JP2018159637A - Magnetic field generating device, nmr analysis device, and mri device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic field generation device capable of generating a magnetic field extremely high in temporal stability even when using the coil of a superconducting wire difficult in superconducting junction and an NMR analysis device and a MRI device provided with the same.SOLUTION: A magnetic field generating device includes: a main coil (11) which is a superconducting coil and generates a magnetic field in the space of an object; and a compensation coil (12) which is a super conduction coil, and connected in series with the main coil. The magnetic field generating device further includes: a first closed circuit (C1) into which an exciting current of the main coil flows; a control coil (21) which is a super conduction coil and magnetically connected to the compensation coil; and a resistor (22) connected in series with the control coil; and a second closed circuit (C2) into which a control current reduced accompanied with a temporal elapse due to power consumption of the resistor flows. An induced electromotive force generated in the compensation coil (12) due to decrease of the control current via an electromagnetic combination between the control coil and the compensation coil has a polarity that induces the current of the same direction as the exciting current of the main coil (11).SELECTED DRAWING: Figure 1

Description

  The present invention relates to a magnetic field generator for generating a magnetic field, and an NMR analyzer and an MRI apparatus provided with the same.

  An NMR (nuclear magnetic resonance) analyzer can examine the molecular structure of a sample by irradiating the sample with an electromagnetic wave and receiving an NMR signal that resonates with the electromagnetic wave while the sample is placed in a magnetic field. The stronger the magnetic field, the higher the frequency of nuclear magnetic resonance, thereby obtaining a precise analysis result. For this reason, development for strengthening the magnetic field of NMR analyzers is currently underway.

  Conventionally, as a magnetic field generator applied to an NMR analyzer, an apparatus using a superconducting coil mainly of LTS (Low-Temperature Superconducting) wire has been put into practical use. By putting the entire circuit including the superconducting coil into a superconducting state, a permanent current can be passed through the circuit and a constant magnetic field that is very stable in time can be generated. On the other hand, the LTS wire needs to be used at a boiling point temperature of liquid helium (4.2 K, hereinafter referred to as “liquid helium temperature”) or an extremely low temperature close to this in order to transition to a superconducting state. In addition, the LTS wire has the property that the superconducting properties are reduced in a strong magnetic field. For this reason, the superconducting coil mainly using the LTS wire has a problem that the operation efficiency is relatively low, and there is a predetermined restriction on the strength of the magnetic field generated.

  On the other hand, the development of superconducting coils using HTS (High-Temperature Superconducting) wire is now underway. The HTS wire has the advantage that it can be operated at a temperature higher than the liquid helium temperature (for example, 30K). Furthermore, some HTS wires, for example, rare earth-based superconducting wires, do not easily deteriorate the superconducting characteristics even in a strong magnetic field. For this reason, the superconducting coil using the HTS wire has the advantage that it has a relatively high operating efficiency and can generate a stronger magnetic field.

  However, the HTS wire has a problem that superconducting joining between the wires is difficult. The superconducting junction means a junction that can bring the connecting portion into a superconducting state. For example, if LTS wires are used, superconducting bonding is possible using solder. The solder portion can also be brought into a superconducting state by setting the temperature to liquid helium. On the other hand, the surface of the HTS wire is coated with a special metal such as silver, and the portion including the metal layer is joined with solder. A metal layer such as silver is essential for improving the characteristics of the HTS wire. However, metals such as silver do not transition to the superconducting state at the high transition temperature of the HTS wire, but also do not transition to the superconducting state at the liquid helium temperature. Currently, development of superconducting joining technology for HTS wire is underway, but it has not been put into practical use at present.

  For this reason, when the superconducting coil of the HTS wire is applied to the magnetic field generator, the connection part in the normal conducting state is included in the circuit including the superconducting coil. Conversely, a closed circuit including a superconducting coil of LTS wire can be used by transferring the entire closed circuit to a superconducting state. Since the entire closed circuit is in a superconducting state, the current of the same magnitude flows semipermanently without attenuating the current flowing in the closed circuit. Such a current is called a permanent current. A permanent magnetic field can generate a very stable magnetic field in time. However, when the superconducting coil of the HTS wire is applied, the current flowing through the circuit does not become a permanent current due to the electrical resistance of the connecting portion, and the current attenuates with time. For this reason, without any device, it is difficult to generate a stable magnetic field whose temporal change is almost zero.

  Conventionally, several techniques for suppressing the attenuation of the current of the superconducting coil or for suppressing the attenuation of the magnetic field generated by the superconducting coil have been proposed. For example, Patent Document 1 discloses an apparatus including a high magnetic field generation unit including a superconducting coil that generates a magnetic field and a power storage unit including a superconducting coil having a larger inductance than the high magnetic field generation unit. The superconducting coil of the high magnetic field generation unit and the superconducting coil of the power storage unit are connected in series. According to such a configuration, the amount of current attenuation of the entire circuit including the high magnetic field generation unit can be reduced by the large energy accumulated in the power storage unit, and a stable magnetic field can be generated by the high magnetic field generation unit. .

  Patent Document 2 further includes a first system superconducting coil for generating a main magnetic field for generating a magnetic field, and a second system superconducting coil for compensating a main magnetic field attenuation for generating a magnetic field opposite to the magnetic field. The device is shown. In this technology, even if the magnetic fields of the two superconducting coils are attenuated, the two magnetic fields are superimposed in opposite directions, thereby canceling each other's attenuation, and the total attenuation of the superimposed magnetic fields. Is expected to be reduced.

JP 2000-323321 A JP 2000-147082 A

  In the NMR analyzer, more precise analysis results can be obtained by strengthening the magnetic field as described above. On the other hand, in the NMR analyzer, a slight change in the magnetic field appears in the analysis result as noise. Therefore, when the superconducting coil of the HTS wire is used, even if a strong magnetic field can be generated so that a precise analysis result can be obtained, the accuracy of the analysis result is lowered due to a decrease in temporal stability of the magnetic field. As a result, there arises a problem that the accuracy of comprehensive analysis is not improved.

  On the other hand, when the technique of Patent Document 1 is applied, the temporal stability of the magnetic field can be increased. However, the superconducting coil mounted on the NMR analyzer is large-scale, and to apply the technique of Patent Document 1, it is necessary to add a superconducting coil of a power storage unit having a larger inductance. Therefore, in the technique of Patent Document 1, there is a problem that the entire apparatus is enlarged and the product cost is remarkably increased. For example, in order to reduce the current decay rate to 1/10, it is necessary to make the inductance of the superconducting coil of the power storage unit 10 times that of the superconducting coil for generating a magnetic field. In addition, if an HTS wire is used for the superconducting coil of the power storage unit, the normal conductive connection portion is included at a constant rate with respect to the wire length, so that the power storage unit itself also has an effect of current attenuation.

  Furthermore, if the technique of Patent Document 2 is applied, it is expected that a time-stable magnetic field can be generated by canceling the magnetic field attenuation generated by the two superconducting coils even if current attenuation occurs. . However, the technique of Patent Document 2 has a problem that it is very difficult to adjust the current of the superconducting coil for main magnetic field attenuation compensation. This is due to the following reason.

  In general, since the current attenuation behavior of a circuit including a coil and a resistor is exponential, if the amount of current attenuation of a superconducting coil of one system is within the compensation coil circuit in accordance with the inductance, The current attenuation can be controlled by selecting the included resistance value. However, in the technique of Patent Document 2, two superconducting coils are arranged to face each other, so that they are magnetically coupled. Accordingly, the behavior of the currents in the superconducting coils of both systems interacts in a complex manner such that the attenuation of the current in the first system superconducting coils affects the current in the second system superconducting coils. For this reason, it is extremely difficult to adjust the current so that the attenuation of the magnetic field is canceled out, unlike simple control in which an independent circuit of one system is adjusted.

