JP7589094B2 - PARTICLE BEAM THERAPY APPARATUS AND METHOD FOR CONTROLLING PARTICLE BEAM THERAPY APPARATUS - Google Patents

Description

The present invention relates to an electromagnet device for beam transport, and in particular to a magnet configuration and operation method with a magnetic field adjustment function.

Particle beam therapy devices are known that accelerate heavy particles such as protons and carbon in an accelerator to form a particle beam, which is then irradiated onto a patient's tumor or other target. In particle beam therapy devices, the beam is accelerated by the accelerator, and the particle beam emitted from the accelerator is transported to an irradiation nozzle while being deflected at required locations by a transport system, and the beam is then irradiated towards the patient from the irradiation nozzle. The accelerator and transport system use numerous electromagnet devices that apply a highly precise magnetic field to the beam.

A commonly used irradiation method in particle beam therapy equipment is to scan a finely focused beam to trace the shape of the tumor (scanning irradiation method), and to scan the beam, a scanning electromagnet that can change the magnetic field at high speed is used.

In recent years, particle beam therapy equipment equipped with a rotating gantry that irradiates a patient with a heavy particle beam, such as heavy carbon ions, from any angle of 360 degrees is beginning to be put into practical use. In order to bend a heavy particle beam in a small radius, a magnetic field stronger than that of a proton is required, so a superconducting bending magnet is used.

The magnetic fields used in accelerators include a dipole magnetic field (deflection magnetic field) that bends the beam, a quadrupole magnetic field that converges and diverges the beam, and a sextupole magnetic field that corrects the chromatic aberration of the beam. In accelerators, the arrangement of these deflection magnets, quadrupole magnets, etc. is determined by optical design to satisfy the specifications of a given beam, and these magnets must generate accurate magnetic fields. For example, in deflection magnets for beam transportation, the error magnetic field is required to be on the order of 1×10 ^-4 or less relative to the strength of the deflection magnetic field.

One example of a superconducting magnet device for beam transport is described in Patent Document 1. In this invention, four or more flat concentrated winding coils are arranged at predetermined positions around the beam duct for beam transport, thereby obtaining a deflection magnetic field distribution with low distortion required for beam transport.

Patent No. 6588849

Electromagnet devices for beam transport require a highly accurate magnetic field. In Patent Document 1, four or more coils with flat concentrated windings are arranged to form a magnetic field. The configuration of Patent Document 1, which combines discrete magnetomotive force sources, can generate an error magnetic field due to errors in the winding shape of each discrete coil and their installation errors (assembly errors) compared to a cosine theta-wound coil, which is an ideal winding method for generating a deflection magnetic field.

The error magnetic field can be minimized by improving the manufacturing precision of the coil; however, in a magnet for beam transport, the error magnetic field must be kept below 1×10 ⁻⁴ of the main magnetic field strength, and it is difficult to meet this requirement through the manufacturing precision of the coil alone.

For error magnetic fields that cannot be controlled during manufacturing, it is necessary to provide a means for correcting the magnetic field, and trim coils (electromagnets for magnetic field correction) are placed. Specifically, the amount of error magnetic field that is expected to occur due to manufacturing errors is estimated, and trim coils are placed to compensate for the error magnetic field. For example, the error magnetic field is decomposed into multi-pole components, and trim coils are installed to generate each of the decomposed multi-pole magnetic field components.

However, since the strength of a multi-pole magnetic field drops sharply as the magnetomotive force source moves away from the center of the magnetic field, it is desirable to place the trim coil toward the center of the magnetic field (inside the main magnetic field magnet). For this reason, space is secured around the beam duct for placing the trim coil, and the main magnetic field magnet is placed outside of that. If the main magnetic field magnet is moved away from the beam duct, the magnetic field strength required for beam transport decreases accordingly, so the magnetomotive force of the magnet needs to be increased. In addition, a power source is required to operate the trim coil. This causes the problem of the device becoming larger in size.

The object of the present invention is to provide a beam transport electromagnet device with a simple configuration that maintains a highly accurate spatial distribution of the magnetic field.

To achieve the above object, the electromagnet device of the present invention has multiple coils arranged so that at least a portion of the magnetic fields they generate overlap, and multiple power sources. By adjusting the amount of current flowing through the multiple coils, unnecessary error magnetic fields are suppressed while maintaining the magnetic field strength required for beam transport.

Specifically, the electromagnet device of the present invention has four coils arranged to surround the beam path, at least two power supplies that supply current to the coils, and a power supply controller that controls the two power supplies. The power supply controller includes a calculation unit that calculates a supply current to the four coils that forms a main magnetic field that crosses the beam path in a diametrical direction at a predetermined angle by superimposing the magnetic fields generated by the four coils, while maintaining the direction and strength of the main magnetic field and suppressing a multipole magnetic field of four or more poles that is generated in association with the main magnetic field, and a supply control unit that causes the power supplies to supply the calculated supply current to the four coils.

The present invention uses a simple configuration to compensate for error magnetic fields caused by manufacturing errors and generate a highly accurate magnetic field in the space through which the beam passes.

