JP4477859B2 - Permanent current switch, superconducting magnet, and magnetic resonance imaging apparatus - Google Patents

Description

  The present invention relates to a superconducting magnet including a superconducting coil and a permanent current switch, and an application device thereof.

  A permanent current switch which is an element of a superconducting magnet operated in the permanent current mode is required to have high electromagnetic stability against disturbances in an operating state. Japanese Patent Laid-Open No. 61-269301 discloses a plurality of permanent current switches as a method for improving the operational stability of a permanent current switch used in a magnet that operates in a high external magnetic field or at a high operating current. A method for reducing the current capacity per switch by electrically connecting them in parallel is described.

JP-A 61-269301

  The instability of the permanent current switch is due to the necessity of increasing the electrical resistance of the switch when the switch is in the OFF state (normally driven state). It is due. In other words, when a local disturbance occurs in the superconducting filament when the switch is in the ON state (superconducting state), the current is commutated to other adjacent superconducting filaments, but at that time, the metal matrix generates a large amount of heat, and as a result A phenomenon occurs in which the entire area of the superconducting wire undergoes normal conduction transition. Therefore, in order to ensure the stability of the switch, a method such as reducing the operating current by thinning the wire or reducing the current load per switch by arranging a plurality of switches in parallel in the permanent current circuit is used. It was.

  However, the method corresponding to the use in a high magnetic field and the large current operation using the plurality of permanent current switches described above prevents the magnet structure from being made compact. In addition, when there is a difference in inductance or connection electrical resistance between terminals due to the electrical wiring structure, a difference occurs in the current value flowing through the switch, so that the current is shared equally among the entire switch. Is difficult.

  An object of the present invention is to provide a permanent current switch having excellent operational stability even at a high current load, and a superconducting magnet incorporating the permanent current switch.

  To achieve the above object, in the present invention, in a permanent current switch comprising a superconducting wire in which a plurality of superconducting filaments are arranged inside a stabilizing metal, and a heating means for heating the superconducting wire, A plurality of superconducting filaments arranged inside were electrically connected to each other by a superconducting material at a predetermined position in the longitudinal direction of the superconducting wire.

  Further, in a permanent current switch comprising a plurality of superconducting wires in which a plurality of superconducting filaments are arranged inside the stabilizing metal and a heating means for heating the superconducting wires, the plurality of superconducting wires are twisted. The superconducting filaments inside the stranded wire were electrically connected to each other by a superconducting material at a predetermined position in the longitudinal direction of the stranded wire.

  According to the present invention, even when a disturbance occurs in the permanent current switch in the operating state, the current flowing through the normally conducting filament is quickly commutated to the adjacent filament, so that no large Joule heat is generated and the entire wire is Return to superconducting state. This is because the time constant τ necessary for redistribution of the current flowing in the superconducting filament inside the superconducting wire is expressed as τ = L / R from the connection resistance R of the filament at both ends of the wire and the inductance L of the wire constituting the switch. In order to reduce the time constant τ, the inductance L should be reduced. In general, a permanent current switch is often non-inductively wound in order to reduce its inductance, and the inductance of the switch depends on the length of superconducting wires arranged in parallel. If it is shortened, the effect of reducing the time constant can be obtained. In other words, if the filaments are connected to each other with a very low electrical resistance such as superconducting connection periodically or partially with respect to the longitudinal direction of the superconducting wire constituting the switch, the time constant of current redistribution is shortened. Overall operational stability can be ensured.

  In addition, when providing the several location which electrically connects superconducting filaments, it is preferable that the space | interval of a connection location is equal with respect to the longitudinal direction of a wire. That is, if the intervals are extremely different, instability is likely to occur in the wire having the longest interval. It should be noted that the interval and the number of locations where these superconducting filaments are electrically connected can be arbitrarily set. In calculation, when the stabilizer for the superconducting wire for the permanent current switch is Cu-Ni and the electric resistance is about 100 times higher than that of oxygen-free copper, it is ideal to divide it into 100. Even when the filaments are divided into two parts as in the case where the filaments are electrically connected at the turning point of the induction winding, the effect can be exhibited.

  Also, a superconducting alloy containing lead was used as means for locally electrically connecting the filaments inside the superconducting wire. A superconducting alloy containing lead such as Pb-Bi has a relatively high critical magnetic field of about 1 T, and can maintain electrical connection between filaments in a superconducting state even when a permanent current switch is placed in the magnetic field, and has a low melting point. Therefore, connection processing can be performed by soldering. Applicable Pb alloys include Pb—Bi, Pb—Sn, Pb—In, and the like.

