Superconducting magnets, particle accelerators, and particle beam therapy devices

Abstract

To provide a superconducting electromagnet, a particle accelerator, and a particle beam therapy device that can reduce the effect of coil magnetization magnetic field.SOLUTION: When a start current value Is is higher than a target current value If, a control unit 20 performs control to decrease a current value from the start current value Is to the target current value If. In this case, the current value descends to reach the target current value If. Further, when the start current value Is is equal to or less than the target current value If, the control unit 20 increases the current value from the start current value Is to a format current value Io higher than the target current value If, and then controls to descend to the target current value If. In this case, the current value descends to reach the target current value If. In this manner, the current value descends to reach the target current value If, regardless of the value of the predetermined current value before changing the current value. As a result, the coil magnetization magnetic field can be kept in the same state when the target current value If is reached.SELECTED DRAWING: Figure 7

Description

[Technical field]

[0001] The present invention relates to a superconducting electromagnet, a particle accelerator, and a particle beam therapy device. [Background technology]

[0002] A superconducting electromagnet includes a coil and a vacuum vessel. The coil is an annular member formed by winding a superconducting wire. The coil is arranged to surround a magnetic pole. The superconducting electromagnet creates a strong magnetic field by creating a vacuum inside the vacuum vessel and passing a current through the coil, which has been made superconducting by a refrigerator. A particle accelerator, such as that described in Patent Document 1, is known as an apparatus using such a superconducting electromagnet. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2020-21901 A Summary of the Invention [Problem to be solved by the invention]

[0004] When a current flows through the coil, a magnetic field is formed around the coil. At this time, the magnetic field formed by the current passing through the coil acts as an external magnetic field, and the superconductor inside the coil is magnetized (coil magnetization magnetic field). This coil magnetization magnetic field affects the magnetic field formed around the coil. In particle accelerators such as those mentioned above, this coil magnetization magnetic field can cause problems such as a phase shift in the acceleration voltage frequency. Therefore, there has been a demand for a superconducting electromagnet that can reduce the effects of the coil magnetization magnetic field.

[0005] Therefore, an object of the present invention is to provide a superconducting electromagnet, a particle accelerator, and a particle beam therapy device that can reduce the influence of the coil magnetization magnetic field. [Means for solving the problem]

[0006] A superconducting electromagnet according to one aspect of the present invention is a superconducting electromagnet that generates a magnetic field by passing a current through a coil, and has a control unit that controls the current value of the coil, and when a predetermined current value of the coil is higher than a target current value, the control unit controls the current value to be reduced from the predetermined current value to the target current value, and when the predetermined current value is equal to or lower than the target current value, the control unit controls the current value to be increased from the predetermined current value to a format current value higher than the target current value, and then reduced to the target current value.

[0007] In a superconducting electromagnet, when the coil current value is changed from a predetermined current value to a target current value, a difference occurs in the coil magnetization magnetic field between the case where the target current value is reached by lowering the current value and the case where the target current value is reached by raising the current value, even though the current value is the same. In response to this, when the predetermined current value of the coil is higher than the target current value, the control unit performs control to lower the current value from the predetermined current value to the target current value. In this case, the current value reaches the target current value in a descending manner. In addition, when the predetermined current value is equal to or less than the target current value, the control unit performs control to raise the current value from the predetermined current value to a format current value higher than the target current value, and then to lower it to the target current value. In this case, the current value reaches the target current value in a descending manner. In this way, regardless of the value of the predetermined current value before changing the current value, the current value reaches the target current value in a descending manner. As a result, the coil magnetization magnetic field can be made the same when the target current value is reached, regardless of the predetermined current value before changing the current value. As a result, the influence of the coil magnetization magnetic field can be reduced.

[0008] A superconducting electromagnet according to one aspect of the present invention is a superconducting electromagnet that generates a magnetic field by passing a current through a coil, and has a control unit that controls the current value of the coil, and when a predetermined current value of the coil is lower than a target current value, the control unit controls the current value to be increased from the predetermined current value to the target current value, and when the predetermined current value is equal to or greater than the target current value, controls the current value to be decreased from the predetermined current value to a format current value lower than the target current value, and then to be increased to the target current value.