1 is a block diagram showing the overall configuration of a particle beam therapy system according to an embodiment; FIG. 2 is a side view of a bending magnet 203 for a heavy particle beam therapy apparatus to which the electromagnetic device according to the embodiment is applied. A-A' cross-sectional view of Figure 2. 3 is a perspective view showing the arrangement of superconducting coils 101 to 104 in a bending magnet 203 in FIG. 2. FIG. 4 is a diagram showing an example of a table showing currents C1 to C4 set in the superconducting coils 101 to 104 of the electromagnetic device according to the embodiment. 10 is a flowchart showing a control operation of a power supply controller 620 according to the embodiment. FIG. 4 is an explanatory diagram showing how to define a coordinate system for a multipole component magnetic field according to the embodiment. FIG. 2 is an explanatory diagram showing the arrangement of superconducting coils and the direction of current in the embodiment. 5 is a graph showing the angular position dependency of the intensity of a normal magnetic field component of a magnetomotive force source of an electromagnetic device according to an embodiment.

An embodiment of the present invention will be described below. However, the present invention is not limited to the embodiment described here, and appropriate combinations and improvements are possible without departing from the spirit and scope of the present invention.
<Particle beam therapy equipment>
First, a particle beam therapy system using the electromagnet device of this embodiment as an electromagnet device for beam transportation will be described with reference to the drawings.

The overall configuration of a particle beam therapy device is shown in Figure 1. As shown in Figure 1, the particle beam therapy device is composed of an accelerator 500 that forms a particle beam, a beam transport system 200 that transports the particle beam, an irradiation device 300 that irradiates a patient 301 with the particle beam, a bed 302 on which the patient 301 is placed, a rotating gantry 400, and a control system 600.

In the example shown in FIG. 1, the accelerator 500 is configured to include a linac 501, which is a pre-stage accelerator, and a synchrotron 502, which further accelerates the particle beam formed by the linac 501, but it is not limited to this configuration and it is of course possible to use a cyclotron or the like.

The beam transport system 200 includes a duct 204 that transports the particle beam formed by the accelerator 500, deflection magnets 201, 202, and 203 that apply a deflection magnetic field to the particle beam (also called a beam) in the duct 204, a quadrupole magnet 211 that applies a magnetic field to focus the particle beam, and an orbit correction magnet 212.

A rotary coupling part 214 is provided in the duct 204, and the tip side of the rotary coupling part 214 is configured to be rotatable around the axis 1213 by the mechanism of the rotary coupling part 214 relative to a fixed part closer to the accelerator 500 than the rotary coupling part 214.

The rotating gantry 400 is arranged around the bed 302 and includes a ring-shaped structure (rotating ring) 401 that rotates around the rotation axis 1213, and a drive unit 402 that drives the rotation of the rotating ring.

The beam transport system 200 and the irradiation device 300, which are located further distal than the rotary connection 214, are mounted on the rotating ring 401 and rotate about the rotation axis 1213 as the rotating ring 401 rotates.

The irradiation device 300 is equipped with a scanning electromagnet 303 for moving the beam irradiation position according to the shape of the affected area.

The beam emitted from the accelerator 500 is bent and transported along the beam transport path by the deflection magnets 201, 202, and 203 while passing through the beam transport system 200, and the shape and position of the beam are adjusted by the quadrupole magnet 211 and the trajectory correction magnet 212. In addition, the beam is deflected in a direction perpendicular to the beam axis direction 216 by the scanning electromagnet 303 installed in the irradiation device 300, so that the affected area is irradiated in a manner that traces the area.

The strength of the magnetic field generated by these deflection magnets 201-203, quadrupole magnet 211, and trajectory correction magnet 212 is changed according to the energy of the beam. The deflection magnet 203 will be explained in detail later.

The energy of the beam may be changed in the accelerator 500, or a degrader may be placed along the transport system 200 to change the energy by attenuating it to the desired amount.

The control system 600 includes a control device 610 that controls the entire device, and a power supply controller 620. The control device 610 controls the intensity of the beam emitted by the accelerator 500 and the drive unit 402 of the rotating gantry 400. The power supply controller 620 controls the power supplies of the deflection magnets 201-203.

<Configuration example of bending electromagnet 203>
2, 3 and 4, the deflection magnet 203 will be described below. The deflection magnet 203 has three electromagnets 1230 arranged along the duct 204, each of which deflects a particle beam by 30 degrees, and is configured to deflect the particle beam by 90 degrees. The deflection magnets 201 and 202 also differ from the deflection magnet 203 in the number of electromagnets 1230 that constitute them, but have the same basic configuration.

Figure 2 is a side view of the deflection magnet 203, and Figure 3 is a cross-sectional view taken along line A-A' of Figure 2. Figure 4 is a perspective view showing the arrangement of superconducting coils in the deflection magnet 203. The deflection magnet 203 comprises an insulating container 110, a refrigerator 120, and three electromagnets 1230 arranged inside the insulating container 110. Each of the three electromagnets 1230 includes at least four superconducting coils 101, 102, 103, and 104 arranged around the beam path (duct 204), at least two or more (four in this example) power supplies 41-44 that supply current to the superconducting coils 101-104, and a power supply controller 620 that controls the power supplies 41-44.

Leads 31a, 31b, 32a, 32b, 33a, 33b, 34a, and 34b are connected to the superconducting coils 101 to 104, respectively, and are connected to power sources 41 to 44 via leads 31a and the like. Leads 31a and the like pass through the insulating container 110.