  According to the present invention, a permanent current switch having excellent operational stability even at a high current load can be obtained. When this supercurrent switch is combined with a superconducting coil to form a superconducting magnet, the stability of the permanent current circuit is improved, and a superconducting magnet having excellent time stability can be obtained.

  Embodiments of the present invention will be described below with reference to the drawings. In order to facilitate understanding, the same functional parts are denoted by the same reference numerals in the following drawings.

FIG. 1 is a schematic diagram showing an electric circuit of a superconducting magnet according to the present invention. The superconducting magnet 10 has a structure in which an excitation power source 15 is connected to a circuit in which a superconducting coil 11, a permanent current switch 13, and a protective resistor 14 are connected in parallel. A portion 16 indicated by a broken line in FIG. 1 represents a structural range of the superconducting magnet. The permanent current switch 13 has a structure in which a wound superconducting wire 17 and a heater 18 for heating the same are built in, and the heater 18 is turned on / off by a heater power source 19 to thereby perform normal conduction / conduction of the superconducting winding 17. It controls superconductivity. The procedure for setting the exciting circuit of the superconducting magnet 10 to the permanent current mode is as follows.
(1) The heater 18 of the permanent current switch 13 is turned on, and the permanent current switch 13 is turned off (normal conduction).
(2) The superconducting coil 11 is energized by the excitation power supply 15.
(3) Turn off the heater 18 of the permanent current switch 13 and turn on the permanent current switch 13 (superconductivity). By this operation, a current flows through a permanent current circuit (closed circuit) composed of the superconducting coil 11 and the permanent current switch 13.
(4) Lower the current of the excitation power supply 15.

  In order to increase the energization speed of the superconducting coil 11, it is necessary to increase the electrical resistance when the permanent current switch 13 connected in parallel is turned off, and to reduce the shunt to the permanent current switch 13, and for the permanent current switch. Cu-Ni is often used as a stabilizer for the superconducting wire 17. However, the use of a high-resistance material as the stabilizing material has a trade-off relationship that reduces electromagnetic stability against disturbance in the operating state.

Example 1
FIG. 2 shows a structural schematic diagram of an example of the permanent current switch according to the present invention. FIG. 3 is an exploded view of the wiring.

  As a superconducting wire constituting the permanent current switch, a superconducting wire 22 having a diameter of 0.6 mm in which 1000 NbTi superconducting filaments 21 having a diameter of 10 μm and a multicore arrangement are arranged inside a Cu-30Ni matrix was used. Using this superconducting wire 22 having a length of 30 m, a permanent current switch 20 having a non-inductive structure in which the superconducting wire 22 was folded in two and wound around a bobbin 23 was produced. Thereafter, Cu-Ni in the region 24 in which the wire is folded in half is chemically dissolved and removed, and then the Nb-Ti superconducting filament exposed in the dissolved and removed portion is soldered with a Pb-Bi alloy, whereby the filaments are mutually bonded. Electrical connection was made with a superconducting material (Pb-Bi alloy). In an actual permanent current switch, a heater wire is provided as a means for heating the superconducting wire 22, but the heating means is not related to the essence of the present invention. Was omitted. Both ends of the superconducting wire rod of the permanent current switch 20 are drawn out from the permanent current switch lead-out portion 25, and are superconductively connected to each other at the end portions of the superconducting wire 12 of the superconducting coil 11 for magnetic field generation and the connecting portions 26a and 26b. A region 27 indicated by a broken line in FIG. 3 indicates a structural range of the permanent current switch 20.

  When the manufactured permanent current switch 20 was immersed in liquid helium, an energization test was performed, and the maximum current that could be energized when the switch was in the ON state (superconducting state) was 800 A under the condition that the external magnetic field was 1 T. The current-carrying performance close to the inherent critical current characteristics could be achieved. In addition, when the permanent current switch having the same structure was prepared for comparison except that the filaments of the non-inductive winding part 24 were not connected with a superconducting material, the comparative permanent current switch showed operational instability, Normal conduction transition was made at a current value of about 500A. That is, it has also been confirmed by experiments that the operation stability of the permanent current switch is improved by locally connecting the filaments in the longitudinal direction of the wire.

  In this example, soldering with a Pb-Bi alloy was used as a method for superconducting the filaments. However, there is no particular limitation on the connection method, and these are mutually superconducting as long as the characteristics of the filaments are not deteriorated. As long as it is connected with low electrical resistance. Further, the interval and the number in the longitudinal direction for connecting the superconducting filaments are not limited to one in the middle of the total length of the superconducting filaments constituting the permanent current switch, as shown in this example. In general, the effect is greater as the distance in the longitudinal direction at the position where the superconducting filaments are connected to each other is shorter, but can be arbitrarily selected according to the switch shape.