  Furthermore, in the technique of Patent Document 2, it is necessary to observe the strength of superposition of the magnetic fields of the superconducting coils of the two systems in order to confirm whether or not the current is correctly adjusted. However, the temporal stability of the magnetic field required for the NMR analyzer is very high. At present, it is difficult to directly measure the magnetic field to confirm whether the magnetic field stability is obtained at the required level. Therefore, it is extremely difficult to adjust the current so that the attenuation of the magnetic field sufficiently cancels from the observation of the magnetic field.

  On the other hand, in order to generate a temporally stable magnetic field with the HTS wire superconducting coil, it can be considered to drive the superconducting coil with a constant current source. However, a superconducting coil mounted on an NMR analyzer has a large scale and a large inductance, and is driven with a low voltage and a large current. A general constant current source is not good at driving a large inductance with a low voltage and a large current. Further, the stability of the magnetic field required for the NMR analyzer is very high. From these facts, it is considered difficult to generate a stable magnetic field as required by an NMR analyzer when a general constant current source is used.

In the above description, if a superconducting coil of an HTS wire is used to enhance the magnetic field, the circuit includes a connection part in a normal conducting state, which causes difficulty in generating a stable magnetic field. did. As an HTS wire that can enhance the magnetic field, there is a REBCO wire (rare earth-based superconducting wire). However, there are several types of superconducting wires that have advantages other than magnetic field enhancement, such as BSCCO wires (bismuth-based superconducting wires) or MgB ₂ superconducting wires. Even when these wires are applied as a superconducting coil, the current problem is that the circuit includes a connection part in a normal conduction state, and the same problem arises that it is difficult to generate a stable magnetic field.

  An object of the present invention is to provide a magnetic field generator capable of generating a magnetic field with high temporal stability even using a coil of a superconducting wire that is difficult to superconducting, and an NMR analyzer and an MRI apparatus equipped with the magnetic field generator. That is.

One embodiment of the present invention provides:
A first closed circuit including a main coil that is a superconducting coil and generates a magnetic field in a target space; and a compensation coil that is a superconducting coil and is connected in series with the main coil;
A control coil, which is a superconducting coil and is magnetically coupled to the compensation coil, and a resistor connected in series with the control coil, a control current that decreases with the passage of time due to power consumption of the resistor flows. A second closed circuit;
With
The induced electromotive force generated in the compensation coil by reducing the control current through the magnetic coupling between the control coil and the compensation coil has a polarity that induces a current in the same direction as the excitation current. A magnetic field generator.

The magnetic field generator of one embodiment of the present invention includes:
A measuring unit for measuring the voltage of the whole or a part of the main coil;
A control unit that changes a resistance value of the resistor based on a measurement result of the measurement unit;
Is further provided.

The magnetic field generator of one embodiment of the present invention includes:
A heater for heating the resistor;
The resistance is a metal having a residual resistance ratio of 30 or more,
The controller is characterized in that the resistance value of the resistor is changed by heating the resistor with the heater.

The magnetic field generator of one embodiment of the present invention includes:
The main coil includes a coil portion of a rare earth-based superconducting wire, a bismuth-based superconducting wire, or a MgB ₂ superconducting wire.

The magnetic field generator of one embodiment of the present invention includes:
It further comprises a ferromagnetic core that magnetically couples the compensation coil and the control coil.

The magnetic field generator of one embodiment of the present invention includes:
The second closed circuit is:
The superconducting coil further includes an inertia coil connected in series with the control coil and the resistor.

The magnetic field generator of one aspect of the present invention includes a plurality of the compensation coils,
A plurality of second closed circuits magnetically coupled to the plurality of compensation coils, respectively.
Each of the plurality of compensation coils is connected in series to the main coil and is connected in parallel to each other.

  One embodiment of the present invention is an NMR analyzer including the magnetic field generator of the present invention.

  One embodiment of the present invention is an MRI apparatus including the magnetic field generator of the present invention.

  According to the present invention, the induced electromotive force can be generated in the first closed circuit including the main coil by the control current of the second closed circuit. The induced electromotive force has a polarity that induces a current in the same direction as the exciting current of the main coil. Therefore, even if there is a portion where a voltage drop that attenuates the excitation current occurs in the first closed circuit, attenuation of the excitation current can be suppressed by the induced electromotive force.

  Further, the first closed circuit is provided with a compensation coil in addition to the main coil, and the compensation coil is magnetically coupled to the control coil of the second closed circuit. Therefore, a high degree of design freedom can be obtained with respect to the inductance value of the compensation coil, the inductance value of the control coil, and the mutual inductance value therebetween. Therefore, these values can be appropriately designed so as to correspond to the decay factor of the excitation current, and the decay of the excitation current can be appropriately suppressed. Further, since the magnitude of the voltage drop of the first closed circuit is very small, the induced electromotive force generated in the control coil is small, and the compensation coil and the control coil do not become large. Furthermore, since the compensation coil and the control coil are superconducting coils, a current can be passed over a long period of time without connecting a power source during the generation of the magnetic field. Therefore, it is possible to generate a magnetic field that is very stable in time over a long period of time.

  Moreover, when the control part which changes the resistance value of the resistance of a 2nd closed circuit based on the measurement result of the voltage of a main coil is further provided, attenuation | damping of an exciting current can be suppressed appropriately by feedback control. Furthermore, the measurement of the voltage of the main coil and the adjustment of the resistance value can be performed at a very fine level even in the present situation. Therefore, the attenuation of the excitation current can be controlled at a fine level to cope with the generation of a magnetic field having a very high stability required by an NMR analyzer or the like.

It is a lineblock diagram showing the magnetic field generator concerning a 1st embodiment of the present invention. It is a block diagram which shows the magnetic field generator which concerns on 2nd Embodiment of this invention. It is a block diagram which shows an example of an inertia coil. It is a circuit diagram which shows the relationship between a compensation coil, a control coil, a variable resistance, and an inertia coil. It is a block diagram which shows the magnetic field generator which concerns on the modification of embodiment of this invention. It is a block diagram which shows the NMR analyzer which concerns on embodiment of this invention. It is a block diagram which shows the MRI apparatus which concerns on embodiment of this invention. It is a block diagram which shows an example of a variable resistance.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the embodiments described below are given various technically preferable limitations for carrying out the present invention, but the scope of the present invention is not limited to the following embodiments and illustrated examples.

(First embodiment)
FIG. 1 is a configuration diagram illustrating a magnetic field generator according to a first embodiment of the present invention. In this circuit diagram, the circuit part with rounded corners indicates that it is a closed circuit in a superconducting state except for the connection part. The same applies to the subsequent circuit diagrams.

  The magnetic field generator 1 according to the first embodiment includes a first closed circuit C1 including a main coil 11, a compensation coil 12, and a permanent current switch (PCS) 13, a control coil 21, and a variable resistor 22. A second closed circuit C2 including a permanent current switch 23 is provided. In the first closed circuit C1, the main coil 11, the compensation coil 12, and the permanent current switch 13 are connected in series. In the second closed circuit C2, the control coil 21, the variable resistor 22, and the permanent current switch 23 are directly connected. Furthermore, the magnetic field generator 1 includes a measuring unit 2, a control unit 3, DC power supplies 15 and 25, protection circuit elements 16 and 26, an iron core 31, a heater 32, a heater power supply 33, and a cooling unit 40. Is provided. The variable resistor 22 corresponds to an example of a resistor according to the present invention.

  The cooling unit 40 is a cryostat, for example, and cools the first closed circuit C <b> 1 and the second closed circuit C <b> 2 to a transition temperature or lower that causes the transition to the superconducting state.