As shown in Figures 2 to 4, superconducting coils 101, 102, 103, and 104 are each flat coils, and superconducting coils 103 and 104 are arranged so that their main planes face each other across the axial direction 216 of the beam. Superconducting coils 101 and 102 are arranged so that their side faces face each other across the axial direction 216 of the beam.

Specifically, the superconducting coils 101 to 104 are wound in a flat shape having a major axis and a minor axis in the main plane, and are arranged so that the major axis is along the axial direction of the duct 204 (axial direction of the beam 216). The major axis direction is curved so as to follow the curvature of the duct 204. The superconducting coils 101 to 104 are arranged in four directions around the duct 204 so that their main planes are parallel to the plane in which the particle beam is deflected. The superconducting coils 101 and 102 are arranged so that their main planes face each other across the axial direction 216 of the beam passing through the duct 204, and the superconducting coils 103 and 104 are arranged so that their side faces face each other across the axial direction 216 of the beam.

The insulated container 110 comprises a vacuum container 111 and a radiation shield 112 arranged inside the vacuum container 111. The refrigerator 120 is mounted on the insulated container 110. For example, the first stage (e.g., 40k) of the refrigerator 120 is thermally connected to the radiation shield and cools the radiation shield 112 to the temperature of the first stage. The second stage (e.g., 4k) is thermally connected to the superconducting coils 101-104 by a copper mesh or the like (not shown) and cools the superconducting coils 101-104 by conduction cooling.

The superconducting coils 101 to 104 constituting the electromagnet 1230 are electrically connected in series with the superconducting coils 101 to 104 at the corresponding positions constituting the other electromagnets 1230 in the thermal insulation container 110. The four sets of superconducting coils connected in series are connected to the ends of the current leads 41a and 41b, 42a and 42b, 43a and 43b, and 44a and 44b installed in the thermal insulation container 110, as shown in FIG. 3. The other ends of the current leads 41a and 41b, 42a and 42b, 43a and 43b, and 44a and 44b are drawn from the inside of the thermal insulation container 110 to the outside and connected to the power sources 41, 42, 43, and 44 installed outside the thermal insulation container 110. As a result, the superconducting coils 101, 102, 103, and 104 are operated by the currents supplied from the power sources 41, 42, 43, and 44, respectively.

The superconducting coils 101 to 104 superimpose the magnetic fields they generate to form a main magnetic field 213 with a predetermined intensity distribution, which is applied to the particle beam transported through the duct 204. Specifically, the main magnetic field 213 is applied in a direction that crosses the traveling direction 216 of the particle beam at a predetermined angle (here, perpendicular). This deflects the particle beam in a direction 215 that is perpendicular to the magnetic field direction 213 and the traveling direction 216 of the particle beam.

2, the power supply controller 620 includes a calculation unit 621 that calculates the supply current to the four superconducting coils 101-104, and a supply control unit 622 that causes the power supplies 41-44 to supply the calculated supply current to each of the four superconducting coils 101-104. The superposition of the magnetic fields generated by the four superconducting coils 101-104 forms a main magnetic field 213 that crosses the beam path (duct 204) in the diametric direction at a predetermined angle, and the calculation unit 621 is configured to calculate a supply current that suppresses a multipole magnetic field of four or more poles that is generated by the main magnetic field 213 while maintaining the direction and strength of the main magnetic field 213.

The four or more pole multipole magnetic field changes with changes in beam intensity and with changes in the positional relationship between the deflection magnet 203 and other components due to the rotation of the rotating gantry 400. Therefore, the calculation unit 621 calculates the supply current value to the superconducting coils 101-104 that can suppress the four or more pole multipole magnetic field while maintaining the direction and intensity of the main magnetic field 213 based on the beam intensity and the rotation angle of the rotating gantry 400. In this embodiment, the power supply controller receives the beam intensity and the rotation angle of the rotating gantry 400 from the control device 610, but the beam intensity and rotation angle may also be measured by a sensor or the like and used by the calculation unit 621 to calculate the supply current value.

Specifically, the calculation unit 621 calculates the currents to be supplied to the four coils using a predetermined formula or table. As an example of the table, one that shows the correspondence between the rotation angle of the rotating gantry 400, the beam intensity, and the current values to be supplied to the superconducting coils (C1 to C4) 101 to 104, as shown in FIG. 5, is used. The current values to be supplied in the table are determined in advance by calculation or experiment.

In addition, when the calculation unit 621 calculates the supply current value based on a formula, the calculation unit 621 can be configured to calculate the supply current value in advance by calculation or experiment for a combination of two or more representative values of the rotation angle of the rotating gantry 400 and two or more representative values of the beam intensity, and to use these values to calculate the supply current value corresponding to the actual beam intensity and the rotation angle of the rotating gantry 400 by interpolation calculation, extrapolation calculation, etc.

<Control operation of power supplies 41 to 44 by power supply controller 620>
The control of the deflection magnet 203 by the power supply controller 620 will be described with reference to the flow chart of FIG.

The power supply controller 620 can be configured by a computer or the like equipped with a processor such as a CPU (Central Processing Unit) or a GPU (Graphics Processing Unit), and a memory 623. In addition to the table 624, programs are pre-stored in the memory 623. The CPU realizes the functions of the calculation unit 621 and the supply control unit 622 by reading and executing the programs stored in the memory 623.

