WO2005117036A1 - Blindage conducteur pour refrigerateur - Google Patents

Blindage conducteur pour refrigerateur Download PDF

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Publication number
WO2005117036A1
WO2005117036A1 PCT/EP2005/005153 EP2005005153W WO2005117036A1 WO 2005117036 A1 WO2005117036 A1 WO 2005117036A1 EP 2005005153 W EP2005005153 W EP 2005005153W WO 2005117036 A1 WO2005117036 A1 WO 2005117036A1
Authority
WO
WIPO (PCT)
Prior art keywords
refrigerator
shield
magnet system
cryogenic
stage
Prior art date
Application number
PCT/EP2005/005153
Other languages
English (en)
Other versions
WO2005117036A8 (fr
Inventor
Timothy John Hughes
Keith White
Stephen Joseph Shelford Lister
Original Assignee
Siemens Magnet Technology Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from GB0411603A external-priority patent/GB0411603D0/en
Application filed by Siemens Magnet Technology Ltd filed Critical Siemens Magnet Technology Ltd
Priority to US11/597,655 priority Critical patent/US8171741B2/en
Priority to CN2005800168586A priority patent/CN1957429B/zh
Priority to GB0619049A priority patent/GB2430024B/en
Publication of WO2005117036A1 publication Critical patent/WO2005117036A1/fr
Publication of WO2005117036A8 publication Critical patent/WO2005117036A8/fr

Links

Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F27/00—Details of transformers or inductances, in general
    • H01F27/34—Special means for preventing or reducing unwanted electric or magnetic effects, e.g. no-load losses, reactive currents, harmonics, oscillations, leakage fields
    • H01F27/36—Electric or magnetic shields or screens
    • H01F27/363—Electric or magnetic shields or screens made of electrically conductive material
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25B—REFRIGERATION MACHINES, PLANTS OR SYSTEMS; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS
    • F25B9/00—Compression machines, plants or systems, in which the refrigerant is air or other gas of low boiling point
    • F25B9/14—Compression machines, plants or systems, in which the refrigerant is air or other gas of low boiling point characterised by the cycle used, e.g. Stirling cycle
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R33/00—Arrangements or instruments for measuring magnetic variables
    • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
    • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
    • G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
    • G01R33/381—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field using electromagnets
    • G01R33/3815—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field using electromagnets with superconducting coils, e.g. power supply therefor
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F6/00—Superconducting magnets; Superconducting coils
    • H01F6/04—Cooling
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F27/00—Details of transformers or inductances, in general
    • H01F27/34—Special means for preventing or reducing unwanted electric or magnetic effects, e.g. no-load losses, reactive currents, harmonics, oscillations, leakage fields
    • H01F27/36—Electric or magnetic shields or screens

