JP6590573B2 - Operation method of superconducting magnet device - Google Patents

Description

  The present invention relates to a method of operating a superconducting magnet device including a superconducting coil having a wound oxide superconducting wire.

  In recent years, the development of superconducting wires in which a superconductor is embedded in a metal sheath or a superconducting film formed on a substrate has been underway. Among these, an oxide superconducting wire provided with a superconducting layer made of an oxide superconductor, which is a high-temperature superconductor having a transition temperature equal to or higher than the liquid nitrogen temperature, has attracted attention.

  Even in superconducting magnet devices that require a high-intensity magnetic field, such as magnetic resonance diagnostic equipment (MRI) and nuclear magnetic resonance analyzer (NMR), an oxide superconducting wire is wound. A technique for generating a magnetic field using the formed superconducting coil has been proposed (see, for example, Non-Patent Documents 1 and 2).

  In the above superconducting magnet apparatus, in order to realize high diagnostic accuracy, it is required to generate a magnetic field that is stable in time and has high spatial uniformity. However, a superconducting coil made of an oxide superconducting wire has a magnetic field distribution due to the influence of an additional magnetic field (hereinafter also referred to as “shielding current magnetic field”) generated by the shielding current induced in the flat superconducting layer. There are technical problems such as the occurrence of distortion or the temporal fluctuation of the magnetic field.

Yoshinori Yanagisawa, Hideaki Maeda, "Mechanism and suppression method of shield current magnetic field in REBCO coil", Special issue: Coiled fundamental technology of RE-based high-temperature superconducting wire, Low temperature engineering, Vol. 48, No. 4, 2013 Kazuhiro Kajikawa and Kazuo Funaki, "A simple method to eliminate shielding currents for magnetization perpendicular to superconducting tapes wound into coils", 2011 Supercond. Sci. Technol. 24 125005

  In Non-Patent Document 1, as methods for reducing the influence of the shield current magnetic field, a current sweep inversion method that is a method for suppressing the temporal drift of the central magnetic field, and a demagnetization method for eliminating the shield current magnetic field ( Demagnetization method).

  Non-Patent Document 2 describes a method of applying an alternating magnetic field perpendicular to the magnetization direction due to the shielding current generated in the superconducting wire.

  However, in the case of a superconducting coil formed by winding a tape-shaped oxide superconducting wire, the magnitude of the shielding current induced in the superconducting layer becomes large, so that the influence of the shielding current magnetic field becomes large. Therefore, even if the method described above is applied, there remains a problem that it takes time until the shielding current is attenuated and the central magnetic field of the coil is stabilized.

  The present invention has been made to solve such a problem, and an object of the present invention is to provide a shielding current in a short time in a superconducting magnet device having a superconducting coil having a wound oxide superconducting wire. To provide an operation method capable of reducing the influence of a magnetic field.

  An operation method of a superconducting magnet apparatus according to an aspect of the present invention, wherein the superconducting magnet apparatus includes a superconducting coil having a wound oxide superconducting wire, a power source for supplying current to the superconducting coil, and energization of the superconducting coil. And a control device for controlling the current. The operation method includes the steps of exciting the superconducting coil by energizing the superconducting coil cooled to the superconducting state, and maintaining the operating current of the superconducting magnet device in the superconducting coil after the step of exciting the superconducting coil. Prepare. The step of exciting the superconducting coil includes a step of heating the superconducting coil so that the energizing current is increased to the operating current value and the first target temperature is lower than the allowable temperature of the superconducting coil. The step of maintaining the operating current includes a step of cooling the superconducting coil so that the second target temperature is lower than the first target temperature.

  According to the above, in the superconducting magnet device provided with the superconducting coil having the wound oxide superconducting wire, it is possible to provide an operation method capable of reducing the influence of the shielding current magnetic field in a short time.

It is sectional drawing which shows schematically the structure of the superconducting magnet apparatus which concerns on embodiment. It is a figure which shows roughly the structural example of the superconducting coil body shown in FIG. It is a figure which shows typically the shielding current induced | guided | derived to an oxide superconducting wire, and a shielding current magnetic field. It is a flowchart explaining the driving | operation method of the superconducting magnet apparatus which concerns on this Embodiment. It is a figure which shows the time change of the energization current of a superconducting coil. It is a figure which shows the operating method of the superconducting magnet apparatus by a comparative example. It is a figure which shows the operating method of the superconducting magnet apparatus by a 1st Example. It is a figure which shows the operating method of the superconducting magnet apparatus by a 2nd Example. It is a figure which shows the operating method of the superconducting magnet apparatus by a 3rd Example.

[Description of Embodiment of the Present Invention]
First, embodiments of the present invention will be listed and described.

  (1) A superconducting magnet device according to one aspect of the present invention (see FIGS. 1 and 2) includes a superconducting coil (10 to 18) having a wound oxide superconducting wire, and a power source for supplying current to the superconducting coil. (132) and a control device (140) for controlling the energization current of the superconducting coil. The operation method of the superconducting magnet device according to one aspect of the present invention includes a step of exciting the superconducting coil by energizing the superconducting coil cooled to the superconducting state (step (S10) in FIG. 3), and exciting the superconducting coil. After the step of performing, the step of holding the operating current of the superconducting magnet device in the superconducting coil (step (S20) in FIG. 3) is provided. The step of exciting the superconducting coil increases the energizing current to the operating current value and heats the superconducting coil to a first target temperature lower than the allowable temperature of the superconducting coil (step (S12 in FIG. 3). ))including. The step of maintaining the operating current includes a step of cooling the superconducting coil so that the second target temperature is lower than the first target temperature (step (S22) in FIG. 3).

