JPH0816695B2 - Magnetic field correction device - Google Patents

Description

TECHNICAL FIELD The present invention relates to a magnetic field correction apparatus used for a nuclear magnetic resonance magnet or the like.

[Prior Art] FIG. 8 shows a conventional magnetic field correction device disclosed in, for example, Japanese Unexamined Patent Publication No. 52-193230, in which reference numeral (10) indicates a cylindrical inner wall of a non-magnetic material. The inner wall of this cylinder (1
A rod-shaped magnetic body (2) is attached to (0). A coil (3) that generates the main magnetic field on the outer circumference of the inner wall (10) of the cylinder
In the inner wall (10) of the cylinder, a region (4) where magnetic field homogeneity is required is indicated by a broken line.

The conventional magnetic field correction device is configured as described above,
The operation will be described below. Generally, the magnetic field generated by the coil (3) does not necessarily have the required magnetic field homogeneity in the region (4). Therefore, as one means to enhance the magnetic field uniformity, the inner wall of the cylinder (10)
It is possible to change the spatial distribution of the generated magnetic field by sticking the rod-shaped magnetic body (2) on the. In the above-mentioned conventional apparatus, experimentally confirming how the magnetic field distribution in the region (4) changes with a plurality of magnetic bodies (2) previously arranged at appropriate positions on the inner wall (10) of the cylinder. Once again, a combination of several magnetic materials has been used empirically to enhance the homogeneity of the magnetic field.

[Problems to be Solved by the Invention] Since the conventional magnetic field correction device is configured as described above, when deciding the size, shape, number of sticking positions, etc. of the magnetic substance, the empirical sense of the person who attaches it. It is not always said that each magnetic field correction device after assembly is not set to the optimum magnetic substance because there is no fixed rule, and that there are variations in characteristics, as well as other components. There was a serious problem that the magnetic material mounting position could not be limited to the range intended by the creator in consideration of the interference with.

The present invention has been made to solve the above problems, and particularly requires the configuration of the magnetic body, the mounting longitude,
It is an object of the present invention to obtain a magnetic field correction device capable of determining the installation latitude and the like according to a certain rule within the installation range intended by the creator.

[Means for Solving the Problems] In the magnetic field correction device according to the first aspect of the present invention, a magnetic body for correcting an inhomogeneous error component included in a main magnetic field has an end face angle and an end face disconnection of the magnetic body. It is configured such that a magnetic field composed of two magnetic material pairs whose area ratio is selected to be an appropriate value based on the mutual relationship is used.

Further, in another invention of the present invention, the relationship between the end face angle and the end face cross-sectional area ratio of the magnetic body is based on a specific concrete relational expression.

[Operation] In the present invention, if the correction amount of the specific magnetic field component to be corrected is known, a predetermined amount of the magnetic material is arranged at a position within the range intended by the creator.

[Embodiment] An embodiment of the present invention will be described below with reference to the drawings.

1 and 2, a rod-shaped magnetic body (2) is provided in a non-magnetic tube (1) made of a non-magnetic material so that it can be selectively attached and detached. The tube (1) and the magnetic body (2) are arranged in a coil (3) having a cylindrical shape and generating a main magnetic field. Further, a portion (4) shown by a broken line is a magnetic field region where uniformity is required.

FIGS. 3 and 4 show a state in which a rod-shaped magnetic body having a cross-sectional area A is arranged at the mounting angle position along the main magnetic field direction (Z-axis direction). The Z-direction component B _Z of the magnetic field produced by the magnetic body (2) at an arbitrary point P (r, θ, φ) in the region (4) requiring uniformity is expressed by the following equation.

Here, K: a constant determined by the magnetic properties of the magnetic substance a: mounting radius of the magnetic substance εm: Neumann coefficient, εm = 1 if m = 0, εm = if m ≠ 0
2, P ^m _n : Legendre polynomial of order m.

The expression is expressed in a polar coordinate system, and the correspondence with a commonly used orthogonal coordinate system is as shown in Table 1.

On the other hand, the z-direction component Bcz of the magnetic field created by the coil (3) in the homogeneity region (4) is expressed by the following equation.

_{Bcz = B 0 + A 1 X} + A 2 Y + A 3 ZX + A 4 ZY + A 5 XY + A 6 (X 2 -Y 2) + where, B ₀ is the magnetic field at the origin (0,0,0), therefore, for example, A ₁ X Indicates an error component proportional to X.

