JP2011000368A - Magnetic field generator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To restrict the increase of a leak magnetic field upon occurrence of quenching concerning an active shield system magnetic field generator.SOLUTION: The magnetic field generator 1 includes a second shield coil 50 and a second shield coil protecting circuit 60. The second shield coil protecting circuit 60 includes a bi-directional diode 61. The ON-voltage Vof the bi-directional diode is set to be larger than electromotive force Vwhich is generated in the second shield coil, by electromagnetic induction caused by magnetic field change in excitation or demagnetization of a main coil protecting circuit 30 and a first shield coil 20 (i), and also is set to be equal to or less than electromotive force Vwhich is generated in the second shield coil 50, by electromagnetic induction caused by the magnetic field change upon occurrence of quenching in the main coil 10 or the first shield coil 20 (ii).

Description

  The present invention relates to a magnetic field generator used as an MRI apparatus.

  When a strong magnetic field is generated by energizing the superconducting coil, a leakage magnetic field is generated around the coil. In particular, since a medical MRI (Magnetic Resonance Imaging) apparatus is installed in a hospital, the leakage magnetic field (leakage magnetic flux density) is desirably small.

  Conventionally, a magnetic field generator is configured to reduce the leakage magnetic field in order to reduce the influence of the leakage magnetic field on equipment and human bodies around the MRI apparatus. Patent Document 1 describes an active shield type device as an example of such a magnetic field generation device. In the active shield system, a superconducting coil (shield coil) that generates a magnetic field in the opposite direction to the main coil is provided outside the superconducting coil (main coil) for generating a magnetic field. The leakage magnetic field is reduced by weakening the magnetic field of the shield coil.

In an active shield type MRI apparatus, a leakage magnetic field (leakage magnetic flux density) measured at a certain reference position is adjusted to be a set value or less. This set value is referred to as “set leakage magnetic field”. The set leakage magnetic field is normally set to a small value of about 5 × 10 ⁻⁴ T (5 gauss) (at a position where the leakage magnetic field is maximized).

JP 2008-177183 A

  In a medical MRI apparatus, generally, energy stored in a superconducting coil is large (1 MJ or more), and the coil may be damaged when a quench occurs. For this reason, the superconducting coil of the MRI apparatus is provided with a protection circuit for preventing damage when a quench occurs. This protection circuit is formed by connecting a resistor, a diode, a thyristor or the like to the superconducting coil.

  In such a device, when a quench occurs in the main coil or the shield coil, the balance between the magnetic field generated in the main coil and the magnetic field generated in the shield coil is lost, and the leakage magnetic field of the device becomes the set leakage magnetic field (at the reference position). Bigger than. However, as described above, in the MRI apparatus, it is necessary to suppress the leakage magnetic field as much as possible, and it is desirable that the leakage magnetic field does not exceed the set leakage magnetic field even when a quench occurs.

  In the superconducting electromagnet apparatus disclosed in Patent Document 1, the resistance of each circuit is set so that the current decay time constant of each circuit is equal between the protection circuit for the main coil and the protection circuit for the shield coil. Electric resistance value is set. Specifically, the ratio of the resistance value of the resistor of each circuit is set to be equal to the ratio of the inductance of each circuit. By this technique, the electric current which flows into a main coil and a shield coil is maintained before and after the occurrence of a quench.

In the technique of Patent Document 1, the resistance value of each resistor is set as a constant value. Therefore, this technique is effective only when the magnetic field change at the occurrence of quenching is constant.
However, the magnetic field change of each coil at the time of occurrence of quenching is not constant and varies depending on the cause of occurrence of quenching and the position of occurrence. Therefore, it is difficult to suppress an increase in the leakage magnetic field at the time of occurrence of quench using the technique of Patent Document 1.

(Task)
The problem to be solved by the present invention is to suppress an increase in the leakage magnetic field when a quench occurs in an active shield type magnetic field generator.