[0009] When the predetermined current value of the coil is lower than the target current value, the control unit controls the current value to be increased from the predetermined current value to the target current value. In this case, the current value reaches the target current value in an increasing manner. When the predetermined current value is equal to or greater than the target current value, the control unit controls the current value to be decreased from the predetermined current value to a format current value lower than the target current value, and then to be increased to the target current value. In this case, the current value reaches the target current value in an increasing manner. In this way, regardless of the predetermined current value before the current value is changed, the current value reaches the target current value in an increasing manner. This allows the coil magnetization magnetic field to be in the same state when the target current value is reached, regardless of the predetermined current value before the current value is changed. As described above, the influence of the coil magnetization magnetic field can be reduced.

[0010] A superconducting electromagnet according to one aspect of the present invention is a superconducting electromagnet that generates a magnetic field by passing a current through a coil, and has a control unit that controls the current value of the coil. The control unit estimates the magnetic field caused by magnetization of the coil that occurs with changes in the current value, and passes a current value through the coil that is shifted from a target current value by the amount of the magnetic field caused by magnetization of the coil.

[0011] The control unit estimates the magnetic field caused by the magnetization of the coil that occurs with the change in the current value, and passes a current value to the coil that is shifted from the target current value by the magnetic field caused by the magnetization of the coil. In this case, when changing from a predetermined current value to the target current value, the control unit can control the current value to be passed to the coil by previously feeding forward the coil magnetization magnetic field. As a result, a magnetic field is formed around the coil in a manner that reduces the coil magnetization magnetic field. As a result, the influence of the coil magnetization magnetic field can be reduced.

[0012] The control unit may keep the coil current value constant at the format current value for a predetermined period of time, thereby ensuring sufficient time for the coil magnetization magnetic field to change.

[0013] The control unit may perform control in the first sweep control of the current value to increase the current value to the maximum current value that can be obtained in the second and subsequent sweep controls. In the first sweep control, the value of the coil magnetization magnetic field can transition over the entire range of the magnetization curve related to the initial magnetization process. Then, in the second and subsequent sweep controls, it is possible to suppress the value of the coil magnetization magnetic field from changing along the magnetization curve related to the initial magnetization process.

[0014] A particle accelerator according to one aspect of the present invention includes the above-described superconducting electromagnet, and accelerates particles to generate a particle beam.

[0015] A particle beam therapy system according to one aspect of the present invention includes the above-described particle accelerator, and performs therapy using particle beams generated by the particle accelerator.

[0016] These particle accelerators and particle beam therapy devices can provide the same effects and advantages as the above-mentioned superconducting electromagnets. Effect of the Invention

[0017] According to the present invention, there are provided a superconducting electromagnet, a particle accelerator, and a particle beam therapy device capable of reducing the influence of the coil magnetization magnetic field. [Brief description of the drawings]

[0018] [Figure 1] 1 is a schematic cross-sectional view showing a particle accelerator equipped with a superconducting electromagnet according to an embodiment of the present invention. [Diagram 2] 1 is a schematic cross-sectional view showing a particle beam therapy device equipped with a superconducting electromagnet according to an embodiment of the present invention. [Diagram 3] FIG. 2 is a schematic diagram for explaining a coil magnetization magnetic field. [Figure 4] 1 is a graph of a magnetization curve showing the relationship between a magnetic field generated by a coil when a current is passed through the coil and a coil magnetization magnetic field. [Diagram 5] FIG. 4 is a diagram showing the relationship between a coil magnetization magnetic field and a current. [Figure 6] 11 is a graph showing a change in current value and a change in coil magnetization magnetic field. [Figure 7]11 is a graph showing a change in current value and a change in coil magnetization magnetic field. [Figure 8] 11 is a graph showing a change in current value and a change in coil magnetization magnetic field. [Figure 9] 11 is a graph showing a change in current value and a change in coil magnetization magnetic field. [Figure 10] FIG. 13 is a diagram for explaining the control for eliminating the magnetization curve related to the initial magnetization process. [Figure 11] 11 is a graph showing a mode in which a current value shifted from a target current value by an estimated coil magnetization magnetic field is passed through a coil. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Various embodiments will be described in detail below with reference to the drawings. Note that the same or corresponding parts in each drawing are denoted by the same reference numerals, and duplicated explanations will be omitted.

