JP2007026818A - Electromagnet forming magnetic field gradient - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electromagnet which forms a magnetic field gradient and in which the number of the power supply for multicoil can be reduced. <P>SOLUTION: This electromagnet 4a is provided with a pair of magnetic poles 6, the main coil 14 which is wound to it to form a uniform magnetic field, and the multicoil 16a which is in its inside and which forms the magnetic field gradient in the radius r direction. Furthermore, a plurality of coils 18 constituting respective multicoils 16a are mutually connected in series and pitches in the radius r direction of juxtaposed part 19 of the coil 18 connected in series are varied. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to, for example, an FFAG (Fixed Field Alternating Gradient) accelerator, an electron beam irradiation apparatus for accelerating electrons using the FFAG accelerator, an electromagnet for deflecting charged particles, and the like. More specifically, the present invention relates to a coil-type electromagnet that forms a magnetic field gradient using a multi-coil.

  One type of electromagnet that forms a magnetic field gradient in the radial direction is a coil type that forms a magnetic field gradient using a multi-coil. Conventionally proposed coil-type electromagnets form a uniform magnetic field by a main coil wound around a magnetic pole, and change the current of multiple coils arranged at the same pitch inside the magnetic field. A gradient is formed (see, for example, pages 6-7 of Non-Patent Document 1).

  An example of such an electromagnet is shown in FIGS.

  The electromagnet 4 has a pair of magnetic poles 6 facing each other with a gap 10 through which the charged particles 2 pass. The two magnetic poles 6 are connected by a yoke 8. The path through which the charged particles 2 pass is surrounded by a vacuum container (not shown) made of a non-magnetic material and kept in a vacuum atmosphere. The same applies to embodiments of the present invention described later.

  The charged particles 2 are, for example, electrons, protons, ions, and the like.

  Around each of the magnetic poles 6, a magnetic field uniform in the radius r direction, which is a direction intersecting with the passing direction of the charged particles 2 (front and back directions in FIG. 5, FIG. 2), between the upper and lower magnetic poles 6. That is, the main coils 14 formed in the gap 10 are wound respectively. The upper and lower main coils 14 are normally connected in series with each other and excited by a DC power source (main coil power source) (not shown). The same applies to embodiments of the present invention described later.

  A multi-coil 16 is provided inside each main coil 14 and in the vicinity of the surface of each magnetic pole 6. Each multi-coil 16 includes a plurality (usually a large number) of coils 18 having side-by-side portions 19 arranged in the direction of the radius r along the opposing surfaces 6 a of the magnetic poles 6. The juxtaposed portions 19 of the coils 18 are arranged at the same pitch in the radius r direction.

  In order to form a magnetic field gradient in the radius r direction between the upper and lower magnetic poles 6 by the multi-coil as described above, the values of currents flowing through the coils 18 constituting the multi-coil 16 are set to different values. This will be described in detail below.

  FIG. 6 shows a simplified example of the coils 18 constituting the upper and lower multi-coils 16 one by one, and an example in which a power source 22 is connected thereto. The upper and lower coils 18 are connected in series with each other, and a DC power source 22 (multi-coil power source) is connected thereto. Such a configuration is provided as many as the number of coils 18 constituting the multi-coil 16.

  With the configuration as described above, a magnetic field gradient in which the magnetic flux density B increases as the radius r increases can be formed in the gap 10 between the upper and lower magnetic poles 6 as shown in FIG. 7, for example. An example of the direction of the magnetic field formed in the gap 10 between the upper and lower magnetic poles 6 is indicated by an arrow M in the figure, but the magnetic field may be formed in the opposite direction.

For example, when the electromagnet 4 is used in an FFAG accelerator, the distribution of the magnetic flux density B in the radius r direction in the gap 10 is set according to the following equation. Here, r ₀ is a reference radius (for example, the radius of the inner end of the magnetic pole 6 in the curvature radius direction of the charged particle 2), B ₀ is a reference magnetic flux density at the reference radius r ₀ , and k is a parameter representing a magnetic field gradient. . The main coil 14 generates this reference magnetic flux density B ₀ , for example.

