EP0145940A1 - Elektrischer Stromkreis für ein Magnetfeld hoher Gleichmässigkeit - Google Patents

Elektrischer Stromkreis für ein Magnetfeld hoher Gleichmässigkeit Download PDF

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Publication number
EP0145940A1
EP0145940A1 EP84113459A EP84113459A EP0145940A1 EP 0145940 A1 EP0145940 A1 EP 0145940A1 EP 84113459 A EP84113459 A EP 84113459A EP 84113459 A EP84113459 A EP 84113459A EP 0145940 A1 EP0145940 A1 EP 0145940A1
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EP
European Patent Office
Prior art keywords
superconductive
coils
current
circuit
coil
Prior art date
Legal status (The legal status 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 status listed.)
Granted
Application number
EP84113459A
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English (en)
French (fr)
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EP0145940B1 (de
Inventor
Thomas Alan Keim
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General Electric Co
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General Electric Co
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Publication date
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Publication of EP0145940A1 publication Critical patent/EP0145940A1/de
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Publication of EP0145940B1 publication Critical patent/EP0145940B1/de
Expired legal-status Critical Current

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F6/00—Superconducting magnets; Superconducting coils
    • H01F6/006—Supplying energising or de-energising current; Flux pumps

Definitions

  • the present invention is related to circuits and methods for providing high uniformity magnetic fields and is particularly relevant to the construction of magnet structures employed in nuclear magnetic resonance (NMR) imaging systems.
  • NMR nuclear magnetic resonance
  • NMR imaging systems particularly those employed in medical diagnostic imaging
  • this magnetic field exhibit an extremely high degree of spacial uniformity.
  • This uniformity requirement typically means that there should not be more than between about 10 and 100 parts per million variation in field strength in the. volume being imaged.
  • magnetic fields for NMR imaging are provided either by permanent magnets, resistive magnets or magnets based on superconductor technology.
  • Superconductive methods for providing the constant, uniform magnetic field provides two distinct advantages. Firstly, superconducting coils which form current loops carrying 1,000 amperes or more can be used to achieve high strength magnetic fields, up to a strength of 1.5 Tesla or more.
  • Present designs for superconductive magnets for use in NMR imaging systems typically employ a set of from about 4 to 6 superconductive coils. These coils are typically disposed on or about a cylindrical surface and are axially aligned so as to provide a relatively uniform magnetic field within a central bore of the NMR magnet.
  • the present approach taken to magnet design is the construction of a multiple coil set with each of the superconductive windings connected in series and carrying the same current.
  • coil geometry and coil current are considered together as the design variables for producing the desired "ideal" uniform magnetic field.
  • a four coil set is considered as a single entity.
  • an electric circuit for providing a high uniformity magnetic field comprises a plurality of superconductive coils together with a plurality of superconductive switch elements connected in parallel with the coils so as to form a number of superconductive current loops. Adjacent loops are connected by a bridging conductor so as to connect the superconducting coils in series. Each superconductive loop may also be constructed in such a way that the loop requires only a single superconductive joint.
  • the bridging conductors may be ohmic or superconductive with the preferable embodiment including an ohmic (for example, copper) conductor connected in parallel with a superconductive conductor with both conductors being disposed within the coolant medium.
  • the circuit of the present invention also preferably includes a number of protective ohmic resistor elements connected in parallel with each of the superconductive coils.
  • the circuit of the present invention is particularly advantageous in that it provides a method for establishing currents in each of the superconductive coils in an independent fashion so that individual adjusting currents may be supplied to each superconductive current loop separately.
  • FIG. 1 illustrates a conventional electric circuit for a superconductive magnet for NMR imaging employing four superconductive coils 10a, lOb, lOc, and 10d. These coils are connected in series, each being joined to a single other coil by a superconducting joint, such as joint lla, llb or llc.
  • Superconductive switch 20 is connected to one end of the series connected coils by means of superconductive joint lid. Switch 20 is also connected to the other end of the series connected coils by means of second superconductive joint 11e. This completes the superconductive current loop in which the same current flows in all of the circuit elements, namely switch 20 and coils 10a-10d.
  • Switch 20 is typically a length of superconductive material disposed in proximity to a heat source which is capable of raising the temperature of the superconductive material to a temperature above its critical value. When thermal energy is applied to the switch, switch 20 is made to exhibit a finite resistance R. The resistance of switch 20 (in its resistive state) produces further resistive (1 2 R) heating of the material in switch 20. This in turn leads to rapid quenching of the current flowing in the superconductive loop.
  • Figure 1 also illustrates the fact that it is highly preferable to dispose ohmic resistance elements in parallel with the superconductive coils. Accordingly, ohmic resistor elements 15a-15d are shown connected in parallel with superconductive coils lOa-d, respectively. Resistance elements 15a-d serve a protective purpose. Under normal (superconductive) operating conditions, all of the loop current flows through the superconductive elements. These elements are maintained below the critical temperature, which is typically above 4.2°K, by immersion in a coolant such as liquid helium. The whole circuit is disposed within a cryostat to maintain the helium in the liquid state. Since the superconductive elements exhibit'zero resistance the preferable current path is the single superconductive loop shown in heavy lines in Figure 1.
  • the protective ohmic elements provide an alternate current path and a location for the dissipation of the electric magnetic energy stored within the corresponding superconductive coil. Since these protective devices do not have to be maintained below a critical temperature, they may be located either inside or outside the cryostat structure.
  • Figures 1 and 2 illustrate circuits employing four superconductive coils, any reasonable number of coils may be employed to provide the desired field homogeneity.
  • the superconductive elements of the circuit comprise an integral assembly.