That is, in order to improve the uniformity of the uniformity region (4),
It can be seen that a magnetic substance (2) that cancels A ₁ X etc. is necessary. The magnetic substance (2) for correcting the X component will be described below as an example. As you can see from the above equation,
There are an infinite number of magnetic field components created by the magnetic body (2), but since a> r in general, the component with a large value of n is Is so small that it can be ignored. Therefore, in practice, it is only necessary to consider the components with small values of n and m. Here, the radial direction component (that is, m
Considering only the following, the following components will be considered.

B ¹¹ ,, B ²¹ , B ²² , B ³¹ , B ³² , B ³³ , B ⁴¹ , B ⁴² , B ⁴³ , B ⁴⁴ , B ⁵¹ , B ⁵² ,
B ⁵³ , B ⁵⁴ . Here, the B ¹¹ component corresponds to the X component according to Table 1. Therefore, the shape and position of the magnetic body (2) that generates only the B ¹¹ component are determined by the following procedure. Set the mounting angle so that the component of m = 2,3,4 is not generated. If we choose, then we have cos (φ −) = 0 in the equation, so B ²² ,
The components of B ³² , B ³³ , B ⁴² , B ⁴³ , B ⁴⁴ , B ⁵² , B ⁵³ , and B ⁵⁴ are lost, and the rest are B ¹¹ , B ²¹ , B ³¹ , B ⁴¹ , and B ⁵¹ .

If one end face angle of the magnetic body (2) is α, and the other end face angle is (π−α), that is, if the magnetic body (2) is symmetric about the Z axis, expression in _{^{B 21 α [P 1 3 (}} cosα) sin 4 α ▲] α π ▼ -α = 0 B 41 α [P 1 5 (cosα) sin 6 α ▲] α π ▼ -α = 0 , and the end, B ²¹ and B ⁴¹ will not occur. Finally, the end face angle of the magnetic body (2) composed of two parts as shown in FIG. 5 and the cross-sectional area ratio thereof are determined so as to satisfy B ⁵¹ = 0 and B ³¹ = 0. Of the end face angles at both ends of the magnetic body (2) composed of two parts, the smaller one (hereinafter referred to as the inner end face angle) and the cross sectional area are α ₁ and A ₁ respectively.
And α ₂ , A ₂ , the output of B ³¹ , B ⁵¹ is B ³¹ ∝A ₁ [P ¹ ₄ (cosα ₁ ) sin ⁵ α ₁ ] ＋ A ₂ [P ¹ ₄ (cosα ₂ ) sin ⁵ α ₂ ] B ⁵¹ ∝ A ₁ [P ¹ ₆ (cos α ₁ ) sin ⁷ α ₁ ] + A ₂ [P ¹ ₆ (cos α ₂ ) sin ⁷ α ₂ ].

If we introduce a quantity g (α) = P ¹ ₆ (cosα) sin ⁷ α / P ¹ ₄ (cosα) sin ⁵ α = P ¹ ₆ (cosα) sin ² α / P ¹ ₄ (cosα), the formula becomes B ⁵¹ ∝ A ₁ g (α ₁ ) [P ¹ ₄ (cos α ₁ ) sin ⁵ α ₁ ] + A ₂ g (α ₂ ) [P ¹ ₄ (cos α ₂ ) sin ⁵ α ₂ ] (4a). FIG. 6 shows a graph when α of the expression is changed.

It can be seen from FIG. 6 that there are α ₁ and α _{2 such} that R = g (α ₁ ) = g (α ₂ ) α ₁ ≠ α ₂ .

Substituting the equation into the equation (4a), B ⁵¹ ∝ R {A ₁ [P ¹ ₄ (cosα ₁ ) sin ⁵ α ₁ ] + A ₂ [P ¹ ₄ (cosα ₂ ) sin ⁵ α ₂ ]} (4b) The {} in (4b) is exactly the same as the right side of the equation. Therefore, in a formula, if you decide that there is a corresponding Is determined, and the outputs of B ³¹ and B ⁵¹ at this time are as follows.

In order that the outputs of B ³¹ and B ⁵¹ both become zero, the cross-sectional area ratio Ie It is sufficient to satisfy the conditions of. Here is a design example. Consider the case where the magnetic material mounting range intended by the manufacturer is 140 ° or less at the end face angle. In this case, if g (α) =-0.5 is selected from Fig. 6, the inner end face angle of each of the two magnetic bodies is α ₁ = 40.1.
0, α ₂ = 66.50 and the corresponding outer end face angles (the larger one of the end face angles of the magnetic substance) is π
−40.1 = 139.90, π−66.50 = 113.50, which is within the range of 140 ° intended by the manufacturer. At this time, from the formula Then, B ³¹ and B ⁵¹ are both zero by using a magnetic body composed of two parts satisfying this ratio and the end face angle.

