JP2005353777A - Protective device for superconducting coil - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a superconducting coil system being capable of avoiding an excessive electromagnetic force by a coil quenching and a high temperature and a high voltage by an energy concentration and realizing a passive operation having a high reliability in a multilayer superconducting coil used for an MRI device, an NMR device or the like. <P>SOLUTION: The shunt resistances of the protective circuit are used as resistance heaters, or the resistance heaters are installed in parallel with the shunt resistance, and the heaters are mounted on the adjacent coils to the coils contained in the protective circuit. The same protective circuit is constituted regarding paired coils at symmetrical positions. Accordingly, when a certain superconducting coil is quenched, a current flows through the resistance heaters combining the shunt resistances of the protective circuit, and each heater heats the adjacent superconducting coils. The protective circuit can be operated surely even when an active quenching is not detected for quenching these superconducting coils. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a protection device for a superconducting coil having a plurality of superconducting coils, and more particularly to a protection device for a superconducting coil that can avoid coil damage during quenching.

Superconducting coils are suitable for steady-state large currents because they have no electrical resistance, and can generate strong magnetic fields with very little time variation. Therefore, taking advantage of their characteristics, NMR (nuclear magnetic resonance) and MRI (magnetic resonance) It has been put to practical use as an electromagnet constituting a tomographic imaging.

The superconducting state can be realized only in a range not exceeding the limit defined by the three conditions of current density, magnetic field, and temperature. Superconducting wires such as NbTi and Nb ₃ Sn put into practical use in the industrial field need to be cooled to an extremely low temperature of about 4K in order to realize a superconducting state. At such a very low temperature, the specific heat of the substance is extremely small, and the temperature rises locally due to minute heat generation, and the superconducting state may be broken. Then, electrical resistance is generated, and the temperature rise region is further expanded by Joule heat generation due to a large current flowing, and the superconducting state is destroyed throughout the coil. This phenomenon is called quenching of the superconducting coil.

  When a quench occurs, the magnetic energy stored in the coil is converted to thermal energy. This can cause damage to the superconducting coil due to local high temperature, high voltage due to voltage drop across the resistor, and excessive electromagnetic force due to induced current. With current technology, the possibility of quenching the superconducting coil during operation cannot be ruled out, and a quench protection device is essential for the superconducting coil.

  In a superconducting coil such as NMR or MRI that is operated for a long time in a permanent current mode that does not use a power source in a steady state, the cryostat has high sealing performance. Therefore, it is common to provide a protective device that releases magnetic energy inside the cryostat during quenching.

  As a method of releasing magnetic energy inside the cryostat, there are an active method using measurement control and a passive method of automatically releasing energy by an electric circuit or a structure. As for the active method, for example, [Non-Patent Document] “IEE Transaction on Applied Superconductivity, Vol. 7, pages 159 to 162, 1997 (IEEE Transaction on Applied Superconductivity, Vol. 7). , pp 159-162 (1997)), and a schematic diagram is shown in FIG.

  FIG. 6 shows the resistance heaters 5a to 5d by turning off the permanent current switch 3 operated by the external heater circuit 4 by turning off the current of the main coil circuit composed of the four superconducting coils 1a to 1d and the permanent current switch 2. FIG. 5 is a schematic diagram of an active protection circuit that prevents local concentration of energy by commutating to 5d and spreading quench across the coil. At the time of excitation and demagnetization of the superconducting coil, a power source is connected to the current leads 6a and 6b to apply a voltage.

  Although not shown in FIG. 6, the coil voltage or the like is constantly monitored, and when any of the coils is quenched, the external heater circuit 4 is activated to drive the permanent current switch 3.

  In addition, there is a technique in which a shunt resistor is inserted in parallel with respect to a plurality of superconducting coils existing as a different conventional technique. In this case, when one superconducting coil is quenched, the coil current is attenuated, and the current of other coils is increased by electromagnetic induction. In addition, the temperature of other coils rises due to the AC loss heat generated inside the superconducting coil due to these current fluctuations, that is, magnetic field fluctuations. These current increases and temperature rises also quench other coils and continue in a chain to passively prevent local concentration of energy by quenching the entire coil.

IEEE Transaction on Applied Superconductivity, Vol. 7, pp. 159 to 162, 1997 (IEEE Transaction on Applied Superconductivity, Vol.7, pp159-162 (1997))

  In the conventional example of FIG. 6 described above, it is necessary to always operate the detection device for detecting the quench. If the detection device and the protective heater driving device fail, the quench protection operation cannot be performed and the coil may be damaged. Further, the detection device and the protective heater power source need to be constantly operated and in a standby state, and there is a problem in that extra power consumption occurs during superconducting coil operation.