  The main coil 11 is a superconducting coil and generates a magnetic field in the target space H. The target space H means a space in which generation of a magnetic field is required, such as a space in which an analysis target or a subject is placed in an NMR analyzer or an MRI apparatus. The main coil 11 is made of, for example, a REBCO wire (rare earth-based superconducting wire). This wire can maintain the superconducting property even in a magnetic field stronger than the LTS wire. Moreover, this wire is an HTS wire, and changes to a superconducting state at a higher temperature than the LTS wire. The main coil 11 does not need to be entirely composed of REBCO wire. For example, a hybrid configuration in which a coil portion of the REBCO wire is provided in the central portion and a coil portion of the LTS wire is provided around the main coil 11 may be adopted. Good. In this case, the coil part of the REBCO wire and the coil part of the LTS wire may be driven by separate circuits. When the coil part of the LTS wire is included, the cooling part 40 is lowered to the liquid helium temperature.

  The compensation coil 12 is a superconducting coil that is magnetically coupled to the control coil 21 and generates an induced electromotive force under the control of the second closed circuit C2. As the compensation coil 12, an HTS wire coil or an LTS wire coil may be applied. When the LTS wire is applied, the cooling unit 40 is lowered to the liquid helium temperature.

  The control coil 21 is a superconducting coil that is magnetically coupled to the compensation coil 12 and generates an induced electromotive force in the compensation coil 12 due to a change in the control current flowing in the second closed circuit C2. The control coil 21 holds magnetic energy and allows a control current to flow for a long period. As the control coil 21, an HTS wire coil or an LTS wire coil may be applied. When the LTS wire is applied, the cooling unit 40 is lowered to the liquid helium temperature. The control coil 21 is referred to as a control coil because it contributes to control that improves temporal stability of the current flowing through the first closed circuit C1. The current flowing through the control coil 21 is also called a control current for the same purpose.

  The iron core 31 is a ferromagnetic core around which the compensation coil 12 and the control coil 21 are wound, and strongly couples the compensation coil 12 and the control coil 21 magnetically. The iron core 31 increases the magnetic energy held by the compensation coil 12 and the control coil 21. The iron core 31 also constitutes a loop-like magnetic circuit and has a role of preventing magnetic flux from leaking into the target space H. The compensation coil 12 and the control coil 21 may be magnetically coupled without using the iron core 31.

  The variable resistor 22 has a very small resistance value, and further changes the resistance value by heating of the heater 32. As the variable resistor 22, a high-purity metal, for example, a metal having a residual resistance ratio (RRR) of 30 or more, such as copper, silver, or aluminum, is employed. In this specification, the “residual resistance ratio” means a ratio of a specific resistance at 4.2K to a specific resistance at room temperature (for example, 300K). A detailed example of the variable resistor 22 will be described in an embodiment described later.

  The DC power source 15 is connected in parallel with the protection circuit element 16 and the permanent current switch 13, and DC-drives the main coil 11 and the compensation coil 12 when the permanent current switch 13 is open (normal conducting state). As a result, an exciting current flows through the main coil 11.

  The permanent current switch 13 is composed of a superconducting wire, and when the DC power source 15 is disconnected and the cooling unit 40 is below the transition temperature, it enters a superconducting state and allows the current of the first closed circuit C1 to flow.

  The protection circuit element 16 applies a current to protect the first closed circuit C1 when the permanent current switch 13 is opened (normal conducting state).

  The first closed circuit C1 includes a normal conducting portion that does not shift to the superconducting state in the connecting portion p where the wire of the main coil 11 is soldered. Therefore, even when the first closed circuit C1 becomes lower than the transition temperature, the resistance of the connection portion p remains in the first closed circuit C1, and the current flowing through the first closed circuit C1 does not become a complete permanent current. However, the resistance value of the connection portion p of the first closed circuit C1 is very small compared to the magnetic energy stored in the main coil 11, and the first closed circuit C1 has a permanent current below the transition temperature. Near current flows. Further, the stability of the current of the first closed circuit C1 is further enhanced by the action of the second closed circuit C2 described later, and a current close to a permanent current flows through the first closed circuit C1. Hereinafter, this current is referred to as “pseudo-permanent current”. The pseudo permanent current corresponds to an excitation current that causes the main coil 11 to generate a magnetic field.

  The DC power supply 25 is connected in parallel to the protection circuit element 26 and the permanent current switch 23, and excites the control coil 21 when the permanent current switch 23 is open (normal conducting state), and supplies the control coil 21 with a control current. Shed.

  The permanent current switch 23 is composed of a superconducting wire, and when the DC power supply 25 is disconnected and the cooling unit 40 is below the transition temperature, the permanent current switch 23 enters a superconducting state and flows the current of the second closed circuit C2.

  The protection circuit element 26 protects the second closed circuit C2 by supplying a current when the permanent current switch 23 is opened (normal conducting state).

  When the second closed circuit C2 becomes lower than the transition temperature, a control current that decreases with the passage of time due to the power consumption of the resistors including the variable resistor 22 flows. Since the magnitude of the resistance including the variable resistance 22 is very small compared to the magnetic energy stored in the control coil 21, the rate of decrease in the control current flowing through the second closed circuit C2 is very small. The current flows for a long period.

  The measuring unit 2 measures the voltage of all or part of the main coil 11. The total voltage means the voltage between both ends of the main coil 11, and the partial voltage means the voltage between both ends of a part of the main coil 11. The measured voltage directly or indirectly represents the induced electromotive force generated in the main coil 11. The induced electromotive force is generated by a change in pseudo permanent current flowing through the main coil 11. The measuring unit 2 may measure a voltage in a section including all or a part of the main coil 11 and the connection part p that is a normal conductor. In this case, the induced electromotive force of the main coil 11 can be estimated by subtracting the voltage drop at the connection portion p from the measured voltage.

  The control unit 3 controls the amount of heat generated by the heater 32 based on the measurement result of the measurement unit 2, thereby controlling the resistance value of the variable resistor 22. Based on the measurement result of the measurement unit 2, the control unit 3 specifically sets the voltage value measured by the measurement unit 2 to be equal to or less than the set value so that the change in the pseudo permanent current is equal to or less than the set value. Perform feedback control.

<Description of operation>
In the magnetic field generator 1 of the first embodiment, first, a predetermined excitation current is caused to flow through the first closed circuit C1 by the DC power source 15, and a predetermined control current is caused to flow through the second closed circuit C2 from the DC power source 25. Thereafter, the cooling unit 40 cools the first closed circuit C1 and the second closed circuit C2 to the transition temperature or lower, and the DC power supplies 15 and 25 reduce their energization currents to zero. Furthermore, the DC power supplies 15 and 25 may be disconnected from the circuit. Thereby, in the 1st closed circuit C1, other than the connection part p will be in a superconducting state, and a pseudo permanent current will flow. Further, in the second closed circuit C2, a control current that decreases with the passage of time flows except for the variable resistor 22 and the connection portion p in a superconducting state.

  The value of the excitation current is set so that a magnetic field having a high intensity (for example, an intensity corresponding to 1.02 GHz) is generated from the main coil 11. Here, the intensity of the magnetic field is represented by the resonance frequency of the proton nucleus in the nuclear magnetic resonance phenomenon. The strength of this magnetic field is difficult to achieve with LTS wire due to the superconducting magnetic field characteristics.

  When the permanent current switch 13 is in a superconducting state and a pseudo permanent current flows through the first closed circuit C1, a voltage drop that attenuates the pseudo permanent current occurs due to the electrical resistance of the connection portion p of the first closed circuit C1. On the other hand, when the control current of the second closed circuit C <b> 2 is decreased by the variable resistor 22, the magnetic flux inside the iron core 31 is decreased by the change in the current of the control coil 21. Then, an induced electromotive force is generated in the compensation coil 12 due to the decrease in the magnetic flux. In other words, an induced electromotive force is generated in the compensation coil 12 by reducing the control current through the magnetic coupling between the control coil 21 and the compensation coil 12. The induced electromotive force has a polarity that induces the same current as the direction in which the pseudo permanent current flows. That is, the direction from the higher potential of the induced electromotive force to the lower one is the same as the direction of the pseudo permanent current. Thereby, attenuation of the pseudo permanent current is suppressed.