  In this way, by heating the superconducting coil during excitation of the superconducting coil, the oxide superconducting wire can be heated to reduce the shielding current induced in the superconducting layer. Then, in the process of flowing the operating current (constant current) to the superconducting coil after exciting the superconducting coil, the superconducting coil is cooled, thereby reducing the electrical resistance value of the oxide superconducting wire and reducing the temporal change of the shielding current. be able to. Thereby, since the shielding current is stabilized at a low current value in a short time after exciting the superconducting coil, the shielding current magnetic field is also stabilized in a state where the intensity is reduced. That is, since the decrease in the strength of the central magnetic field of the coil is suppressed and the temporal drift of the central magnetic field is suppressed, the influence of the shield current magnetic field can be reduced in a short time.

  Further, since the superconducting magnet device is configured such that the magnitude of the generated magnetic field can be adjusted by the energization current supplied to the superconducting coil, the magnitude of the shielding current magnetic field varies depending on the magnitude of the generated magnetic field. According to the operation method according to one aspect of the present invention, the influence of the shield current magnetic field is reduced by controlling the temperature of the superconducting coil. It is possible to generate a magnetic field having uniformity.

  (2) Preferably, in the step of heating the superconducting coil (step (S12)), heating is performed until the end portion of the superconducting coil in the axial direction reaches the first target temperature.

  In this case, the shielding current is effectively reduced by heating the end portion where a relatively large shielding current is induced compared to the axial central portion of the superconducting coil until the first target temperature is reached. can do.

  (3) Preferably, the step of maintaining the operating current (step (S20)) includes a step of vibrating the energizing current around the operating current value before the step of cooling the superconducting coil (step (S22)). Including.

  In this way, since the AC loss (hysteresis loss) due to the change of the magnetic field increases by vibrating the energizing current, the superconducting coil can be heated without using a heating device such as a heater.

  (4) Preferably, in the step of oscillating the energization current, the oscillation of the energization current after the superconducting coil reaches a third target temperature lower than the allowable temperature of the superconducting coil and higher than the first target temperature. Attenuate the amplitude.

  In this way, the intensity of the shield current magnetic field can be further reduced by using the hysteresis effect of the shield current magnetic field in addition to the heating of the superconducting coil.

  (5) Preferably, in the step of heating the superconducting coil (step (S12)), the superconducting coil is heated by an AC loss caused by increasing the energizing current.

  In this way, the rate of change of the conduction current in the excitation process of the superconducting coil can be increased, so that the excitation time can be shortened. As a result, the influence of the shield current magnetic field can be reduced in a short time after the excitation of the superconducting coil is started. Further, the superconducting coil can be heated without using a heating device such as a heater.

  (6) Preferably, the change rate of the energization current is set on condition that the heat generation amount per unit time due to the AC loss generated in the superconducting coil does not exceed the allowable heat generation amount of the superconducting coil.

  By doing so, it is possible to prevent the heat generated by the superconducting coil from affecting the magnetic characteristics of the superconducting coil.

  (7) Preferably, the step of heating the superconducting coil (step (S12)) includes a heating device (heaters 10a, 10b, 12a, 12b, 14a, 14b, 16a, 16b, 18a, 18b, 20a, 20b). And using to heat the superconducting coil.

  In this way, the shielding current induced in the oxide superconducting wire during excitation of the superconducting coil can be reduced. The superconducting coil may be heated by attaching a heating device to the cooling head of the cooling device.

(8) Preferably, the said heating apparatus is provided in the edge part of the axial direction of a superconducting coil.
In this way, the shielding current can be effectively reduced by intensively heating the end portion where a relatively large shielding current is induced compared to the axial central portion of the superconducting coil.

  (9) Preferably, in the step of heating the superconducting coil (step (S12)), the superconducting coil is further heated by an AC loss caused by increasing the energizing current.

  In this way, the rate of change of the conduction current in the excitation process of the superconducting coil can be increased, so that the excitation time can be shortened. As a result, the influence of the shield current magnetic field can be reduced in a short time after the excitation of the superconducting coil is started.

[Details of the embodiment of the present invention]
Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following drawings, the same or corresponding parts are denoted by the same reference numerals, and description thereof will not be repeated.

(Configuration of superconducting magnet device)
FIG. 1 is a cross-sectional view schematically showing a configuration of a superconducting magnet device according to an embodiment. The superconducting magnet apparatus according to the embodiment can be applied to, for example, an MRI apparatus. Referring to FIG. 1, a superconducting magnet device 100 according to an embodiment includes a superconducting coil body 91, a heat insulating container 111, a cooling device 121, a hose 122, a compressor 123, a cable 131, a power source 132, And a control device 140.

  The heat insulating container 111 has a hollow cylindrical shape, and accommodates the superconducting coil body 91 therein. The inside of the heat insulating container 111 is maintained in a vacuum state. The vacuum state means a reduced pressure state capable of maintaining heat insulation from atmospheric pressure.