(1) In order to solve the above problems, a magnetic field generator according to the present invention reduces a leakage magnetic field by an active shield system, and includes a main coil composed of a superconducting coil and a superconducting coil. The first shield coil disposed concentrically with the main coil on the outer side in the radial direction and electrically connected in series to the main coil, and electrically connected in parallel to the main coil to generate a quench A main coil protection circuit that sometimes protects the main coil, a first shield coil protection circuit that is electrically connected in parallel to the first shield coil and that protects the first shield coil when a quench occurs, and a superconducting coil The second shield disposed concentrically with the first shield coil on the radially outer side of the first shield coil. And Rudokoiru, so as to short-circuit both ends of the second shield coil, and a second shield coil protection circuit electrically connected to the second shield coil.
The second shield coil protection circuit includes a bidirectional diode, and the on-voltage of the bidirectional diode is caused by (i) a magnetic field change during excitation or demagnetization of the main coil and the first shield coil. The electromagnetic force is set to be larger than the electromotive force generated in the second shield coil, and (ii) the electromagnetic induction due to the magnetic field change caused by the quenching of the main coil or the first shield coil It is set below the electromotive force generated in the two shield coils.

In this configuration, the magnetic field of the main coil generated outside the first shield coil is weakened by the magnetic field of the first shield coil by the active shield method, and the leakage magnetic field (magnetic flux density) is reduced.
Further, in this configuration, the active shield type magnetic field generator is provided with the second shield coil, and the second shield coil suppresses an increase in the leakage magnetic field by electromagnetic induction. Therefore, when a quench occurs in the main coil or the first shield coil, it is possible to suppress an increase in the magnetic field leaking from the magnetic field generator regardless of the position and cause of the quench.
Further, in this configuration, when a quench occurs in the main coil or the first shield coil, a current flows through the bidirectional diode, so that the second shield coil functions when a quench occurs. On the other hand, during excitation and demagnetization (excitation or demagnetization) of the main coil and the first shield coil, current does not flow through the bidirectional diode, and thus the second shield coil does not function during excitation and demagnetization. Therefore, stable excitation and demagnetization can be performed without disturbing the magnetic field by the second shield coil.

The “superconducting coil” is formed by winding a wire made of a superconducting material into a solenoid shape (cylindrical shape). As a material for the superconducting coil, niobium titanium (NbTi), niobium tin (Nb ₃ Sn), and other high-temperature superconducting substances can be used.

  Each of the “main coil protection circuit” and the “first shield coil protection circuit” includes a conductor and a resistor. A resistor, a diode, a thyristor, or the like can be used as this resistor.

  The main coil protection circuit and the second shield coil protection circuit may be completely independent, or the main coil protection circuit and the second shield coil protection circuit may overlap part of the conductor. Good.

  (2) In the magnetic field generator of (1) according to the present invention, the axial length of the second shield coil is the maximum length in the axial direction of the main coil and the first shield coil. It may be the above.

  In this configuration, since the second shield coil has a sufficient length, an increase in the leakage magnetic field is reliably suppressed by the second shield coil.

Moreover, in the magnetic field generator according to the present invention, a method using a refrigerant is preferable as a cooling method for the superconducting coil. In this case, the superconducting coil is disposed in a refrigerant tank into which a refrigerant has been injected.
Moreover, it is preferable that the main coil, the first shield coil, and the second shield coil are arranged in the same cooling tank in terms of arrangement efficiency and the like.

  In order to satisfy the magnetic field uniformity required for the MRI apparatus and suitable as a magnetic field generator for the MRI apparatus, the main coil and the first shield coil are preferably composed of a plurality of coils.

It is an electric circuit diagram of the magnetic field generator concerning one embodiment of the present invention. It is sectional drawing of the coil group of FIG. FIG. 3 is a cross-sectional view taken along line E-E ′ of FIG. 2. It is a graph which shows the change of the leakage magnetic field at the time of generation | occurrence | production of quenching in the magnetic field generator of FIG. It is a graph which shows the change of the leakage magnetic field at the time of generation | occurrence | production of quenching in the conventional magnetic field generator.

  Hereinafter, an embodiment of the present invention will be described with reference to FIGS. In the following description, the vertical direction is the vertical direction in the figure. FIG. 2 corresponds to the G-G ′ cross-sectional view of FIG. 3.