  The change in the pseudo permanent current causes an induced electromotive force to be generated in the main coil 11, and a voltage at a part or both ends of the main coil 11 including the induced electromotive force is measured by the measurement unit 2. Based on the measurement result, the control unit 3 controls the resistance value of the variable resistor 22 so that the magnitude (absolute value) of the induced electromotive force of the main coil 11 is not more than a set value close to zero. Depending on the magnitude of the resistance value of the variable resistor 22, the magnitude of the induced electromotive force generated in the compensation coil 12 changes, and the amount of change in the pseudo permanent current can be controlled.

  At present, it is difficult to accurately measure the magnetic field strength at the stability level required for the NMR analyzer, but with a voltage, it can be measured with an accuracy of the order of nV. The induced voltage generated in the main coil 11 is substantially proportional to the deviation of the resistance value of the variable resistor 22 from the optimum value. For this reason, the control part 3 can maintain the induced electromotive force of the main coil 11 at zero by highly accurate feedback control. As a result, the pseudo permanent current flowing through the first closed circuit C1 is very stable so as not to change from the permanent current.

In 1st Embodiment, the measurement result of the voltage of the main coil 11 was employ | adopted as a feedback signal, and it demonstrated that the control part 3 controlled the resistance value of the variable resistance 22 based on this feedback signal. However, as a feedback signal for controlling the resistance value of the variable resistor 22, a control sample showing a known reaction may be set near the sample to be measured, and a signal from the control sample may be adopted. If the resistance value of the variable resistor 22 is controlled so that this signal takes a predetermined value, the main coil 11 can continue to form a constant magnetic field. Specifically, heavy water (D ₂ O) may be used as a control sample, and its NMR signal may be used as a signal.

  Here, the energy aspect is supplemented. In the first closed circuit C1 and the second closed circuit C2 including electric resistance, Joule heat is generated by the electric resistance. The fact that the decay of the pseudo permanent current is suppressed requires that energy lost as Joule heat be supplemented from somewhere. This supplemented energy is extracted from the magnetic energy stored in the compensation coil 12 and the control coil 21. Therefore, the duration in which the pseudo permanent current flowing through the first closed circuit C1 is compensated by the compensation coil 12 and kept constant is limited by the accumulated magnetic energy. Therefore, the compensation coil 12, the control coil 21, and the iron core 31 are designed so as to satisfy the duration required by the system.

  As described above, according to the magnetic field generator 1 of the first embodiment, the magnetic field generator 1 is a strong magnetic field that cannot be generated from the main coil 11 to the target space H by the superconducting coil of the LTS wire, and has very high temporal stability. A magnetic field can be generated.

(Second Embodiment)
FIG. 2 is a block diagram showing a magnetic field generator according to the second embodiment of the present invention. FIG. 3 is a configuration diagram illustrating an example of an inertia coil.

  The magnetic field generator 1A of the second embodiment is different from the first embodiment in that an inertia coil 24 is added to the second closed circuit C2A, and other parts are the same as those of the first embodiment. About the same structure, the same code | symbol is attached | subjected and detailed description is abbreviate | omitted.

  The inertia coil 24 is connected in series with the control coil 21, the variable resistor 22, and the permanent current switch 23 in the second closed circuit C2A. As shown in FIG. 3, if the inertia coil 24 can be magnetically isolated, it may have a ferromagnetic core 35 (iron core or the like), thereby increasing the self-inductance. The core 35 is preferably hard to be magnetized. If the inertia coil 24 is not magnetically isolated, the inertia coil 24 is affected by a change in the external magnetic field, which may disturb the stability of the pseudo permanent current of the first closed circuit C1. In order to avoid such a situation, a ring-shaped core 35 may be employed so that the magnetic flux generated by the inertia coil 24 is confined.

<Description of operation>
First, the circuit of the first embodiment that does not have the inertia coil 24 will be considered. Hereinafter, L ₁ is the self-inductance of the compensation coil 12, L ₂ is the self-inductance of the control coil 21, R is the resistance value of the variable resistor 22, I ₁ is a pseudo permanent current flowing through the main coil 11 and the compensation coil 12, and I ₂ is The control current V ₁ flowing in the control coil 21 will be described as an induced electromotive force generated in the compensation coil 12. The description will be made assuming that the degree of magnetic coupling between the compensation coil 12 and the control coil 21 is 100%.

但し、ｔは時間、変数上のドットは時間微分を示す。 When the current I ₁ of the first closed circuit C ₁ is stable in time, the induced electromotive force V ₁ generated in the compensation coil 12 is expressed by the following equation (1) based on the mutual inductance relationship between the compensation coil 12 and the control coil 21. It is expressed as Further, since the current flowing through the compensation coil 12 is stable over time, the voltage balance of the second closed circuit C2 is expressed as the following equation (2).

However, t represents time, and the dot on the variable represents time differentiation.

Here, consider a situation where the time derivative of the current I ₁ at a certain time t = t _s swings slightly negative from zero. Then, a term derived from the mutual inductance between the control coil 21 and the compensation coil 12 appears in the induced electromotive force of the control coil 21, and the voltage balance of the second closed circuit C2 changes as in the following equation (3).

Moment the time t _s, the temporal change rate of the current I ₂ of the second closed circuit C2 may vary, current itself does not change discontinuously. Therefore, “I ₂ (t _s ) = I ₂ (t <t _s )”. Here, “t <t _s ” indicates the time when the current I ₁ before “t _s ” was stable. Then, when the third term of the equation (3) is rewritten using the equation (2), the following equation (4) is obtained.

Thus, first decay rate of the current I ₂ of the second closed circuit C2 becomes smaller by the amount of the second term on the right side of equation (4). As a result, the induced electromotive force V ₁ (t _s ) generated in the compensation coil 12 is expressed by the following equation (5).

In this way, the compensation coil 12 is induced electromotive force V ₁ supplied to the first closed circuit C1 is kept stable without change. If the change in the time derivative of the current I ₁ at time t _s as described above are those produced by the external magnetic field fluctuations in the primary coil 11, induced electromotive force changes in the time derivative of the current I ₁ brings the main coil 11, external It is offset by the effect of magnetic field fluctuations. For this reason, even if the voltage across the main coil 11 is monitored, this change cannot be captured. Thereafter, when the external magnetic field fluctuation of the main coil 11 disappears (or when the speed of the external magnetic field fluctuation is restored), the same change as described above occurs, but this change also appears in the voltage across the main coil 11. It does n’t come.

Further, if an external magnetic field fluctuation occurs during a certain period, the time derivative of the current I ₁ continues to swing negatively during this period. As a result, the pseudo permanent current I flowing in the main coil 11 without being detected. ₁ will decrease.

  FIG. 4 is a circuit diagram showing the relationship between the compensation coil, the control coil, the variable resistor, and the inertia coil.

Next, the behavior of the circuit of the second embodiment will be described. The second closed circuit C2 of the second embodiment includes an inertia coil 24 in series as shown in FIG. In FIG. 4, the main coil 11 is omitted. Further, the compensation coil 12 and the control coil 21 are magnetically coupled via the iron core 31, and the inertia coil 24 is not magnetically coupled to any coil. The variables L ₁ , L ₂ , I ₁ , I ₂ , and V ₁ are as described above, and L ₃ indicates the self-inductance of the inertia coil 24.

次に、或る時刻ｔ＝ｔ_ｓに電流Ｉ_１の時間微分がゼロからわずかに負に振れる状況を考えると、上記の式（４）、（５）は、慣性コイル２４の存在により、次式（７）、（８）のように変化する。

When the calculation proceeds in the same manner as described above, first, when the current of the first closed circuit C1 is stable in time, the voltage balance of the second closed circuit C2 of the second embodiment is expressed by the following equation (6). It is expressed as follows.