  Superconducting coil body 91 includes a superconducting coil formed by winding an oxide superconducting wire having a tape-like shape. The oxide superconducting wire has, for example, a bismuth (Bi) -based superconductor extending in the extending direction, and a sheath covering the superconductor. The sheath is made of, for example, silver or a silver alloy. The oxide superconducting wire has a characteristic that the AC loss increases as a magnetic field perpendicular to the tape-like surface is applied. The detailed configuration of the superconducting coil will be described later.

  Superconducting coil body 91 further includes a heat transfer plate that connects the superconducting coil and cooling device 121 so that heat can be transferred. The superconducting coil is cooled by the cooling device 121. The cooling device 121 has a cooling head 120 thermally connected to the superconducting coil. The cooling device 121 is, for example, a Gifford-McMahon refrigerator, a pulse tube refrigerator, or a Stirling refrigerator. The cooling device 121 is connected to the compressor 123 via the hose 122. The cooling device 121 causes the cooling head 120 to generate a cryogenic temperature that is not higher than the critical temperature of the oxide superconducting material constituting the superconducting coil. The cryogenic temperature obtained by the cooling head 120 is transferred to the superconducting coil through the heat transfer plate. In addition, as a cooling part which cools a superconducting coil, it is good also as a structure which immerses a superconducting part in refrigerant | coolants, such as liquid helium or liquid nitrogen accommodated in the heat insulation container 111, without using the cooling device 121. FIG.

  The cable 131 is disposed between the superconducting coil body 91 and the power source 132. When a current is applied to the superconducting coil from the power supply 132 via the cable 131, the superconducting coil generates a magnetic field (magnetic flux). An imaging region FOV (Field of View) of the MRI apparatus is formed in a space at the center of the cylinder of the heat insulating container 111 within a range indicated by a dotted line in the drawing. Since the imaging region FOV is located outside the heat insulating container 111 and can be maintained at room temperature and atmospheric pressure, the subject can store his / her inspection region in the imaging region FOV.

  The control device 140 controls the energization current of the superconducting coil. As an example, the control device 140 is mainly configured by a microcomputer including a CPU (Central Processing Unit) and a storage unit such as a ROM (Read Only Memory) and a RAM (Random Access Memory).

  Control device 140 controls the temperature of the superconducting coil (hereinafter also referred to as coil temperature) in addition to the control of the energization current of the superconducting coil. Specifically, the control device 140 is configured to control the operation of the cooling device 121 and to control the start and stop of the heater (see FIG. 2).

  FIG. 2 is a diagram schematically showing a configuration example of the superconducting coil body 91 shown in FIG. Referring to FIG. 2, superconducting coil body 91 includes a plurality of superconducting coils, gradient magnetic field coil 21, shims 22, 24, and a plurality of heaters.

  In the configuration example of FIG. 2, the plurality of superconducting coils are composed of six superconducting coils 10, 12, 14, 16, 18, and 20. The six superconducting coils 10, 12, 14, 16, 18, and 20 are arranged at a distance from each other with a common coil central axis. The coil center axis is set so as to coincide with the Z axis perpendicular to the equator plane 8.

  The superconducting coils 14 and 16 are opposed to each other with the equator plane 8 as a symmetry plane. The superconducting coils 12 and 18 are arranged opposite to each other with the equator plane 8 as a symmetry plane. The superconducting coils 10 and 20 are opposed to each other with the equator plane 8 as a symmetry plane. By flowing a constant current through each of the six superconducting coils 10, 12, 14, 16, 18, and 20, a magnetic field in the Z-axis direction can be generated in the imaging region FOV.

  Inside the superconducting coils 10, 12, 14, 16, 18, and 20, a gradient magnetic field coil 21 is disposed with the coil central axis coinciding with the Z axis. The gradient magnetic field coil 21 generates a gradient magnetic field in which the magnetic field is spatially changed so as to be superimposed on the uniform magnetic field of the imaging region FOV for the purpose of obtaining positional information in the imaging region FOV.

  The shims 22 and 24 are provided between the gradient magnetic field coil 21 and the superconducting coils 10, 12, 14, 17, 18, and 20. The shim 22 is disposed along the outer peripheral side wall of the gradient coil 21. The shim 24 is disposed along the inner peripheral side wall of the heat insulating container 111. The shims 22 and 24 are magnetic material adjusting iron materials for further improving the uniformity of the uniform magnetic field in the imaging region FOV. The uniformity of the imaging area FOV can be adjusted by adjusting the position where the shim is attached and the thickness of the shim based on the measurement result of the magnetic field generated in the imaging area FOV.

  When the superconducting magnet device 100 is operated, a static magnetic field Bc in the direction of the white arrow is generated in the imaging region FOV. When the superconducting magnet apparatus 100 is an MRI apparatus, the static magnetic field Bc is required to have a high intensity of about 3T, a high uniformity of about 5 ppm, and a temporal stability. .

  However, the magnetic field distribution is distorted due to the influence of an additional magnetic field (shielding current magnetic field) generated inside the coil by the shielding current induced in the flat superconducting layer of the oxide superconducting wire, and the magnetic field varies with time. There are technical issues such as.