The power supply controller 620 can be configured in whole or in part by hardware. For example, a circuit can be designed to realize the functions of the calculation unit 621 and the supply unit 622 using a custom IC such as an ASIC (Application Specific Integrated Circuit) or a programmable IC such as an FPGA (Field-Programmable Gate Array).

When the calculation unit 621 receives a signal from the control device 610 indicating that beam extraction has started (step 61), it acquires the current gantry rotation angle and beam intensity from the control device 610 (step 62). The calculation unit 621 acquires the gantry rotation angle by acquiring a control signal that indicates the rotation angle output by the control device 610 to the driver 402 of the rotating gantry 400. The calculation unit 621 acquires the beam intensity by acquiring a control signal that the control device 610 uses to set the beam intensity for the accelerator 500.

The calculation unit 621 refers to table 624 in memory 623 and determines the current values C1, C2, C3, and C4 corresponding to the gantry rotation angle and beam intensity acquired in step 62 (step 63). The calculation unit 621 sets the current values C1, C2, C3, and C4 in the supply control unit 622 (step 64). As a result, the supply control unit 622 controls the output currents of the power supplies 41 to 44 to be the current values C1, C2, C3, and C4.

This forms a main magnetic field 213 that crosses the beam path (duct 204) in the diametric direction at a predetermined angle. In addition, the current values C1, C2, C3, and C4 change in response to changes in the rotation angle of the rotating gantry 400 and the beam intensity, thereby suppressing the four- or more-pole multipole magnetic field that is generated by the main magnetic field 213 while maintaining the direction and intensity of the main magnetic field 213.

The calculation unit 621 determines whether the control device 610 has finished beam extraction, and if extraction is continuing, returns to step 62 and continues current control. When beam extraction has ended, the power supply controller 610 also ends current control of the power supplies 41 to 44 (step 65).

Effects of the embodiment
In this manner, in the deflection magnet 203 of this embodiment, the coil that generates the magnetic field 213 required for deflecting the particle beam is divided into superconducting coils 101 to 104, and the currents supplied independently from the power supplies 41 to 44 are controlled to current values calculated by the power supply controller 621. This makes it possible to reduce the multipole magnetic field while forming the main magnetic field 213 in the desired direction in the duct 204 by superimposing the magnetic fields created by the superconducting coils 101 to 104.

In other words, the superconducting coils 101-104 are positioned to generate a characteristic multi-pole expanded magnetic field within the duct 204, and therefore it is possible to mix a multi-pole magnetic field into the magnetic field 213 by adjusting the magnetomotive force of those coils. In other words, the unwanted multi-pole magnetic field generated by magnet manufacturing errors can be canceled by adjusting the magnetomotive force.

This makes it possible to provide an electromagnet for beam transport that can compensate for manufacturing errors, changes in beam intensity, and unnecessary multipole magnetic fields generated by the rotation of the rotating gantry 400, without the need to install magnetic field correction means such as trim coils.

<Calculation method of current values C1, C2, C3, C4 of superelectric coils 101 to 104>
As described above, the main magnetic field 213 needs to be kept constant according to the energy of the beam. In this embodiment, the beam is deflected by the main magnetic field 213, which is a combination of magnetic fields generated by the superconducting coils 101 to 104 excited by the four power supplies (41, 42, 43, 44), and the current values C1, C2, C3, and C4 of the power supplies (41, 42, 43, 44) are appropriately controlled so that the magnetic field 213 is at a predetermined value.

Magnetic field stability is required for electromagnets used in beam transport. If the strength of the deflection magnetic field in a magnet with a deflection radius of 2.4 m differs from the design value by 1×10 ⁻⁴ , the beam position at the exit of the deflection magnet will shift by 0.24 mm. For this reason, magnets for rotating gantries in particle beam therapy devices are required to have a main magnetic field stability better than 1×10 ⁻⁴ .

In magnets used for beam transport, the strength of the main magnetic field must be changed according to the beam intensity, and in magnets for heavy ion radiotherapy devices, the magnetic field strength is changed between 20% and 100% of the rated magnetic field strength. In air-core magnets, the magnetic field uniformity changes slightly due to deformation of the coil caused by electromagnetic force, and in magnets with magnetic materials such as an iron core and magnetic shield, the magnetic field uniformity also changes (multi-pole magnetic field components are generated) due to changes in the magnetization of the magnetic material. In the case of iron cryostats (magnetic shields), due to symmetry, mainly hexapole magnetic field components are generated, and their strength is on the order of 1× ^10-4 relative to the integrated strength of the main magnetic field, requiring correction.

At the most downstream of the rotating gantry is a bending magnet that bends the beam by 90 degrees, which consists of three superconducting magnets. The superconducting magnets are supported insulated in an insulated vacuum vessel (cryostat), but as the rotating gantry rotates, the weight of the magnets causes slight positional deviation and rotation. The quadrupole and hexapole magnetic fields generated by a 0.1 degree magnet rotation are about 1.5 x ^10-5, but since a magnetic field precision of ^10-4 is required, this amount cannot be ignored.