(overall structure)
First, the overall configuration of the magnetic field generator 1 will be described. The magnetic field generator 1 is used as an MRI apparatus. Further, in the magnetic field generator 1, (i) the leakage magnetic field is reduced by the active shield method, and (ii) the increase of the leakage magnetic field due to quenching is suppressed by the second shield coil 50.

  The magnetic field generator 1 includes a first circuit 5 and a second circuit 6. The first circuit 5 includes a main coil 10, a first shield coil 20, a main coil protection circuit 30, a first shield coil protection circuit 40, a permanent current switch 71, and a power source 72. The second circuit 6 includes a second shield coil 50 and a second shield coil protection circuit 60. The first circuit 5 and the second circuit 6 are electrically separated, but are magnetically connected via the coils of both circuits.

  The magnetic field generator 1 includes a coil group (five coils of the main coil 10, two coils of the first shield coil 20, and the second shield coil 50). The coil included in the coil group is a superconducting coil. That is, the main coil 10, the first shield coil 20, and the second shield coil 50 are made of a superconducting material wire.

  A power source 72 is connected in parallel to the permanent current switch 71. The permanent current switch 71 is connected in parallel to the main coil 10 and the first shield coil 20, and the power source 72 is also connected in parallel to the main coil 10 and the first shield coil 20. Has been.

(First circuit)
The first circuit 5 will be described. As described above, the first circuit 5 includes the main coil 10, the first shield coil 20, the main coil protection circuit 30, the first shield coil protection circuit 40, the permanent current switch 71, and the power source 72.

(Main coil and first shield coil)
The main coil 10 includes five coils (coil 10a, coil 10b, coil 10c, coil 10d, and coil 10e). The first shield coil 20 includes two coils (coil 20a and coil 10e). 20b). The main coil 10 and the first shield coil 20 are cooled to a transition temperature or lower using a liquid refrigerant (or gas refrigerant) in a cooling tank (not shown).

The main coil 10 is disposed such that the axial direction C (see the arrow C direction in FIG. 2) coincides with the vertical direction. The direction of the magnetic field B ₀ generated inside the main coil 10 is parallel to the axial direction C. The axial length of the main coil 10 is Lm, and the axial length of the first shield coil 20 is L1 (see FIG. 2). Lm is larger than L1. The coil length relationship is not limited to this. Further, the axial direction C may coincide with the horizontal direction.

  The inner diameter of the first shield coil 20 is larger than the maximum outer diameter of the main coil 10. The first shield coil 20 is disposed outside the main coil 10 with respect to the radial direction D of the main coil 10 (see the arrow D direction in the figure). That is, the main coil 10 is disposed inside the first shield coil 20 (see FIG. 2).

  The first shield coil 20 is arranged concentrically with the main coil 10, and the central axis of the main coil 10 (see the one-dot chain line in FIG. 2) coincides with the central axis of the first shield coil 20. Further, the axial direction of the first shield coil 20 coincides with the axial direction C of the main coil 10. The radial direction D is perpendicular to the axial direction C.

  The first shield coil 20 is electrically connected to the main coil 10 in series. Specifically, the five coils of the main coil 10 are connected in series, the two coils of the first shield coil 20 are connected in series, and the main coil 10 and the first shield coil 20 are They are electrically connected in series (see FIG. 1).

In the magnetic field generator 1, magnetic field leakage to the outside of the first shield coil 20 is suppressed by the active shield method. Specifically, the magnetic field of the main coil 10 generated outside the first shield coil 20 in the radial direction D is weakened by the magnetic field of the first shield coil 20 in the direction opposite to the magnetic field.
In the magnetic field generator 1, the leakage magnetic field (leakage magnetic flux density) of the device is adjusted to be equal to or less than a set value (setting leakage magnetic field) at a certain reference position. During normal operation, the leakage magnetic field of the device is maintained at the set leakage magnetic field (at this reference position).

(Main coil protection circuit)
The main coil protection circuit 30 is electrically connected to the main coil 10 in parallel. The main coil protection circuit 30 protects the main coil 10 when a quench occurs in the main coil 10.