Then, when the time derivative of the current I ₁ at a certain time t = t _s consider a situation swing slightly negative from zero, the above equation (4), (5), the presence of inertia coil 24, the following It changes like Formula (7) and (8).

That is, the induced electromotive force V ₁ supplied from the compensation coil 12 to the first closed circuit C ₁ is changed by the second term on the right side of Equation (8) due to the presence of the inertia coil 24. This change is not only captured by monitoring the voltage across the main coil 11, but also has an opposite sign to the change in the current in the first closed circuit C1, so the change in the current in the first closed circuit C1 is detected. Work in the direction of relaxation.

This is the role of the inertia coil 24 in the second closed circuit C2. Due to the presence of the inertia coil 24, a spontaneous negative feedback works with respect to the current change of the first closed circuit C1. Negative feedback strength, i.e. the size of the relaxation effects of current change depends on the magnitude of self-inductance L ₃ of the inertial coil 24, a strong relaxation is achieved larger the value. That is, due to the presence of the inertia coil 24, the amount of change in the pseudo permanent current due to a sudden factor can be reduced.

  As described above, according to the magnetic field generation device 1A of the second embodiment, even if some factor that suddenly changes the pseudo permanent current occurs due to the presence of the inertia coil 24, this factor becomes pseudo permanent. The influence on the current can be reduced. As a result, the stability of the pseudo permanent current can be further increased, and a temporally stable magnetic field can be generated.

(Third embodiment)
FIG. 6 is a configuration diagram showing an NMR analyzer according to the embodiment of the present invention.

  The NMR analyzer 100 of this embodiment includes a sample placement unit 101 on which a sample is placed, the magnetic field generation device 1 of the first embodiment or the magnetic field generation device 1A of the second embodiment, the probe 102, the spectroscopic unit 103, and the like. And an analysis device 104. The sample placement unit 101 corresponds to an example of a target space according to the present invention.

  The magnetic field generator 1 causes the sample placement unit 101 to generate a static magnetic field. The probe 102 irradiates the sample placement unit 101 with a radio wave including a frequency component of nuclear magnetic resonance, and detects an NMR signal output from the sample. The spectroscopic unit 103 and the analysis device 104 perform spectrum analysis of the NMR signal detected by the probe 102.

  According to the NMR analyzer 100 of this embodiment, the magnetic field generator 1 (or 1A) can generate a magnetic field having a high intensity (for example, an intensity equivalent to 1.02 GHz) that could not be generated by the superconducting coil of the LTS wire. . Furthermore, even if the first closed circuit C1 includes the connection portion p that does not shift to the superconducting state, a magnetic field with very high stability is generated. As a result, the accuracy of the nuclear magnetic resonance signal can be increased, and the sample can be analyzed more precisely.

(Modification)
The magnetic field generators 1 and 1A of the first and second embodiments still have room for improvement when applied to an NMR analyzer or an MRI apparatus. This is because the magnetic field generators 1 and 1A of the first and second embodiments have a finite duration of the pseudo-permanent current, and the control coil 21 needs to be re-excited after the time has elapsed. The inconvenience at the time of excitation again appears relatively large in a superconducting coil using an HTS wire having a flat cross-sectional shape. When a relatively strong voltage is applied to the superconducting coil of the HTS wire and its current changes, a shield current distribution is formed that cancels the change in the magnetic field caused thereby. This is because the change with time causes a magnetic field fluctuation which adversely affects the NMR analyzer or the MRI apparatus.

  In the pseudo-permanent current of the first and second embodiments, the duration limit is caused by the current decay of the second closed circuit C2. In order to resume the driving of the pseudo permanent current of the main coil 11, the current of the second closed circuit C <b> 2 must be recovered, but at that time, the control coil 21 is re-excited. Excitation of the control coil 21 brings an induced voltage to the compensation coil 12 magnetically coupled thereto, and a voltage is applied to the main coil 11 connected to the compensation coil 12 through a current circuit. That is, the current restoration operation of the second closed circuit C2 brings about voltage application to the main coil 11, and as a result, a shielding current that causes fluctuations in the magnetic field is formed in the main coil 11. Subsequently, a modified example for solving such a problem will be described.

  FIG. 5 is a configuration diagram showing a magnetic field generator according to a modification of the embodiment of the present invention. The modified magnetic field generator 1Z includes two compensation coils 12 and 12Z and two second closed circuits C2 that are magnetically coupled to the two compensation coils 12 and 12Z, respectively (see FIG. 1 and shown in FIG. 5). Abbreviation). Instead of the second closed circuit C2 of the first embodiment, a second closed circuit C2A (see FIG. 2) of the second embodiment may be employed. The two compensation coils 12 and 12Z are each connected in series with the main coil 11 and connected in parallel to each other. The two parallel current paths rt1 and rt2 including the two compensation coils 12 and 12Z are respectively provided with permanent current switches 13 and 13Z. That is, the two parallel current paths rt1 and rt2 can be opened and closed independently by opening and closing the permanent current switches 13 and 13Z. The DC power supply 15 of the main coil 11 is connected in parallel with two parallel current paths rt1 and rt2.

<Description of operation>
In the magnetic field generator 1Z of such a modification, when the voltage drop of the main coil 11 is compensated by the action of one compensation coil 12 and one second closed circuit C2, the current path of the other compensation coil 12Z. rt2 is opened by the permanent current switch 13Z. When the limit of the duration of the second closed circuit C2 corresponding to one compensation coil 12 comes, the current path rt2 of the other compensation coil 12Z is closed by the permanent current switch 13Z, so The voltage drop of the main coil 11 is compensated by the action of the second closed circuit C2.

  When the total current of the first closed circuit C1 is transferred from the path of one of the compensation coils 12 to the path of the other compensation coil 12Z, the current of the control coil 21 magnetically coupled to the compensation coil 12 before the change is increased. At the same time, the current of the control coil 21Z magnetically coupled to the replacement compensation coil 12Z is lowered. If the speeds of these current changes are made to coincide, the induced voltages brought from both the compensation coils 12 and 12Z to the main coil 11 cancel each other, and the current flowing through the main coil 11 without applying a voltage to the main coil 11 Can be switched from the current path rt1 to the current path rt2. If the current of the control coil 21 before the change is arbitrarily increased, the voltage at both ends of the main coil 11 is measured, and the current decay speed of the control coil 21Z at the change destination is controlled so that it becomes zero, the current path During the transition between rt1 and rt2, the voltage drop of the connection resistance on the current path of the pseudo permanent current of the main coil 11 can be canceled.

  As described above, by repeating the operation of switching the current path when the limit of the duration of the second closed circuit C2 is reached, it becomes possible to realize literally permanent current driving in a pseudo manner.

(Fourth embodiment)
FIG. 7 is a configuration diagram showing an MRI apparatus according to an embodiment of the present invention.

  The MRI apparatus 200 of the present embodiment includes a subject placement unit 201 where a subject is placed, the magnetic field generation device 1 of the first embodiment or the magnetic field generation device 1A of the second embodiment, a gradient magnetic field coil 202, and a radio wave transmission / reception unit. 203, a data collection unit 204, and a data processing unit 205. The subject placement unit 201 corresponds to an example of a target space according to the present invention.

  The magnetic field generator 1 causes the subject placement unit 201 to generate a static magnetic field. The gradient magnetic field coil 202 causes the subject placement unit 201 to generate a gradient magnetic field. The radio wave transmission / reception unit 203 transmits and receives radio waves that undergo nuclear magnetic resonance with protons. The data collection unit 204 collects the position where nuclear magnetic resonance occurs due to the gradient magnetic field and the intensity of the received radio wave. The data processing unit 205 generates a three-dimensional image based on the collected information.

