WO2013145933A1 - Superconducting magnet device, superconducting coil protection method, and magnetic resonance imaging device - Google Patents

Abstract

A superconducting magnet device is characterized by comprising forcible quench coils (30A to 40B) that cause AC loss in superconducting coils (10A to 20B) and a switching element (52) that is a switching means for controlling the actuation of the forcible quench coils, wherein the forcible quench coils (30A to 40B) are each disposed corresponding to each of the superconducting coils (10A to 20B). The superconducting magnet device is further characterized in that the turn-on voltage of the switching element (52) is higher than the voltage applied across both ends of the superconducting coil group (10A to 20B) while the superconducting magnet device is performing a rated operation.

Description

Superconducting magnet apparatus, superconducting coil protection method, and magnetic resonance imaging apparatus

The present invention relates to a superconducting magnet device, a superconducting coil protection method, and a nuclear magnetic resonance apparatus.

A magnetic resonance imaging apparatus (hereinafter referred to as an MRI (Magnetic Resonance Imaging) apparatus) utilizes the fact that the nuclear magnetic resonance (NMR) phenomenon of hydrogen nuclei constituting most of a living body varies depending on tissues. Thus, a living tissue is imaged, and the strength of resonance and the speed of temporal change of resonance appear as image contrast.
In the MRI apparatus, in order to obtain a high-quality image, it is necessary to generate a magnetic field having high intensity and high static magnetic field uniformity in the imaging region. In general, a superconducting magnet device is used to generate this high-intensity static magnetic field.

Superconducting magnet devices are required to have a highly uniform, high magnetic field, and temporally stable magnetic field. For cooling the superconducting magnet device, immersion cooling with liquid helium or conduction cooling with a refrigerator is used.
Note that the cooling by the refrigerator has an advantage that the operation is simple.

A superconducting magnet device used in an MRI apparatus suppresses leakage of a single or a plurality of superconducting main coils for generating a magnetic field in a magnetic field space of an imaging region and a magnetic field generated by the superconducting main coil to the outside of the apparatus. Therefore, a single or a plurality of superconducting shield coils that have a current opposite to the coil current of the superconducting main coil, a magnetic material that forms a magnetic path for passing the magnetic flux generated by the coil current, and the superconducting main coil And a cooling vessel containing liquid helium for cooling the superconducting shield coil, a radiation shield that surrounds the cooling vessel and is provided substantially on the same axis, and further provided on the outer periphery of the radiation shield for vacuum insulation. And a vacuum vessel.

By the way, there is a possibility that a part of the superconducting wire used for the superconducting coil (superconducting main coil, superconducting shield coil) of the superconducting magnet device is changed from the superconducting state to the normal conducting state (normal conducting transition). This process is called quenching.
In this process, the current flowing through the superconducting main coil and the current flowing through the superconducting shield coil are attenuated in time, and a dynamic magnetic field is generated during quenching with respect to the static magnetic field during rated operation.
In addition, when an unspecified superconducting coil is quenched, a certain time is required until the other superconducting coils are quenched. For this reason, the superconducting coil is arranged symmetrically in the axial direction with respect to the magnet center, but the decay current after quenching is not symmetric, and the leakage magnetic field is expanded by the magnetic field created by each, and the maximum applied to the coil There are coils that also increase electromagnetic force. Depending on the direction of the magnetic field, the direction of the electromagnetic force during the rated operation is opposite to that of the rated operation, so that the coil support structure becomes large.

As a method for solving such a problem, a heater is installed to be thermally coupled to the superconducting main coil and the superconducting shield coil, and when a certain unspecified coil is quenched, the heaters of other coils are turned on to perform heat. In particular, a technique for quenching other superconducting coils has been disclosed (see, for example, Patent Document 1).

JP 2011-71515 A JP 2003-197418 A

The superconducting magnet device is composed of a plurality of superconducting coils. In the quenching process, the total magnetic energy is converted into thermal energy by Joule heat generation, so if the total magnetic energy is consumed with a single coil, there is a high possibility of damaging the superconducting coil. For this reason, the superconducting coil is divided into several electric circuits, and a means for avoiding burning of the superconducting coil is taken by connecting a protective resistor in parallel.

However, by dividing the electric circuit, the value of the flowing current is not symmetric even if the superconducting coils are geometrically the same and arranged at symmetrical positions. As a result, the voltage across the superconducting coil that consumes more magnetic energy increases, and the coil temperature also increases. The magnetic field generated by the superconducting main coil and the superconducting shield coil is also unbalanced and the leakage magnetic field is expanded, and the magnitude and direction of the electromagnetic force generated by the magnetic field and the coil current are different from those during rated operation.