  Specifically, the static magnetic field Bc generated in the superconducting coil is divided into an axial component and a radial component of the coil. As shown in FIG. 3, when the radial component of the magnetic field (corresponding to the vector Bc1 in the figure) is applied in the vertical direction to the superconducting layer of the oxide superconducting wire 30, the radial component Bc1 is repelled. A shielding current Is is induced in the superconducting layer.

  The shield current Is generates a shield current magnetic field (vector Bs1 in the figure) inside the coil. As shown in FIG. 2, the shield current magnetic field Bs is generated in the opposite direction to the central magnetic field Bc of the coil. Therefore, the value of the central magnetic field Bc of the coil becomes smaller by the amount of the shielding current magnetic field Bs than the design value due to the shielding current magnetic field Bs generated in the excitation process of the superconducting coil. That is, since the shielding current magnetic field Bs decreases the strength of the central magnetic field Bc of the coil, the spatial distribution of the magnetic field Bc near the coil center becomes non-uniform.

  Further, the shielding current Is induced in the superconducting layer continues to flow as a permanent current, but gradually relaxes over a long time due to a magnetic flux creep phenomenon or the like. Thereby, the shielding current magnetic field Bs decreases with time in the coil center. As a result, the central magnetic field Bc drifts in the positive direction with time.

  As described above, the shielding current magnetic field Bs is a physical phenomenon that occurs due to the tape shape of the oxide superconducting wire, and various reduction methods have been proposed, but there is no effective method at present. Therefore, in the operation method of the superconducting magnet device according to the present embodiment, the influence of the shield current magnetic field Bs is reduced by controlling the excitation process of the superconducting coil and the temperature of the superconducting coil after the excitation.

(Operation method of superconducting magnet device)
Hereinafter, a method of operating superconducting magnet device 100 according to the present embodiment will be described.

  FIG. 4 is a flowchart for explaining a method of operating the superconducting magnet device according to the present embodiment. Note that the flowchart shown in FIG. 4 can be realized by executing a program stored in advance in the control device 140.

  Referring to FIG. 4, the operating method of the superconducting magnet device includes a step of exciting the superconducting coil (S10) and a step of maintaining the operating current of the superconducting coil (S20).

  First, a step of exciting the superconducting coil (S10) is performed. Specifically, the superconducting coil is excited by energizing the superconducting coil cooled to the superconducting state. FIG. 5 shows the time change of the conduction current I of the superconducting coil. As shown in FIG. 5, the energization current I supplied from the power source 132 (FIG. 1) to the superconducting coil is increased from 0 to the operating current value Iop of the superconducting magnet device 100. In this specification, the time from when the energizing current I is 0 (time t = 0) to when the energizing current I reaches the operating current value Iop (time t1) is also referred to as excitation time.

  During excitation of the superconducting coil, a shielding current Is is induced in the superconducting layer of the oxide superconducting wire. The shield current Is generates a shield current magnetic field Bs in the direction opposite to the central magnetic field Bc of the coil.

  When the energization current I reaches the operating current value Iop (time t1), subsequently, as shown in FIG. 5, a step (S20) of maintaining the operating current Iop of the superconducting coil is performed.

  After time t1, the shielding current Is attenuates with time due to a magnetic flux creep phenomenon or the like. Assuming that the inductance of the superconducting coil is L and the electric resistance value is R, the shielding current Is attenuates based on a time constant represented by L / R.

  The step of exciting the superconducting coil (S10) includes the step of heating the superconducting coil (S12). In the step of heating the superconducting coil (S12), the superconducting coil is heated to a first target temperature T1 * that is lower than the allowable temperature of the superconducting coil. This reduces the magnitude of the shielding current Is induced in the superconducting layer.

  Specifically, the magnitude of the shielding current Is flowing through the superconducting coil depends on the critical current Ic. The critical current Ic has a temperature dependency such that the higher the temperature of the oxide superconducting wire (the closer to the critical temperature), the smaller the critical current Ic. Therefore, the shielding current Is also decreases as the temperature of the oxide superconducting wire increases. Therefore, in the step of heating the superconducting coil (S12), the magnitude of the shielding current Is is reduced by heating the superconducting coil when exciting the superconducting coil using the temperature dependence of the shielding current Is. To do. By reducing the magnitude of the shielding current Is, the magnitude of the shielding current magnetic field Bs1 (see FIG. 3) generated inside the coil can be reduced.

  In the step of heating the superconducting coil (S12), it is preferable to heat the superconducting coil until the axial end of the superconducting coil reaches the first target temperature T1 *. In the superconducting coil, the magnitude of the shielding current Is increases from the central part to the end part in the axial direction of the superconducting coil. In the present embodiment, in each of the six superconducting coils 10, 12, 14, 16, 18, 20 (see FIG. 2), the shield current Is is larger at the end portion than at the central portion in the axial direction. Become. This is because the end of the superconducting coil in the axial direction has a higher strength of the vertical magnetic field than the central portion in the axial direction, and a larger shielding current Is is induced to repel the vertical magnetic field. Therefore, the shielding current Is can be effectively reduced by heating the end of the superconducting coil in the axial direction until the first target temperature T1 * is reached.

