U.S. patent application number 10/908752 was filed with the patent office on 2006-11-30 for apparatus for thermal shielding of a superconducting magnet.
This patent application is currently assigned to GENERAL ELECTRIC COMPANY. Invention is credited to Neil Clarke, Longzhi Jiang, Gregory Alan Lehmann.
Application Number | 20060266053 10/908752 |
Document ID | / |
Family ID | 36637429 |
Filed Date | 2006-11-30 |
United States Patent
Application |
20060266053 |
Kind Code |
A1 |
Jiang; Longzhi ; et
al. |
November 30, 2006 |
APPARATUS FOR THERMAL SHIELDING OF A SUPERCONDUCTING MAGNET
Abstract
A thermal shield for a superconducting magnet includes a shield
body having an annular shape. The shield body includes a material
having a thermal conductivity greater than about 1000 W/mK at about
70K.
Inventors: |
Jiang; Longzhi; (Florence,
SC) ; Lehmann; Gregory Alan; (Brookfield, WI)
; Clarke; Neil; (Oxford, GB) |
Correspondence
Address: |
CANTOR COLBURN, LLP
55 GRIFFIN ROAD SOUTH
BLOOMFIELD
CT
06002
US
|
Assignee: |
GENERAL ELECTRIC COMPANY
1 River Road
Schenectady
NY
|
Family ID: |
36637429 |
Appl. No.: |
10/908752 |
Filed: |
May 25, 2005 |
Current U.S.
Class: |
62/51.1 ;
165/132; 165/135 |
Current CPC
Class: |
F25D 3/00 20130101; F25D
19/00 20130101; H01F 6/04 20130101; F25D 19/006 20130101; F25B 9/10
20130101; G01R 33/3815 20130101 |
Class at
Publication: |
062/051.1 ;
165/132; 165/135 |
International
Class: |
F25B 19/00 20060101
F25B019/00; F28D 1/06 20060101 F28D001/06; F28F 13/00 20060101
F28F013/00 |
Claims
1. A thermal shield for a superconducting magnet, the thermal
shield including: a shield body having an annular shape and
comprising a material having a thermal conductivity greater than
about 1000 W/mK at about 70K.
2. The thermal shield of claim 1, wherein the shield body comprises
a material having an electrical resitivity in a range of about 3
10-6 .OMEGA.m to about 3 10-3 .OMEGA.m.
3. The thermal shield of claim 1, wherein the shield body comprises
a material having a density less than about about 2.4 g/cm3.
4. The thermal shield of claim 1, wherein the shield body comprises
at least one of thermal pyrolitic graphite and pyrolitic boron
nitride.
5. The thermal shield of claim 1, wherein the shield body
comprises: a first sidewall; a second sidewall disposed
substantially parallel to the first sidewall and facing the first
sidewall; a third sidewall disposed substantially perpendicular to
the first and second sidewalls and extended between corresponding
edges of the first and second sidewalls; and a fourth sidewall
disposed substantially parallel to the third sidewall and extending
between corresponding opposite edges of the first and second
sidewalls.
6. The thermal shield of claim 5, wherein the first, second, third
and fourth sidewalls are connected to each other via mechanical
joints.
7. The thermal shield of claim 6, wherein the mechanical joints
include a support ring disposed at an interior portion of each
mechanical joint.
8. The thermal shield of claim 7, wherein the mechanical joints
include a high thermal conductive epoxy.
9. The thermal shield of claim 5, wherein the first and second
sidewalls each comprise a first and second sub-member.
10. The thermal shield of claim 9, wherein the first and second
sub-members are each connected via a support ring.
11. The thermal shield of claim 1, wherein a high conductive
aluminum tape is disposed at an inner surface of the thermal
shield.
12. The thermal shield of claim 1, further comprising thermal links
providing thermal communication between the thermal shield and at
least one of: a coldhead sleeve disposed at the thermal shield; and
a penetration through the thermal shield that contains at least one
of wires and instrumentation in communication with the
superconducting magnet.