When the direction of the electromagnetic force applied to the coil is reversed as compared with the rated operation, it is necessary to have a support structure that can withstand the reversed electromagnetic force in order to ensure the soundness of the magnet. This reversal electromagnetic force is an electromagnetic force generated only at the time of quenching, and is an unnecessary support structure during rated operation, which causes an increase in cost.

In the case of a quench protection circuit using a heater according to the prior art, it is necessary to thermally couple the heater and the superconducting coil. The superconducting coil is molded from resin, etc., but in order to efficiently input the heater, it is necessary to devise a heater in the vicinity of the superconducting wire. It's not easy. Furthermore, since the magnetic energy stored in the superconducting coil is very high compared to the heat generation allowable amount in the heater, a commutation mechanism for preventing the heater from burning or a fuse for cutting off the current is required. Larger scale or fuse replacement work occurs.

In order to avoid damage to the heater, it is possible to prepare a separate power supply for energizing the heater circuit, detect the quench and close the heater circuit, but the additional facilities will be large.

Therefore, the present invention provides a superconducting magnet device, a superconducting coil protection method, and a magnetic resonance imaging apparatus that can easily propagate quenching, based on a principle different from the method of quenching a superconducting coil using a heater. The issue is to provide.

In order to solve such a problem, the present invention includes a forced quenching coil that causes an AC loss in a superconducting coil, and a switching unit that operates the forced quenching coil, and the forced quenching coil includes a plurality of forced quenching coils. The superconducting coils are installed respectively.

According to the present invention, a superconducting magnet device, a superconducting coil protection method, and a magnetic resonance imaging apparatus that can easily propagate the quench can be provided by a principle different from the method of quenching the superconducting coil with a heater. be able to.

It is a conceptual diagram of the MRI apparatus with which the superconducting magnet apparatus of this invention is applied. It is a cross-sectional schematic diagram of the gantry of the MRI apparatus to which the superconducting magnet apparatus of 1st Embodiment is applied. It is a figure which shows schematic layout of the coil for forced quenching of FIG. 2, (a) is typical sectional drawing, (b) is a typical perspective view. It is a circuit diagram which shows the protection circuit of the superconducting magnet apparatus of 1st Embodiment. It is a circuit diagram which shows the protection circuit of the superconducting magnet apparatus of 2nd Embodiment. It is a circuit diagram which shows the circuit for forced quenching among the circuits of FIG. 5A. It is a time development figure of the electric current of the superconducting coil group from the quench start in a comparative example. It is a time development figure of the electric current of the superconducting coil group from the quench start in this embodiment. It is a time development figure of the coil temperature of the superconducting coil group from the quench start in a comparative example. It is a time development figure of the coil temperature of the superconducting coil group from the quench start in this embodiment. It is a time development figure of the voltage between coil terminals of the superconducting coil group from the quench start in a comparative example. It is a time development figure of the voltage between coil terminals of the superconducting coil group from the quench start in this embodiment. It is a time development figure of the electromagnetic force impressed to coil 12B at the time of quenching in the superconducting magnet device in a comparative example. It is a time development figure of the electromagnetic force impressed to coil 12B at the time of quenching in the superconducting magnet device in this embodiment (2nd embodiment). It is a figure which shows the maximum area | region of the leakage magnetic field (0.5mT) at the time of quenching in the superconducting magnet apparatus in a comparative example. It is a figure which shows the maximum area | region of the leakage magnetic field (0.5mT) at the time of quenching in the superconducting magnet apparatus in this embodiment (2nd Embodiment). It is a cross-sectional schematic diagram of the gantry of the MRI apparatus with which the superconducting magnet apparatus of 3rd Embodiment is applied. It is typical sectional drawing which shows the installation condition of the coil for quenching in the superconducting magnet apparatus of 4th Embodiment.

Hereinafter, an MRI apparatus to which a superconducting magnet apparatus according to an embodiment of the present invention is applied will be described in detail with reference to the drawings as appropriate. In the following, a horizontal magnetic field type MRI apparatus will be described, but the present invention is not limited to this, and can also be applied to other superconducting magnet apparatuses other than the MRI apparatus.

<< First Embodiment >>
FIG. 1 is a conceptual diagram of an MRI apparatus to which the superconducting magnet apparatus of the present invention is applied.
This MRI apparatus 1 uses a nuclear magnetic resonance (NMR) phenomenon to obtain a tomographic image of a subject (not shown, the same applies hereinafter). As shown in FIG. A gantry 1a that houses various devices for inducing NMR phenomena and receiving NMR signals, a bed 2 on which the subject is placed, and the subject placed on the bed 2 to the imaging region F in the magnetic field space Conveying means 2a for conveying, a power supply for controlling various devices in the gantry 1a, a control device 3 containing various control devices, a processing device 4 such as a computer for processing detected nuclear magnetic resonance signals, and processed nuclear magnetism A display device 5 that displays a tomographic image based on the resonance signal is included, and each is connected by a power source / signal line 6. The gantry 1a, the bed 2 and the transport means 2a are disposed in a shield room (not shown) that shields high-frequency electromagnetic waves and static magnetic fields, and the control device 3, the processing device 4, and the display device 5 are disposed outside the shield room.