(Example 2)
FIG. 4 is a wiring development view of another example of the permanent current switch according to the present invention. Here, a superconducting wire 22 similar to that of Example 1 was used as the superconducting wire constituting the permanent current switch 40 in a length of 30 m, and the following processing was performed before winding as a permanent current switch. Every 6 m in the longitudinal direction, the copper alloy stabilizing material of the superconducting wire 22 is removed by about 5 mm in length, and the superconducting filaments 21 exposed in the regions 41a to 41d from which the stabilizing material has been removed are electrically connected by Pb-Bi solder. Connected. Thereafter, non-inductive winding was performed to produce a permanent current switch 40.

  When the manufactured permanent current switch was immersed in liquid helium, an energization test was performed. As a result, the maximum current that could be energized in the ON state (superconducting state) of the permanent current switch was 825 A under the condition that the external magnetic field was 1T. . Also in this example, it was possible to achieve energization performance close to the critical current characteristics inherent to the superconducting wire 22. That is, it has been confirmed by experiments that even if a permanent current switch is manufactured after locally connecting the superconducting filaments at a predetermined position in the longitudinal direction of the superconducting wire, the operation stability is improved. This is because the Pb alloy is relatively soft and has excellent deformability.

(Example 3)
FIG. 5 is a wiring development view of another example of the permanent current switch according to the present invention. Here, as a superconducting wire constituting the permanent current switch 50, a stranded wire 52 in which three wires similar to those in Example 1 were twisted over a length of 30 m was used. The twisted wire 52 was folded in half and wound around a bobbin to produce a non-inductive permanent current switch 50. Thereafter, the Cu—Ni alloy in the folded region 24 of the wire is chemically dissolved and removed with nitric acid, and the exposed superconducting filament is soldered with a Pb—Bi alloy, thereby electrically connecting the filaments with the superconducting material. did.

  When the energization test was performed with the manufactured permanent current switch 50 immersed in liquid helium, the maximum current that could be energized when the permanent current switch was in the ON state (superconducting state) was 2300 A under the condition that the external magnetic field was 1T. The current-carrying performance close to the critical current characteristics inherent in the superconducting wire was achieved. That is, not only in the strands but also in the stranded wire 52, the superconducting filaments are partially electrically connected to each other at a predetermined position in the longitudinal direction, so that the time constant required for commutation between the superconducting filaments is shortened, resulting in operation. It was confirmed that the stability was improved. This is because, as in the prior art, the permanent current switches are electrically connected in parallel, and the current capacity of the wire itself is made by twisting the superconducting wire without taking the method of reducing the current sharing ratio per switch. It is shown that a switch having excellent operational stability can be provided even by increasing.

Example 4
The permanent current switch shown in Example 3 was incorporated into a superconducting magnet operated in the permanent current mode to produce the superconducting magnet shown in the circuit diagram of FIG. 4, and an excitation test was performed. After the superconducting coil 11 is excited by the external excitation power source 15 when the permanent current switch 13 is in the OFF state (the switch is heated to normal conduction), the permanent current switch 13 is in the ON state (heater heating is stopped and the superconducting state is The permanent current mode.

  The current flowing through the permanent current circuit was 700 A, and the magnetic field generated by the superconducting coil 11 was 4.0 T at the coil center. Although the magnetic field at the place where the permanent current switch 13 was disposed was about 1T, quenching due to the disturbance of the permanent current switch portion 13 did not occur, and highly stable characteristics were obtained.

(Example 5)
FIG. 6 is an external view of a magnetic resonance imaging apparatus incorporating a superconducting magnet according to the present invention. FIG. 7 shows a configuration block diagram of the devices constituting these magnetic resonance imaging apparatuses.

  The superconducting magnets 61a and 61b obtained in Example 4 were vertically arranged opposite to each other, and a uniform magnetic field was generated in the gap portion. The patient (subject) is inserted into the uniform magnetic field generation space between the superconducting magnets 61 a and 61 b while lying on the moving table 62. Further, a pair of gradient magnetic field applying means, a high frequency transmission / reception means for transmitting a high frequency magnetic field to the subject and receiving a magnetic resonance signal from the subject, a high frequency transmission / reception means and a gradient magnetic field application means are connected to the high frequency magnetic field and the gradient magnetic field. A magnetic resonance imaging apparatus as shown in FIG. 6 is configured by providing control means 65 for controlling and taking in nuclear magnetic resonance signals and performing image processing. Note that the gradient magnetic field applying means and the high-frequency transmitting / receiving means are integrated with the superconducting magnet.