  Further, in the conventional technique in which a shunt resistor is inserted into the superconducting coil, the reduction in safety shown in FIG. 6 and unnecessary power consumption during operation do not occur. However, when current increase due to electromagnetic induction and temperature rise due to AC loss are used, there has been a problem of generation of excessive electromagnetic force due to current exceeding the rating. Therefore, when this method is used, the structural strength of the coil winding and coil winding frame must be designed to withstand the excessive electromagnetic force during quench protection. Decrease and the structure of the equipment has become complicated.

  The problem to be solved by the present invention is to provide a protection device for a superconducting coil that does not cause a decrease in safety, generates excessive electromagnetic force, and restricts the superconducting wire.

  In order to solve the above-described problems, the present invention provides a protection device for a superconducting coil comprising a plurality of superconducting coils and a shunt resistor that forms a protection circuit corresponding to each of the plurality of superconducting coils. The resistor is characterized by heating other superconducting coils other than the superconducting coils that form a protection circuit during quenching.

  In the protection device for a superconducting coil of the present invention, the shunt resistor heats the superconducting coil adjacent to the superconducting coil forming the protection circuit at the time of quenching.

  In the protection device for a superconducting coil according to the present invention, the shunt resistor is constituted by a divided resistor.

  Furthermore, in order to solve the above-mentioned problem, the present invention provides a superconducting coil protection device comprising a plurality of superconducting coils and a protection circuit corresponding to each of the plurality of superconducting coils. A resistance heater for heating the conduction coil is provided, and the resistance heater is characterized by heating a superconducting coil other than the superconducting coil that forms a protection circuit at the time of quenching.

  In the superconducting coil protection apparatus of the present invention, the resistance heater heats the superconducting coil adjacent to the superconducting coil forming the protection circuit at the time of quenching.

  In the protection device for a superconducting coil according to the present invention, the resistance heater is composed of divided resistors.

  In the protection device for a superconducting coil according to the present invention, the resistance heater is composed of a parallel resistor.

  In the protection device for a superconducting coil of the present invention, a shunt resistor is arranged in parallel with the resistance heater.

  Furthermore, in order to solve the above-mentioned problems, the present invention provides a superconducting coil protection device comprising a plurality of superconducting coils and a protection circuit corresponding to each of the plurality of superconducting coils. A heating element for heating the conduction coil is provided, and the heating element heats a superconducting coil other than the superconducting coil that forms a protection circuit at the time of quenching.

  In the superconducting coil protection device of the present invention, a shunt resistor is provided as the heating element.

  In the superconducting coil protection device of the present invention, a resistance heater is provided as the heating element.

  Further, in the superconducting coil protection device of the present invention, the shunt resistor or the resistance heater heats the superconducting coil adjacent to the superconducting coil forming the protection circuit at the time of quenching. is there.

  Furthermore, the present invention employs the above-described superconducting coil protection device in an NMR apparatus.

  The present invention employs the above-described superconducting coil protection device in an MRI apparatus.

  According to the present invention, it is possible to provide a protection device for a superconducting coil that is capable of preventing generation of an excessive electromagnetic force without causing a decrease in safety.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a circuit diagram of an embodiment of a protection device for a superconducting coil according to the present invention, and shows a protection circuit for a multilayer coil composed of four coils.

  In this embodiment, a shunt resistor of a divided protection circuit is used as a protective heater for the heating element, and this is installed mutually in adjacent coils. In FIG. 1, superconducting coils 1a to 1d, permanent current switch 2, and current leads 6a and 6b are the same as those of the conventional circuit. As the shunt resistors, the resistors 10a to 10d divided into two are used as both a resistor and a heater, and each resistance heater is installed in an adjacent superconducting coil. In FIG. 1, a broken line arrow indicates thermal coupling, that is, when a current flows through each resistance heater, the coil at the tip of the arrow is heated.

  In the embodiment of FIG. 1 of the present invention, when one superconducting coil, for example, the superconducting coil 1b, is quenched, a current flows through the shunt resistor and the heater combined resistor 10b of the protection circuit, and the superconducting coils 1a and 1c are adjacent to each other. To quench them. Therefore, even if there is no active quench detection, the protection circuit operates reliably. In addition to the increase in current due to electromagnetic induction and the temperature increase due to AC loss, the coil temperature can be increased more quickly by the heater, so that the generation of excessive electromagnetic force can be suppressed. Moreover, since the heating rate of the other coils can be adjusted by adjusting the resistance value and heat generation density of the heater, it is not necessary to limit the selection of the superconducting wire.