FIG. 2 is a schematic cross-sectional view including the central axis of the gantry of the MRI apparatus to which the superconducting magnet apparatus of the first embodiment is applied.
The gantry 1a has a cylindrical shape centered on the central axis Z. A superconducting magnet device is provided in the gantry 1 a of the MRI apparatus 1, and an imaging region F exists at the center on the central axis Z.
The superconducting magnet device provided in the gantry 1a suppresses the leakage of the generated magnetic field to the outside of the superconducting main coils 10A, 10B, 11A, 11B, 12A, and 12B for generating a magnetic field in the imaging region F. Superconducting shield coils 20A and 20B, and forcibly quenching coils 30A, 40B, 31A, 32A, 32B, 31B, 40A installed in the vicinity of a part or a plurality of portions of the superconducting main coil and the superconducting shield coil in the circumferential direction. 30B.
The superconducting main coils 10A, 10B, 11A, 11B, 12A, 12B and the superconducting shield coils 20A, 20B are arranged coaxially with the central axis Z as an axis.
The superconducting main coil 10A and the superconducting main coil 10B are arranged symmetrically with respect to a symmetry plane (not shown) that passes through the center O of the imaging region F and has the central axis Z as a normal line. Similarly, the superconducting main coils 11A and 11B are arranged symmetrically with respect to the symmetry plane, the superconducting main coils 12A and 12B are arranged symmetrically with respect to the symmetry plane, and the superconducting shield coils 20A and 20B are symmetrical with respect to the symmetry plane. Is arranged.

The superconducting magnet device is formed so as to cover the cooling container 62 and the cooling container 62 that accommodates the superconducting main coils 10A, 10B, 11A, 11B, 12A, and 12B, and the superconducting shield coils 20A and 20B together with the refrigerant 61. And a vacuum vessel 64 surrounding the cooling vessel 62 and the radiation shield 63 and having a vacuum inside.
As the refrigerant 61 accommodated in the cooling container 62, for example, a liquefied refrigerant such as liquid helium is used, and a refrigerator (not shown) for cooling the refrigerant 61 and the radiation shield 63 is installed in the vacuum container 64. ing. The inside of the cooling container 62 is maintained at, for example, about 4.2K by such a refrigerant 61.

A constant current flows through the superconducting main coils 10A, 10B, 11A, 11B, 12A, and 12B, and the superconducting main coils 10A, 10B, 11A, 11B, 12A, and 12B pass through the superconducting shield coils 20A and 20B. A constant current in the reverse direction flows.
These superconducting main coils 10A, 10B, 11A, 11B, 12A, and 12B, and the superconducting shield coils 20A and 20B suppress the electromagnetic force, the leakage magnetic field, the maximum experience magnetic field, the magnetic field uniformity, and the magnetic field strength within an allowable range. As described above, the position, shape, and number of installations can be changed.

In addition, as shown in FIG. 2, each superconducting main coil 10A, 10B, 11A, 11B, 12A, 12B includes a forced quenching coil 30A, 31A, 32A, 32B, 31B, 30B, respectively, on the outer peripheral side and the inner peripheral side. It is installed so as to be sandwiched from the side. In addition, forcibly quenching coils 40A and 40B are also installed in the superconducting shield coils 20A and 20B so as to be sandwiched from the outer peripheral side and the inner peripheral side.

Taking the superconducting main coil 10A as an example, in the first embodiment, the forced quenching coil 30A sandwiches the inner peripheral side and the outer peripheral side (that is, the outer side) of the superconducting main coil 10A at one place on the circumference. It is installed as follows. This is shown in FIGS. 3 (a) and 3 (b). Incidentally, the forced quenching coil 30A may be either the outer peripheral side or the inner peripheral side, but in order to efficiently generate an AC loss (AC loss) in the superconducting main coil 10A, the inner peripheral side and the outer peripheral side are arranged. It is preferable to install so that it may oppose as a pair. Further, it is preferable that the forced quenching coil 30A be positioned as close to the superconducting main coil 10A as possible, but the position is not particularly limited as long as it is within a range that causes an AC loss. In addition, although the forced quenching coil 30A is illustrated as one pair, it may be two pairs or three pairs. Moreover, you may install one or more things which are not paired. Alternatively, the superconducting main coils 10A, 10B, 11A, 11B, 12A, 12B or the superconducting shield coils 20A, 20B may be installed on the side surface side.
Since these points are the same in the other superconducting main coil 10B and the like, and the other superconducting shield coil 20A and the like, the description thereof is omitted.