  The main coil protection circuit 30 includes five circuits (a circuit 30a, a circuit 30b, a circuit 30c, a circuit 30d, and a circuit 30e). The five circuits (sections) are respectively included in the five coils included in the main coil 10. It corresponds to. Each of the five circuits of the main coil protection circuit 30 includes a resistor (a resistor 31a, a resistor 31b, a resistor 31c, a resistor 31d, or a resistor 31e) and a conductor. When quenching occurs in the main coil 10, a current flows through the resistor, so that damage to the main coil 10 can be prevented.

(First shield coil protection circuit)
The first shield coil protection circuit 40 is electrically connected in parallel to the first shield coil 20. The first shield coil protection circuit 40 protects the first shield coil 20 when a quench occurs in the first shield coil 20.

  The first shield coil protection circuit 40 includes two circuits (a circuit 40a and a circuit 40b), and these two circuits (sections) correspond to the two coils included in the first shield coil 20, respectively. In addition, each of the two circuits of the first shield coil protection circuit 40 includes a resistor (the resistor 41a or the resistor 41b) and a conductor. When quenching occurs in the first shield coil 20, a current flows through the resistor, so that the first shield coil 20 can be prevented from being damaged.

(About permanent current switch)
Next, the operation of the magnetic field generator 1 using the permanent current switch 71 will be described.
First, while increasing the current flowing through the main coil 10 and the first shield coil 20 using the power source 72, the permanent current switch 71 is turned off (the permanent current switch 71 is electrically connected to the permanent current switch 71 by heating the superconducting wire). (Resistance is generated.)
When the currents of the main coil 10 and the first shield coil 20 reach the rated values, the permanent current switch 71 is turned on (the state where the permanent current switch 71 is in a superconducting state by cooling). As a result, a current also flows through the permanent current switch 71.
In this state, when the current of the power source 72 is lowered to zero and then the power source 72 is disconnected from the circuit, the current continues to flow through the closed circuit including the permanent current switch 71, the main coil 10, and the first shield coil 20.

(Second circuit)
Next, the second circuit 6 will be described. As described above, the second circuit 6 includes the second shield coil 50 and the second shield coil protection circuit 60. The second circuit 6 is provided separately from the first circuit 5 and is a closed circuit.

(Second shield coil)
The second shield coil 50 will be described. The axial length L2 of the second shield coil 50 is larger than the maximum length (that is, Lm) in the axial direction C of the main coil 10 and the first shield coil 20 (see FIG. 2). The second shield coil 50 is arranged so as to cover the range from the upper end to the lower end of the main coil 10 and the first shield coil 20. The second shield coil 50 is cooled below the transition temperature using a liquid refrigerant (or gas refrigerant). Moreover, the main coil 10, the 1st shield coil 20, and the 2nd shield coil 50 are arrange | positioned in the same cooling tank.

  The inner diameter of the second shield coil 50 is larger than the outer diameter of the first shield coil 20. The second shield coil 50 is arranged outside in the radial direction D of the first shield coil 20.

  The second shield coil 50 is disposed concentrically with the main coil 10 and the first shield coil 20, and the central axis of the second shield coil 50 is the central axis of the main coil 10 (see the one-dot chain line in FIG. 2). Matches. Further, the axial direction of the second shield coil 50 coincides with the axial direction C of the main coil 10 and the first shield coil 20.

  In the magnetic field generator 1, when a quench occurs in the main coil 10 or the first shield coil 20, the magnetic field balance between the main coil 10 and the first shield coil 20 is lost, and leakage at a reference position outside the first shield coil 20 occurs. The magnetic field may become larger than the set leakage magnetic field. Even in this case, an induced electromotive force is generated in the second shield coil 50 so as to cancel the change in the magnetic flux, and as a result, an increase in the leakage magnetic field outside the second shield coil 50 is suppressed.