Definitions

  • the present invention relates to cryogenic magnet apparatus for producing uniform magnetic fields.
  • the present invention relates to a shield to be placed around a cryogenic refrigerator, to reduce the influence of the cryogenic refrigerator on the stability of the resultant magnetic field.
  • MRI magnet systems typically include cryogenic magnet apparatus and are used for medical diagnosis.
  • a requirement of an MRI magnet is a stable, homogeneous, magnetic field.
  • a superconducting magnet system which operates at very low temperature, the temperature being maintained by cooling the superconductor, typically by immersion, in a low temperature cryogenic fluid, typically 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 liquid used.
  • the superconducting magnet system typically comprises a set of superconductor windings for producing a magnetic field, a cryogenic fluid vessel which contains the superconductor windings and the cryogenic fluid, one or more thermal shields completely surrounding the cryogenic fluid vessel, and a vacuum jacket completely enclosing the one or more thermal shields.
  • a refrigerator In order to further reduce the heat load onto the fluid vessel, and thus the loss of liquid cryogen due to boil-off, it is common practice to use a refrigerator to cool the thermal shields to a low temperature. It is also known to use a refrigerator to directly refrigerate the cryogen vessel, thereby reducing the cryogen fluid consumption to zero. In both cases it is necessary to achieve good thermal contact between the refrigerator and the object to be cooled.
  • any magnetic material in the vicinity of the magnet will be magnetized by the field surrounding the magnet, and its magnetism will affect the homogeneity and magnitude of the imaging field in the centre of the magnet.
  • the disturbance can be compensated by a process known as shimming, in which extra fields are created in the imaging region which cancel the effect of the disturbing field. If there are moving magnetic materials in the vicinity of the magnet, shimming cannot compensate, and the imaging field is disturbed with a resulting degradation of the MRI image. It is evidently desirable to reduce such time varying interferences to a minimum.
  • a Faraday cage around the magnet can shield it from high frequency interference, and a magnetically soft steel cage will ameliorate the effects of low frequency magnetic interference, outside the cages.
  • refrigerators which are used on superconducting MRI magnet systems may contain magnetic materials in their heat exchangers, known as regenerators, which move during the operation of the refrigerator.
  • regenerators which move during the operation of the refrigerator.
  • regenerators As these refrigerators are used to cool the MRI system, they are in close proximity to the magnet, and are usually situated partially inside the vacuum jacket of the magnet, and therefore cannot be shielded by the conventional means mentioned before. It is desirable to find a means of reducing the interference.
  • the refrigerator is subject to wear, and must be replaced after a certain time in order to maintain adequate performance. It must therefore be removably interfaced to the magnet system.
  • the moving magnetic materials of the refrigerator move in the field of the magnet, and the moving magnetization degrades the MRI image.
  • United States Patent 5,701,744 describes a superconductive shield of bismuth alloy placed around a rare-earth displacement cryocooler. Such a shield has disadvantages in that the bismuth alloy shield may itself become permanently magnetised; the bismuth alloy used is relatively expensive, and does not have sufficient thermal conductivity.
  • the present invention accordingly provides apparatus as defined in the appended claims to address at least some of the disadvantages of the prior art.
  • the present invention provides an electrically conductive shield placed in the vacuum space surrounding that part of the refrigerator where moving magnetic parts are situated, so that magnetic field disturbances of the homogeneous field due to the moving magnetic parts of the refrigerator are reduced.
  • Fig. 1 shows a cross-section of a cryogenic magnet system which may benefit from the present invention
  • Fig. 2 shows part of a refrigerator and interface, suitable for use in a system such as that illustrated in Fig. 1, modified according to the present invention
  • Figs. 3A and 3B shows isometric and plan diagrams, respectively, useful for discussing the theoretical effects of the present invention.
  • FIG. 1 shows a schematic of a cryogenic magnet system fitted with a refrigerator 4 in an interface sock (also known as an interface sleeve) 5.
  • the particular cryogenic magnet system illustrated is an MRI magnet system.
  • Liquid cryogen vessel 1, containing superconductor magnet (not shown) is surrounded by one or more thermal shields 2, which are in turn completely surrounded by a vacuum jacket 3.
  • a refrigerator 4 thermally and mechanically interfaced by interface sock 5 so as to cool the thermal shields 2 through a thermal link 5 a, which may be of braided copper or any other suitable known thermal link.
  • the interior of the interface sock 5 may be in communion with the interior of the cryogen vessel 1 , for example through a tube 6.
  • the refrigerator 4 may then serve to recondense evaporated cryogen gas and deliver it back to the cryogen vessel 1 through the tube 6.
  • certain magnetic material may be brought into motion.
  • the regenerator material in a Gifford-McMahon (GM)-type refrigerator may oscillate as shown by arrow 7.
  • FIG. 2 shows an example of part of a refrigerator and interface sock in more detail.
  • the refrigerator is a two-stage refrigerator.
  • a first stage 21 of the refrigerator 4 cools a first stage cooling stage 22, which is connected to a first stage thermal station 23 of the interface sock.
  • This first stage thermal station 23 is thermally linked to the thermal shield(s) 2 by thermal link 5 a, thereby, providing a heat path for the cooling of the shield(s) by the refrigerator.
  • a second stage 8 of the refrigerator 4 is situated in the lower part 9 of the interface sock 5.