FIG. 4 is a circuit diagram showing a protection circuit of the superconducting magnet device according to the first embodiment.
Superconducting main coils 10A, 10B, 11A, 11B, 12A, 12B, superconducting shield coils 20A, 20B, and forced quenching coils 30A, 40B, 31A, 32A, 32B, 31B, 40A, 30B have a circuit as shown in FIG. Connected with.
In the circuit in this embodiment, as shown in the prior art of Patent Document 2, when considering a circuit in which a superconducting main coil and a superconducting shield coil are connected in series, a superconducting main coil 10A and Superconducting shield coil 20B is paired and connected in series to form superconducting coil group A1, and superconducting main coil 11A and superconducting main coil 12A are paired and connected in series to form superconducting coil group B1. The main coil 12B and the superconducting main coil 11B are connected in series to form a superconducting coil group C1, and the superconducting shield coil 20A and the superconducting main coil 10B are connected in series to form a superconducting coil group D1. These superconducting coil groups A1, B1, C1, and D1 are connected in series. A permanent current switch 51 is connected in parallel to the superconducting coil groups A1, B1, C1, and D1 connected in series (collectively referred to as “superconducting coil group 100” as appropriate), and the main circuit of the superconducting magnet device Is formed. Further, the forced quenching coils 30A, 40B, 31A, 32A, 32B, 31B, 40A, and 30B are connected in series (referred to as “forced quenching coil group 110” as appropriate). The forced quenching coil group 110 is further connected in series with a switching element 52 (switching means) to form a forced quenching circuit 53. The forced quench circuit 53 is connected in parallel with the superconducting coil group 100 (A1, B1, C1, D1).

Examples of the switching element 52 include semiconductor elements such as bidirectional diodes, thyristors, and transistors. Here, the bidirectional diode is formed by connecting two diodes in parallel in opposite directions.
The superconducting main coils 10A, 10B, 11A, 11B, 12A, 12B, the superconducting shield coils 20A, 20B, the switching element 52 and the forced quenching coils 30A, 40B, 31A, 32A, 32B, 31B, 40A, 30B. The number of components and the number of divisions of the protection circuit can be appropriately set so that these are within the allowable range in consideration of the coil voltage at the time of quenching, temperature rise, electromagnetic force, leakage magnetic field, and the like. Incidentally, in the first embodiment, the protection circuit and the like are not divided.

In such a circuit, during rated operation of the MRI apparatus 1, a current flows on a circuit composed of the superconducting coil groups A 1, B 1, C 1, D 1 and the permanent current switch 51. That is, no current flows through the switching element 52. Note that, by adjusting the number of switching elements 52 as necessary, a voltage exceeding the turn-on voltage of the switching element 52 is generated when the superconducting coil groups A1, B1, C1, and D1 are excited and demagnetized. Even in such a case, it is possible to prevent the current from being shunted to the switching element 52 side. Thus, no current flows through the forced quenching coils 30A, 40B, 31A, 32A, 32B, 31B, 40A, 30B during rated operation.
On the other hand, at the time of quenching, a voltage exceeding the turn-on voltage of the switching element 52 is generated in the superconducting main coils 10A, 10B, 11A, 11B, 12A, 12B and the superconducting shield coils 20A, 20B (the turn-on voltage of the switching element is Because the voltage is higher than the voltage applied to both ends of the superconducting coil group during rated operation of the superconducting magnet device), a current flows to the switching element 52 side and the forced quenching coils 30A, 40B, 31A, 32A, 32B, 31B. , 40A, and 30B, a fluctuating magnetic field generated by the forced quenching coil current can be applied to the superconducting coil.

Incidentally, each coil of the forced quenching coil group 110 is manufactured by winding a low resistance conductor, and its inductance is smaller (for example, two digits or more) than the inductance of each coil of the superconducting coil group 100, and is further superconducting. Since the coils of the coil group 100 are not magnetically coupled, a current substantially equal to the short-circuit current flows through the coils of the forced quenching coil group 110 immediately after the switching element 52 is turned on (circuit closed). The amount of time change of the magnetic field generated by the coil increases, and the AC loss necessary for forcibly quenching each coil of the superconducting coil group 100 can be locally generated. Thereby, when quenching occurs in an unspecified coil of the superconducting coil group 100, other coils can be quenched almost immediately. The forced quenching coil group 110 is connected in parallel to the superconducting coil group 100, and a current for operating the forced quenching coil group 110 flows from the superconducting coil side during quenching. There is no need for a separate power supply for operation, and it is possible to avoid an increase in the size of incidental facilities.