  According to the MRI apparatus 200 of the present embodiment, the magnetic field generator 1 (or 1A) can generate a magnetic field having a high intensity (for example, an intensity equivalent to 1.02 GHz) that could not be generated by the LTS wire superconducting coil. Furthermore, even if the first closed circuit C1 includes the connection portion p that does not shift to the superconducting state, a magnetic field with very high stability is generated. As a result, the S / N ratio of the received signal can be improved, and the frequency of nuclear magnetic resonance can be increased, so that more precise tomography of the subject can be performed.

The embodiments of the present invention have been described above. However, the present invention is not limited to the above embodiment. For example, in the above embodiment, the main coil 11 is configured to include the coil portion of the REBCO wire in order to generate a strong magnetic field. However, the main coil 11 may be configured to include, for example, a coil portion of a BSCCO wire or a MgB ₂ superconducting wire. Even in such a case, a portion that cannot be transferred to the superconducting state is included in the connecting portion that connects the wires, but the effect of providing a very temporally stable magnetic field with this influence is obtained.

  In the above-described embodiment, the control unit 3 has been described by taking as an example a configuration in which the variable resistor 22 is controlled by feedback control. However, if the resistance value of each connection portion of the first closed circuit C1 and the second closed circuit C2 is known in advance, the temporal change of the resistance value to be controlled can be obtained by calculation. Therefore, the control unit 3 may be configured to control the variable resistor 22 according to a calculation result obtained in advance without using the footback control. Further, if the duration of the generated magnetic field may be short, the resistance value of the variable resistor 22 may be set to a constant value. Even in this case, the time-stable magnetic field is reduced by reducing the attenuation of the pseudo permanent current. Can be generated. The details shown in each embodiment can be changed as appropriate without departing from the spirit of the invention.

  Next, a more specific example 1 of the magnetic field generator 1 of the first embodiment will be described. In the first embodiment, an example of the value of each parameter of the component is shown, and the duration of generation of the magnetic field of the magnetic field generator 1 is also described.

<Cooling section and main coil>
Conductive cooling can be applied as a cooling method of the cooling unit 40. The cooling unit 40 is assumed to be cooled to around 30K in a state where the temperature adjustment by the heater 32 is not performed.

  The self-inductance of the main coil 11 is set to 10H. However, the present invention can be applied regardless of the self-inductance of the main coil 11. Further, the current value at which the main coil 11 is DC driven is set to 300A. Note that when the drive current of the main coil 11 increases, heat generation due to the resistance of the connection portion in the circuit increases. In that case, the present invention can be applied by taking measures such as increasing the current of the second closed circuit C2 together. Here, as the main coil 11, the coil part of the HTS wire contained in the hybrid type superconducting coil utilized with an NMR analyzer may be assumed. The first closed circuit C1 including the main coil 11, the compensation coil 12, and the permanent current switch 13 is composed of a REBCO wire. In this case, the critical current of the first closed circuit C1 when cooled to around 30K is sufficiently larger than 300A (for example, 400A). Therefore, according to the setting of the current value, there is no possibility that the superconducting quench occurs in the first closed circuit C1.

  The first closed circuit C1 including the main coil 11 includes solder connection portions at a plurality of locations. Electrical resistance remains in the connection portion even during cooling. The electrical resistance of the connection portion p of the REBCO wire is 0.01 μ to 0.02 μΩ per location, and the total resistance value of the connection portions p at a plurality of locations is assumed to be about 0.1 μΩ. Therefore, when a direct current of 300 A flows through the first closed circuit C1, it is assumed that the total voltage drop at all the connections of the first closed circuit C1 is about 30 μV and the total Joule heat is about 9000 μW = 9 mW.

<Compensation coil and control coil>
The compensation coil 12 and the control coil 21 are wound around a ring-shaped iron core 31, as shown in FIG. Each self-inductance between the compensation coil 12 and the control coil 21 is set to 20 mH. The self-inductance value defines the time during which the pseudo permanent current of the first closed circuit C1 can be kept constant. If this duration may be short, this self-inductance may be made smaller. To increase the value of the self-inductance, the number of turns of each coil may be increased. However, the amount of superconducting wire required increases, resulting in disadvantages such as an increase in manufacturing cost. Since the self-inductance includes an increase due to the magnetization of the iron core 31, the volumes of the compensation coil 12 and the control coil 21 are small. The iron core 31 is designed to have a large cross-sectional area so that the magnetic flux density passing through the iron core 31 is large enough to avoid magnetization of the iron core 31. Further, the iron core 31 is arranged so as not to be affected by the magnetic fluxes of coils other than the compensation coil 12 and the control coil 21 (for example, the main coil 11), and is shielded if necessary.

<Variable resistance>
FIG. 8 is a configuration diagram illustrating an example of a variable resistor.

  The variable resistor 22 is a metal with HTS wire soldered to both ends, and for example, a block-like high-purity metal can be adopted. Generally, in a cryogenic environment, a high-purity metal has a temperature-dependent electrical resistivity. Due to this property, the electric resistance of the variable resistor 22 changes by changing the temperature. As described above, a metal having a residual resistance ratio of 30 or more can be used as the material of the variable resistor 22. For example, as a material of the variable resistor 22, C1020 (JIS standard) copper can be used. This material has a residual resistance ratio of, for example, around 50. In this specification, “residual resistance ratio” means the ratio of the specific resistance at 4.2K to the specific resistance at room temperature (for example, 300K).

  The lower limit of the range in which the temperature of the variable resistor 22 can be controlled by the heater 32 in the cooling unit 40 is defined by the cooling temperature of the cooling unit 40. The upper limit is defined as a temperature at which the superconducting wire connected to both ends of the variable resistor 22 can maintain the superconducting state. When a relatively inexpensive one-stage GM refrigerator (Gifford-McMahon cooler) is employed, the cooling temperature of the cooling unit 40 is about 30K. Further, if the REBCO wire is connected to both ends of the variable resistor 22, 70 to 80 K at which the wire can maintain a superconducting state and can maintain a sufficient critical current is a practical upper limit of temperature. Therefore, the controllable variable resistor 22 has a temperature range of 30 to 80K.

  Here, C1020 copper is adopted for the variable resistor 22 and the temperature of the variable resistor 22 is set within a controllable range from 40 K to 77 K (the boiling point temperature of liquid nitrogen at 1 atm, hereinafter referred to as “liquid nitrogen temperature”). Suppose you change the In this case, the electrical resistivity of the variable resistor 22 is 2 μΩ · mm at the liquid nitrogen temperature, and changes to 0.5 μΩ · mm at 40K. Further, the resistance value of the variable resistor 22 increases four times.

  The temperature control of the variable resistor 22 is performed by bringing the first surface S1 and the second surface S2 of the variable resistor 22 facing in opposite directions into contact with the heater 32 and the cooling plate, respectively. Then, the output (heat amount) of the heater 32 is changed. Of the variable resistor 22, the surface to which the HTS wire M1 is soldered is a pair of third surface S3 and fourth surface S4 facing opposite to each other, and is different from the first surface S1 and the second surface S2. In this configuration, a temperature gradient is generated inside the variable resistor 22, but the output of the heater 32 and the resistance value of the variable resistor 22 can be associated with each other. It is preferable that the first surface S1 with which the heater 32 is contacted and the second surface S2 with which the cooling plate is contacted are narrow and have a large area. Thereby, the precision which controls the resistance value of the variable resistance 22, or the response speed required for control can be raised.

<Duration of pseudo permanent current compensation effect>
Next, the duration of the magnetic field generated by the magnetic field generator 1 having the above configuration will be described.