  • the regenerator of the second stage of the refrigerator may contain magnetic material.
  • the second-stage regenerator material may move in the field generated by the magnet system. The movement of this material during operation of the refrigerator creates a disturbance in the magnetic field produced by the magnet system. This disturbance will then cause disruption of the uniformity of the magnetic field of the system, and disruption of images produced by an MRI system using the magnet. In systems other than MRI systems, otherwise undesirable disruptions to the homogeneity of the magnetic field will result.
  • an electrically conductive shield 10 at least substantially surrounds the second stage 8 of the refrigerator 4, and is mechanically and thermally attached to the interface sock 5 near to the cold end 24.
  • the body of the shield 10 is cylindrical, and is preferably closed at one end by a base 11 which is in good thermal contact with the body of the shield.
  • the shield includes hole allowing tube 6 to protrude through the shield.
  • the body of the shield 10 extends as far as possible along the refrigerator second stage 8 but not so as to touch the higher temperature regions of the refrigerator sock, such as the first stage thermal station 23.
  • the shield 10 may be secured using screws 12 or studs and nuts 13 through, or around the periphery of, the base 11, or by other means to provide mechanical support and thermal contact between the shield 10 and the cold end 24 of the refrigerator interface sock 5.
  • the refrigerator sock is filled with cryogen gas, and is in communion with the cryogen vessel 1.
  • the shield 10 is located outside of the interface sock 5, in the vacuum between cryogen vessel 1 and vacuum jacket 3.
  • Shield 10 is located within the vacuum space of the magnet system because it is typically a thermally conductive element as well as an electrically conductive element. If the shield 10 were placed inside the refrigerator interface sock, where there is cryogen gas in the illustrated example, the shield 10 would conduct heat by contact with the cryogen gas from near the upper regions of the second stage 8 of the refrigerator, which are at a temperature near that of the first stage heat stage 22, to the lower region of the second stage 8 of the refrigerator which are at a much lower temperature. This would seriously reduce the overall cooling ability of the refrigeration.
  • the interface sock 5 may be sealed from the cryogen vessel 1, and the refrigerator may be in a vacuum space within the sock.
  • the shield 10 could also be placed inside the refrigerator interface sock, in close proximity to the second stage of the refrigerator.
  • Figure 3 A-3B show the distortion of a field of the magnet system, modified according to an embodiment of the present invention, as a result of the presence and motion of magnetic material 14 such as within a regenerator of the refrigerator 4. Only the most distorted field lines are shown. The distortion is shown for a magnetic material 14 of a material which locally increases the magnetic field strength, but other types of magnetic material used in regenerators are of a type which decrease the local magnetic field strength.
  • the present invention may be applied to embodiments in which either type of magnetic material is present.
  • the magnetic material 14 is within the electrically conductive shield 10 and produces a distortion of the local magnetic field.
  • the field distortion intersects the wall of shield 10 in the area 15 indicated.
  • the inventors believe that the following explanation gives an accurate understanding of the operation of the present invention.
  • the magnetic material moves during the operation of the refrigerator, as shown by arrow 7, the magnetic field distortion moves and the magnetic flux distribution intersecting the wall of the shield 10 changes. It is well known that if the magnetic flux intersecting a conductor changes, eddy currents are set up which oppose the change of flux.
  • the overall effect of these eddy currents, which oppose changes in the magnetic flux, is that if the electrical conductivity of the shield 10 is large, the changes of magnetic field inside the shield 10 when the regenerator moves will be greatly reduced on the outside of the shield.
  • the shield 10 accordingly reduces the effect of the moving magnetic material 14 on the magnetic field of the system.
  • the magnetic shielding effect of electrically conductive shields for cyclically time varying magnetic fields depends on the electrical resistivity p and thickness of the shield and the frequency / of the time variation.
  • the frequency / of the refrigerator is typically about 1-2 Hz.
  • the resistivity p of C101 copper is 17.9xl0 "9 ⁇ -m, and of 1200 aluminium is 28.6xl0 "9 ⁇ -m.
  • the permeability of free space ⁇ 0 4 ⁇ xl0 "7 H/m.
  • the skin depth is respectively 0.048m and 0.060m for copper and aluminium.
  • the resistivity p of electrical conductors such as copper and aluminium decreases as the temperature is reduced; and that the reduction of resistivity increases as the purity and softness of the conductor increases.
  • the resistivity reduces by a factor of up to 5000 if the temperature is reduced to 4.2 K, and the skin depth at 2Hz decreases to 0.85 mm.
  • the shielding will not be as effective as calculated above, because of the finite length of the shield. It is to be understood that, although aluminium has been used as an example, other materials having similar electrical properties, for example copper, can also be used.
  • the magnetic flux changes are in the areas indicated 15, aligned with the external field direction indicated by arrow Bo, and eddy currents will be set up in these regions. It is possible therefore with little effect on the shielding properties of the shield to cut shield 10 along its length, perpendicular to the field direction, as indicated at 16 in Fig. 3B.
  • assembly of the shield around the refrigerator interface sock 5 is made much more simple as compared to the process required for assembling a single piece shield around the refrigerator interface sock.