That is, according to the first embodiment, when quenching occurs in any of the superconducting coil group 100, that is, the superconducting main coils 10A, 10B, 11A, 11B, 12A, 12B, and the superconducting shield coils 20A, 20B, The superconducting coils 30A, 40B, 31A, 32A, 32B, 31B, 40A, and 30B generated by the forced quenching coils are subjected to AC loss caused by the varying magnetic field, thereby causing the other superconducting main coils 10A, 10B, 11A, 11B, and 12A. , 12B and the superconducting shield coils 20A, 20B can be quickly propagated, and as a result, the soundness of the superconducting magnet apparatus (MRI apparatus 1) can be maintained. That is, it is possible to prevent the coils of the superconducting coil group 100 from being burned out.
Also, since the operating principle is different from the quench protection circuit using a heater, there is no need for a commutation mechanism to prevent the heater from burning, or a fuse to cut off the current. No replacement work occurs. That is, each inconvenience as seen in the quench protection circuit using the heater described in the problem column does not occur.
Further, since the quench can be quickly propagated to all the coils constituting the superconducting coil group 100, even if a reversal electromagnetic force is generated during the quench, the magnitude can be reduced, and the reversal electromagnetic force The support structure for withstanding the load can be simplified.

As for the leakage magnetic field, the superconducting magnet apparatus (MRI apparatus 1) has a superconducting coil group 100 arranged symmetrically in the axial direction with respect to the central axis Z, which is the center of the magnet, as shown in FIG. During operation, the leakage magnetic field is suppressed by the superconducting shield coils 20A and 20B in the superconducting coil group 100. However, the decay current after quenching is not symmetric (see FIG. 6A described later), and each coil creates. The leakage magnetic field is expanded by the magnetic field. However, in this embodiment, the quench can be promptly propagated to each coil of the superconducting coil group 100, so that the attenuation current is symmetrically arranged, that is, the superconducting coils arranged geometrically at the same symmetrical position in the quenching process. All superconducting coils can be quenched almost simultaneously so that the currents are the same as much as possible (see FIG. 6B described later), thereby suppressing the leakage magnetic field.
In addition, each coil of the forced quenching coil group 110 is arranged in each coil of the superconducting coil group 100 so as to correspond to each coil of the superconducting coil group 100 in a one-to-one manner. It can be thinned out as appropriate or increased as appropriate.

<Supplement of action and principle>
Supplementary explanation will be given on the operation of the superconducting magnet device according to the present embodiment. That is, as a first embodiment, a superconducting state is used by using a circular forced quenching coil (30A) that is arranged so that a pair of two superconducting coils (superconducting main coil 10A) is sandwiched as shown in FIG. It is estimated whether or not the superconducting main coil 10A can be transferred to normal conduction.

According to the literature [Wilson MN, Superconducting Magnets, Oxford University Press, 1983], the AC loss Q per unit volume is
Q = (dBi / dt) ^ 2 / μ0 * 2 * τ
The coupling time constant τ is
τ = 1/2 * (p / 2π) ^ 2 * μ0 / ρ
It is. Here, Bi is the average magnetic field inside the superconducting wire, μ0 is the magnetic permeability in air, p is the twist pitch of the superconducting multifilament filament, and ρ is the specific resistance of the stabilized copper between the filaments. Here, it is assumed that a current of 100 A (ampere) flows in the forced quenching coil 30A in 0.1 s from the start of the quench, the twist pitch p is 0.05 m, and the specific resistance ρ of the stabilized copper is 1.0E-9 Ωm. did. The forced quenching coil 30A (see FIG. 3A) is a circular coil having a diameter of 100 mm, a coil cross section of 40 mm × 16 mm, and a number of turns of 40 turns so that a variable magnetic field can be applied to the length of the twist pitch of the superconducting main coil 10A. It was. For the coil winding, a low-resistance conductive wire cooled to a very low temperature, a low-temperature superconducting wire, a high-temperature superconducting wire, or the like can be used. Since the size of the forced quenching coil 30A is smaller than that of the superconducting main coil 10A and the inductance differs by two digits or more, the switching element 52 is closed and immediately after the forced quenching coil 30A is energized, it is almost equivalent to a short-circuit current. Current flows. Under this condition, the AC loss in the divided volume of the circumferential length of 0.05 m of the superconducting main coil 10A is estimated to be about 2.5 J in 0.1 s.
On the other hand, the energy required for the normal conducting transition of the superconducting main coil 10A is known to be about 1.5 J in the same volume from the analysis and experimental results so far. From the above results, it is possible to cause the superconducting main coil 10A to undergo normal conduction transition using the forced quenching coil 30A and the energizing current. If the forced quenching coil 30A has an electrical resistance, further energy is injected by thermal coupling.

As described above, according to the present invention, in the quench process, for example, the superconducting magnet device, the magnetic resonance imaging device, and the nucleus that can effectively suppress the expansion of the area of the leakage magnetic field and suppress the electromagnetic force applied to the coil. A magnetic resonance apparatus can be provided.