(Second shield coil protection circuit)
The second shield coil protection circuit 60 will be described. The second shield coil protection circuit 60 is electrically connected to the second shield coil 50 so as to short-circuit both ends of the second shield coil 50. Thereby, the second circuit 6 is a closed circuit. The second shield coil protection circuit 60 includes a bidirectional diode 61 and a conductor.

(Bidirectional diode)
Next, the bidirectional diode 61 will be described. The bidirectional diode 61 is included in the second shield coil protection circuit 60. The bidirectional diode 61 includes two diodes (a diode 61a and a diode 61b), and these two diodes are arranged in opposite directions (see FIG. 1). The two diodes are the same component, and their on-voltages V _on are the same.

The on-voltage V _on of the bidirectional diode 61 is an electromotive force (induction) generated in the second shield coil 50 due to electromagnetic induction caused by a magnetic field change in excitation and demagnetization (excitation or demagnetization) of the main coil 10 and the first shield coil 20. It is set to be larger than the potential difference that causes current.
The on-voltage V _on of the bidirectional diode 61 is set to be equal to or lower than the electromotive force generated in the second shield coil 50 due to electromagnetic induction caused by the magnetic field change when the main coil 10 or the first shield coil 20 is quenched. ing.

Thus, the on-voltage V _on of the bidirectional diode 61 is larger than the induced electromotive force (maximum electromotive force when changed) generated at the time of excitation and demagnetization and equal to or less than the induced electromotive force generated when the quench occurs. It is set to become. Therefore, no current flows through the bidirectional diode 61 when the main coil 10 and the first shield coil 20 are excited and demagnetized. However, when quenching occurs in the main coil 10 and the first shield coil 20, the bidirectional current is not generated. A current flows through the diode 61.
The on voltage V _on corresponds to the on voltage of the diode 61a and the diode 61b.

Here, the setting of the ON voltage V _on of the bidirectional diode 61 will be described in more detail. The magnitude of the induced electromotive force generated in the second shield coil 50 (electromotive force generated in the direction to cancel the change in magnetic flux) is proportional to the magnetic field change rate (see Faraday's law of electromagnetic induction).
Therefore, the magnetic field change rate (magnetic field change amount per unit time) at the time of quench occurrence and excitation demagnetization is compared. First, excitation (or demagnetization) of the main coil 10 and the first shield coil 20 takes about one hour. On the other hand, when a quench occurs, the superconducting coil (the main coil 10 or the first shield coil 20) becomes a normal state in about several seconds. Therefore, when a quench occurs, the magnetic field change of the superconducting coil is completed in a few seconds. In addition, the amount of change (change width) in the magnetic field is about the same between excitation and demagnetization and when quenching occurs.
From the above, the magnetic field change rate when quenching occurs is larger than the magnetic field change rate during excitation and demagnetization. As a result, the induced electromotive force V _q generated when quenching occurs is larger than the induced electromotive force V _m generated during excitation demagnetization. The ON voltage V _on is set so as to satisfy the following equation.
V _m <V _on ≦ V _q (Formula 1)

Thus, in the magnetic field generator 1, since the second shield coil 50 does not function during excitation and demagnetization, stable excitation and demagnetization is possible. Next, the reason why the second shield coil 50 is not allowed to function during excitation and demagnetization will be described.
When the second shield coil 50 functions during excitation and demagnetization, an induced current flows through the second shield coil 50, and the magnetic field strength and magnetic field uniformity required for the MRI apparatus cannot be obtained.
Therefore, in the configuration of the magnetic field generator 1, the second shield coil 50 functions only when a quench occurs and does not function when excited and demagnetized.

  Further, when quenching occurs in the second shield coil 50, current flows through the bidirectional diode 61, so that damage to the second shield coil 50 can be prevented.

(Operation example)
Next, an operation example of the magnetic field generator 1 will be described. Here, as an example, a case where quenching occurs in the coil 10e of the main coil 10 will be described.

  During normal operation, the balance of the magnetic field between the main coil 10 and the first shield coil 20 is maintained. As a result, in the magnetic field generator 1, the magnetic field at the reference position outside the first shield coil 20 is maintained so as not to exceed the set leakage magnetic field.