  • testing of the circuit requires the simultaneous testing of all of superconductive coils 10a-d, all of the superconductive joints lla-e and superconductive switch 20. If unexpected quenching or field drift is perceived, it is difficult to determine which of the elements of the superconductive circuit is at fault. If a problem exists in one of the superconductive joints, the specific problem is difficult to isolate since any of the five joints could in fact be causing a problem.
  • the first current loop comprises superconductive coil 10a, superconductive joint 21b, superconductive switch 20a, superconductive joint 21a and the associated connecting superconductive wire.
  • the loops including coils lOb, 10c and lOd, respectively.
  • bridging conductors 25a-c are shown in Figure 2 as ohmic components, it is also possible to employ superconductive components for these bridging conductors.
  • bridging conductor 25a could comprise a superconductive conductor extending between superconductive joints 21b and 21c.
  • the bridging conductors of the present invention are preferably ohmic and positioned as shown in Figure 2.
  • the circuit of Figure 2 also preferably includes ohmic protective resistive devices 15a-d connected in parallel with superconductive coils loa-d, respectively.
  • the circuit of Figure 2 also preferably includes terminals T 1 , T 2 , T 3 , T4, and T 5 to provide independent current adjustment in the four current loops shown.
  • the superconductive coils of the circuit of Figure 1 are usually fabricated separately, assembled and joined by means of special superconductive joints, as shown.
  • this conventional approach requires n + 1 superconductive joints each of which must be tested simultaneously.
  • the circuit of Figure 2 as shown illustrates the presence of 2n superconductive joints, each separate superconductive loop may be tested separately so that if a defect is detected, it is immediately known, with high likelihood, that the problem lies in either one or the other of the two superconductive joints in each separate loop.
  • the circuit of Figure 2 is particularly amenable to the construction of superconductive loops having only a single superconductive joint.
  • switch 20a by forming switch 20a from the same length of superconductive wire as used to form coil 10a, it is possible to eliminate either joint 21a or 21b.
  • the circuit of Figure 2 possesses only n superconductive joints. Each such joint is found within a distinct, separately energizable loop. Accordingly, if a defect is found within the loop, the problem may be immediately determined to be within a single superconductive joint.
  • FIG 3 illustrates only a single superconductive loop circuit, it being understood that the modification indicated in Figure 3 is applicable to each of the four superconductive loops shown in Figure 2.
  • the protective function of resistors 15a-d is at least partially defeated by the fact that switches 20a-d, respectively, are normally in the superconductive state during a quench in coils loa-d, respectively. This current would tend to be shunted through switch 20a rather than protective resistor 15a.
  • Resistors 16x and 17x are preferably implemented by providing a center tap in a single, integrated resistor structure.
  • center tap does not imply connection to the exact midpoint of the structure (coil or resistor) to which the term is applied.
  • conductors which are necessarily superconductive are shown by heavier lines.
  • the operation of the circuit of Figure 2 is also significantly different from the operation of the circuit of Figure 1, particularly with respect to persistent current initiation.
  • superconductive coils loa-d are reduced to a temperature below their critical temperature so as to be superconductive.
  • Switches 20a-d are placed in their resistive states and a main current power supply is connected to terminals T 1 and T 5 .
  • the current is slowly increased until the nominal design current is reached.
  • switches 20a-d are switched to the superconductive state so as to establish a plurality of superconductive current loops each of which possess the same nominal current.
  • the stability of the power supply and the switching time sequence for switches there may be some slight variation in the currents in the four loops.
  • the modular and independent design of the present invention precludes this aspect of the circuit from posing any problems.
  • the main power supply is usually disconnected from terminals T 1 and T 5 1
  • conventional measurement methods may be employed to determine the uniformity of the magnetic field. Calculations may then be performed to produce coil current corrections which would produce a more uniform field.
  • the current variations are typically seen to be in the order of 100 milliamperes, rather than 1,000 amperes. Power supplies for providing these relatively small levels of adjusting currents can be controlled much more accurately than the main power supply. Accordingly, in accordance with the present invention adjusting currents are provided for coils loa-d independently.
  • the main power supply is reconnected across terminals T 1 and T 5 .
  • a correction power supply is then connected across terminals T 2 and T 3*
  • the current from the main supply is then returned to its previously applied value, so that the currents in the superconductive switches are ⁇ then approximately zero.
  • the switches are turned to their resistive states and the current in the adjusting power supply is adjusted to its desired value.
  • the switches are then returned to their superconductive states.
  • the current from all power supplies is set to zero and they are removed from the circuit. In this manner, the currents in all n of the coils may be adjusted independently.
  • This method offers the advantage that the same main current power supply is used repeatedly. Accordingly, less stringent requirements on the accuracy of the main power supply are required. Furthermore, stability of the main current power supply is only a factor over the length of time it takes to fine tune the current in the independent loops. Typically, this is only a matter of minutes.
  • the superconductive material of the present invention may comprise any material exhibiting superconductive properties
  • superconductors comprising niobium-titanium filaments disposed within a copper or aluminum matrix have been found to be particularly useful in the design and construction of NMR magnet coils.
  • liquid helium is the preferred coolant for use in the cryostat to maintain the superconductive material below its critical temperature.
  • the circuit of the present invention can act to reduce or eliminate the need for correction coils that are often employed in NMR imaging magnets. Furthermore, it is seen that the present invention provides an opportunity for limiting the testing requirements for the superconductive joints in a multicoil magnet. The circuit of the present invention also provides an opportunity for the construction of a superconductive coil and superconductive switch from the same length of superconductive conductor. Another very significant advantage of the circuit of the present invention is that the spacial homogeneity of the magnetic field may be accurately and precisely controlled by means of independently establishing correcting currents in the superconductive loops.