  The step (S20) of maintaining the operating current Iop of the superconducting coil includes a step (S22) of cooling the superconducting coil. In the step of cooling the superconducting coil (S22), the superconducting coil is cooled to a second target temperature T2 * that is lower than the first target temperature T1 *.

  The oxide superconducting wire has a state in which the electrical resistance is almost as low as 0 below the critical temperature, but even in this state, the temperature dependence is such that the electrical resistance value decreases as the temperature decreases (away from the critical temperature). Have. In the step of cooling the superconducting coil (S22), the electric resistance value of the oxide superconducting wire is lowered by cooling the oxide superconducting wire in which the shielding current Is is reduced by heating. When the electrical resistance value of the oxide superconducting wire is reduced, the time constant (L / R) determined by the ratio of the inductance L and the electrical resistance value R is increased in the superconducting coil. Thereby, since the time change from the state where the magnitude | size was reduced is reduced, the shielding current Is is stabilized in time.

  As described above, in the method of operating the superconducting magnet device according to the present embodiment, the superconducting coil is heated during the excitation of the superconducting coil, so that the oxide superconducting wire is heated and the shielding current Is induced in the superconducting layer. Reduce. Then, in the process of flowing the operating current (constant current) to the superconducting coil after exciting the superconducting coil, the superconducting coil is cooled, thereby reducing the electrical resistance value of the oxide superconducting wire and reducing the temporal change of the shielding current Is. Let Thereby, since the shielding current Is is stabilized at a low current value in a short time after exciting the superconducting coil, the shielding current magnetic field Bs is also stabilized in a state where the intensity is reduced. That is, since the decrease in the strength of the central magnetic field Bc of the coil is suppressed and the temporal drift of the central magnetic field Bc is suppressed, the influence of the shield current magnetic field can be reduced in a short time.

  Hereinafter, the operation method of the superconducting magnet device according to the present embodiment will be described in detail by way of examples and comparative examples. However, the present embodiment is not limited to these.

<Comparative example>
FIG. 6 is a diagram illustrating a method of operating the superconducting magnet device according to the comparative example. FIG. 6 shows the time change of the fluctuation of the magnetic field in the imaging region FOV, where t = 0 is the time when the excitation of the superconducting coil is started, and the energization current of the superconducting coil, the temperature of the superconducting coil (coil temperature). Further, FIG. 6 shows a time change of heater operation (on) / stop (off).

  In the following description, it is assumed that the operating current value Iop of the superconducting magnet device is 200 A in both the comparative example and the example. In both the comparative example and the example, the coil temperature indicates the temperature at the end of the superconducting coil in the axial direction (Z-axis direction in FIG. 1). The oxide superconducting wire has a characteristic that the AC loss increases as a magnetic field perpendicular to the tape-like surface is applied. Therefore, in the superconducting coil, the coil positioned at the end in the axial direction has a higher vertical magnetic field strength than the coil positioned at the center in the axial direction, so that the AC loss is large and the heat generation tends to increase. Therefore, by controlling the temperature at the end in the axial direction as the coil temperature, the end of the superconducting coil is prevented from becoming normal and quenching is prevented.

  The operation method according to the comparative example includes a step of exciting the superconducting coil and a step of holding the operating current Iop of the superconducting coil after the step of exciting the superconducting coil. However, in the comparative example, the step of exciting the superconducting coil does not include the step of heating the superconducting coil. Therefore, the heater is fixed off.

  As shown in FIG. 6, in the comparative example, the excitation time of the superconducting coil is 4 hours. That is, the control device 140 increases the energization current of the superconducting coil from 0 A to 200 A that is the operating current value (Iop) over 4 hours.

  By increasing the energizing current with time, the magnetic field generated by the superconducting coil changes. As the magnetic field changes with time, heat loss due to AC loss, particularly hysteresis loss, increases, so that the coil temperature rises with time. As a result, the coil temperature becomes the highest when the excitation time has elapsed (time t = 4H), indicating 14K.

  After the energized current reaches the operating current value Iop, the control device 140 fixes the magnetic field at a constant value by keeping the energized current at the operating current value Iop. Since the heat generation due to the change of the magnetic field is suppressed, the coil temperature decreases with time. The control device 140 controls the cooling device 121 so as to keep the coil temperature at 10K.

  The magnitude of the magnetic field fluctuation in the imaging region FOV exceeds 20 ppm when the excitation time has elapsed (time t = 4H). Although not shown, the magnitude of the magnetic field fluctuation at this time reaches 30 ppm. After this point, the shielding current Is decreases with time due to the magnetic flux creep phenomenon. As the shielding current Is decreases, the shielding current magnetic field Bs also decreases, so that the magnitude of the magnetic field fluctuation also decreases with time.

  In the comparative example, the magnitude of the magnetic field fluctuation is 9 hours after the start of excitation (time t = 0), in other words, 5 hours after the end of excitation (time t = 4H). Has reached 0.5 ppm. Thus, in the comparative example, it takes about 9 hours from the start of excitation of the superconducting coil until the magnetic field fluctuation in the imaging region FOV is stabilized at about 0.5 ppm. Accordingly, it takes about 9 hours from the start of excitation of the superconducting coil until a temporally and spatially accurate static magnetic field is generated in the imaging region FOV and a tomographic image of the subject can be captured. It will be.