Regarding the dielectric electromotive force generated in the compensation coil 12 and the control coil 21, the above relational expression (1) is established. Induced electromotive force V ₁ is, when matching the voltage drop of the normal conductive portion included in the first closed circuit C1, the attenuation amount of the pseudo persistent current flowing through the first closed circuit C1 becomes zero. When the main coil 11 is driven with a pseudo permanent current with zero attenuation, the voltage drop at the connection portion p of the first closed circuit C1 becomes constant. When this condition is added to the equation (1), the following equation (9) is derived.

Then, the critical time _{t c,} if defined as the time a I 2 (t) = 0, the critical time _{t c} is expressed by the following equation (10).

ここで、Ｒ（ｔ）は可変抵抗２２を含んだ第２閉回路Ｃ２の全抵抗値を示す。 Further, when the pseudo-persistent current in the first closed circuit C1 is constant, relationship between the control current I ₂ and the voltage drop of the second closed circuit C2 is represented by the following formula (11.1). From this, the value R (t) of the resistance component of the second closed circuit C2 is derived as shown in the following equation (11.2).

Here, R (t) indicates the total resistance value of the second closed circuit C 2 including the variable resistor 22.

From the equation (11.2), it can be seen that the resistance value R (t = t _c ) diverges at the critical time t _c . For this reason, the duration during which a constant pseudo-permanent current can be maintained is shorter than the critical time t _c . The ratio of the actual duration to the critical time t _c is determined by how much the resistance value R (t) including the variable resistor 22 can be increased. Further, an initial value of the resistance value is determined from the equation (11.2).

The first embodiment assumes a configuration in which the inductances of the compensation coil 12 and the control coil 21 are equal (L ₁ = L ₂ ), and the initial currents of the first closed circuit C1 and the second closed circuit C2 are equal. In this configuration, the initial value R (0) of the resistance value is equal to the total resistance value of the first closed circuit C1, and is 0.1 μΩ. If it is assumed that the resistance value of the second closed circuit C2 can be increased to twice the initial value R (0) by the variable resistor 22, the duration is half of the critical time. If the calculation is performed by substituting the values of the parameters in the first embodiment, the duration can be obtained as 100,000 seconds≈28 hours.

The measurement time of the NMR analyzer usually does not require a long time such as 28 hours. Therefore, according to the magnetic field generator 1 shown in Example 1, a stable magnetic field required for the NMR analyzer can be generated for a sufficiently long time. On the other hand, depending on the sample, analysis may take several days. Even in this case, to increase the self-inductance of the compensation coil 12 and the control coil 21, by extending the critical time t _c, it can be extended in length to accommodate the duration. In order to increase the inductance, the iron core 31 is enlarged and the number of turns of the compensation coil 12 and the control coil 21 is increased.

<Dimensions of variable resistance>
Here, the dimension of the variable resistor for realizing the change of the resistance value R (t) (0.1 μ to 0.2 μΩ) described in the description of the duration of the magnetic field will be described.

  The initial resistance value of 0.1 μΩ is not the resistance value of the copper block portion 22 a of the variable resistor 22. Although the block portion 22a and the HTS wire M1 are soldered, a non-zero connection resistance is generated in the soldered portion. In the case of a REBCO wire having a width of 4 mm, the typical connection resistance is about 0.2 μΩ for both the third surface S3 and the fourth surface S4 with a connection length of 1 cm. The connection resistance is inversely proportional to the connection length. When the connection length between the variable resistor 22 and the HTS wire M1 is 10 cm (= 100 mm), the connection resistance is 0.02 μΩ. Furthermore, the resistance of other connection parts p such as the permanent current switch 23 of the second closed circuit C2 is about 0.03 μΩ. From these, the initial value of the resistance value of the block portion 22a of the variable resistor 22 may be the remaining 0.05 μΩ.

  The length d and width w of the connecting portion of the HTS wire M1 are defined as 100 mm and the wire width 4 mm. Then, the length d of one side of the block portion 22a of the variable resistor 22 is determined to be 100 mm, and the width w of the other side is determined to be 4 mm. Assuming that the initial temperature of the variable resistor 22 is 40K, as described above, the electrical resistivity of copper at the initial temperature of 40K is 0.5 μΩ · mm. Therefore, when the initial value of the resistance of the variable resistor 22 is 0.05 μΩ, it can be seen that the length h of the remaining one side of the block portion 22a of the variable resistor 22 may be 40 mm.

  The connection resistance of the HTS wire M1, which is a REBCO wire, has almost no temperature dependence. Therefore, in order to change the overall resistance of the second closed circuit C2 from 0.1 μ to 0.2 μΩ, the resistance value of the block portion 22a of the variable resistor 22 is changed from the initial value 0.05 μΩ to 3 by temperature change. It is necessary to increase it to 0.15 μΩ. This increase corresponds to a change from 40K to 70K in copper having a residual resistance ratio of 50 when converted to a temperature change.

  Furthermore, the magnitude of the control current of the second closed circuit C2 is assumed to be an initial value of 300A and 150A after elapse of the duration. That is, a current of 300A at 40K and 150A at 70K flows through the HTS wire M1 close to the variable resistor 22. In the case of a REBCO wire having a width of 4 mm, these current values are below the critical current at any temperature, and there is no fear that energization becomes difficult.

<Control of heater by control unit>
The control unit 3 monitors the voltage of the main coil 11 and controls the output of the heater 32. At this time, the control unit 3 may not monitor the temperature of the variable resistor 22. Here, when the connection part p is included in the middle of the wire which comprises the main coil 11, it is good to set it as the structure which the control part 3 monitors the voltage of the part of the main coil 11 which does not include the connection part p. If the pseudo permanent current is strictly constant, no voltage is generated in the main coil 11. Therefore, the control unit 3 controls the output of the heater 32 so that no voltage is generated in the main coil 11.

In the field of NMR analyzers, 10 ⁻¹¹ Ω is one criterion for stability, which is a value obtained by converting the stability required for a permanent current that generates a magnetic field into a resistance value included in a circuit. When the resistance of this value is included in the first closed circuit C1, if a current of 300 A is passed through the first closed circuit C1, a voltage drop of 3 nV occurs, and the same induced electromotive force is generated in the main coil 11. become. Therefore, the control unit 3 controls the heater 32 by feedback control so that a voltage of 3 nV or more is not generated in the main coil 11. When the output of the heater 32 is increased, the temperature of the variable resistor 22 is increased, the resistance value is increased, and the current decrease rate of the control coil 21 is increased. As a result, the induced electromotive force generated in the magnetically coupled compensation coil 12 increases. When the output of the heater 32 is decreased, the temperature of the variable resistor 22 is decreased, the resistance value is decreased, and the current decrease rate of the control coil 21 is decreased. This reduces the induced electromotive force generated in the magnetically coupled compensation coil 12. By the feedback control of the control unit 3 based on the above 3 nV or less, the stability of the pseudo permanent current required for the NMR analyzer can be achieved. And the stability of the magnetic field required for the NMR analyzer is achieved.

<Measurement unit>
The measuring unit 2 measures a minute voltage on the order of nV. A measuring instrument for measuring a voltage on the order of nV has been put into practical use, but the measuring unit 2 of Example 1 has the following structure in a wiring serving as a probe. There are two types of noise generated in voltage measurement wiring: thermoelectromotive force and induced voltage. The thermoelectromotive force is a voltage generated by the temperature difference between the ends of the two wirings of both poles. In the case where the cooling unit 40 is configured to cool the main coil 11 by immersion cooling of the refrigerant, a temperature difference is unlikely to occur, but in the case of conduction cooling, a temperature difference can occur. Therefore, in the case of conduction cooling, it is preferable to employ a configuration in which cooling is performed so that there is no temperature difference between the two portions of the main coil 11 to which the wiring of the measurement unit 2 is connected. In the case where a temperature difference occurs, a configuration may be adopted in which the temperature difference is measured using a thermocouple or the like, and the thermoelectromotive force due to the temperature difference is subtracted from the measurement result of the measurement unit 2.