  A magnetic field of about 2.0 T was generated in a permanent current state and highly stable against disturbance in the space at the center of the pair of superconducting magnets 61a and 61b arranged opposite to each other, and could be held. As a result, it has become possible to stably carry out the operation of repeating imaging photography using the magnetic resonance principle over a long period of time.

Schematic which shows the electric circuit of the superconducting magnet by this invention. 1 is a schematic structural diagram of an example of a permanent current switch according to the present invention. The wiring expanded view of an example of the permanent current switch by this invention. The wiring expanded view of the other example of the permanent current switch by this invention. The wiring expanded view of the other example of the permanent current switch by this invention. 1 is an external view of a magnetic resonance imaging apparatus incorporating a superconducting magnet according to the present invention. 1 is a block diagram of components of a magnetic resonance imaging apparatus incorporating a superconducting magnet according to the present invention.

Explanation of symbols

  10: Superconducting magnet, 11: Superconducting coil, 12: Superconducting wire of superconducting coil, 13: Permanent current switch, 14: Protection resistor, 15: Excitation power source, 17: Superconducting wire, 18: Heater, 19: Heater power source, 20: Permanent current switch, 21: Superconducting filament, 22: Superconducting wire, 23: Bobbin, 24: Area where wire rod is folded in half, 25: Permanent current switch opening part, 26a, 26b: Connection part, 40: Permanent current switch, 41a- 41d: Area from which stabilizing material has been removed, 50: Permanent current switch, 52: Twisted wire, 61a, 61b: Superconducting magnet, 62: Moving table, 65: Control means

Claims (8)

In a permanent current switch comprising a superconducting wire in which a plurality of superconducting filaments are arranged inside a stabilizing metal, and heating means for heating the superconducting wire,
A plurality of superconducting filaments arranged inside the superconducting wire are electrically connected to each other by a superconducting material at a predetermined position in the longitudinal direction of the superconducting wire ,
The superconducting wire has a non-inductive winding structure, and the predetermined position in the longitudinal direction of the superconducting wire is a non-inductive winding wire folding position . In a permanent current switch comprising a plurality of superconducting wires in which a plurality of superconducting filaments are arranged inside a stabilizing metal, and heating means for heating the superconducting wires,
The plurality of superconducting wires are stranded, and the superconducting filaments inside the stranded wire are electrically connected to each other by a superconducting material at a predetermined position in the longitudinal direction of the stranded wire ,
The superconducting wire has a non-inductive winding structure, and the predetermined position in the longitudinal direction of the superconducting wire is a non-inductive winding wire folding position .   3. The permanent current switch according to claim 1, wherein the superconducting material for electrically connecting the superconducting filaments to each other is a superconducting alloy containing lead. The permanent current switch according to any one of claims 1 to 3 , wherein the stabilizing metal is locally replaced by the superconducting material at a predetermined position in the longitudinal direction. In a permanent current switch comprising a superconducting wire in which a plurality of superconducting filaments are arranged inside a stabilizing metal, and heating means for heating the superconducting wire,
A plurality of superconducting filaments arranged inside the superconducting wire are electrically connected to each other by a superconducting material at a predetermined position in the longitudinal direction of the superconducting wire,
The permanent current switch, wherein the stabilizing metal is locally replaced by the superconducting material at a predetermined position in the longitudinal direction. In a permanent current switch comprising a plurality of superconducting wires in which a plurality of superconducting filaments are arranged inside a stabilizing metal, and heating means for heating the superconducting wires ,
The plurality of superconducting wires are stranded, and the superconducting filaments inside the stranded wire are electrically connected to each other by a superconducting material at a predetermined position in the longitudinal direction of the stranded wire,
The permanent current switch, wherein the stabilizing metal is locally replaced by the superconducting material at a predetermined position in the longitudinal direction. In a superconducting magnet including a superconducting coil for generating a magnetic field and a permanent current switch connected in parallel with the superconducting coil,
Superconducting magnet, comprising the persistent current switch of any one of claims 1-6 as the permanent current switch. A superconducting magnet for generating a magnetic field; a pair of gradient magnetic field applying means; a high frequency transmitting / receiving means for transmitting a high frequency magnetic field to the subject and receiving a magnetic resonance signal from the subject; A magnetic resonance imaging apparatus including means for controlling the gradient magnetic field applying means and controlling the capture of the magnetic resonance signal to perform image processing,
A magnetic resonance imaging apparatus comprising the superconducting magnet according to claim 7 as the superconducting magnet.

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