According to the current control of this embodiment, the intensity of the quadrupole magnetic field, sextupole magnetic field, and octupole magnetic field can each be made 1×10 ⁻⁵ or less while maintaining the intensity of the main magnetic field at the intensity required for beam deflection.

The current values of C1, C2, C3, and C4 can be calculated as follows. When a unit current (density) is passed through each coil, the magnetic field that each coil creates on the beam path is calculated in advance and expanded into a multipole. Then, the multipole expanded magnetic field is integrated along the path to obtain integrated multipole magnetic field data. Furthermore, current is passed through the individual magnet before it is installed in the rotating gantry housing to measure the magnetic field distribution inside the beam duct. By comparing the two, data on the integrated multipole magnetic field strength when a unit current is passed through each coil used for magnetic field correction (sensitivity data) is generated. Using this sensitivity data, the current distribution that corrects the observed magnetic field distribution or beam spot shape is determined by solving an inverse problem, and the current values of C1, C2, C3, and C4 are calculated.

After the magnets are assembled into the rotating gantry housing, adjustments are made so that the beam passes through to a certain extent, and then a current correction table is created. The beam energy is set to a certain value, the rotating gantry is fixed at a certain angular position, and the beam is passed through. The beam shape is observed with a beam monitor, and the current value is fine-tuned to correct the residual error multipole magnetic field estimated from the shape. This process is performed for representative gantry angles and beam energy intensities to build a correction current table. For intermediate gantry positions and energy states, table interpolation processing is performed to determine the correction current amount.

<Multipole magnetic field and coil arrangement>
(Magnetic field design)
Below, we explain the magnetic field expressions and magnetic field design concepts for the design of electromagnets for beam transport. Even for electromagnets that transport beams in an arc, we first design the magnetic field using an infinite linear magnetomotive force source, and then use that as a base to create a three-dimensional design. We explain the magnetic field created by an infinite linear magnetomotive force (current).

Figure 7 is an explanatory diagram of how to set up a coordinate system to explain a multi-pole component magnetic field. Here, we consider a coordinate system like the one shown in Figure 7. The magnetomotive force source is a linear current I, which flows perpendicular to the paper surface from the front to the back. The direction of the magnetic field is taken to be the z-axis direction. The distance from the origin of the coordinate system to the position of the current is given by f, and the angle it makes with the x-axis is given by φ. Similarly, the position of the magnetic field evaluation point is given by r and θ.

と、原点からの距離ｒのべき乗で展開した表式で書くことができる。ここでｎは展開次数である。 At this time, the magnetic field Bz(r, θ) around the origin is

This can be written as an expression expanded in powers of the distance r from the origin, where n is the order of expansion.

のようにＢｎ、ｎｌ（ｒ、θ）とＢｎ、ｓｗ（ｒ、θ）の２成分の和の形で表現する。 Bn(r,θ) for the magnetic field of order n

It is expressed as the sum of two components, Bn,nl(r,θ) and Bn,sw(r,θ), as shown below.

の表式で書き表され、それぞれをノーマル２（ｎ＋１）極磁場、スキュー２（ｎ＋１）極磁場と呼ぶことにする。ｎ＝０の場合にはスキュー磁場はゼロになるからノーマル磁場しか存在しない。ｎ＝０の磁場がビーム輸送に使われる偏向磁場であり、ｎ＝１のノーマル４極磁場がビームの収束・発散に利用される。 Bn,nl(r,θ) and Bn,sw(r,θ) are

These are called the normal 2(n+1) pole magnetic field and the skewed 2(n+1) pole magnetic field, respectively. When n=0, the skew magnetic field is zero, so only the normal magnetic field exists. The magnetic field with n=0 is the deflection magnetic field used for beam transport, and the normal quadrupole magnetic field with n=1 is used for beam convergence and divergence.

A uniform magnetic field is required to deflect the beam flight path with a constant curvature, and the deflection electromagnet is designed to have a magnetic field that basically only has a uniform magnetic field of order n=0 (a dipole magnetic field) and zeros out the magnetic field components of other orders (also called multipole component magnetic fields).

Cosine theta winding is often used as a coil winding method (current arrangement method) to obtain a two-pole magnetic field. This is an arrangement method in which the current intensity (current distribution) is distributed in a cos(θ) shape with respect to the angle θ from the x-axis in the coordinate system in Figure 7.

In Equation 3 and Equation 4, the dependence of the multipole expansion magnetic field strength on the angular position of the current source is the cos[(n+1)φ] and sin[(n+1)φ] parts, but if the current strength distribution is given a cos(φ) distribution and integrated, all terms other than n=0 become zero, which shows that only a two-pole magnetic field can be obtained with a cosine theta winding.

In the present invention and/or this embodiment, we aim to realize a deflection magnet using a system in which coils with rectangular cross sections are discretely arranged, rather than a distributed winding coil in which the current density distribution of the so-called cosine theta winding changes continuously.

Even in this case, as in the case of cosine theta winding, the coils must be arranged so that the multipole magnetic field components other than n=0 are zero (or a value close to zero, a negligible value, etc.).