13. A shielding system for shielding and cooling a superconducting
magnet coil via a cryogen, the system comprising: a cryogen vessel
containing the cryogen and disposed proximate to the
superconducting magnet coil to enclose the superconducting magnet
coil; a thermal shield including a shield body comprising a
material having a thermal conductivity greater than about 1000 W/mK
at about 70K, the thermal shield being disposed proximate to the
cryogen vessel to enclose the cryogen vessel; and a vacuum vessel
disposed proximate to the thermal shield to enclose the thermal
shield.
14. The system of claim 13, wherein the shield body comprises a
material having an electrical resitivity in a range of about 3 10-6
.OMEGA.m to about 3 10-3 .OMEGA.m.
15. The system of claim 13, wherein the shield body comprises a
material having a density less than about about 2.4 g/cm3.
16. The system of claim 13, wherein the shield body comprises at
least one of thermal pyrolitic graphite and pyrolitic boron
nitride.
17. A thermal shield for a superconducting magnet, the thermal
shield including: a shield body having an annular shape; and a
cladding comprising a material having a thermal conductivity
greater than about 1000 W/mK at about 70K, the cladding being
disposed at a surface of the shield body.
18. The thermal shield of claim 17, wherein the cladding comprises
a material having an electrical resitivity in a range of about 3
10-6 .OMEGA.m to about 3 10-3 .OMEGA.m.
19. The thermal shield of claim 17, wherein the cladding comprises
a material having a density less than about about 2.4 g/cm3.
20. The thermal shield of claim 17, wherein the cladding comprises
at least one of thermal pyrolitic graphite and pyrolitic boron
nitride and a thickness of the cladding is selected in response to
thermal gradient and average shield temperature variations.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a thermal shield and, more
particularly, to a thermal shield for use with a superconducting
magnet of, for example, a magnetic resonance imaging (MRI)
system.
[0002] MRI systems are commonly used in medical imaging
applications since MRI scans produce detailed images of soft
tissues. MRI systems produce images by excitation of selected
dipoles within a subject and receiving magnetic resonance signals
emanating from the dipoles. In order to produce the excitation of
selected dipoles within the subject, a powerful and uniform
magnetic field is required. The powerful and uniform magnetic field
may be produced by superconducting magnet coils.
[0003] Superconducting magnetic coils operate under cryogenic
temperatures and therefore require robust cooling systems. The
cooling systems typically require a cryogen or refrigerant, for
example, liquid helium, in order to achieve cryogenic temperatures.
However, cryogens are not abundant and often add significant cost
to a cryostat portion of an MRI system. Thus, it is desirable to
thermally isolate the superconducting magnet coils to the greatest
extent possible to minimize cooling requirements.
[0004] In order to thermally isolate the superconducting magnet
coils, thermal shields have been disposed around the
superconducting magnet coils. Aluminum alloys have typically been
employed for the thermal shields. Aluminum is considered
advantageous for having properties including a low density and a
light weight, while maintaining good strength and thermal
conductivity characteristics. However, a disadvantage of aluminum
is that aluminum has a high electrical conductivity. High
electrical conductivity creates large mechanical stresses on the
thermal shields when the magnet becomes normal or quenched.
Additionally, aluminum thermal shields suffer from field
instability that may reduce a quality of images obtained by the MRI
system.
[0005] Two types of field instability, gradient and vibration
induced field instability affect aluminum thermal shields by
producing eddy currents in the thermal shield that reduce image
quality. Vibration induced field instability is caused by vibration
from a cooling engine (coldhead), environmental excitation and
gradient pulse. Such vibration produces eddy currents that reduce
image quality and are complex and costly to avoid. Gradient field
instability is a result of magnetic fields generated during
gradient pulse that may cause image artifacts.