<First embodiment>
FIG. 7 is a diagram showing a method of operating the superconducting magnet device according to the first embodiment. FIG. 7 shows the time when the excitation of the superconducting coil is started at t = 0, the energization current of the superconducting coil, the coil temperature, the magnetic field fluctuation in the imaging region FOV, and the heater on / off time change.

  The operation method according to the first embodiment includes a step of exciting the superconducting coil (S10 in FIG. 3), a step of maintaining the operating current Iop of the superconducting coil after the step of exciting the superconducting coil (S20 in FIG. 3), Is provided. In the first embodiment, the excitation time of the superconducting coil is set to 4 hours as in the comparative example. That is, the energization current of the superconducting coil is increased from 0 A to 200 A that is the operating current value Iop over 4 hours.

  In the first embodiment, the heater is further turned on during the excitation time. As shown in FIG. 2, a heater is provided at the axial end of each superconducting coil. Specifically, heaters 10 a and 10 b are provided at both ends of the superconducting coil 10 in the axial direction. Heaters 12a and 12b are provided at both axial ends of the superconducting coil 12, respectively. Heaters 14a and 14b are provided at both axial ends of the superconducting coil 14, respectively. Heaters 16a and 16b are provided at both ends of the superconducting coil 16 in the axial direction. Heaters 18a and 18b are provided at both axial ends of the superconducting coil 18, respectively. Heaters 20a and 20b are provided at both axial ends of the superconducting coil 20, respectively. Each superconducting coil can be heated by attaching these heaters to the cooling head 120. Thus, by providing a heater at the end portion in the axial direction of the superconducting coil where the shielding current Is becomes relatively large, the end portion can be heated preferentially. Therefore, the shielding current Is can be effectively reduced.

  Thus, in the first embodiment, the superconducting coil is heated using the heater as the step of heating the superconducting coil (S12 in FIG. 3). Therefore, in the excitation process, the superconducting coil is heated by receiving heat applied from the heater in addition to an increase in heat generation due to AC loss (mainly hysteresis loss). The coil temperature becomes the highest at the time when the excitation time has elapsed (time t = 4H), indicating 23K.

  Here, 1st target temperature T1 * in the process (S12 of FIG. 3) of heating a superconducting coil is set to the temperature below the allowable temperature of a superconducting coil. The allowable temperature of the superconducting coil is a temperature at which the magnetic characteristics of the superconducting coil are not affected by the heat generated by the superconducting coil. In the first embodiment, the allowable temperature is set to 24K, and the first target temperature T1 * is set to 23K, which is 1K lower than the allowable temperature. When the heat generation amount of the superconducting coil being excited exceeds the cooling capacity of the cooling device 121, the coil temperature may exceed the allowable temperature. In order to keep the coil temperature below the allowable temperature, the amount of heat generated by the superconducting coil during excitation is controlled.

  In the first example, the magnitude of the magnetic field fluctuation in the imaging region FOV is 15 ppm when the excitation time has elapsed (time t = 4H), and is suppressed to about ½ of the magnetic field fluctuation in the comparative example. It has been. This is because in the first embodiment, the coil temperature at the time when the excitation time has elapsed is higher than that in the comparative example, so that the shielding current Is induced in the superconducting layer is smaller than that in the comparative example, resulting in shielding. It shows that the intensity of the current magnetic field Bs has been reduced.

  In the first embodiment, as a step of cooling the superconducting coil (S22 in FIG. 3), after exciting the superconducting coil, the heater is turned off and the coil temperature is lower than the first target temperature T1 * (23K). The superconducting coil is cooled using the cooling device 121 so that the target temperature T2 * is 2. The second target temperature T2 * is set to 10K, for example.

  Thereby, after excitation, as the coil temperature decreases from the first target temperature T1 * (23K) with time, the electrical resistance value of the superconducting coil also decreases with time. Since the time constant increases with time due to the decrease in the electrical resistance value, the time change of the shielding current Is decreases.

  As described above, when the intensity of the shield current magnetic field Bs at the time when the excitation time has elapsed is reduced, and when the intensity of the shield current magnetic field Bs after the excitation is stabilized in a reduced state, the excitation is started. At the time when 7.2 hours have elapsed from (time t = 0), in other words, at the time when 3.2 hours have elapsed from the time when excitation is completed (time t = 4H), the magnitude of the magnetic field fluctuation reaches 0.5 ppm. is doing. According to the first embodiment, compared with the comparative example, the time required from the start of excitation of the superconducting coil to the stabilization of the magnetic field fluctuation in the imaging region FOV to about 0.5 ppm is shortened by about 2 hours. . This shortens the time required from when excitation of the superconducting coil starts until a magnetic field with high temporal and spatial uniformity is generated in the imaging region FOV and a tomographic image of the subject can be captured. can do.

<Second embodiment>
FIG. 8 is a diagram showing a method of operating the superconducting magnet device according to the second embodiment. In FIG. 8, the time when the excitation of the superconducting coil is started is t = 0, and the energization current of the superconducting coil, the coil temperature, the magnetic field fluctuation in the imaging region FOV, and the heater on / off time change are shown.