  Here, when quenching occurs in the coil 10e, the coil 10e enters a normal conduction state, and the electrical resistance of the coil 10e increases rapidly. For this reason, the current flowing through the coil 10e decreases rapidly, and the current flowing through the resistor 31e increases. Thereby, the coil 10e is protected.

  In addition, when quenching occurs in the coil 10e, the current flowing through the coil 10e of the main coil 10 decreases compared to before the quenching (during normal operation), and therefore flows through the first shield coil 20 by electromagnetic induction. The current increases. For this reason, when a quench occurs, the balance between the magnetic field generated by the first shield coil 20 and the magnetic field generated by the main coil 10 is lost, and thereby the magnetic field at the reference position outside the first shield coil 20 is set. More than the leakage magnetic field.

  At this time, an induced electromotive force is generated in the second shield coil 50 so as to cancel the change in the magnetic flux. As a result, an increase in the leakage magnetic field outside the second shield coil 50 is suppressed.

  The resistance values of the resistors of the main coil protection circuit 30 and the first shield coil protection circuit 40 are set sufficiently smaller than the resistance values of the main coil 10 and the first shield coil 20 when the quench occurs. Has been. Therefore, when a quench occurs in the main coil 10 or the first shield coil 20, most of the energy stored in the main coil 10 and the first shield coil 20 is consumed by the resistor.

In addition, although the case where quenching generate | occur | produced in the coil 10e of the main coil 10 was demonstrated here, also when quenching generate | occur | produces in the 1st shield coil 20 (coil 20a or coil 20b), the 1st shield coil 20 of the same The outer magnetic field is increased, and the increase of the leakage magnetic field is suppressed by the configuration of the magnetic field generator 1.
Similarly, when quenching occurs in the coils 10a to 10d, this configuration suppresses an increase in the leakage magnetic field.

(Change in leakage magnetic field in conventional magnetic field generator)
Next, changes in the leakage magnetic field in the conventional active shield type magnetic field generator (the circuit of FIG. 1 excluding the second circuit 6) will be described with reference to FIG.

FIG. 5 shows the time change of the leakage magnetic field at a certain reference position outside the conventional magnetic field generator (outside the first shield coil). In addition, the vertical axis (leakage magnetic field) in FIG. 5 is an absolute value. As shown in the figure, the set leakage magnetic field at this reference position during normal operation (stable state) is 5 × 10 ⁻⁴ T (5 gauss).

  For example, when quenching occurs in the main coil, the current flowing through the main coil decreases, and the current flowing through the shield coil (first shield coil) increases due to electromagnetic induction. Therefore, the magnetic field of the shield coil becomes larger than that during normal operation, and the magnetic field in the main coil and the shield coil is disturbed (that is, the balance of the magnetic field is lost). As a result, as shown in FIG. 5, when quenching occurs in the main coil, the leakage magnetic field increases more than the set leakage magnetic field.

  In FIG. 5, the decrease after the leakage magnetic field increases is that the induced current flowing in the shield coil exceeds the critical current, so that quenching occurs in the shield coil, and current flows in both the main coil and the shield coil. This is because it stops flowing.

  Although the case where quenching occurs in the main coil has been described here, the same magnetic field change occurs when quenching occurs in the shield coil.

(Leakage magnetic field change in the magnetic field generator of this embodiment)
Next, the leakage magnetic field change in the magnetic field generator 1 will be described with reference to FIG. FIG. 4 shows the change over time of the leakage magnetic field at a certain reference position (the position of the point F in FIGS. 2 and 3) outside the magnetic field generator 1 (outside the second shield coil 50). In addition, the vertical axis (leakage magnetic field) in FIG. 4 is an absolute value. As shown in the figure, the set leakage magnetic field at the position of the point F during normal operation is 5 × 10 ⁻⁴ T (5 gauss).

