JPS62139304A - Magnetic circuit with excellent uniformity of magnetic field - Google Patents

Abstract

PURPOSE:To make the lines of magnetic force uniform over a wide range of area in an air gap by a method wherein an auxiliary magnet is arranged at the end part of a pole piece, and it is magnetized in the direction reverse to the lines of magnetic force of the air gap. CONSTITUTION:A box type yoke is formed with soft iron yokes 11, 12 and 13. Main magnets 21 and 22 and pole pieces 31 and 32 are arranged inside the yokes 11 and 13 respectively. Said main magnets are magnetized in the direction as shown by the arrow M in the diagram, and the pole piece 31 becomes N-pole and the pole piece 32 becomes S-pole. Besides, an auxiliary magnet 41, magnetized in the direction reverse to the direction of the magnetic field in the magnetic gap, is arranged on the end part of the pole pieces 31 and 32. According to this constitution, the deterioration in magnetic field strength can be prevented as the lines of magnetic force is absorbed by the yoke 12 at the end part of the pole pieces even when the intervals of the pole pieces is made wider, and the lines of magnetic force is made uniform in the air gap over a wide range. The auxiliary magnet 12 is opposed to the pole pieces 32 and 33, and the ferrite magnet of the relative magnetic permeability mu of almost 1 is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a magnetic circuit having a highly uniform magnetic field distribution, such as used in NMR-CT (Nuclear Magnetic Resonance Computer Topography).

(Prior Art) As is well known, a magnetic circuit used in an NMR apparatus requires a very large magnetic field strength and a highly uniform magnetic field distribution. The required magnetic field strength is determined by the frequency of nuclear magnetic resonance such as protons, but it is 1,000
Usually used at ~20,000 Gauss. If the uniformity of the magnetic field is poor, the frequency range in which this resonance occurs will also become wider, and the so-called half-width of resonance will become wider, resulting in poor measurement accuracy. Therefore, the uniformity of the magnetic field is ultimately 1o at the place where it is used.
A value between -6 and 10-8 is required.

A magnetic circuit made of permanent magnets or electromagnets is used in this NMR device. Various improvements have been made to the shape of the pole piece, but with only a magnetic circuit, the required uniformity of the magnetic field within the magnetic gap cannot be achieved even when the gap is made small. Even if the length is within 20 mm, it is about 1o-5 to 10-6, and in order to calibrate this non-uniformity of the magnetic field, a calibration coil is installed on the pole piece to obtain the required uniformity of the magnetic field. .

However, in an NMR-CT device where the object to be examined has a size of 300 to 400 mm, a uniformity of the magnetic field of about 70 mm is required in a portion of this size within the magnetic gap. ing. In order to obtain good magnetic field uniformity in such a large air gap, first, the magnetic circuit of the permanent magnet alone should be set to about 10-2, and then the shape of the pole piece should be devised to make it 10-4 to 10-5. Make it. Furthermore, it is generally considered to use a coil to improve uniformity.

In order to obtain such uniformity of the magnetic field in a permanent magnet circuit,
Attempts have been made to cover the periphery of this gap with permanent magnets magnetized in the same direction (Japanese Patent Application Publication No. 59-6501995). That is, it is made by stacking block-shaped permanent magnets with magnetic gaps left. In this case, a relatively uniform magnetic field can be obtained, but in order to obtain the required magnetic field strength, an extremely large number of magnets are required, which increases the weight of the magnetic circuit and reduces the overall equipment cost. It had become something big.

(Problems to be Solved by the Invention) The present invention aims to provide a magnetic circuit that has a highly uniform magnetic field even when the magnetic gap is relatively large, and requires only a relatively small amount of permanent magnets. purpose.

(Means for Solving the Problems) In the magnetic circuit of the present invention, two magnetism generating means are arranged in a soft magnetic yoke with a magnetic gap in between, and each of the magnetism generating means is connected to a main magnet. and a parallel pole piece provided on one of the magnetic poles, these pole pieces are arranged to face each other across the magnetic gap, and the main magnet applies a unidirectional magnetic field within the magnetic gap. an auxiliary magnet magnetized in a direction opposite to the direction of the magnetic field in the magnetic gap is arranged at an end of the gap close to the yoke; It is.

In carrying out the present invention, it is desirable that the auxiliary magnet be arranged so as to be in contact with the two pole pieces.

The main magnet is a ferrite magnet or Nd-Fe-
Rare earth-iron magnets such as 8-base magnets and rare earth cobalt magnets such as Sm-Co are used. Block-shaped magnets such as ferrite magnets, rare earth iron magnets, and rare earth cobalt magnets can be used by stacking them.

Further, it is desirable that the auxiliary magnet placed close to the yoke be one in which the relative magnetic permeability μ of the atmosphere is close to 1. Such permanent magnets include rare earth/iron, rare earth/cobalt magnets, and ferrite magnets.