  FIG. 2 is a second embodiment of the present invention, and shows a protection circuit for a multilayer coil composed of four coils as in the first embodiment. In FIG. 2, superconducting coils 1a to 1d and permanent current switch 2 constitute a main circuit. Further, the shunt resistors 8a to 8d constitute a divided protection circuit. Two heater combined resistors 10a to 10d are connected to each protection circuit, and are installed in adjacent coils 1a to 1d. Furthermore, diode pairs 9a to 9d are provided in series with the shunt resistors 8a to 8d.

  During steady operation of the superconducting coil, the current flows through a circuit constituted by the permanent current switch 2 and the superconducting coils 1a to 1d, and the current flows through the shunt resistors 8a to 8d or the heater combined resistors 10a to 10d corresponding to the heating elements. Absent. Further, at the start and end of operation, that is, when the superconducting coil is excited and demagnetized, a power source is connected to the current leads 6a and 6b and a voltage is applied to both ends of the coil, and both ends of the superconducting coils 1a to 1d are connected. Voltage is generated. In a superconducting coil for NMR, this voltage is usually about several volts depending on the size of the superconducting coil. Since the forward voltage of the diode used for the diode pair is about 0.5 to 1.0 V per diode, if necessary, two or more diodes are connected in series to excite and demagnetize the superconducting coil. Even with the voltage generated at times, it is possible to prevent current from flowing through the shunt resistor and the resistance heater.

When one superconducting coil is quenched, the voltage generated in that coil immediately exceeds several tens of volts and greatly exceeds the forward voltage of the diode, so that current flows through the shunt resistor and the resistance heater. At this time, the sizes of the shunt resistor and the resistance heater are determined based on the magnitude of a desired current flowing through the resistance heater. For example, if the resistance value of the resistance heater is made ten times larger than the resistance value of the shunt resistance, the current flowing through the resistance heater becomes 1/10 of the current flowing through the shunt resistance, and the wiring to the resistance heater can be made thinner. It becomes possible.

  FIG. 3 shows an attachment diagram of the resistance heater according to the present invention to the multilayer superconducting coil shown in FIG. In FIG. 3, only three superconducting coils 1a to 1c are shown and only a protection circuit for the superconducting coil 1b is shown in order to avoid complexity. Each superconducting coil is connected to an adjacent coil by superconducting connections 11a to 11d using rising wires 12 from the coil.

  Superconducting coil 1b is connected to an adjacent coil by superconducting connections 11b and 11c, and is connected to diode pair 9b and shunt resistor 8b by wiring 13 from the connecting portion. The heater combined resistor 10b divided into two has a U-shape as shown in FIG. 3 in order to reduce electromagnetic induction, and is connected in parallel with the shunt resistor 8b by the wiring 14.

  According to the embodiment of the present invention, when the superconducting coil 1b is quenched, a current having the same magnitude as the coil current flows through the wiring 13, so that the wiring 13 is a superconducting wire equivalent to the coil wire. By making the resistance value of the heater combined resistor 10b larger than the value of the shunt resistor 8b, the current value of the resistance heater can be made much smaller than the coil current value, and the wiring 14 can be a copper wire that is easy to handle. it can. The material of the heater combined resistor 10b can be stainless steel having a small temperature dependency and magnetic field dependency of the resistance value.

  According to this embodiment, a passive protection circuit that does not require quench detection can be realized, and adjacent coils can be sequentially quenched by heating the heater, so excessive electromagnetic force due to an increase in current more than necessary, High temperature and high voltage due to proper heating can be suppressed.

The current decay time constant of the protection circuit shown in FIGS. 1 and 2 is almost determined by the ratio of the inductance value of the coil of the protection circuit and the shunt resistance value. In the circuit of FIG.
By adjusting 8d and the values of the heater combined resistors 10a to 10d, it is possible to appropriately select a current value to be supplied to the heater without changing the time constant and to set an optimal heater heat generation amount.

  FIG. 4 shows a third embodiment of the present invention.