[0020] FIG. 1 is a schematic cross-sectional view showing a particle accelerator equipped with a superconducting electromagnet according to the present embodiment. The particle accelerator 1 shown in FIG. 1 is a cyclotron used to generate and emit a charged particle beam by accelerating charged particles supplied from an ion source (not shown) in a neutron capture therapy system for cancer treatment using boron neutron capture therapy (BNCT). Examples of the charged particles include protons, heavy particles (heavy ions), and electrons. The particle accelerator 1 can also be used as a cyclotron for PET, a cyclotron for RI production, and a cyclotron for atomic nucleus experiments. The particle accelerator 1 may be adopted in a particle beam therapy device 100 as shown in FIG. 2. The particle beam therapy device 100 shown in FIG. 2 includes the particle accelerator 1, a transport line 121, an irradiation unit 102, and a rotating gantry 105. The irradiation unit 102 emits a particle beam generated in a particle accelerator and transported by a transport line 121. The irradiation unit 102 irradiates a patient on a treatment table 104. The irradiation unit 102 is attached to a rotating gantry 105, and the irradiation position in the rotation direction is adjusted by the rotating gantry 105. Note that the particle accelerator 1 is not limited to a cyclotron, and may be a synchrocyclotron.

[0021] Returning to FIG. 1, the particle accelerator 1 includes a yoke 2, a superconducting electromagnet 3, a pair of magnetic poles 4A and 4B, and a vacuum vessel 5.

[0022] The yoke 2 supports the superconducting electromagnet 3, a pair of magnetic poles 4A, 4B, a vacuum vessel 5, etc. The yoke 2 is a hollow disk-shaped block, and inside it are provided a pair of magnetic poles 4A, 4B that form a magnetic field necessary for accelerating charged particles. The magnetic poles 4A, 4B are circular in plan view, and are arranged opposite each other with the median plane MP (the acceleration plane along which the charged particles are accelerated) in between. The superconducting electromagnet 3 is arranged around the magnetic poles 4A, 4B.

[0023] The superconducting electromagnet 3 includes a pair of coils 30A, 30B, a vacuum vessel 5, and a control unit 20. Each of the coils 30A, 30B is annular and formed by winding a superconducting wire. The coils 30A, 30B are disposed so as to surround the magnetic poles 4A, 4B, respectively. The material of the superconducting wire is not particularly limited, but NbTi, Nb3Sn, MgB2, Bi-based superconductors (Bi2223, Bi2212, etc.), rare earth-based superconductors, iron-based superconductors, etc. may be used.

[0024] The vacuum vessel 5 is a vessel that houses the coils 30A, 30B in a vacuum state. The vacuum vessel 5 and a refrigerator (not shown) constitute a cryostat that can cool the coils 30A, 30B until they reach a superconducting state. For example, a GM refrigerator (Gifford-McMhon cooler) can be used as the refrigerator. Note that the type of refrigerator is not limited to a GM refrigerator, and may be another refrigerator such as a Stirling refrigerator.

[0025] In the particle accelerator 1, a strong magnetic field is generated by creating a vacuum inside the vacuum vessel 5 and then passing a current through the coil of the superconducting electromagnet 3, which is made superconducting by a refrigerator. Charged particles supplied from an ion source (not shown) are accelerated by the influence of the magnetic field on the median plane MP in the space between the magnetic poles 4A and 4B, and are emitted as a charged particle beam.

[0026] The control unit 20 is a device that controls the current values ​​of the coils 30A and 30B. Here, in order to explain the contents of the control by the control unit 20, the coil magnetization magnetic field of the coil 30 will be explained with reference to FIG. 3. In addition, when the coils 30A and 30B are not particularly distinguished, they are referred to as "coil 30". When a current flows through the coil 30, a magnetic field MF is formed around the coil 30. At this time, in the coil 30, the magnetic field MF is used as an external magnetic field, and the superconductor inside the coil 30 generates magnetization (coil magnetization magnetic field). Such a coil magnetization magnetic field affects the magnetic field formed around the coil 30. In FIG. 3, "Berror" indicates the coil magnetization magnetic field. "Bext" is the magnetic field formed by the coil 30 by current flow, excluding Berror. Here, as shown in FIG. 3(a)(b), the value and behavior of the coil magnetization magnetic field are different from each other due to the influence of the mutual inductance M when the coil 30 is excited by increasing the current value and when the coil 30 is demagnetized by decreasing the current value. Specifically, FIG. 4 is a graph of a magnetization curve showing the relationship between the magnetic field formed by the coil 30 by energization and the coil magnetization magnetic field. The horizontal axis of FIG. 4 indicates the external magnetic field, and the vertical axis indicates the magnetization of the superconductor. Note that even if the relationship between the current value and the coil magnetization magnetic field is plotted by regarding the current value as the horizontal axis, the shape will be approximately the same as that of FIG. 4. Therefore, the horizontal axis of FIG. 4 may be regarded as the current value. Also, the value on the vertical axis may be simply referred to as the "coil magnetization magnetic field value." As shown in FIG. 4, when the current value of the coil 30 is increased in the positive region, the value of the coil magnetization magnetic field moves along the magnetization curve E1 on the negative side of the graph. When the current value of the coil 30 is decreased in the positive region, the value of the coil magnetization magnetic field moves along the magnetization curve E2 on the positive side of the graph. In this way, the coil magnetization magnetic field due to the influence of the coil magnetization magnetic field depends on the sweep pattern of the current.