This auxiliary magnet is desirably a ferrite magnet, since it only has to function to prevent the magnetic field from spreading outside at the outer periphery between the pole pieces, and preventing the lines of magnetic force from being attracted to the yoke and causing poor uniformity.

(Example) Hereinafter, the present invention will be described in detail based on drawings of examples.

FIG. 1 shows a front view of a magnetic circuit according to an embodiment of the present invention. Here, 11.13 is an end yoke, 12 is a side yoke, end yoke 11, side yoke 12) end yoke 1
3. The side yokes 12 form a box-shaped yoke as a whole. Each of these yokes is made of a soft magnetic material, such as soft iron or 3341 material. End yoke 11
A main magnet 21 is arranged inside the main magnet 21, and a pole piece 31 is arranged in the gap between the main magnet 21. In addition, the end yoke 1
A main magnet 22 is arranged inside the main magnet 3, and a pole piece 32 is arranged near the air gap of the main magnet 22. These main magnets are magnetized in the direction of arrow M, and the pole piece 31 is magnetized in the direction of arrow M.
? fi, the pole piece 32 has an S pole. With this arrangement, the lines of magnetic force pass from the main magnet 22 to the end yokes 13, 12) through the end yokes 11, through the main magnet 21, through the pole piece 31, through the air gap, and into the pole piece 32) the main magnet. 22 and back. The lines of magnetic force are almost parallel between the pole piece 31 and the pole piece 32, but even if the pole piece diameter is as large as 1000 mm, when the distance between the pole pieces becomes as large as 500 to 700 mm, the lines of magnetic force therebetween become disordered. Even if the magnetic lines of force are perpendicular in the center of the pole piece, they bulge to the left and right as they approach the ends, and the magnetic field strength decreases.

In particular, when the side yokes 12 are arranged on the left and right sides of the magnetic gap, there is a strong tendency for the lines of magnetic force in the magnetic gap to be attracted to the side yokes, so there is a strong tendency for the magnetic field strength at the end of the pole piece to decrease. becomes.

In the present invention, auxiliary magnets 41 are arranged at the ends of the pole pieces 31 and 32, and the auxiliary magnets are magnetized in a direction opposite to the direction of the magnetic lines of force in the air gap. By doing this, it was possible to make the lines of magnetic force uniform over a wide range within the air gap.

Here, the main magnet is a ferrite magnet (residual magnetic flux density 3r
: 3980 Gauss, Coercive force Hc: 3100 Oersted, 1) 1c: 3160 Oersted, Maximum energy (BH) ma
I made it to be. A ferrite magnet with the same characteristics was also used as an auxiliary magnet. At this time, the magnetic field strength at the center of the air gap Wmm was 4R Gauss. In FIG. 2, a shows the distribution of magnetic field strength at the center of the air gap without an auxiliary magnet, and b shows the distribution of magnetic field strength at the center of the air gap when an auxiliary magnet is attached. In graph b, the magnetic field strength distribution at the center of the gap is 400 m from the center to the left and right.
It is almost within 1/100 up to a+,
It can be seen that the uniformity of the magnetic field is significantly improved.

The above is a case where a ferrite magnet is used as the main magnet, but the main magnet is Nd-Fe-B (Br:11
．． Gauss to 3, Oersted to Hc:10.8,
When a ferrite magnet was used as an auxiliary magnet, the air gap magnetic flux was 4536 Gauss at the center.

(Effects of the Invention) As described above, even when the present invention has a large magnetic gap, the amount of permanent magnets is relatively small and a highly uniform magnetic field distribution can be obtained. It becomes a circuit.

[Brief explanation of drawings]

FIG. 1 is a front view of a magnetic circuit according to an embodiment of the present invention, and FIG. 2 is a diagram showing the magnetic field distribution within the magnetic gap. 11, 13: End yoke, 12: Side yoke, 21.22
: Main magnet, 31.32 pole piece, 41: Auxiliary magnet 2nd Figure shell 11 air disturbance, 5L (m)

Claims (4)

[Claims] (1) Two magnetism generating means are arranged within the soft magnetic yoke with a magnetic gap in between, and each magnetism generating means consists of a main magnet and a pole piece provided on one of the magnetic poles. , these pole pieces are arranged to face each other across the magnetic gap, and the main magnet is magnetized to generate a unidirectional magnetic field within the magnetic gap, A magnetic circuit with good magnetic field uniformity, characterized in that an auxiliary magnet magnetized in a direction opposite to the direction of the magnetic field in the magnetic gap is arranged at an end close to the yoke. (2) A magnetic circuit with good magnetic field uniformity according to claim 1, wherein the auxiliary magnet is arranged so as to be in contact with both pole pieces. (3) A magnetic circuit with good magnetic field uniformity according to claim 2, wherein the auxiliary magnet has a relative magnetic permeability μ of approximately 1. (4) A magnetic circuit with good magnetic field uniformity according to claim 3, wherein the auxiliary magnet is a ferrite magnet.