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Magnetic Resonance Imaging Apparatus (AREA)
EP84113459A 1983-11-18 1984-11-08 Elektrischer Stromkreis für ein Magnetfeld hoher Gleichmässigkeit Expired EP0145940B1 (de)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US55321183A 1983-11-18 1983-11-18
US553211 1995-11-07

Publications (2)

Publication Number Publication Date
EP0145940A1 true EP0145940A1 (de) 1985-06-26
EP0145940B1 EP0145940B1 (de) 1988-04-27

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EP84113459A Expired EP0145940B1 (de) 1983-11-18 1984-11-08 Elektrischer Stromkreis für ein Magnetfeld hoher Gleichmässigkeit

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EP (1) EP0145940B1 (de)
JP (1) JPS60137005A (de)
DE (1) DE3470815D1 (de)
IL (1) IL73353A (de)

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2193323A (en) * 1986-06-26 1988-02-03 Nat Res Dev Electrical coils
EP0601648A1 (de) * 1992-12-11 1994-06-15 Koninklijke Philips Electronics N.V. Kernspinresonanzapparat mit einem supraleitenden Magneten
GB2471325A (en) * 2009-06-26 2010-12-29 Siemens Magnet Technology Ltd Quench energy dissipation for superconducting magnets

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2766957B2 (ja) * 1990-03-16 1998-06-18 日本電信電話株式会社 直列接続された複数個の電磁石

Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3263133A (en) * 1966-07-26 Superconducting magnet
GB1179740A (en) * 1966-06-22 1970-01-28 Hitachi Ltd Superconducting Magnet Apparatus
GB1226597A (de) * 1967-06-28 1971-03-31
FR2112054A1 (de) * 1970-08-14 1972-06-16 Commissariat Energie Atomique
GB1404682A (en) * 1972-01-12 1975-09-03 Oxford Instr Co Ltd Superconducting magnets and leads thereto
DE2153562B2 (de) * 1970-10-29 1979-10-25 Compagnie Generale D'electricite S.A., Paris Schutzschaltung für eine supraleitende Spule

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3263133A (en) * 1966-07-26 Superconducting magnet
GB1179740A (en) * 1966-06-22 1970-01-28 Hitachi Ltd Superconducting Magnet Apparatus
GB1226597A (de) * 1967-06-28 1971-03-31
FR2112054A1 (de) * 1970-08-14 1972-06-16 Commissariat Energie Atomique
DE2153562B2 (de) * 1970-10-29 1979-10-25 Compagnie Generale D'electricite S.A., Paris Schutzschaltung für eine supraleitende Spule
GB1404682A (en) * 1972-01-12 1975-09-03 Oxford Instr Co Ltd Superconducting magnets and leads thereto

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2193323A (en) * 1986-06-26 1988-02-03 Nat Res Dev Electrical coils
GB2193323B (en) * 1986-06-26 1991-03-27 Nat Res Dev Electrical coils
EP0601648A1 (de) * 1992-12-11 1994-06-15 Koninklijke Philips Electronics N.V. Kernspinresonanzapparat mit einem supraleitenden Magneten
GB2471325A (en) * 2009-06-26 2010-12-29 Siemens Magnet Technology Ltd Quench energy dissipation for superconducting magnets
GB2471325B (en) * 2009-06-26 2011-05-18 Siemens Magnet Technology Ltd Quench energy dissipation for superconducting magnets
US8345392B2 (en) 2009-06-26 2013-01-01 Siemens Plc Quench energy dissipation for superconducting magnets

Also Published As

Publication number Publication date
JPS60137005A (ja) 1985-07-20
EP0145940B1 (de) 1988-04-27
IL73353A (en) 1988-11-30
DE3470815D1 (en) 1988-06-01
IL73353A0 (en) 1985-01-31

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