<< Second Embodiment >>
Next, a second embodiment will be described. 5A and 5B are circuit diagrams showing the protection circuit of the superconducting magnet device of the second embodiment. FIG. 5A shows the entire circuit, and FIG. 5B shows the protection circuit (forced quenching circuit) in the circuit of FIG. 5A. FIG.

In the first embodiment, all the coils of the superconducting coil 100, that is, the superconducting main coils 10A, 10B, 11A, 11B, 12A, 12B and the superconducting shield coils 20A, 20B are connected in series. For this reason, the maximum voltage generated between the terminals of the superconducting main coils 10A, 10B, 11A, 11B, 12A, 12B or the superconducting shield coils 20A, 20B may be increased. Therefore, in the second embodiment, as shown in FIG. 5A, the circuit is divided so that each of the superconducting coil groups A1 to D1 is connected in parallel with the forced quench circuit 53 so that the maximum voltage does not increase. For example, it is set within 2000V. By doing so, the superconducting magnet device of the present embodiment can avoid a situation where the conversion of magnetic energy to thermal energy concentrates on a single coil and the coil burns out when a quench occurs. .

The forced quench circuit 53 has the same configuration as that of the first embodiment as shown in FIG. 5B. In the second embodiment, the forced quench circuits 53 are connected in a daisy chain as shown in FIG. 5A. ing. Incidentally, if the superconducting main coil 10A is taken as an example, in the first embodiment, one forced quenching coil 30A is installed on the circumference of the superconducting main coil 10A. Although not shown in this second embodiment, since the number of circuit divisions is 4, four forced quenching coils 30A are installed on the circumference of the superconducting main coil 10A. It is what is done. The four points are the same in the other superconducting main coils 10B, 11A, 11B, 12A, 12B and the superconducting shield coils 20A, 20B.

Next, the effect of this embodiment will be described. The superconducting main coils 10A, 10B, 11A, 11B, 12A, 12B and the superconducting shield coils 20A, 20B shown in FIG. 2 are configured as four superconducting coil groups A1, B1, C1, D1, as shown in FIG. The protection circuit is also shown in FIG. 5A, and simulation (numerical analysis) was performed under the condition that each coil of each forced quenching coil group 110 was installed on a part of the circumference of each coil of each superconducting coil group 100. Note that the current value initially energized in the superconducting main coils 10A, 10B, 11A, 11B, 12A, 12B, and the superconducting shield coils 20A, 20B was a coil current value that generates a magnetic field of about 3 T in the imaging region F.

Assuming that quenching occurred in the superconducting shield coil 20B (superconducting coil group A1), the following simulation (numerical analysis) was performed.

6A and 6B are simulation results of time development diagrams of currents of superconducting coil groups A1, B1, C1, and D1 (see FIG. 5A) from the start of quenching. For comparison, FIG. 6A shows a simulation result according to a comparative example to which the present invention is not applied, and FIG. 6B shows a simulation result according to the present embodiment. The circuit diagram used is that of FIG. 5A as described above. The comparative example is a divided circuit including only the bidirectional diode 52 in the quench protection circuit 53. In this comparative example, when a superconducting coil is quenched, the current of the circuit including the coil is attenuated and the magnetic flux changes with time, so the other superconducting coils experience the fluctuation magnetic field and the quench propagates, Protected.
In FIG. 6A which is a comparative example, when a quench occurs in the superconducting main coil 10A (when it starts), the current of the superconducting coil group A1 including the superconducting main coil 10A which is the quench starting coil gradually attenuates from the start of the quench, and is caused by resistance Magnetic energy is dissipated by Joule loss. In the other superconducting coil groups B1, C1, and D1, each of the superconducting shield coil 20B and the superconducting coil groups B1, C1, and D1, which are other coils, is caused by the AC loss caused by the fluctuating magnetic field caused by the superconducting main coil 10A that is quenched first. Magnetic energy is consumed after the coil has transitioned to normal conduction. For this reason, the current attenuation waveform of each coil differs.
On the other hand, in the case of the present embodiment of FIG. 6B, the propagation of the quench from the quench start to the other superconducting coil groups B1, C1, D1 occurs immediately (in the example, assuming propagation after 0.1 s), and the current of each circuit The values are almost equal in attenuation.

FIG. 7A and FIG. 7B show time evolution diagrams of coil temperatures of superconducting coils (superconducting main coils 10A, 10B, 11A, 11B, 12A, 12B, superconducting shield coils 20A, 20B) from the start of quenching. FIG. 7A shows a comparative example, and FIG. 7B shows this embodiment.
The vertical axis in FIG. 7A is 220K full scale, and the vertical axis in FIG. 7B is 180K full scale.
In the comparative example of FIG. 7A, the magnetic energy is converted into thermal energy by the coil (superconducting shield coil 20B) that has undergone normal conduction transition, and the temperature rises. On the other hand, in the simulation result according to the present embodiment of FIG. 7B, the quench is propagated by the forced quenching coil 110, and magnetic energy is distributed and consumed by as many superconducting coil groups 100 as possible. That is, although the temperature rise of a specific coil can be avoided, in the comparative example of FIG. 7A, the propagation of quenching to other superconducting coils is delayed, and the temperature of the superconducting coil where quenching has started is rising. Further, the local maximum attainable temperature is about 160K compared to about 190K in the comparative example, and is lower in the embodiment.