[0027] FIG. 5 shows a schematic diagram of the positive region of the horizontal axis of the graph in FIG. 4. However, the horizontal axis is replaced with the current value for the coil 30. For example, when the current value of the coil 30 is directly changed from the starting current value to the target current value (Ib), the value of the coil magnetization magnetic field will differ depending on the magnitude relationship between the starting current value and the target current value. That is, when the starting current value is "Ia" which is smaller than "Ib", the value of the coil magnetization magnetic field will move along the negative magnetization curve E1. At this time, the value of the coil magnetization magnetic field moves from the value of the start point S1 along the magnetization curve E1 to the value of the target point F1. When the starting current value is "Ic" which is larger than "Ib", the value of the coil magnetization magnetic field moves along the positive magnetization curve E2. At this time, the value of the coil magnetization magnetic field moves from the value of the start point S2 along the magnetization curve E2 to the value of the target point F2. In this way, although the final target current value is the same at "Ib", the value of the coil magnetization magnetic field at the target point F1 and the value of the coil magnetization magnetic field at the target point F2 are different values ​​from each other.

[0028] Therefore, in controlling the current value to change from a predetermined current value (hereinafter referred to as the start current value) to the target current value, the control unit 20 controls the current value so that the value of the coil magnetization magnetic field at the target current value is the same regardless of the magnitude of the start current value. Just before the target current value is reached, the control unit 20 reduces the current value (demagnetizes) to reach the target current value. In this case, just before the target current value is reached, the control unit 20 controls the current value so that the value of the coil magnetization magnetic field moves along the positive magnetization curve E2.

[0029] The control contents of the control unit 20 will be described with reference to Figs. 6 and 7. The graphs on the left side of Figs. 6(a) and (b) are graphs showing the relationship between the transition of time and the transition of the current value. The graphs on the right side of Figs. 6(a) and (b) are graphs showing the relationship between the current value and the value of the coil magnetization magnetic field. The same applies to Figs. 7 and onwards. In the following description, the "start current value Is" and the "target current value If" are used. The current value immediately before the start current value Is is used as the "previous current value Ip". The current value immediately before the target current value If for shifting the value of the coil magnetization magnetic field on the positive magnetization curve E2 is used as the "format current value Io". In the graph showing the current value and the value of the coil magnetization magnetic field, the point corresponding to the start current value Is is used as the start point S, the point corresponding to the format current value Io is used as the format point O, and the point corresponding to the target current value If is used as the target point F.

[0030] 6 and 7 show the control contents of the control unit 20 when the format current value Io is higher than the target current value If. In this case, no matter how the value of the coil magnetization magnetic field has changed up to the format point O, when it moves from the format point O to the target point F, it changes along the positive magnetization curve E2.

[0031] 6 shows the control contents when the start current value Is is higher than the target current value If and the format current value Io. As shown in FIG. 6, when the start current value Is is higher than the target current value If, ​​the control unit 20 performs control to decrease the current value from the start current value Is to the target current value If.

[0032] FIG. 6(a) shows the control contents when the previous current value Ip is larger than the start current value Is. At this time, the control unit 20 lowers the current value to change from the previous current value Ip to the start current value Is (graph on the left), so the start point S exists on the positive magnetization curve E2 (graph on the right). The control unit 20 lowers the current value from the start current value Is to the format current value Io. As a result, the value of the coil magnetization magnetic field transitions from the start point S to the format point O on the positive magnetization curve E2. The control unit 20 also lowers the current value from the format current value Io to the target current value If. As a result, the value of the coil magnetization magnetic field transitions from the format point O to the target point F on the positive magnetization curve E2.