As described above, the end face angle (α), the mounting angle (ψ), and the cross-sectional area intended by the manufacturer could be determined for the X component magnetic body. It can be seen that such a combination makes it possible to secure design freedom by changing R.

The magnetic body (2) of FIG. , The negative output of X (A ₁ X <0) is generated, and the positive output of X (A ₁ X>) when each →→ π.
0). Further, the output of A ₁ X can be adjusted by changing the total cross-sectional area under the condition that the cross-sectional area ratio of the two constituent parts of the magnetic body (2) is not changed, as can be seen from the above equation.

Therefore, before correction, several kinds of rods having the same length and cross-sectional area ratio under the above conditions but different cross-sectional areas are prepared and selectively used, whereby the magnetic field can be easily adjusted.

Further, for the present invention, prior to adjusting the homogeneity of the magnetic field,
A non-magnetic tube (1) made of a non-magnetic material in which the magnetic material (2) is previously inserted in a position where the magnetic material (2) for the X component is attached.
The length of this non-magnetic tube (1) is equivalent to the length of the magnetic body (2), and the inner diameter of this non-magnetic tube (1) is the absolute output of the required component. The magnetic field correction device is configured to be determined by the value.

Furthermore, rod-shaped magnetic materials for Y, ZX, ZY, XY, X ² − Y ² (2)
As for the structure of, the end surface angle and the end surface cross-sectional area ratio of the magnetic body composed of two parts are selected to be appropriate values based on the mutual relationship as in the case of the X component. It becomes possible to attach the magnetic body and to generate a necessary compensating magnetic field.

Table 2 shows an example, and the cross-sectional area ratio and the end face angle change depending on the design conditions.

In addition, FIG. 7 shows a non-magnetic tube for inserting a magnetic body arranged on one circumference to correct the non-uniform magnetic field components of X, Y, ZX, ZY, XY, Z ² -Y ^2. It is shown.

[Effects of the Invention] As described above, according to the present invention, the magnetic body can be set for each error component within the range intended by the manufacturer, and a non-magnetic tube for the component can be installed to provide the magnetic substance. Since the body can be selectively attached to or detached from the non-magnetic tube, there is an effect that the uniformity can be adjusted easily and surely.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of an embodiment of the present invention, and FIG.
3 and 4 are schematic views for explaining the magnetic field generated by the magnetic material, FIG. 5 is a perspective view of the X component correcting magnetic material, and FIG. 6 is end face angle characteristics. FIG. 7 is a layout view of magnetic components for correcting each component, and FIG. 8 is a perspective view of a conventional magnetic field correction device. (1), (1A), (1B), (1C) ... Non-magnetic tube, (2)
…… Magnetic material, (3) …… Coil, (4) …… Uniformity area. In each drawing, the same reference numerals indicate the same or corresponding parts.

Claims (2)

[Claims] 1. A magnetic field correction device using a magnetic material used for a cylindrical magnet, which requires a spatially uniform magnetic field distribution, for correcting an inhomogeneous error component contained in a main magnetic field. A specific position in the circumferential direction of the inner peripheral surface of the magnet so that the magnetic body is arranged at a specific position according to each component that appears when the magnetic field is developed in series in the orthogonal coordinate system in the region where uniformity is required. Can be selectively attached to and detached from the inside of a plurality of non-magnetic tubes arranged in the above-mentioned manner, and the magnetic body has an appropriate value based on the mutual relationship between the end face angle and the end face sectional area ratio of the magnetic body. The magnetic field correction device is characterized in that the magnetic field correction device is composed of two magnetic body pairs selected to prevent generation of a specific nonuniform magnetic field component. 2. Two magnetic material pairs are mounted at positions in the circumferential direction. The sum of the end face angles of both ends of each magnetic body of this magnetic body pair is π, and the smaller one of the end face angles of both ends of each magnetic body forming the magnetic body pair. And the cross-sectional area ratio of each magnetic body (Wherein A _1, A ₂ is the cross-sectional area, α _1, α ₂ is the end face angle of the smaller one of the end surface angle of both ends of the magnetic body, P _n ^m denotes the Legendre polynomials.) And characterized by a The magnetic field correction device according to claim 1.