  The operation method according to the second embodiment is similar to the operation method according to the first embodiment, after the step of exciting the superconducting coil (S10 in FIG. 3) and the step of exciting the superconducting coil, the operating current of the superconducting coil. A step of holding Iop (S20 in FIG. 3). However, in the second embodiment, the excitation time of the superconducting coil is set to 0.5 hours, which is shorter than that of the first embodiment. That is, the energization current of the superconducting coil is increased from 0 A to the operating current value Iop (200 A) over 0.5 hours. The change rate of the energization current is constant during the excitation time.

  In the second embodiment, as in the first embodiment, the superconducting coil is heated during the excitation time. Specifically, as a step of heating the superconducting coil (S12 in FIG. 3), in addition to heating the superconducting coil using a heater, AC loss (mainly hysteresis loss) resulting from increasing energization current To heat the superconducting coil. That is, the superconducting coil is heated to the first target temperature T1 * by both the heating by the heater and the AC loss generated in the superconducting coil. The first target temperature T1 * is the same temperature (23K) as in the first embodiment.

  Specifically, in the second embodiment, the change rate of the energization current during excitation is made faster than in the first embodiment. As a result, the time change of the magnetic field generated by the superconducting coil increases, so that heat generation due to AC loss (hysteresis loss) increases and the amount of heat generation per unit time increases compared to the first embodiment. As a result, the superconducting coil can be heated to the first target temperature T1 * (23K) with a shorter excitation time.

  However, when the heat generation amount per unit time exceeds the cooling capacity of the cooling device 121, the coil temperature may exceed the allowable temperature. Therefore, the change rate of the energization current is set so that the heat generation amount per unit time due to heating by the heater and AC loss does not exceed the allowable heat generation amount of the superconducting coil.

  Also in the second embodiment, as in the first embodiment, after exciting the superconducting coil, the heater is turned off, and the second target whose coil temperature is lower than the first target temperature T1 * (23K). The superconducting coil is cooled using the cooling device 121 so that the temperature becomes T2 * (10K).

  As shown in FIG. 8, in the second embodiment, the excitation time is significantly shortened compared to the first embodiment, and therefore, when 4 hours have elapsed since the excitation was started (time t = 0). At (time t = 4H), the magnitude of the magnetic field fluctuation reaches 0.5 ppm. As a result, in the second embodiment, compared with the first comparative example, the time required from the start of excitation of the superconducting coil to the stabilization of the magnetic field fluctuation in the imaging region FOV to about 0.5 ppm is about 3 hours. It is getting shorter. Thus, according to the second embodiment, after excitation of the superconducting coil is started, a magnetic field having high temporal and spatial uniformity is generated in the imaging region FOV, and a tomographic image of the subject is captured. The time required to reach a possible state can be further shortened.

<Third embodiment>
FIG. 9 is a diagram showing a method of operating the superconducting magnet device according to the third embodiment. FIG. 9 shows the energization current of the superconducting coil, the coil temperature, the magnetic field fluctuation in the imaging region FOV, and the heater on / off time change with the time point when the excitation of the superconducting coil is started as t = 0.

  The operation method according to the third embodiment is similar to the operation methods according to the first and second embodiments, after the step of exciting the superconducting coil (S10 in FIG. 3) and the step of exciting the superconducting coil. Step (S20 in FIG. 3) of maintaining the operating current Iop. However, in the third embodiment, the excitation time of the superconducting coil is 0.25 hours, which is shorter than that of the second embodiment. That is, the energization current of the superconducting coil is increased from 0 A to the operating current value Iop (200 A) over 0.25 hours. The change rate of the energization current is constant during the excitation time.

  In the third embodiment, in the step of exciting the superconducting coil (S10 in FIG. 3), the superconducting coil is heated during the excitation time as in the first and second embodiments. However, in the third embodiment, unlike the first and second embodiments, the first target temperature is not increased by using an AC loss (mainly hysteresis loss) caused by increasing the energization current without using a heater. The superconducting coil is heated to T1 *. The first target temperature T1 * is set to a temperature (for example, 20K) lower than that in the first and second embodiments.

  In the third embodiment, the change rate of the energization current during excitation is further increased compared to the second embodiment. As a result, the time change of the magnetic field generated by the superconducting coil increases, so that heat generation due to AC loss (hysteresis loss) increases and the amount of heat generation per unit time increases as compared to the second embodiment. However, the change rate of the energization current is set so that the heat generation amount per unit time due to AC loss does not exceed the allowable heat generation amount of the superconducting coil.

  Further, in the third embodiment, the step of maintaining the operating current Iop of the superconducting coil (S20 in FIG. 3) is centered on the operating current value Iop before the step of cooling the superconducting coil (S22 in FIG. 3). Including a step of vibrating the energizing current. Specifically, as shown in FIG. 9, the energization current is vibrated for a predetermined time from the time when the excitation time has elapsed (time t = 0.25H). The predetermined time is, for example, 0.25H. It is assumed that the energized current is vibrated within a range of, for example, an operating current value Iop ± 5A (± 2.5%).

  By oscillating the energizing current, AC loss (hysteresis loss, etc.) due to a change in magnetic field increases. As a result, the coil temperature further increases from the first target temperature T1 * (20K).