[0006] Thus, it is desirable to design a thermal shield that
improves upon existing art.
BRIEF DESCRIPTION OF THE INVENTION
[0007] Exemplary embodiments of the invention include a thermal
shield for a superconducting magnet. The thermal shield for a
superconducting magnet includes a shield body having an annular
shape. The shield body includes a material having a thermal
conductivity greater than about 1000 W/mK at about 70K.
[0008] Further exemplary embodiments of the invention include a
shielding system for shielding and cooling a superconducting magnet
coil via a cryogen. The shielding system includes a cryogen vessel,
a thermal shield, and a vacuum vessel. The cryogen vessel contains
the cryogen and is disposed proximate to the superconducting magnet
coil to enclose the superconducting magnet coil. The thermal shield
includes a shield body comprising a material having a thermal
conductivity greater than about 1000 W/mK at about 70K. The thermal
shield is disposed proximate to the cryogen vessel to enclose the
cryogen vessel. The vacuum vessel is disposed proximate to the
thermal shield to enclose the thermal shield.
[0009] Still further exemplary embodiments of the invention include
a thermal shield for a superconducting magnet. The thermal shield
includes a shield body having an annular shape and a cladding. The
cladding includes a material having a thermal conductivity greater
than about 1000 W/mK at about 70K. The cladding is disposed at a
surface of the shield body.
[0010] The above, and other objects, features and advantages of the
present invention will become apparent from the following
description read in conjunction with the accompanying drawings, in
which like reference numerals designate the same elements.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Referring now to the drawings wherein like elements are
numbered alike in the several FIGURES:
[0012] FIG. 1 shows a sectional view of a shielding system
according to an exemplary embodiment;
[0013] FIG. 2 shows a sectional view of an composite thermal shield
according to an exemplary embodiment; and
[0014] FIG. 3 shows an expanded view of a joint section of the
composite thermal shield of FIG. 2.
DETAILED DESCRIPTION OF THE INVENTION
[0015] FIG. 1 shows a sectional view of a shielding system
according to an exemplary embodiment. A shielding system 10 for a
superconducting magnet coil 12 includes a helium vessel 14 (or
cryogen vessel), a low temperature thermal shield 16, a high
temperature thermal shield 18 and a vacuum vessel 20. The shielding
system 10 extends around an imaging space (not shown) to form an
annular shape around a circumference of the imaging space. Each of
the superconducting magnet coil 12, the helium vessel 14, the low
temperature thermal shield 16, the high temperature thermal shield
18 and the vacuum vessel 20 similarly extend around the
circumference of the imaging space in an annular shape.
[0016] The superconducting magnet coil 12 may be any suitable
superconducting coil known in the art. The helium vessel 14 is
disposed proximate to the superconducting magnet coil 12 to enclose
the superconducting magnet coil 12. The helium vessel 14 is filled
with a cryogenic coolant, for example, liquid helium. The cryogenic
coolant provides cooling to the superconducting magnet coil 12 to
allow the superconducting magnet coil 12 to achieve
superconductivity at cryogenic temperatures. In this exemplary
embodiment, the helium vessel 14 has a shape of a hollow
rectangular prism extended to form an annular shape, however, any
suitable shape is envisioned.
[0017] The low temperature thermal shield 16 is disposed proximate
to the helium vessel 14 to enclose the helium vessel 14. The high
temperature thermal shield 18 is disposed proximate to the low
temperature thermal shield 16 to enclose the low temperature
thermal shield 16. The low and high temperature thermal shields 16
and 18 function to thermally isolate the superconducting magnet
coil 12 to reduce the cooling requirements on the cryogenic
coolant. A shape of the low and high temperature thermal shields 16
and 18 is substantially similar to that of the helium vessel 14.
Although FIG. 1 shows two thermal shields, it should be noted that
either more or less thermal shields may be employed depending on
operational requirements of the shielding system 10.