When the current magnetomotive force sources are arranged symmetrically (with respect to the x-axis) in a two-dimensional plane as in this embodiment, all of the magnetic field skew components are cancelled out as can be seen from Equation 4, allowing magnets to be designed with a focus only on the normal components. Furthermore, by arranging the magnetomotive force sources antisymmetrically (with respect to the z-axis), the magnetic field components where n is an odd number among the magnetic field components of the normal components are cancelled out. Antisymmetric here means that the cross-sectional shape of the coil is symmetrical with respect to the z-axis, and the current flows in opposite directions. In Figure 8, a circle with a dot in the middle indicates that the current is flowing from the back of the paper to the front, and a circle with a cross indicates that the current is flowing toward the back of the paper.

Therefore, the only magnetic fields that need to be considered in the design are those where n is an even order, and the magnetic field design involves canceling out the 6-pole magnetic field where n=2, the 10-pole magnetic field where n=4, the 14-pole magnetic field where n=6, etc., other than the 2-pole magnetic field where n=0 that should be retained. The strength of the n-th order magnetic field decreases as the nth power of (r/f) the higher the order, so there is no need to cancel out magnetic fields of infinitely higher orders, and the magnetic fields that need to be canceled are determined by the magnetic field precision required in the area through which the beam passes.

(Coil arrangement principle)
9 is a graph showing the angular position dependence of the normal magnetic field component strength of the magnetomotive force source. The angular dependence of the normal magnetic field component is cos[(n+1)φ], and is shown for n=0, 2, and 4.

Since the n=0th order magnetic field has a cos[φ] dependency on the angular position of the current, it is efficient to place the current (coil) as close to the x-axis as possible, and the main positive magnetomotive force is placed in the area where cos[φ] is between 0.5 and 1, and the main negative magnetomotive force is placed in the area where cos[φ] is between -1 and -0.5. Considering the first quadrant where x>0, z>0, the positive magnetomotive force is placed within a range of 60 degrees from the x-axis to generate a deflecting magnetic field. (In other words, the reverse magnetomotive force source placed on the outside to reduce leakage magnetic field is not taken into consideration.)

Next, consider the n=2 order magnetic field. In the first quadrant, in the range from 0 to 60 degrees, cos[3φ] changes sign at φ=30 degrees. Since only positive magnetomotive forces are placed in the range from 0 to 60 degrees, the magnetomotive force source is placed across this φ=30 degree line to cancel the n=2 order magnetic field.

Therefore, in order to realize a deflection magnet with the minimum number of four coils as in this embodiment, as shown in Figure 8, coils of the same shape are placed in the first and second low-angle regions bounded by the lines φ = ±30 degrees and ±150 degrees, while taking into consideration top-bottom symmetry and left-right antisymmetry.

<Normal 4-pole and 8-pole magnetic field adjustment>
The dependence of the current source of the normal quadrupole magnetic field of n=1 on the angular position is cos(2φ), and since a set of anti-bilaterally symmetric magnetomotive force sources (with magnetomotive force whose sign is opposite and whose position is 180 degrees-φ) does not have an output, the normal quadrupole magnetic field cannot be adjusted by adjusting the currents in the anti-bilaterally symmetric coils (103, 104) located in the first and second high angle regions. A normal quadrupole magnetic field can be generated by adjusting the amount of current in the coils (101, 102) located in the first and second low angle regions.

The dependence of the current source of the normal octupole magnetic field with n=3 on the angular position is cos(4φ), and since a set of anti-symmetric magnetomotive force sources (with magnetomotive force of opposite sign and position 180 degrees -φ) does not have an output, the normal octupole magnetic field cannot be adjusted by adjusting the currents in the symmetric coils (103, 104) placed in the first and second high angle regions. A normal octupole magnetic field can be generated by adjusting the amount of current in the coils (101, 102) placed in the first and second low angle regions.

There is insufficient degree of freedom to independently adjust the magnetic field components for n=1 and n=3, but generally, a quadrupole magnet is placed separately in the electromagnet device for beam transport to adjust the beam shape, and by adjusting the output of this external quadrupole magnet, magnetic field correction for the normal quadrupole and normal octupole magnetic fields is possible.

For magnetic field components with n=5 or more, the magnetic field strength generated by the magnet itself is small, so the amount of error magnetic field is also small and is not subject to magnetic field correction. Furthermore, the slight magnetic field generated by magnetomotive force adjustment does not affect beam transport and can be ignored.

<Normal 6-pole and 10-pole magnetic field adjustment>
The dependence of the current source of the normal sextupole magnetic field of n=2 on the angular position is cos(3φ), and the output is also available in a set of left-right symmetric magnetomotive force sources (where the sign of the magnetomotive force is opposite and the position is 180 degrees-φ). Therefore, a normal sextupole magnetic field can be generated by adjusting the currents in all the coils (101, 102) in the first and second high angle regions (103, 104) and the first and second low angle regions.

The dependence of the current source on the angular position of the normal 10-pole magnetic field for n=4 is cos(5φ), and similar to the normal 6-pole magnetic field, a normal 10-pole magnetic field can be generated by adjusting the currents in all coils (101-104) in the first and second high angle regions and the first and second low angle regions.

There is insufficient degree of freedom to independently adjust the magnetic field components for n=2 and n=4, but for magnetic field components for n=4 and above, the magnetic field strength generated by the magnet itself is small, so the amount of error magnetic field is also small and is not subject to magnetic field correction, and the slight magnetic field generated by magnetomotive force adjustment does not affect beam transport and can be ignored.