[Equation 1]
B = B ₀ (r / r ₀ ) ^k

Assuming that the current value necessary for forming the magnetic flux density B is I, generally, B∝I. Therefore, the following equation is derived from this and the above equation (1). I ₀ is the current value at the reference radius r ₀ .

[Equation 2]
I = I ₀ (r / r ₀ ) ^k

  In Equation 2, the current value I changes continuously, but actually, the plurality of coils 18 constituting the multi-coil 16 are arranged in a discrete manner (discretely), and the current value I is Since this is the total current flowing through the coil 18, the current value I changes stepwise. This will be described in detail below.

Assuming that the current value at the reference radius r ₀ is I ₀ and the total current value from there to the n-th coil 18 (radius r _n ) is I _n , the following equation is derived from Equation 2. n is an integer of 0 or more.

[Equation 3]
I _n = I ₀ (r _n / r ₀ ) ^k

If the constant pitch of the plurality of coils 18 constituting the multi-coil 16 is dr, the coil 18 is not present at the position of the reference radius r ₀ , and the next coil 18 is the first, the radius rn of the _n- th coil 18 is It is expressed by the following formula.

[Equation 4]
r _n = r ₀ + n · dr

  Substituting Equation 4 into Equation 3 yields the following equation.

[Equation 5]
I _n = I ₀ (1 + n · dr / r ₀ ) ^k

The total current value I _n−1 from the above equation 5 to the (n−1) th coil 18 is expressed by the following equation.

[Equation 6]
I _n-1 = I ₀ {1+ (n-1) · dr / r ₀ } ^k

Therefore, the current value in of the _nth coil 18 is expressed by the following equation.

[Equation 7]
_{i n = I n -I n-} 1
= I ₀ (1 + n · dr / r ₀ ) ^k −I ₀ {1+ (n−1) · dr / r ₀ } ^k

  The magnetic field gradient as described above can be formed by using the multi-coil 16 having a constant pitch dr by changing the current values passed through the coils 18 constituting the multi-coil 16 to different values according to the equation (7). it can.

"Accelerator Basics and Advanced Accelerators", OHO '03 High Energy Accelerator Seminar, High Energy Accelerator Science Research Promotion Committee, August 25, 2003, pp. 1-29

  In the electromagnet 4, in order to form a magnetic field gradient, it is necessary to set different current values to flow through the coils 18 constituting the multi-coil 16. (See FIG. 6). That is, since a large number of power supplies for the multi-coil are required, the configuration of the power supply becomes complicated and the control thereof becomes complicated.

  Accordingly, the main object of the present invention is to provide an electromagnet that forms a magnetic field gradient and can reduce the number of multi-coil power supplies.

  In the electromagnet according to the present invention, at least some of the coils constituting each multi-coil are connected in series to each other, and the pitch in the radial direction of the parallel portions of the series-connected coils is changed. It is characterized by letting it.

  According to the electromagnet, currents of the same magnitude can be supplied from a single power source to a plurality of coils connected in series among the coils constituting each multi-coil. Even in that case, since the pitch in the radial direction of the juxtaposed portion of the plurality of coils connected in series is changed, a magnetic field gradient corresponding to the change in the pitch can be formed in the radial direction.

  All of the coils constituting each multi-coil may be connected in series with each other, and the pitch in the radial direction of the juxtaposed portions of the coils connected in series may be changed.

  The pitch in the radial direction of the juxtaposed coils of the series-connected coils may be reduced as the radius increases.

  According to the first aspect of the present invention, currents of the same magnitude can be supplied from a single power source to a plurality of coils connected in series among the coils constituting each multi-coil. Even in that case, since the pitch in the radial direction of the juxtaposed portion of the plurality of coils connected in series is changed, a magnetic field gradient corresponding to the change in the pitch can be formed in the radial direction. Therefore, the number of multi-coil power supplies can be reduced. Along with this, control of the power supply becomes easy.

  According to the second aspect of the present invention, since all the coils constituting each multi-coil are connected in series with each other and the pitch of the juxtaposed portion is changed, the number of power sources for the multi-coil is further reduced. This has the further effect that the power supply can be easily controlled.