[0018] The low and high temperature thermal shields 16 and 18 are
each in thermal contact with a portion of a coldhead sleeve 24. In
an exemplary embodiment, the low and high temperature thermal
shields 16 and 18 may each be in physical contact with the coldhead
sleeve 24. Alternatively, a thermal link 26 may provide thermal
contact between the low and high temperature thermal shields 16 and
18 and the coldhead sleeve 24.
[0019] The coldhead sleeve 24 provides a means for cooling the low
and high temperature thermal shields 16 and 18. In an exemplary
embodiment, a cooling engine (not shown) provides cooling to the
low and high temperature thermal shields 16 and 18 to cool the low
and high temperature thermal shields 16 and 18 to a temperature of
about 45K to about 70K, which varies in response to a conductance
of a thermal shield. The cooling engine may be, for example, a
stirling or pulse tube type, but is not limited to any particular
engine. The shielding system 10 may alternatively include a
plurality of coldhead sleeves 24, as shown in FIG. 1. The coldhead
sleeve 24 may include a recondenser 28 in thermal communication
with the cryogenic coolant.
[0020] The vacuum vessel 20 is disposed proximate to the high
temperature thermal shield 18 to enclose the high temperature
thermal shield 18 and maintain interior portions of the vacuum
vessel 20 substantially at a vacuum with respect to regions
exterior to the vacuum vessel 20. A shape of the vacuum vessel 20
is substantially similar to that of the low and high temperature
thermal shields 16 and 18 and the helium vessel 14.
[0021] In an exemplary embodiment, a penetration 28 passes through
the vacuum vessel 20, the high temperature thermal shield 18, the
low temperature thermal shield 16, and the helium vessel 14. The
penetration 28 provides a conduit to pass wires for electrical
communication with the superconducting magnet coil 12 or
instrumentation to monitor a characteristic of the superconducting
magnet coil 12. The penetration 28 may include thermal links 26
providing thermal communication with the low and high temperature
thermal shields 16 and 18.
[0022] In an exemplary embodiment, the low and high temperature
thermal shields 16 and 18 are each a composite thermal shield 40
(see FIG. 2). Alternatively, one of the low and high temperature
thermal shields 16 and 18 may be a composite thermal shield 40,
while the other is a conventional thermal shield. Additionally, if
the shielding system 10 comprises a plurality of thermal shields,
it should be understood that any combination of conventional
thermal shields and composite thermal shields 40 may be
employed.
[0023] FIG. 2 shows a sectional view of a composite thermal shield
40 according to an exemplary embodiment. The composite shield 40
according to this exemplary embodiment employs a shape of a hollow
rectangular prism extended annularly around the circumference of
the imaging space, but any suitable shape may be employed. The
composite thermal shield 40 includes a shield body 41 having a
first sidewall 42, a second sidewall, 44, a third sidewall 46, and
a fourth sidewall 48. Each of the first to fourth sidewalls 42 to
48 is made of a composite material to be described in greater
detail below. When viewed in a cross section, the first sidewall 42
is disposed substantially perpendicular to the third sidewall 46
and the fourth sidewall 48. End portions of the third and fourth
sidewalls 46 and 48 are disposed proximate to opposite end portions
of the first sidewall 42, such that the third and fourth sidewalls
46 and 48 are substantially parallel to each other and face each
other. The second sidewall 44 is disposed substantially
perpendicular to the third and fourth sidewalls 46 and 48. The
second sidewall 44 is disposed proximate to opposite end portions
of each of the third and fourth sidewalls 46 and 48, such that the
second sidewall 44 is substantially parallel to the first sidewall
42 and faces the first sidewall 42. The first to fourth sidewalls
42 to 48 define a receiving space to receive, for example, the
superconducting magnet coil 12, the helium vessel 14 or another
thermal shield.
