy resin based journal 26
bonds effectively with an epoxy based thermosetting material. Other
combination of thermosetting or thermoplastic material with a
similar material for the journals may also be expected to provide
good bonding characteristics.
[0027] In a particularly preferred embodiment, the journals are
made of aluminium or copper open celled foam.
[0028] In prior art arrangements such as shown in FIG. 4, a
difficulty has existed in leading conductors from the various coils
12 to a termination board. As has been discussed above, even a
small amount of movement in a superconductor wire may generate
enough local heating to bring about a quench. Although
superconductor wire within coils 12 is held relatively immobile by
tension and adjacent layers of superconductor wire, coil
terminations are made by single superconducting wires which are led
as cable runs along the former or other suitable structure to a
termination board. These wires tend to move under the influence of
the generated magnetic field, since they carry large currents. In
order to prevent movement, and so avoid local heating and a quench,
it has become normal practice to restrain the conductors to the
former or other mechanical support at very short intervals, of the
order of 5 mm with nylon cable ties or the like. Such operation is
very time consuming, and not always effective. It is also time
consuming to remove and replace these ties if required for
servicing. According to an advantage of the present invention, such
conductors leading from coils 12 to a termination board 24, and
indeed the termination board 24 itself, may be embedded within the
thermosetting or thermoplastic material. This will prevent movement
of the conductors, and may assist in servicing, as will be
discussed further below. The termination board itself may be
embedded within the thermosetting or thermoplastic material, or may
be placed to be accessible from outside of the thermoplastic or
thermosetting material. According to this aspect of the present
invention, it is no longer necessary to provide tie bars, the
mechanical structures formerly provided for supporting the
superconducting wires in cable runs.
[0029] The present invention provides an electromagnet comprising
coils embedded in a thermoplastic or thermosetting material which
is solid at the temperature of operation of the electromagnet.
Since current superconductive electromagnets typically operate at a
temperature of 4K, certain materials not normally considered to be
thermoplastic solids have been found suitable for use as the
embedding material in the present invention.
[0030] Examples of effective thermoplastic materials include
room-temperature organic thermoplastic resins and solids such as
polyethylene and paraffin wax, along with other materials such as
water and nitrogen. Water and nitrogen have been found to be
particularly strong in compression. Water should be purified before
use, for example by boiling. Examples of effective thermosetting
materials include organic thermosetting resins. The thermoplastic
or thermosetting embedding material may be filled with a suitable
filler material. Examples include glass fibre, glass beads, sand,
and gravel.
[0031] If, following the monolithic embedding of coils 12 in a
thermoplastic material 22, it is necessary to remove one of the
coils for servicing, this may achieved simply by raising the
temperature of the electromagnet structure above the melting or
boiling point of the thermoplastic material. The coils then become
simple to remove. Once the coil is replaced, or a substitute is
provided, the embedding process is performed again, with the coils
being held in position at least until the thermoplastic material
has solidified. For nitrogen, the electromagnet need only be raised
above about 80K, for water above about 273K, and for paraffin wax,
above about 350K. This operation is not so simple if a
thermosetting material 22 is used. However, the use of a
thermosetting material may be preferred in examples where it is not
desired for the material to revert to its liquid state on
warming.
[0032] The present invention provides superconducting coils
embedded in a monolithic solid piece of embedding material, being a
thermosetting or thermoplastic material. When such an assembly is
to be brought to operating temperature, a difficulty may arise in
cooling the monolithic solid piece. If an embedding material of low
thermal conductivity is used, for example nitrogen, an outer
surface of the embedding material may be cooled to a very low
temperature while material deeper within the solid piece is still
at a higher temperature. The outer surface may then thermally
contract, while the inner regions are not contracting. This will
set up stresses within the piece, and a risk of fracture or
delamination will exist. To avoid or alleviate this problem,
cooling tubes may be added in to the structure before the embedding
material is added. In this case, cryogen may be circulated through
the cooling tubes to assist with cooling the piece to operating
temperature. Since cooling may then be applied both to the exterior
and the interior of the embedding material, the thermally induced
stresses and the resultant risk of fracture or delamination may be
avoided. A more even cooling of the piece may be achieved.
[0033] As mentioned earlier, the journals 26 may incorporate
channels 28. These channels may be used as cooling tubes as
described in the preceding paragraph. These channels, if provided,
must have continuous walls. That is, even in embodiments where the
journals are formed from open celled foam, the channels must have
continuous walls to prevent the ingress of the embedding material
into the channels 28. A skinned foam material may be useful in this
application. A particular advantage of providing cooling channels
28 integrally within the journals 26 as shown in FIG. 3 is that
cooling is provided in close proximity to the superconducting
coils. This will help to assure effective cooling of the coils to
operating temperature in operation. A heat exchanger manifold such
as shown at 24 in FIG. 1 is employed to carry cryogenic fluid to
the channels 28.