[0033] FIG. 6(b) shows the control contents when the previous current value Ip is smaller than the start current value Is. At this time, the control unit 20 increases the current value to change from the previous current value Ip to the start current value Is (graph on the left), so the start point S exists on the negative magnetization curve E1 (graph on the right). The control unit 20 increases the current value from the start current value Is to the format current value Io. At this time, the value of the coil magnetization magnetic field jumps from the start point S of the negative magnetization curve E1 to the positive magnetization curve E2, and transitions to the format point O on the positive magnetization curve E2. In addition, the control unit 20 decreases the current value from the format current value Io to the target current value If. As a result, the value of the coil magnetization magnetic field transitions from the format point O to the target point F on the positive magnetization curve E2.

[0034] Fig. 7 shows the control contents when the start current value Is is equal to or less than the target current value If and the format current value Io. As shown in Fig. 7, when the start current value Is is equal to or less than the target current value If, ​​the control unit 20 performs control to increase the current value from the start current value Is to the format current value Io, which is higher than the target current value If, ​​and then decrease the current value to the target current value If.

[0035] FIG. 7(a) shows the control contents when the previous current value Ip is larger than the start current value Is. At this time, the control unit 20 lowers the current value to change from the previous current value Ip to the start current value Is (graph on the left), so the start point S is on the positive magnetization curve E2 (graph on the right). The control unit 20 increases the current value from the start current value Is to the format current value Io. As a result, the value of the coil magnetization magnetic field jumps from the positive magnetization curve E2 to the negative magnetization curve E1, transitions along the negative magnetization curve E1, and transitions to the position of the current value corresponding to the format point O. The control unit 20 lowers the current value from the format current value Io to the target current value If. As a result, the value of the coil magnetization magnetic field jumps from the negative magnetization curve E1 to the positive magnetization curve E2, and transitions from the format point O to the target point F on the positive magnetization curve E2.

[0036] FIG. 7(b) shows the control contents when the previous current value Ip is equal to or less than the start current value Is. At this time, the control unit 20 increases the current value to change the previous current value Ip to the start current value Is (graph on the left), so that the start point S exists on the negative magnetization curve E1 (graph on the right). The control unit 20 increases the current value from the start current value Is to the format current value Io. As a result, the value of the coil magnetization magnetic field moves from the start point S along the negative magnetization curve E1 to the position of the current value corresponding to the format point O. The control unit 20 decreases the current value from the format current value Io to the target current value If. As a result, the value of the coil magnetization magnetic field jumps from the negative magnetization curve E1 to the positive magnetization curve E2, and moves from the format point O to the target point F on the positive magnetization curve E2.

[0037] The control contents by the control unit 20 are not limited to those shown in Fig. 6 and Fig. 7, and the control contents shown in Fig. 8 and Fig. 9 may be adopted. Fig. 8 and Fig. 9 show the control contents of the control unit 20 when the format current value Io is lower than the target current value If. In this case, no matter how the value of the coil magnetization magnetic field has changed up to the format point O, when it moves from the format point O to the target point F, it moves along the negative magnetization curve E1. That is, the control unit 20 controls the current value so that the value of the coil magnetization magnetic field moves along the negative magnetization curve E1 immediately before reaching the target current value.

[0038] Fig. 8 shows the control contents when the start current value Is is equal to or greater than the target current value If and the format current value Io. As shown in Fig. 8, the control unit 20 performs control to decrease the current value from the start current value Is to the format current value Io, which is lower than the target current value If, ​​and then to increase the current value to the target current value If.

[0039] FIG. 8(a) shows the control contents when the previous current value Ip is larger than the start current value Is. At this time, the control unit 20 lowers the current value to change from the previous current value Ip to the start current value Is (graph on the left), so the start point S exists on the positive magnetization curve E2 (graph on the right). The control unit 20 lowers the current value from the start current value Is to the format current value Io. As a result, the value of the coil magnetization magnetic field shifts from the start point S to the position of the current value corresponding to the format point O on the positive magnetization curve E2. In addition, the control unit 20 lowers the current value from the format current value Io to the target current value If. As a result, the value of the coil magnetization magnetic field jumps from the positive magnetization curve E2 to the negative magnetization curve E1, and shifts from the format point O to the target point F on the negative magnetization curve E1.