Landscapes

  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Physics & Mathematics (AREA)
  • Mechanical Engineering (AREA)
  • Condensed Matter Physics & Semiconductors (AREA)
  • General Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Thermal Sciences (AREA)
  • General Engineering & Computer Science (AREA)
  • Containers, Films, And Cooling For Superconductive Devices (AREA)
  • Shielding Devices Or Components To Electric Or Magnetic Fields (AREA)
  • Magnetic Resonance Imaging Apparatus (AREA)
  • Devices That Are Associated With Refrigeration Equipment (AREA)

Abstract

Un système magnétique cryogénique comprend une cuve cryogénique (1) contenant un enroulement magnétique, une enveloppe à vide isolant (3) contenant la cuve cryogénique, et un réfrigérateur (4) au moins partiellement enfermé dans l'enveloppe à vide isolant et relié thermiquement (6) à la cuve cryogénique. Plus particulièrement, ce système comprend un blindage électromagnétique.
PCT/EP2005/005153 2004-05-25 2005-03-12 Blindage conducteur pour refrigerateur WO2005117036A1 (fr)

Priority Applications (3)

Application Number Priority Date Filing Date Title
US11/597,655 US8171741B2 (en) 2004-05-25 2005-03-12 Electrically conductive shield for refrigerator
CN2005800168586A CN1957429B (zh) 2004-05-25 2005-05-12 用于制冷机的导电屏蔽罩
GB0619049A GB2430024B (en) 2004-05-25 2005-05-12 Electrically conductive shield for refrigerator

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
GB0411603.4 2004-05-25
GB0411603A GB0411603D0 (en) 2004-05-25 2004-05-25 Electromagnetic shield for refrigerator
GB0426534A GB2414539B (en) 2004-05-25 2004-12-03 Electrically conductive shield for refrigerator
GB0426534.4 2004-12-03

Publications (2)

Publication Number Publication Date
WO2005117036A1 true WO2005117036A1 (fr) 2005-12-08
WO2005117036A8 WO2005117036A8 (fr) 2006-03-16

Family

ID=34967896

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/EP2005/005153 WO2005117036A1 (fr) 2004-05-25 2005-03-12 Blindage conducteur pour refrigerateur

Country Status (2)

Country Link
GB (1) GB2430024B (fr)
WO (1) WO2005117036A1 (fr)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN101752050A (zh) * 2010-03-24 2010-06-23 哈尔滨工业大学 高温超导线圈的磁场屏蔽装置

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5701744A (en) * 1996-10-31 1997-12-30 General Electric Company Magnetic resonance imager with helium recondensing
JPH1022118A (ja) * 1996-07-01 1998-01-23 Sumitomo Heavy Ind Ltd 超電導コイルの熱シールド板

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH1022118A (ja) * 1996-07-01 1998-01-23 Sumitomo Heavy Ind Ltd 超電導コイルの熱シールド板
US5701744A (en) * 1996-10-31 1997-12-30 General Electric Company Magnetic resonance imager with helium recondensing

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
PATENT ABSTRACTS OF JAPAN vol. 1998, no. 05 30 April 1998 (1998-04-30) *

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN101752050A (zh) * 2010-03-24 2010-06-23 哈尔滨工业大学 高温超导线圈的磁场屏蔽装置

Also Published As

Publication number Publication date
GB0619049D0 (en) 2006-11-08
GB2430024A (en) 2007-03-14
GB2430024B (en) 2008-08-20
WO2005117036A8 (fr) 2006-03-16
GB2430024A8 (en) 2007-07-11

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