8A and 8B show the voltage between terminals of the superconducting coil from the start of quenching. Like the coil temperature, in the case of the comparative example of FIG. 8A, the propagation of quenching is delayed. Although the voltage rises, according to the present invention of FIG. 8B, the voltage between the coil terminals is suppressed by about 30 to 40% compared to the comparative example. Incidentally, also in this figure, the scale of the graph is different between the comparative example of FIG. 8A and the embodiment of FIG. 8B.

9A and 9B show time development diagrams of electromagnetic force applied to the coil of the superconducting coil 12B from the start of quenching. In the case of the comparative example of FIG. 9A, as shown in FIG. 6A, since the currents flowing through the superconducting coil groups A1, B1, C1, and D1 are greatly different, an electromagnetic force whose sign is reversed is applied particularly to the axial electromagnetic force. Is done. In order to ensure the soundness of the magnets (each coil of the superconducting coil group 100), a support structure for supporting the above reversal electromagnetic force is required, and the direction of the electromagnetic force applied to the coils during the rated operation is opposite to the direction It is necessary to have a support structure for the electromagnetic force. Moreover, since the electromagnetic force higher than the electromagnetic force applied at the time of rated operation is applied after quenching as for the radial electromagnetic force, a structure for supporting this maximum electromagnetic force is required. Incidentally, as shown in FIG. 9B, it can be seen that in this embodiment, the electromagnetic force is suppressed and smooth.

10A and 10B show the maximum region of the magnetic field region (0.5 mT line is displayed in FIG. 10) leaking from the superconducting magnet from the start of quenching (note that 0.5 mT (millitesla) is 5 G (Gauss)). Is). FIG. 10A is a comparative example, and FIG. 10B is an embodiment. 10A and 10B both show the superconducting magnet device vertically, and the vertical axis shows the distance in the horizontal direction (see FIG. 2) from the center of the imaging region F in meters. That is, the horizontal direction in FIG. 2 (the direction of the central axis Z) is the vertical axis in FIGS. 10A and 10B.
As is clear from FIGS. 10A and 10B, in this embodiment, the leakage magnetic field can be reduced as compared with the comparative example. That is, in the comparative example, as shown in FIG. 6A, since the currents flowing through the superconducting coil groups A1, B1, C1, and D1 are greatly different, the magnetic field canceled by the superposition of the magnetic fields becomes non-equilibrium, and the leakage magnetic field region is expanded. .

<< Third Embodiment >>
Next, a superconducting magnet device according to a third embodiment will be described. FIG. 11 is a schematic cross-sectional view including the symmetry axis of the gantry of the MRI apparatus to which the superconducting magnet apparatus of the third embodiment is applied. The difference between the forced quenching coil according to the third embodiment and the forced quenching coil according to the first and second embodiments is that the forced quenching coil according to the first and second embodiments is all different. The forced quenching coil according to the third embodiment is different from that of the superconducting coil in that it is installed only in the superconducting coil that greatly affects the soundness of the magnet.

That is, in the third embodiment, only the superconducting coils 20A, 20B, 10A, 10B (superconducting main coils 10A, 10B, superconducting shield coils 20A, 20B) having particularly large magnetic energy are forcibly quenched coils 30A, 30B, 40A, 40B. Is installed.
Also in the third embodiment, the electromagnetic force applied to the expansion region of the leakage magnetic field and the superconducting coil is caused by the superconducting coil current and the magnetic field generated by the current, and the soundness of the magnet can be improved.

<< Fourth Embodiment >>
Next, a superconducting magnet device according to a fourth embodiment will be described.
The difference between the forced quenching coil according to the fourth embodiment and the forced quenching coil according to the first embodiment is that the forced quenching coil according to the first embodiment is located beside the superconducting coil (on the outer periphery / inner periphery). The forced quenching coil according to the fourth embodiment is different in that it is installed at the lead portion of the superconducting coil winding.

FIG. 12 is a schematic cross-sectional view showing the installation situation of the quenching coil in the superconducting magnet device of the fourth embodiment, where reference numeral 70 is a bobbin, reference numeral 10A is a superconducting main coil, and reference numeral 30A is This is a forced quenching coil, and reference numeral 80 is a lead portion. As described above, in the fourth embodiment, the opening portion 80 is provided.
Incidentally, although FIG. 12 shows an open type MRI apparatus, it can also be applied to the tunnel type MRI apparatus 1 shown in FIG. In the case of the tunnel type, as shown in FIG. 2, for example, a forced quenching coil 30A sandwiching the superconducting main coil 10A from above and below is installed at a side lead portion (not shown in FIG. 2) of the superconducting main coil 10A. The Rukoto.