  According to the third aspect of the present invention, since the pitch of the parallel portions of the coils connected in series is reduced as the radius increases, a magnetic field gradient in which the magnetic flux density increases as the radius increases is formed. can do.

  FIG. 1 is a schematic perspective view showing an embodiment of an electromagnet according to the present invention. FIG. 2 is a schematic longitudinal sectional view of the electromagnet shown in FIG. Parts identical or corresponding to those in the conventional example shown in FIGS. 4 and 5 are denoted by the same reference numerals, and differences from the conventional example will be mainly described below.

  The electromagnet 4a includes the following multicoil 16a in place of the conventional multicoil 16. That is, the multi-coil 16 a is provided inside the main coil 14 and in the vicinity of the surface of each magnetic pole 6. Each multi-coil 16a is composed of a plurality (usually a large number) of coils 18 having juxtaposed portions 19 arranged in the direction of the radius r along the opposing surface 6a of each magnetic pole 6.

  In this embodiment, all of the plurality of coils 18 constituting each multi-coil 16a are connected in series with each other. This is shown in a simplified manner in FIG. In FIG. 3, only three coils 18 constituting each multi-coil 16a are shown. However, this is only for simplifying the illustration, and each multi-coil 16a actually has more coils 18. is doing.

  Furthermore, in this embodiment, the upper and lower multi-coils 16a are connected in series with each other, and a DC power source 24 (multi-coil power source) is connected thereto. In this embodiment, one such configuration is provided for one electromagnet 4a.

  Furthermore, in this embodiment, in each of the multi-coils 16a, the pitch (interval) in the radius r direction of the juxtaposed portions 19 of the coils 18 connected in series is changed. More specifically, as can be seen from FIGS. 1 and 2, in this embodiment, the pitch is reduced as the radius r increases.

  According to the electromagnet 4a, currents of the same magnitude can be supplied from one power source 24 to all the coils 18 constituting each multi-coil 16a. Even in that case, since the pitch in the radius r direction of the juxtaposed portions 19 of all the coils 18 connected in series is changed, a magnetic field gradient corresponding to the change in the pitch can be formed in the radius r direction. Specifically, in this embodiment, the pitch in the radius r direction of the juxtaposed portions 19 of the coils 18 is decreased as the radius r increases, so that the magnetic flux density increases as the radius r increases. A gradient can be formed.

  Therefore, according to this electromagnet 4a, the number of power supplies for multi-coils can be greatly reduced as compared with the prior art. Along with this, control of the power supply becomes easy and the cost of the power supply can be reduced. In particular, in this embodiment, the power source for the upper and lower multi-coils 16a is only one power source 24, so the above effect becomes more remarkable.

  An example of the relationship between the pitch of the coil 18 constituting each multi-coil 16a and the magnetic field gradient will be described in detail below.

Taking the case where this electromagnet 4a is used for, for example, an FFAG accelerator as an example, when a magnetic field gradient close to the distribution of magnetic flux density B shown in the above equation 1 is formed, the following equation is derived from the above equation 2. The meanings of r _n , r ₀ , I _n , I ₀ and k are as described above.

[Equation 8]
r _n = r ₀ (I _n / I ₀ ) ^{1 / k}

Further, when the value of the current flowing to the plurality of coils 18 constituting the multi-coil 16a and di, the current value di becomes the same value in all the coils 18, the total current value I _n of up to n-th coil 18 It is expressed by the following formula.

[Equation 9]
I _n = I ₀ + n · di

  Substituting Equation 9 into Equation 8 gives the following equation.

[Equation 10]
r _n = r ₀ (1 + n · di / I ₀ ) ^{1 / k}

Therefore, from this equation 10, the pitch P _{n of the} juxtaposed portions 19 of the plurality of coils 18 constituting the multi-coil 16a can be expressed by the following equation.