For example, when quenching occurs in the main coil 10, the current flowing through the main coil 10 decreases, and the current flowing through the first shield coil 20 increases due to electromagnetic induction. Therefore, the magnetic field of the first shield coil 20 becomes larger than that during normal operation, and the magnetic fields in the main coil 10 and the first shield coil 20 are disturbed. That is, the balance between the magnetic field generated by the main coil 10 and the magnetic field generated by the first shield coil 20 is lost. As a result, the magnetic field at the position outside the first shield coil 20 and inside the second shield coil 50 (the position indicated by the point H in FIG. 3) increases compared to that during normal operation. At this time, an induced current flows through the second shield coil 50. As a result, a magnetic field is generated in the second shield coil 50 so as to suppress the increased leakage magnetic field. Therefore, as shown in FIG. 4, after the occurrence of quenching, the set leakage magnetic field (5 × 10 ⁻⁴ T) is maintained at the reference position (the position of the point F) without increasing the leakage magnetic field.

  When the operation of the magnetic field generator 1 is continued, the current flowing through the main coil 10 and the first shield coil 20 is attenuated until it becomes zero. Accordingly, the induced current generated in the second shield coil 50 is also attenuated accordingly. Therefore, as shown in FIG. 4, the leakage magnetic field finally becomes zero. The cause of the decrease in the leakage magnetic field is not directly related to the occurrence of quenching in the main coil 10 (or the first shield coil 20).

  Although the case where quenching occurs in the main coil 10 has been described here, the same magnetic field change occurs when quenching occurs in the first shield coil 20.

(effect)
Next, the effect obtained by the magnetic field generator 1 according to this embodiment will be described.
The magnetic field generator 1 reduces a leakage magnetic field by an active shield system, and includes a main coil 10 including five superconducting coils (coil 10a, coil 10b, coil 10c, coil 10d, and coil 10e) and two superconducting coils. A first shield coil 20 composed of coils (coil 20a and coil 20b), arranged concentrically with the main coil 10 on the radially outer side of the main coil 10, and electrically connected in series to the main coil 10, A main coil protection circuit 30 (circuit 30a, circuit 30b, circuit 30c, circuit 30d, and circuit 30e) that is electrically connected in parallel to the coil 10 and protects the main coil 10 when a quench occurs, and a first shield coil 20 is electrically connected in parallel to the first shield core when a quench occurs. The first shield coil protection circuit 40 (the circuit 40a and the circuit 40b) for protecting the cable 20 and a superconducting coil, and is arranged on the outer side in the radial direction of the first shield coil 20 and concentrically with the first shield coil 20. A shield coil 50 and a second shield coil protection circuit 60 electrically connected to the second shield coil 50 so as to short-circuit both ends of the second shield coil 50 are provided.
The second shield coil protection circuit 60 includes a bidirectional diode 61, and the ON voltage V _on of the bidirectional diode 61 is (i) a magnetic field in excitation or demagnetization of the main coil 10 and the first shield coil 20. Due to electromagnetic induction caused by the change, it is set to be larger than the electromotive force V _m generated in the second shield coil 50, and (ii) caused by the magnetic field change at the time of quenching of the main coil 10 or the first shield coil 20. It is set below the electromotive force V _q generated in the second shield coil 50 by electromagnetic induction.

In this configuration, the magnetic field of the main coil 10 generated outside the first shield coil 20 is weakened by the magnetic field of the first shield coil 20 by the active shield method, and the leakage magnetic field (magnetic flux density) is reduced.
In this configuration, the second shield coil 50 is provided in the active shield type magnetic field generator, and the second shield coil 50 suppresses an increase in the leakage magnetic field by electromagnetic induction. Therefore, when a quench occurs in the main coil 10 or the first shield coil 20, it is possible to suppress an increase in the magnetic field leaking from the magnetic field generator regardless of the position or cause of the quench.
In this configuration, when a quench occurs in the main coil 10 or the first shield coil 20, a current flows through the bidirectional diode 61. Therefore, the second shield coil 50 functions when a quench occurs. On the other hand, when the main coil 10 and the first shield coil 20 are excited and demagnetized (excitation or demagnetization), no current flows through the bidirectional diode 61, and therefore the second shield coil 50 does not function during excitation and demagnetization. Therefore, stable excitation and demagnetization can be performed without disturbing the magnetic field by the second shield coil 50.