<Adjustment of skewed 4-, 6-, and 8-pole magnetic fields>
The dependence of the current source of the skewed quadrupole magnetic field of n=1 on the angular position is sin(2φ), and since a magnetomotive force source symmetrical above and below (with the same sign of magnetomotive force and located at -φ degrees) does not have an output, the skewed quadrupole magnetic field cannot be adjusted by adjusting the current of the coils (101, 102) symmetrical with respect to the equatorial plane and located in the first and second low angle regions. The skewed quadrupole magnetic field can be generated by adjusting the amount of current of the coils (103, 104) located in the first and second high angle regions.

The dependence of the current source of the skewed sextupole magnetic field with n=2 on the angular position is sin(3φ), and since a vertically symmetrical magnetomotive force source (with the same sign of magnetomotive force and located at -φ degrees) does not have its output, the skewed sextupole magnetic field cannot be adjusted by adjusting the current of the coils (101, 102) that are symmetrical with respect to the equatorial plane and placed in the first and second low-angle regions. Furthermore, since a set of left-right symmetrical magnetomotive force sources (with opposite signs of magnetomotive force and located at 180 degrees -φ) does not have its output, the skewed sextupole magnetic field cannot be adjusted by adjusting the current of the left-right symmetrical coils (103, 104) that are placed in the first and second high-angle regions. Therefore, the skewed sextupole magnetic field cannot be corrected in this embodiment, which has a left-right symmetrical and vertically symmetrical magnetomotive force arrangement.

The dependence of the current source of the skewed 8-pole magnetic field with n=3 on the angular position is sin(4φ), and since a magnetomotive force source that is symmetrical above and below (with the same sign of magnetomotive force and located at -φ degrees) does not have an output, the skewed 8-pole magnetic field cannot be adjusted by adjusting the current of the coils (101, 102) that are symmetrical with respect to the equatorial plane and located in the first and second low-angle regions. The coils (103, 104) that are symmetrical left and right and located in the first and second high-angle regions originally have an output for the skewed 8-pole magnetic field, but in the magnet of this embodiment, the coils (103, 104) that are located in the first and second high-angle regions are symmetrical above and below and symmetrical left and right to the coils located in the range of 30 degrees to 60 degrees in the first quadrant. The output of the skewed 8-pole magnetic field is zero when the source is located at 45 degrees, so in this embodiment, there is almost no ability to correct the skewed 8-pole magnetic field.

As such, the magnet of this embodiment does not essentially have the ability to correct magnetic fields other than a four-pole magnetic field with respect to skew components. However, skewed multi-pole magnetic fields are generated by magnetomotive force sources that are asymmetric in the top and bottom, and by manufacturing magnets with strict control over top and bottom symmetry through pairing, the generation of error magnetic fields from skew components of n=2 or more can be suppressed and does not pose a problem.

<Summary of magnetic field correction>
In this embodiment, the following magnetic field correction of multipole magnetic field components can be performed.
1) Normal quadrupole magnetic field 2) Normal hexapole magnetic field 3) Normal octapole magnetic field 4) Skewed quadrupole magnetic field For the normal quadrupole magnetic field and normal octapole magnetic field, a corrective magnetic field can be generated by making a difference in the magnetomotive forces of the coils (101, 102) arranged in the first and second low-angle regions. However, due to a lack of freedom, they can be independently adjusted by utilizing an externally installed quadrupole magnet that adjusts the beam shape and the normal quadrupole magnetic field.

For a normal 6-pole magnetic field, a difference is made between the total magnetomotive force of the coils (101, 102) arranged in the first and second low angle regions and the total magnetomotive force of the coils (103, 104) arranged in the first and second high angle regions, and the magnetomotive forces of the coils arranged in the first and second high angle regions are changed by the same amount, thereby making it possible to adjust the normal 6-pole magnetic field independently.

The skewed quadrupole magnetic field can be adjusted independently by creating a difference in the magnetomotive forces of the coils (103, 104) arranged in the first and second high angle regions.

<Skewed quadrupole auxiliary magnetic field correction means>
The error magnetic field can be corrected independently for the four multipole magnetic field components, and the magnetic field can be corrected by adding a correction current, which is a superposition of the currents required to generate each multipole correction magnetic field, to each coil. However, there is a limit to the current that can be passed through the superconducting coils, and the magnetic field correction capability is limited by the amount of correction current that can be added.

In particular, for the coils (103, 104) located in the first and second high angle regions that generate the corrective magnetic fields of the skewed quadrupole magnetic field and normal hexapole magnetic field, the load factor of these coils is large due to the magnet design, so it is not possible to add a large correction current.

In order to maintain a large correction capability for the normal 6-pole magnetic field, it is necessary to reduce the allocation of the adjustment capability for the skewed quadrupole magnetic field by adjusting the magnetomotive force. To achieve this, it is effective to utilize an auxiliary skewed quadrupole magnetic field correction means. A normal conducting quadrupole magnet 211 for correcting the beam shape is installed outside the superconducting bending magnet. This quadrupole magnet is configured to generate a normal quadrupole magnetic field, and is also used to correct the error magnetic field of the normal quadrupole magnetic field, but it is also possible to generate a skewed quadrupole magnetic field by slightly rotating and installing this magnet.