[Equation 11]
P _n = r _n −r _n−1
= R ₀ (1 + n · di / I ₀ ) ^{1 / k} −r ₀ {1+ (n−1) · di / I ₀ } ^{1 / k}

Even if the current value di flowing through the multi-coil 16a is constant, the magnetic field gradient close to the distribution of the magnetic flux density B shown in Equation 1 can be formed by changing the pitch _Pn according to Equation 11.

  A part or all of each coil 18 constituting each multi-coil 16 a may be embedded in each magnetic pole 6, for example, in the vicinity of the surface of each magnetic pole 6. The same applies to each main coil 14.

  In addition, it is preferable to connect the upper and lower multi-coils 16a in series as shown in the example of FIG. 3 from the viewpoint that only one multi-coil power source 24 is required, but the upper and lower multi-coils 16a are connected in series. Instead, a power source may be provided for each multi-coil 16a. In that case, although two power supplies for the multi-coil are required, the number of power supplies can be significantly reduced as compared with the conventional power supply. Alternatively, the upper and lower multicoils 16a may be connected in parallel to one power source 24.

  In the above, the embodiment has been described in which all of the plurality of coils 18 constituting each multi-coil 16a are connected in series with each other and the pitch is changed. However, the present invention is not limited thereto, and as required. In addition, some of the plurality of coils 18 constituting each multi-coil 16a may be connected in series to each other, and the pitch of the juxtaposed portions 19 of the series-connected coils 18 may be changed. . For example, the plurality of coils 18 constituting each multi-coil 16a may be divided into a plurality of groups (for example, two to three groups), the coils 18 in each group may be connected in series, and the pitch may be changed at a desired ratio. . In this case, a multi-coil power source is provided for each group. However, the number of multi-coil power sources can be reduced as compared with the case where a power source is provided for each coil 18 as in the prior art. it can.

  A device including the above-described electromagnet 4a and a power source therefor can be referred to as an electromagnet device. Therefore, for example, the upper and lower main coils 14 constituting the electromagnet 4a are connected in series with each other and a DC power source for the main coil is connected to the upper and lower multi coils 16a in series with each other as shown in FIG. Then, a multi-coil DC power supply 24 may be connected to the electromagnet device to form a magnetic field gradient.

1 is a schematic perspective view showing an embodiment of an electromagnet according to the present invention. It is a schematic longitudinal cross-sectional view of the electromagnet shown in FIG. It is a figure which shows the example which connected the power supply to it while simplifying and showing the upper and lower multicoil which comprises the electromagnet shown to FIG. 1 and FIG. It is a schematic perspective view which shows one Embodiment of the conventional electromagnet. It is a schematic longitudinal cross-sectional view of the electromagnet shown in FIG. FIG. 6 is a diagram showing a simplified example of coils constituting the upper and lower multi-coils of the electromagnet shown in FIGS. 4 and 5 and an example in which a power source is connected thereto. It is a figure which shows an example of the magnetic field gradient formed in the radial direction.

Explanation of symbols

2 charged particle 4a electromagnet 6 magnetic pole 14 main coil 16a multi-coil 18 coil 19 juxtaposed part 24 power source (power source for multi-coil)

Claims (3)

A pair of magnetic poles facing each other with a gap through which charged particles pass;
A main coil that is wound around each of the magnetic poles, and that forms a uniform magnetic field between the magnetic poles in a radial direction that is a direction that intersects the direction of passage of the charged particles;
A magnetic field gradient in the radial direction between the magnetic poles is provided inside the main coils and includes a plurality of coils having juxtaposed portions arranged in the radial direction along opposing surfaces between the magnetic poles. In an electromagnet comprising a multi-coil forming
A plurality of at least some of the coils constituting each multi-coil are connected in series with each other, and the pitch in the radial direction of the juxtaposed portions of the series-connected coils is changed. An electromagnet that forms a magnetic field gradient.   2. The electromagnet for forming a magnetic field gradient according to claim 1, wherein all the coils constituting each of the multi-coils are connected in series with each other, and the pitch in the radial direction of the juxtaposed portions of the series-connected coils is changed. .   The electromagnet for forming a magnetic field gradient according to claim 1 or 2, wherein a pitch in the radial direction of the juxtaposed coils of the series-connected coils is reduced as the radius increases.

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