  The induced voltage is caused by a change in magnetic flux passing through a surface surrounded by a closed curve formed by the wiring of the measurement unit 2. As a measure for preventing the induced voltage, it is possible to adopt a method of twisting two wires serving as probes of the measuring unit 2 to form a double helix structure and frequently inverting the sign of the magnetic flux passing between them to cancel. Or the method etc. which use the shield wire surrounded by the conductor for the wiring of the probe of the measurement part 2 are employable.

  The change of the pseudo permanent current flowing through the main coil 11 is proportional to the time integration of the applied voltage. Therefore, even if a relatively large voltage is applied to the main coil 11, if it is a spike voltage that can be eliminated in a short time, it hardly affects the change of the pseudo permanent current. On the contrary, if the control unit 3 reacts excessively, there is a possibility that an adverse effect may be brought about in stabilizing the pseudo permanent current. Therefore, the measurement unit 2 may include a low-pass filter that removes a short-time voltage change from the measurement result.

  As described above, according to the magnetic field generator 1 of Example 1, a strong magnetic field that cannot be generated by the superconducting coil of the LTS wire can be generated, and further, the magnetic field can be generated over a long time with high stability required for the NMR analyzer. Can continue to occur.

  Next, a more specific example 2 of the magnetic field generator 1A of the first embodiment will be described. The magnetic field generator 1A according to the second embodiment is different from the first embodiment in that an inertia coil 24 is added. The inertia coil 24 spontaneously relieves sudden current changes that occur in the first closed circuit C1, but here, the influence of the inertia coil 24 other than the relaxation action will be described.

この式（１２）を前述の式（１１．２）と比較すると係数（１＋Ｌ_３／Ｌ_２）の分、抵抗値が大きくなることが分かる。上記の設定では、慣性コイル２４の自己インダクタンスＬ_３は制御コイル２１の自己インダクタンスＬ_２の半分なので、抵抗値Ｒ（ｔ）は実施例１の１．５倍になる。係数（１＋Ｌ_３／Ｌ_２）は、時間的に変化しないので、抵抗値Ｒ（ｔ）の初期値、初期値における温度に依存しない成分の大きさ、温度に依存する成分の大きさを、共に実施例１の１．５倍にすれば、式（１２）が満たされる。この場合、可変抵抗２２のブロック部２２ａの抵抗値は、初期値を０．０７５μΩ、接続抵抗に由来する温度に依存しない成分を０．０７５μΩ、合計を０．１５μΩとすればよい。 The optimum value of the self-inductance of the inertia coil 24 should be determined by the characteristics of the magnetic field generator 1A and the system using the magnetic field generator 1A, but is set to 10 mH here, for example. Even if the inertia coil 24 is added, the above-mentioned formulas (9), (10), and (11.1) hold. However, by adding the inertia coil 24, the equation of the resistance value R (t) of the second closed circuit C2 is expressed as the following equation (12), unlike the equation (11.2).

When this equation (12) is compared with the above equation (11.2), it can be seen that the resistance value increases by the coefficient (1 + L ₃ / L ₂ ). In the above configuration, the self-inductance L ₃ of the inertial coil 24 so half of the self-inductance L ₂ of the control coil 21, the resistance value R (t) becomes 1.5 times that of Example 1. Since the coefficient (1 + L ₃ / L ₂ ) does not change with time, the initial value of the resistance value R (t), the size of the component that does not depend on the temperature at the initial value, and the size of the component that depends on the temperature are both If it is 1.5 times that of the first embodiment, the expression (12) is satisfied. In this case, as for the resistance value of the block portion 22a of the variable resistor 22, the initial value may be 0.075 μΩ, the temperature-independent component derived from the connection resistance may be 0.075 μΩ, and the total may be 0.15 μΩ.

  The resistance value of the block portion 22a of the variable resistor 22 can be obtained, for example, by setting the width of the block portion 22a to 60 mm, which is 1.5 times that of the first embodiment. Further, it is expected that the increase in the connection resistance independent of the temperature will be compensated by the increase in the connection portion p of the HTS wire due to the addition of the inertia coil 24. That is, when the number of devices constituting the second closed circuit C2 is increased 1.5 times from two to three, the number of connection portions p is also increased 1.5 times, and the total connection resistance is also 1 Expected to be about 5 times. If the increase amount of the resistance value due to the increase in the connection portion p is smaller than 1.5 times, the resistance of the block portion 22a of the variable resistor 22 may be increased to compensate. In this case, since the range of the resistance value that can be changed in the same temperature range as in the first embodiment becomes larger, it is more preferable. Conversely, if the amount of increase in the resistance value due to the increase in the connection portion p is larger than 1.5 times, the resistance of the block portion 22a of the variable resistor 22 may be reduced to compensate, or the self-inductance of the inertia coil 24 may be compensated. You may cope by the method of enlarging. In the latter case, the increase in the resistance component is larger than 1.5 times, so that the resistance value can be supplemented by the above-described method. Further, when the self-inductance of the inertia coil 24 is increased, the effect of spontaneously mitigating the sudden current change occurring in the first closed circuit C1 becomes stronger, so that the stability of the pseudo permanent current can be further improved. Benefits arise.

  As described above, according to the magnetic field generator 1A of Example 2, it is possible to generate a magnetic field having very high stability as required for an NMR analyzer. Furthermore, even if a sudden factor causing a change in current occurs, the effect of reducing this influence can be obtained.

DESCRIPTION OF SYMBOLS 1 Magnetic field generator 2 Measuring part 3 Control part C1 1st closed circuit C2, C2A 2nd closed circuit 11 Main coil 12 Compensation coil 13, 23 Permanent current switch 15, 25 DC power supply 16, 26 Protection circuit element 21 Control coil 22 Variable Resistance (resistance)
24 Inertial coil 32 Heater 33 Heater power supply 100 NMR analyzer 200 MRI apparatus

Claims (9)

A first closed circuit including a main coil that is a superconducting coil and generates a magnetic field in a target space; and a compensation coil that is a superconducting coil and is connected in series with the main coil;
A control coil, which is a superconducting coil and is magnetically coupled to the compensation coil, and a resistor connected in series with the control coil, a control current that decreases with the passage of time due to power consumption of the resistor flows. A second closed circuit;
With
The induced electromotive force generated in the compensation coil by reducing the control current through the magnetic coupling between the control coil and the compensation coil has a polarity that induces a current in the same direction as the excitation current. Magnetic field generator. A measuring unit for measuring the voltage of the whole or a part of the main coil;
A control unit that changes a resistance value of the resistor based on a measurement result of the measurement unit;
The magnetic field generator according to claim 1, further comprising: A heater for heating the resistor;
The resistance is a metal having a residual resistance ratio of 30 or more,
The magnetic field generator according to claim 2, wherein the control unit changes the resistance value of the resistor by heating the resistor with the heater. 4. The magnetic field generator according to claim 1, wherein the main coil includes a coil portion of a rare earth-based superconducting wire, a bismuth-based superconducting wire, or an MgB ₂ superconducting wire. 5.   5. The magnetic field generation apparatus according to claim 1, further comprising a ferromagnetic core that magnetically couples the compensation coil and the control coil. 6. The second closed circuit is:
The magnetic field generator according to any one of claims 1 to 5, further comprising an inertia coil that is a superconducting coil and is connected in series with the control coil and the resistor. A plurality of the compensation coils;
A plurality of second closed circuits magnetically coupled to the plurality of compensation coils, respectively.
The magnetic field generator according to claim 1, wherein each of the plurality of compensation coils is connected in series with the main coil and connected in parallel to each other.   An NMR analyzer comprising the magnetic field generator according to any one of claims 1 to 7.   An MRI apparatus comprising the magnetic field generator according to any one of claims 1 to 7.

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