[0024] In an exemplary embodiment, the first and second sidewalls
42 and 44 each include sub-members 50. Although FIG. shows two
sub-members 50 for each of the first and second sidewalls 42 and
44, it should be understood that additional sub-members 50 may be
employed. Each sub-member 50 forms an opposite end portion of the
first and second sidewalls 42 and 44. Additionally, adjacent ends
of each sub-member 50 are joined by a center support ring 54. The
center support ring 54 extends around an interior portion of the
composite thermal shield 40 to seal a joint between each sub-member
50.
[0025] Joints between each of the first to fourth sidewalls 42 to
48 are sealed by a corner support ring 58. Each corner support ring
58 extends around an interior portion of the composite thermal
shield 40 to seal the joints between each of the first to fourth
sidewalls 42 to 48.
[0026] FIG. 3 shows an expanded view of a joint section "A" of the
composite thermal shield 40 of FIG. 2. FIG. 3 shows the joint
section "A" between the first and fourth sidewalls 42 and 48, but
each other joint section is substantially identical. The first and
fourth sidewalls 42 and 48 are sealed by the corner support ring
58. Screws 60 may engage the corner support ring 58 via holes in
each of the first and fourth sidewalls 42 and 48. Additionally, a
high thermal conductive epoxy 64 may be disposed between the first
and fourth sidewalls 42 and 48 at a location of contact between the
first and fourth sidewalls 42 and 48.
[0027] The composite material used to make each of the first and
fourth sidewalls 42 and 48 is chosen for its low density, high
thermal conductivity and low electrical conductivity relative to
aluminum and aluminum alloys. For example, thermal pyrolytic
graphite (TPG) and pyrolitic boron nitride (PBN) may be used. The
composite material is selected to have a thermal conductivity
greater than about 1000 W/mK at about 70K. Since aluminum has a
thermal conductivity of about 300 W/mK, thickness of a thermal
shield may be reduced by three times and still achieve similar
thermal performance to an aluminum thermal shield. The composite
material is selected to have an electrical resistivity in a range
of about 3 10-6 .OMEGA.m to about 3 10-3 .OMEGA.m. Since the
composite material has a high electrical resistivity as compared to
a conventional thermal shield, eddy currents induced in a thermal
shield having the high electrical resistivity are negligible. Thus,
vibration and gradient coil induced field instability are
negligible. The composite material is selected to have density less
than about 2.4 g/cm3 or about 10% less than aluminum, thereby
decreasing weight of the thermal shield.
[0028] A surface of the composite thermal shield 40 that faces the
helium vessel 14 must have emissivity control to reduce heat
radiation from the composite thermal shield 40 to the helium vessel
14. To achieve the emissivity control, high conductive aluminum
tape 70 may be applied to an inner surface of the composite thermal
shield 40.
[0029] As stated above, any combination of composite thermal
shields 40 and conventional thermal shields may be employed.
Additionally, a conventional thermal shield may be disposed in
contact and enclosing a composite thermal shield 40. Alternatively,
a conventional thermal shield may be clad with TPG or PBN to
provide improved performance. Furthermore, a thickness of TPG or
PBN cladding may be varied to optimize thermal gradient and average
shield temperature reductions.
[0030] In addition, while the invention has been described with
reference to exemplary embodiments, it will be understood by those
skilled in the art that various changes may be made and equivalents
may be substituted for elements thereof without departing from the
scope of the invention. In addition, many modifications may be made
to adapt a particular situation or material to the teachings of the
invention without departing from the essential scope thereof.
Therefore, it is intended that the invention not be limited to the
particular embodiment disclosed as the best mode contemplated for
carrying out this invention, but that the invention will include
all embodiments falling within the scope of the appended claims.
Moreover, the use of the terms first, second, etc. do not denote
any order or importance, but rather the terms first, second, etc.
are used to distinguish one element from another. Furthermore, the
use of the terms a, an, etc. do not denote a limitation of
quantity, but rather denote the presence of at least one of the
referenced item.
* * * * *