The strength of the skewed quadrupole magnetic field generated by the superconducting deflection magnet is measured in advance, and the quadrupole magnet 211 is rotated and installed so as to roughly cancel this magnetic field. Since the amount of skewed quadrupole magnetic field that needs to be compensated for by adjusting the magnetomotive force of the superconducting deflection magnet is reduced, it becomes possible to flow more additional correction current to correct the normal hexapole magnetic field.

As described above, it is possible to compensate for normal 4-pole, 6-pole, 8-pole magnetic fields and skewed 4-pole magnetic fields that arise due to manufacturing errors by adjusting the magnetomotive force of the deflection magnet without installing any additional correction magnetic field generating means.

The electromagnet device of the present invention is a superconducting magnet for beam transport and can be applied to various devices that require beam transport. For example, it is useful as an electromagnet for a particle beam therapy device, and can improve the performance and reduce the size of the particle beam therapy device.

20, 21... Coil, 20m, 21m... Magnetic field,
31a, 31b, 32a, 32b, 33a, 33b, 34a, 34b...leads,
41, 42, 43, 44...Power supply (excitation power supply)
101 to 104: coil; 110: heat insulating container; 120: refrigerator; 200: beam transport system;
201, 202, 203... deflection magnet, 204... duct, 211... quadrupole magnet, 212... trajectory correction magnet, 214... rotation coupling part, 301... patient, 300... inspection device, 302... bed, 303... scanning electromagnet, 400... rotating gantry, 500... accelerator, 501... linac, 502... synchrotron, 1213... rotation axis, 1230... electromagnet

Claims (8)

A particle beam therapy device comprising: an accelerator that emits a particle beam; a transport system that transports the beam; an irradiation device that irradiates a subject with the beam; and a control device that controls the accelerator or the irradiation device,
the transport system includes an electromagnetic device that deflects the beam and a quadrupole magnet that adjusts the beam shape;
The quadrupole magnet has an output adjusted to generate a normal quadrupole magnetic field and is slightly rotated to generate a skew quadrupole magnetic field;
the electromagnetic device includes at least four coils arranged around a beam path, at least two power supplies that supply current to the coils, and a power supply controller that controls the two power supplies;
The power supply controller is a particle beam therapy device characterized in that it comprises: a calculation unit that calculates a supply current to the four coils such that a main magnetic field that crosses the beam path at a predetermined angle is formed by superimposing the magnetic fields generated by the four coils, and a supply control unit that causes the power supply to supply the calculated supply current to the four coils while maintaining the direction and strength of the main magnetic field and suppressing a multipole magnetic field of four or more poles that is generated in association with the main magnetic field. 2. The particle beam therapy system according to claim 1, wherein the calculation unit calculates the supply current based on an intensity set for the accelerator by the control device . 2. The particle beam therapy system according to claim 1, further comprising a rotation device,
the irradiation device and the four coils are mounted on the rotation device, and the rotation device rotates the four coils about a predetermined axis while maintaining a positional relationship between the four coils;
The particle beam therapy device according to claim 1, wherein the calculation unit calculates the supply current in accordance with an angle of rotation of the rotation device. 2. The particle beam therapy system according to claim 1, wherein the calculation unit calculates the currents to be supplied to the four coils using a predetermined formula and/or table. 5. The particle beam therapy system according to claim 4, further comprising a rotation device,
the irradiation device and the four coils are mounted on the rotation device, and the rotation device rotates the four coils about a predetermined axis while maintaining a positional relationship between the four coils;
The particle beam therapy device according to claim 1, wherein the table indicates a correspondence relationship between the rotation angle of the rotation device, the intensity set by the control device to the accelerator , and the value of the supply current. 2. The particle beam therapy device according to claim 1, wherein the four coils are flat coils, and among the four coils, a first and a second coil are arranged such that their main planes face each other across an axial direction of a beam passing through the beam path, and a third and a fourth coil are arranged such that their side surfaces face each other across the axial direction of the beam . A method for controlling a particle beam therapy apparatus comprising: an accelerator for emitting a particle beam; a transport system for transporting the beam; an irradiation device for irradiating a subject with the beam; and a control device for controlling the accelerator or the irradiation device, wherein the transport system includes an electromagnet device for deflecting the beam and a quadrupole magnet for adjusting a beam shape, and the electromagnet device has at least four coils arranged around a beam path and at least two power sources for supplying current to the coils, the method comprising:
The output of the quadrupole magnet is adjusted to generate a normal quadrupole magnetic field, and a skewed quadrupole magnetic field is generated by slightly rotating the magnet.
A main magnetic field is formed by superposing the magnetic fields generated by the four coils in a diametrical direction across the beam path at a predetermined angle, and a current to be supplied to the four coils is calculated so that a multipole magnetic field having four or more poles generated in association with the main magnetic field is suppressed while maintaining the direction and strength of the main magnetic field;
A method for controlling a particle beam therapy system, comprising: supplying the calculated supply current from the power supply to the four coils. 8. A method for controlling a particle beam therapy apparatus according to claim 7, wherein the particle beam therapy apparatus further comprises a rotation device on which the irradiation device and the four coils are mounted,
A method for controlling a particle beam therapy apparatus , characterized in that the calculation calculates the supply current based on an intensity set by the control device to the accelerator and an angle of rotation of the rotation device .

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