[0040] FIG. 8(b) shows the control contents when the previous current value Ip is smaller than the start current value Is. At this time, the control unit 20 increases the current value to change from the previous current value Ip to the start current value Is (graph on the left), so the start point S exists on the negative magnetization curve E1 (graph on the right). The control unit 20 increases the current value from the start current value Is to the format current value Io. At this time, the value of the coil magnetization magnetic field jumps from the start point S of the negative magnetization curve E1 to the positive magnetization curve E2, and transitions to the position of the current value corresponding to the format point O on the positive magnetization curve E2. In addition, the control unit 20 decreases the current value from the format current value Io to the target current value If. As a result, the value of the coil magnetization magnetic field jumps from the positive magnetization curve E2 to the negative magnetization curve E1, and transitions from the format point O to the target point F on the negative magnetization curve E1.

[0041] 9 shows the control content when the start current value Is is lower than the target current value If and the format current value Io. As shown in FIG 9, the control unit 20 performs control to increase the current value from the start current value Is to the target current value If.

[0042] FIG. 9(a) shows the control contents when the previous current value Ip is larger than the start current value Is. At this time, the control unit 20 lowers the current value to change from the previous current value Ip to the start current value Is (graph on the left), so the start point S exists on the positive magnetization curve E2 (graph on the right). The control unit 20 increases the current value from the start current value Is to the format current value Io. As a result, the value of the coil magnetization magnetic field jumps from the positive magnetization curve E2 to the negative magnetization curve E1, transitions along the negative magnetization curve E1, and transitions to the format point O. The control unit 20 increases the current value from the format current value Io to the target current value If. As a result, the value of the coil magnetization magnetic field transitions from the format point O to the target point F on the negative magnetization curve E1.

[0043] FIG. 9(b) shows the control contents when the previous current value Ip is smaller than the start current value Is. At this time, the control unit 20 increases the current value to change from the previous current value Ip to the start current value Is (graph on the left), so the start point S exists on the negative magnetization curve E1 (graph on the right). The control unit 20 increases the current value from the start current value Is to the format current value Io. As a result, the value of the coil magnetization magnetic field moves from the start point S along the negative magnetization curve E1 to the format point O. The control unit 20 increases the current value from the format current value Io to the target current value If. As a result, the value of the coil magnetization magnetic field moves from the format point O to the target point F on the negative magnetization curve E1.

[0044] Here, as shown in FIG. 10(a), the current sweep pattern has a magnetization curve E3 related to the initial magnetization process on the negative side of the coil magnetization magnetic field, in addition to the magnetization curve E1. The magnetization curve E3 related to the initial magnetization process is a magnetization curve along which the value of the coil magnetization magnetic field transitions when a current value that increases for the first time is passed through the coil 30. For example, when the value of the coil magnetization magnetic field transitions along the magnetization curve E1 in the case of rising from the format current value Io to the target current value If, ​​the value transitions from the format point O to the target point F. However, when the value of the coil magnetization magnetic field transitions along the magnetization curve E3, the value transitions from a different format point O' to a different target point F'.

[0045] Here, if the value of the coil magnetization magnetic field passes through the magnetization curve E3 related to the initial magnetization process at a certain current value even once, it will move along the magnetization curve E1 from the next time onwards. Therefore, as shown in FIG. 10(c), the control unit 20 performs control in the first sweep control of the current value to increase it to the maximum current value Imax that can be taken in the second and subsequent sweep controls. As a result, as shown in FIG. 10(b), the value of the coil magnetization magnetic field can move through the entire range of the magnetization curve E3 related to the initial magnetization process in the first sweep control. Then, after the above process (after time t1 in FIG. 10(c)), by performing the processes shown in FIG. 6 to FIG. 9, it is possible to perform control such that the value of the magnetization magnetic field moves along the magnetization curves E1 and E2.