  The two resistance heaters installed in parallel with the shunt resistor are connected in series in FIG. 3, but the value of the current flowing through each pair of heater resistances may be changed as a parallel connection. FIG. 4 shows a circuit diagram in the case where the two resistors of the heater combined resistor pairs 10a to 10d are connected in parallel as a third embodiment of the present invention. According to this embodiment, the resistance values of the resistance heaters can be changed from each other so that the current value of each resistance heater can be set independently, so that the optimum current value for the coil to be heated, and thus the heater heat is generated. It becomes possible to control.

  FIG. 5 shows a fourth embodiment of the present invention.

  FIG. 5 shows a circuit diagram when the present invention is applied to a symmetric multilayer coil. The circuit is configured so that the coils at symmetrical positions form the same protection circuit. Thereby, even at the time of quenching, the current values of the coils at the symmetrical positions are the same, so that the unbalanced electromagnetic force is completely cancelled.

  Therefore, according to the present Example, the electromagnetic force balance of the coil in a symmetrical position is not lost, generation of electromagnetic force that is not an axis target can be suppressed, and the coil structure can be simplified.

  As described above, according to the present invention, it is possible to realize a superconducting coil protection device that is highly safe and that can reduce the operating cost. By applying the present invention to an apparatus or an MRI (magnetic resonance tomography) apparatus, it is possible to provide an NMR apparatus or an MRI apparatus that is improved in safety and prevents an excessive electromagnetic force from being generated.

1 is a circuit diagram of a first embodiment of the present invention. The circuit diagram of the 2nd example of the present invention. The figure which shows the attachment state and electric wiring of the circuit of 2nd Example of this invention. The circuit diagram of the 3rd example of the present invention. The circuit diagram of the 4th example of the present invention. The circuit diagram of a prior art example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1a-1d ... Superconducting coil, 2 ... Permanent current switch, 3 ... Permanent current switch, 4 ... External heater circuit, 5a-5d ... Resistance heater, 6a, 6b ... Current lead, 7a-7d ... Resistance heater drive circuit, 8a ˜8d: shunt resistor, 9a˜9d… diode pair, 10a˜10d… resistor, 11a˜11d ... superconducting connection, 12 ... coil start-up wire, 13 ... wiring, 14 ... wiring.

Claims (14)

In a superconducting coil protection device comprising a plurality of superconducting coils and a shunt resistor that forms a protection circuit corresponding to each of the plurality of superconducting coils,
The shunt resistor heats a superconducting coil other than the superconducting coil that forms a protection circuit at the time of quenching. The protection device for a superconducting coil according to claim 1,
The superconducting coil protection device is characterized in that the shunt resistor heats a superconducting coil adjacent to the superconducting coil forming the protection circuit during quenching. The protection device for a superconducting coil according to claim 1 or 2,
The protective device for a superconducting coil, wherein the shunt resistor is composed of divided resistors. In a superconducting coil protection device comprising a plurality of superconducting coils and a protection circuit corresponding to each of the plurality of superconducting coils,
A resistance heater for heating the superconducting coil as the protection circuit;
The resistance heater heats a superconducting coil other than the superconducting coil that forms a protection circuit at the time of quenching. The protection device for a superconducting coil according to claim 7,
The resistance heater heats the superconducting coil adjacent to the superconducting coil forming the protection circuit at the time of quenching. The protection device for a superconducting coil according to claim 4 or 5,
The resistance heater is composed of a divided resistor, and a superconducting coil protection device. The protection device for a superconducting coil according to claim 6,
The superconducting coil protection device, wherein the resistance heater is composed of parallel resistors. The protection device for a superconducting coil according to claim 6,
A superconducting coil protection device, wherein a shunt resistor is arranged in parallel to the resistance heater. In one of claims 4 to 5,
An NMR apparatus comprising a superconducting coil protection device. In one of claims 4 to 5,
An MRI apparatus comprising a protective device for a superconducting coil. In a superconducting coil protection device comprising a plurality of superconducting coils and a protection circuit corresponding to each of the plurality of superconducting coils,
A heating element for heating the superconducting coil as the protection circuit,
A superconducting coil protection device, wherein the heating element heats a superconducting coil other than the superconducting coil that forms a protection circuit during quenching. The protection device for a superconducting coil according to claim 11,
A superconducting coil protection device comprising a shunt resistor as the heating element. The protection device for a superconducting coil according to claim 11,
A superconducting coil protection device comprising a resistance heater as the heating element. The protection device for a superconducting coil according to claim 12 or 13,
The superconducting coil protection device, wherein the shunt resistor or the resistance heater heats the superconducting coil adjacent to the superconducting coil forming the protection circuit at the time of quenching.

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