  In the step of oscillating the energized current, after the superconducting coil reaches a third target temperature T3 * lower than the allowable temperature of the superconducting coil and higher than the first target temperature T1 * (20K), the oscillation of the energized current is performed. Attenuate the amplitude. The third target temperature T3 * is set to 23K which is 1K lower than the allowable temperature (24K) of the superconducting coil.

  By attenuating the vibration amplitude of the energized current, the energized current approaches the operating current value Iop while repeating overshoot and undershoot with respect to the operating current value Iop. By this operation, the shielding current loop in the oxide superconducting wire can be gradually reduced, and finally, the operating current value Iop can be reached when the shielding current magnetic field Bs is substantially zero. Such a method of eliminating the shield current magnetic field Bs uses a hysteresis effect of the shield current magnetic field Bs, and is also called a demagnetization method (demagnetization method).

  In the third embodiment, by performing the demagnetization method, the magnitude of the magnetic field fluctuation at the time when the predetermined time (0.25H) has elapsed (time t = 0.5H) is about 2 ppm. Has been reduced.

  After the step of vibrating the energization current, a step of cooling the superconducting coil (S22 in FIG. 3) is performed. Specifically, the energizing current is fixed to the operating current value Iop, and the cooling device 121 is set so that the coil temperature becomes the second target temperature T2 * (10K) lower than the first target temperature T1 * (23K). Is used to cool the superconducting coil.

  As described above, in the third embodiment, in the step of exciting the superconducting coil and the step of vibrating the energized current, the superconducting coil is heated by the AC loss (hysteresis loss) due to the time change of the magnetic field. This eliminates the need to install a heater for heating the superconducting coil, thereby reducing the size of the superconducting coil.

  Further, the shielding current magnetic field Bs can be further reduced by attenuating the oscillation amplitude of the energized current in the step of oscillating the energized current. As a result, in the third embodiment, the magnitude of the magnetic field fluctuation reaches 0.5 ppm at the time (time t = 1H) after 1 hour has passed since the time when excitation was started (time t = 0). As a result, in the third embodiment, as compared with the first comparative example, the time required from the start of excitation of the superconducting coil to the stabilization of the magnetic field fluctuation in the imaging region FOV to about 0.5 ppm is about 6 hours. It is getting shorter. As a result, according to the third embodiment, after starting the excitation of the superconducting coil, a magnetic field having high temporal and spatial uniformity is generated in the imaging region FOV, and a tomographic image of the subject is captured. The time required to reach a possible state can be further shortened.

  The embodiments and examples disclosed herein are illustrative in all respects and should not be construed as being restrictive. The scope of the present invention is shown not by the embodiments and examples described above but by the scope of claims, and is intended to include meanings equivalent to the scope of claims and all modifications within the scope.

10, 12, 14, 16, 18, 20 Superconducting coils 10a, 10b, 12a, 12b, 14a, 14b, 16a, 16b, 18a, 18b, 20a, 20b Heater 21 Gradient magnetic field coil 22, 24 Shim 30 Oxide superconducting wire 91 Superconducting coil body 100 Superconducting magnet device 111 Thermal insulation container 120 Cooling head 121 Cooling device 122 Hose 123 Compressor 131 Cable 132 Power supply 140 Controller Bc Static magnetic field (coil center magnetic field)
Bs Shielding current magnetic field FOV Imaging region I Energizing current Is Shielding current Iop Operating current

Claims (7)

A method of operating a superconducting magnet device,
The superconducting magnet device is
A superconducting coil having a wound oxide superconducting wire;
A power supply for supplying current to the superconducting coil;
A control device for controlling an energization current of the superconducting coil,
The driving method is as follows:
Exciting the superconducting coil by energizing the superconducting coil cooled to a superconducting state;
After the step of exciting the superconducting coil, holding the operating current of the superconducting magnet device in the superconducting coil,
Wherein the step of energizing the superconducting coil, with increasing the energizing current to the operating current, comprising the step of heating the superconducting coil so that the first target temperature,
The step of holding the operating current is seen containing a step of cooling the superconducting coil such that the first lower than the target temperature second target temperature,
The method of operating a superconducting magnet device , wherein in the step of heating the superconducting coil, heating is performed until an axial end of the superconducting coil reaches the first target temperature . The method of operating a superconducting magnet device according to claim 1, wherein the step of maintaining the operating current includes a step of oscillating the energizing current around the operating current value before the step of cooling the superconducting coil. In the step of vibrating the energizing current, the superconducting coil, after reaching a high third target temperature than before Symbol first target temperature, to attenuate the vibration amplitude of the energizing current, according to claim 2 Operation method of superconducting magnet device. The method of operating a superconducting magnet device according to claim 3 , wherein, in the step of heating the superconducting coil, the superconducting coil is heated by an AC loss resulting from increasing the energization current. The method of operating a superconducting magnet device according to claim 1 , wherein the step of heating the superconducting coil includes a step of heating the superconducting coil using a heating device. The operation method of the superconducting magnet device according to claim 5 , wherein the heating device is provided at an end portion of the superconducting coil in the axial direction. The method of operating a superconducting magnet device according to claim 5 or 6 , wherein, in the step of heating the superconducting coil, the superconducting coil is further heated by an AC loss caused by increasing the energization current.

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