[0046] Next, the target current value If and the format current value I O The magnitude of the difference between the magnetic field and the magnetic field is explained below. The magnitude of this difference ΔI is set by the following formula (1). μ0 is the magnetic permeability of a vacuum, J c is the critical current value of the superconductor, r f indicates the filament radius. I0 is an arbitrary current value, B m is the strongest magnetic flux density in the coil for the current value. The critical current value J c is determined by temperature and empirical magnetic field, so J c has a range depending on the operating temperature and the experienced magnetic field. For example, the value of α may be the value calculated by the formula (2).

number

[0047] The control unit 20 keeps the current value of the coil 30 constant at the format current value Io for a predetermined time. The predetermined constant time is not particularly limited, but it is sufficient that, for example, when the control unit 20 changes the current value from the start current value Is to the format current value Io, a sufficient time is ensured for the value of the coil magnetization magnetic field to transition from the start point to the format point O. For example, the predetermined constant time may be set to about 30 to 600 seconds.

[0048] 6 to 9, the control unit 20 controls the current value so that the value of the coil magnetization magnetic field transitions along the negative magnetization curve E1 or the positive magnetization curve E2 immediately before reaching the target current value. Alternatively, as shown in Fig. 11, in the control for changing the coil current value from the start current value to the target current value, the control unit 20 may estimate the coil magnetization magnetic field (magnetic field caused by magnetization of the coil 30 generated with the change in the current value) and pass a current value shifted from the target current value by the amount of the coil magnetization magnetic field through the coil 30.

[0049] Specifically, the control unit 20 passes a current value to the coil 30 that is shifted by an estimated value ΔI` from the target current value I. For example, as shown in FIG. 11(a), the control unit 20 passes a current value of "I+ΔI`" to the coil 30. Alternatively, as shown in FIG. 11(b), the control unit 20 passes a current value of "I-ΔI`" to the coil 30. ΔI` is set by equation (3). Each parameter is the same as equation (1) above. α needs to satisfy equation (4). α may be found by calculating the sum of the magnetic fields of each unit volume created at any point around the coil 30 by the magnetic moment per unit volume of the superconductor.

number

[0050] Next, the action and effect of the superconducting electromagnet 3 according to this embodiment will be described.

[0051] As described above, in the superconducting electromagnet 3, the coil 30 uses the magnetic field MF as an external magnetic field, and the superconductor inside the coil 30 generates magnetization (coil magnetization magnetic field). As a result, a coil magnetization magnetic field is generated in the coil 30. Such a coil magnetization magnetic field cannot be ignored when the absolute value of the magnetic field of the coil 30 is highly required. As shown in FIG. 1, the influence of the coil magnetization magnetic field as described above may be a problem in the particle accelerator 1. Since there is a correlation between the coil magnetization magnetic field and the phase shift of the acceleration voltage frequency, the coil magnetization magnetic field affects the acceleration of particles. Therefore, the coil magnetization magnetic field that depends on the sweep pattern of the current may cause a decrease in the acceleration efficiency of the particle accelerator 1. Since the particle accelerator 1 requires precision, a superconducting electromagnet that can reduce the influence of the coil magnetization magnetic field due to the coil magnetization magnetic field has been desired.

[0052] Here, in the superconducting electromagnet 3, when the current value of the coil 30 is changed from the start current value Is to the target current value If, ​​a difference occurs in the coil magnetization magnetic field due to the coil magnetization magnetic field between the case where the current value is lowered to reach the target current value If and the case where the current value is raised to reach the target current value If, ​​even though the current value is the same (see FIG. 5). In response to this, as shown in FIG. 6 and FIG. 7, when the start current value Is is higher than the target current value If, ​​the control unit 20 performs control to lower the current value from the start current value Is to the target current value If. In this case, the current value reaches the target current value If in a descending manner. In addition, when the start current value Is is equal to or lower than the target current value If, ​​the control unit 20 performs control to raise the current value from the start current value Is to a format current value Io higher than the target current value If, ​​and then to lower it to the target current value If. In this case, the current value reaches the target current value If in a descending manner. In this way, regardless of the value of the predetermined current value before the current value is changed, the current value reaches the target current value If in a descending manner. This makes it possible to keep the coil magnetizing magnetic field in the same state when the target current value If is reached, regardless of the start current value Is before the current value is changed. As a result, the influence of the coil magnetizing magnetic field can be reduced.