The forced quenching coil according to the fourth embodiment does not require a space for installing the forced quenching coil beside the superconducting coil, so that the gantry 1a of the MRI apparatus 1 (see FIG. 1) to which the superconducting magnet device is applied is used. There is an advantage that the opening area can be expanded. Further, if the opening area is the same, the height can be suppressed. This is the same for the opener type MRI apparatus.

It should be noted that the present invention is not limited to the techniques described in the first to fourth embodiments described above, and can be implemented with appropriate modifications within a range where the effects are exhibited.

1 MRI system (superconducting magnet system)
1a Gantry 2 Bed 2a Transport means 3 Control device (analysis means)
4 processing equipment (analysis means)
5 Display Device 6 Power / Signal Lines 10A, 10B, 11A, 11B, 12A, 12B Superconducting main coil (superconducting coil)
20A, 20B Superconducting shield coil (superconducting coil)
30A, 30B, 31A, 31B, 32A, 32B, 40A, 40B Forced quench coil 51 Permanent current switch 52 Switching element (semiconductor element, switching means)
53 Forced quench circuit 61 Refrigerant 62 Cooling vessel 63 Radiation shield 64 Vacuum vessel 100 Superconducting coil group A1, B1, C1, D1 Superconducting coil group 110 Forced quenching coil group F Imaging region (magnetic field space)
O Center Z Center axis

Claims (10)

1. A superconducting magnet device comprising a plurality of superconducting coils,
   A coil for forced quenching that generates AC loss in the superconducting coil;
   Switching means for operating the forced quenching coil,
   The superconducting magnet device, wherein the forcible quenching coil is installed for each of the plurality of superconducting coils.
2. The switching means is a switching element,
   A forced quench circuit comprising the switching element and the forced quench coil connected in series;
   The forced quenching circuit is connected in parallel to a superconducting coil group in which one or more superconducting coils are connected in series,
   2. The superconducting magnet device according to claim 1, wherein a turn-on voltage of the switching element is higher than a voltage applied to both ends of the superconducting coil group when the superconducting magnet device is rated.
3. The forced quench circuit includes a plurality of forced quench coils,
   The superconducting magnet device according to claim 2, wherein the plurality of forced quenching coils are connected in series and are installed with respect to the plurality of superconducting coils.
4. In the case where the superconducting magnet device includes a plurality of the superconducting coil groups,
   The superconducting magnet device according to claim 2, wherein a plurality of the forced quenching circuits are provided, and are connected in parallel to the superconducting coil group.
5. In the case where the superconducting magnet device includes a plurality of the superconducting coil groups,
   The superconducting magnet device according to claim 3, wherein a plurality of the forced quenching circuits are provided and connected in parallel to the superconducting coil group.
6. A pair of forced quenching coils are installed so as to sandwich the superconducting coil,
   The superconducting magnet device according to any one of claims 2 to 5, further comprising the forced quenching circuit in which the pair of forced quenching coils are connected in series.
7. The superconducting magnet apparatus according to any one of claims 1 to 5, wherein the forced quenching coil is installed at a lead portion of a winding of the superconducting coil.
8. A method of protecting the superconducting coil in a superconducting magnet device comprising a plurality of superconducting coils,
   The superconducting magnet device is:
   A forced quenching coil for generating an AC loss in the superconducting coil, and a switching means for operating the forced quenching coil,
   The forced quenching coil is installed for each of the plurality of superconducting coils,
   When a quench occurs in any of the plurality of superconducting coils,
   The switching means actuates the forced quenching coil;
   The activated forced quenching coil generates a fluctuating magnetic field,
   AC loss is generated in the plurality of superconducting coils to which the varying magnetic field is applied,
   A method of protecting a superconducting coil, wherein the quench is propagated to a plurality of the superconducting coils to which the varying magnetic field is applied.
9. When a quench occurs in any of the plurality of superconducting coils,
   The switching means, the forced quenching coil, and the superconducting coil form a closed circuit,
   The current flowing in the superconducting coil included in the closed circuit flows into the forced quenching coil,
   The method for protecting a superconducting coil according to claim 8, wherein the forced quenching coil into which the current flows generates a fluctuating magnetic field.
10. The superconducting magnet device according to any one of claims 1 to 5,
    A bed on which the subject is placed;
    Transport means for transporting the subject placed on the bed to the imaging region;
    Analyzing means for analyzing a nuclear magnetic resonance signal from the subject transported to the imaging region by the transporting means. A magnetic resonance imaging apparatus comprising:

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