[0034] In certain particularly advantageous embodiments, cooling of
the coils may be provided solely by circulation of cryogen through
the channel 28. In such embodiments, it may no longer be necessary
to provide a cryogen vessel 14, or a quantity of liquid cryogen for
immersion of the coils. A small quantity of liquid cryogen, for
example 35 litres, may be provided in a small sump, and may be
caused to circulate around the channels 28 of each coil, providing
more direct cooling to the coils, and avoiding the need to supply
and maintain a large quantity of liquid cryogen. In such
embodiments, the cryogen tank 14 need not be provided, and the
coils embedded in the embedding material with the sump may be
placed in the outer vacuum chamber 16. Thermal shields 18 may still
be provided. The overall system may be made smaller, lighter and
less costly. In addition, the patient bore may be made wider and/or
shorter since there would be no cryogen vessel to accommodate.
[0035] Manufacture of the embedded coil assembly of the present
invention may proceed as follows. An appropriate mould may be
provided. For the embodiment illustrated in FIG. 1, a cylindrical
mould is required. This may be provided, for example, as two
concentric cylinders of stainless steel coated in a suitable
release agent. End pieces would be needed. The coils 12 are
arranged in the required relative positions within the mould. As
described herein, this may be arranged by mechanical attachment of
the coils, optionally within journals 26, to mechanical spacers.
Alternatively, mechanical spacers may be integrally formed with
journals 26. The mould is then filled with the chosen embedding
material. If the chosen embedding material is a thermoplastic
material, the temperature of the mould and its contents must be
kept below the solidifying temperature of the thermoplastic
material. It may be found convenient to arrange the mould with its
axis vertical. It may also be found convenient to provide one of
the cylinders, most probably the outer cylinder, as several pieces
which may be assembled around the coils and disassembled to reveal
the moulded article. Alternatively, the mould may be left in place,
a permanent feature which may add to the mechanical strength of the
finished article.
[0036] Alternatively, particularly where thermosetting materials
are used as the embedding material, it may be found possible to
apply the embedding material to the coils and the journals and
spacers as appropriate without the use of a mould, for example by
brushing epoxy resin onto the coils, laying resin-impregnated glass
fibre matting over the structure, out-gassing in a vacuum then
compressing the structure by any suitable means, such as an
inflatable cuff.
[0037] The coils are wound in a conventional manner, onto journals
such as shown in FIG. 3, or a former such as shown in FIG. 4. In
certain embodiments, a former such as shown in FIG. 4 may be
provided, but made of an open celled foam material, which is later
filled by the embedding material.
[0038] Suspension of the magnet assembly within the outer vacuum
chamber 16 can be provided by attaching suspension members directly
to the embedded coil assembly. This may be particularly suitable in
embodiments using an embedding material of very low thermal
conductivity.
[0039] Further properties of the resultant electromagnet may be
determined by appropriate selection of the materials used. The
embedding material may be chosen to be lightweight, for example
paraffin wax. Stronger solid materials may be preferred if the
electromagnet is to be transported at ambient temperature. On the
other hand, the embedding material may be chosen to be heavy, for
example sand and/or gravel filled resin. A heavy system provides
advantages in reduced floor vibration. If required, embedding of
the coils with such materials may be done at the installation site,
to facilitate transport.
[0040] Advantageously, according to certain embodiments of the
invention, the electrical conductivity of the journals or former
may be controlled by selection of the materials used. For example,
a copper or aluminium foam former or journals may be used, and the
degree of porosity may be controlled to provide the required
electrical conductivity. The electrical conductivity may be
selected to provide a required behaviour during a quench, for
example to limit a burst of magnetic field during a quench, or to
control the magnitude of eddy current which may flow during
imaging. Such eddy currents may reduce the quality of the final
images.
[0041] While the invention has been described with particular
reference to thermosetting and thermoplastic materials, at least
some of the advantages of the present invention may be obtained by
monolithically embedding the plurality of coils of a
superconducting magnet in any material which becomes solid and is
solid at the operating temperature of a superconductive magnet.
Examples of such materials include, but need not be limited to,
thermoplastics, thermosetting resins, ceramic slurries, cements,
concrete, and plaster. These latter materials undergo irreversible
reactions to become solid, having much the same effect as
thermosetting resins.
* * * * *