[0053] Also, as shown in FIG. 8 and FIG. 9, when the start current value Is is lower than the target current value If, ​​the control unit 20 performs control to increase the current value from the start current value Is to the target current value If. In this case, the current value reaches the target current value If in an increasing manner. Also, when a predetermined current value is equal to or higher than the target current value If, ​​the control unit 20 performs control to decrease the current value from the start current value Is to a format current value Io lower than the target current value If, ​​and then to increase it to the target current value If. In this case, the current value reaches the target current value If in an increasing manner. In this way, regardless of the value of the start current value before changing the current value, the current value reaches the target current value If in an increasing manner. As a result, the coil magnetization magnetic field can be made the same when the target current value If is reached, regardless of the start current value Is before changing the current value. As a result, the influence of the coil magnetization magnetic field can be reduced.

[0054] As shown in FIG. 11, the control unit 20 estimates the magnetic field caused by magnetization of the coil 30 resulting from a change in the current value, and supplies to the coil 30 a current value shifted from the target current value by the amount of the coil magnetization magnetic field.

[0055] In this case, when changing from the start current value to the target current value, the control unit 20 can control the current value flowing to the coil 30 by previously feeding forward the coil magnetizing magnetic field. As a result, a magnetic field is formed around the coil 30 in a state in which the coil magnetizing magnetic field is reduced. As a result, the influence of the coil magnetizing magnetic field can be reduced.

[0056] The control unit 20 may keep the current value of the coil 30 constant at the format current value Io for a predetermined time. In this case, it is possible to ensure a sufficient time for the coil magnetization magnetic field to change.

[0057] In the first sweep control of the current value, the control unit 20 may perform control to increase the current value to the maximum current value Imax that can be obtained in the second and subsequent sweep controls. In the first sweep control, the value of the coil magnetization magnetic field can transition over the entire range of the magnetization curve E3 related to the initial magnetization process. Then, in the second and subsequent sweep controls, it is possible to suppress the value of the coil magnetization magnetic field from changing along the magnetization curve E3 related to the initial magnetization process.

[0058] The particle accelerator 1 according to this embodiment includes the above-mentioned superconducting electromagnet 3, and accelerates particles to generate a particle beam.

[0059] The particle beam therapy system 100 according to this embodiment includes the above-mentioned particle accelerator 1, and performs therapy using particle beams generated by the particle accelerator 1.

[0060] According to the particle accelerator 1 and the particle beam therapy system 100, the same actions and effects as those of the above-mentioned superconducting electromagnet 3 can be obtained.

[0061] The present invention is not limited to the above-described embodiments.

[0062] For example, superconducting electromagnets have been used in particle accelerators, but they may also be used in silicon single crystal pulling devices and the like. [Explanation of symbols]

[0063] 1...particle accelerator, 3...superconducting electromagnet, 20...control unit, 30, 30A, 30B...coil, 100...particle beam therapy device.

Claims

1. A superconducting electromagnet that generates a magnetic field by passing a current through a coil. A control unit for controlling a current value of the coil, the control unit performs control to reduce the current value from the predetermined current value to the target current value when the predetermined current value of the coil is higher than the target current value, a superconducting electromagnet that, when the predetermined current value is equal to or lower than the target current value, controls so that the current value is increased from the predetermined current value to a format current value that is higher than the target current value, and then decreased to the target current value.

2. A superconducting electromagnet that generates a magnetic field by passing a current through a coil. A control unit for controlling a current value of the coil, the control unit performs control to increase a current value from the predetermined current value to the target current value when a predetermined current value of the coil is lower than the target current value, a superconducting electromagnet that, when the predetermined current value is equal to or greater than the target current value, controls the current value to be lowered from the predetermined current value to a format current value that is lower than the target current value, and then to be raised to the target current value.

3. A superconducting electromagnet that generates a magnetic field by passing a current through a coil. A control unit for controlling a current value of the coil, The control unit estimates a magnetic field caused by magnetization of the coil that occurs with a change in current value, and passes a current value to the coil that is shifted from the target current value by an amount corresponding to the magnetic field caused by the magnetization of the coil.

4. 3. The superconducting electromagnet according to claim 1, wherein the control unit keeps a current value of the coil constant at the format current value for a predetermined time.

5. 5. The superconducting electromagnet according to claim 1, wherein the control unit performs control in a first sweep control of the current value to increase the current value to a maximum current value that can be obtained in a second or subsequent sweep control.

6. A particle accelerator comprising the superconducting electromagnet according to any one of claims 1 to 5, for accelerating particles to generate a particle beam.

7. A particle beam therapy system comprising the particle accelerator according to claim 6, for performing therapy using a particle beam generated by the particle accelerator.

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