Moreover, in the magnetic field generator 1, the axial length L2 of the second shield coil 50 is not less than the maximum length Lm in the axial direction of the main coil 10 and the first shield coil 20.
In this configuration, since the second shield coil 50 has a sufficient length, an increase in the leakage magnetic field is reliably suppressed by the second shield coil 50.

  Moreover, in the magnetic field generator 1, each of the main coil 10 and the 1st shield coil 20 is a split type coil, and each includes a plurality of coils. This makes it possible to satisfy the magnetic field uniformity required for the MRI apparatus.

(About other embodiments)
The embodiment of the present invention is not limited to the above embodiment. For example, the number of diodes included in the bidirectional diode is not limited to two. As an example, a bidirectional diode includes four diodes connected in series, of which two diodes are arranged in one direction and the other two are arranged in the opposite direction. May be. In this case, the value of the on-current of the bidirectional diode is the sum of the on-currents of the two diodes.

  Moreover, in said embodiment, the cooling system of a coil is a refrigerant | coolant use type, and the same refrigerant tank is used as a refrigerant tank of the main coil 10, the 1st shield coil 20, and the 2nd shield coil 50. Yes. However, each coil may be disposed in a corresponding refrigerant tank. Further, the cooling method of the coil may be a refrigerator use type that does not use a refrigerant.

  Moreover, in said embodiment, the main coil 10 and the 1st shield coil 20 are each a split type coil, and are comprised from the some coil. Each of these may be composed of only one coil instead of the split type. In the above-described embodiment, the second shield coil 50 is composed of one coil, but the second shield coil may also be composed of a plurality of coils (divided coil).

The on-state voltage V _on of the bidirectional diode is set so as to satisfy the above formula 1, and varies depending on the operating current, the magnetic field strength, the size of the measurement space, etc., but is about 5 to 10 [V], for example. be able to.

  The magnetic field generator according to the present invention can be used as a medical MRI apparatus.

DESCRIPTION OF SYMBOLS 1 Magnetic field generator 5 1st circuit 6 2nd circuit 10 Main coil 10a, 10b, 10c, 10d, 10e Coil 20 which comprises main coil 1st shield coil 20a, 20b Coil 30 which comprises 1st shield coil 30 Main coil protection Circuits 30a, 30b, 30c, 30d, 30e Circuits 31a, 31b, 31c, 31d, 31e constituting the main coil protection circuit Resistor 40 First shield coil protection circuits 40a, 40b Circuit 41a constituting the first shield coil protection circuit , 41b Resistor 50 Second shield coil 60 Second shield coil protection circuit 61 Bidirectional diode 61a, 61b Diode 71 Permanent current switch 72 Power supply

Claims (2)

A magnetic field generator (1) for reducing a leakage magnetic field by an active shield method,
A main coil (10) comprising a superconducting coil;
A first shield coil (20) comprising a superconducting coil, arranged concentrically with the main coil on the radially outer side of the main coil, and electrically connected in series to the main coil;
A main coil protection circuit (30) electrically connected in parallel to the main coil and protecting the main coil when a quench occurs;
A first shield coil protection circuit (40) connected in parallel to the first shield coil and protecting the first shield coil when a quench occurs;
A second shield coil (50) consisting of a superconducting coil and arranged concentrically with the first shield coil on the radially outer side of the first shield coil;
A second shield coil protection circuit (60) electrically connected to the second shield coil so as to short-circuit both ends of the second shield coil;
The second shield coil protection circuit includes a bidirectional diode (61),
The on-voltage of the bidirectional diode is set to be larger than (i) the electromotive force generated in the second shield coil due to electromagnetic induction caused by magnetic field change in excitation or demagnetization of the main coil and the first shield coil. And (ii) the electromotive force generated in the second shield coil is set to be equal to or less than the electromotive force generated in the second shield coil by electromagnetic induction caused by a magnetic field change when the main coil or the first shield coil is quenched. Magnetic field generator.   2. The magnetic field generator according to claim 1, wherein an axial length of the second shield coil is equal to or greater than a maximum length in the axial direction of the main coil and the first shield coil.

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