JPS6229113A - Superconducting device - Google Patents

Abstract

PURPOSE:To reduce an eddy current without impairing the cooling effect of a thermal shielding structure by a method wherein a thermal shielding plate made of a material of good thermal conductivity for the part where a heat concentration takes place and one made of a material of poor electric conductivity, for the other parts. CONSTITUTION:The evaporator of a refrigerator and a thermal shielding plate 62 are connected through the intermediary of the thermal shielding part 61 consisting of a shielding cylinder 63 and a thermal shielding plate 64. As all heat load is absorbed into a refrigerator 8 and it is concentrated in the cylinder 63 and the temperature equalizable thermal shielding plate 64, a material of good thermal conductivity is used for the cylinder 63 and the shielding plate 64. Also, the thermal shielding plate 62 conducts heat in axial direction only as far as to the temperature equalizable thermal shielding plate 64, and the heat load is dispersed in the circumferential direction. As a result, there generates no temperature difference even when a material of bad heat conduction of a certain degree is used. The excellent thermal conducting part where an eddy current runs easily is only a very small part, and the effect of the eddy current generated at that part is small.

Description

Detailed Description of the Invention [Field of Application of the Invention] The present invention relates to a superconducting device, and particularly to a superconducting device having a structure for cooling a superconducting coil by immersing it in liquid helium, having a refrigerator, and having an improved heat shield structure. The present invention relates to a superconducting device using a cryostat equipped with a body.

[Background of the invention]

It is known that when certain metals (NbTi, NbaSn, etc.) are kept at extremely low temperatures, they exhibit a superconducting phenomenon in which electrical resistance disappears. If a coil made of the above-mentioned metal is formed by utilizing the characteristics of the superconducting phenomenon, and this coil is kept at an extremely low temperature and used as a superconducting coil, a high magnetic field and highly stable static magnetic field can be generated without power loss. Can be done. In order to obtain such a superconducting coil, a cryostat (cryogenic constant temperature chamber) for accommodating the coil has conventionally been provided.

The above-mentioned superconducting device for generating a static magnetic field using such a cryostat has come to be applied to a variety of devices, and in particular, has recently been applied to medical nuclear magnetic resonance computer diagnostic equipment (NM).
Application to R-CT devices) is attracting attention. As a cryostat for NMR, as described in Japanese Patent Application Laid-open No. 59-32758 and US Pat. No. 4,277,949,
A liquid helium tank that accommodates a superconducting coil, a heat shield structure and a liquid nitrogen tank provided around the liquid helium tank to reduce heat intrusion into the liquid helium tank, and an outer wall that surrounds the outside with a vacuum. , stored in a space within the outer wall,
It has a helium refrigerator equipped with a heat exchanger and an expansion machine that generates low temperature, and the refrigerator cools the liquid nitrogen and liquid helium in the liquid nitrogen tank and the liquid helium tank. be.

However, the technology related to the above-mentioned cryostat is
In short, the main concern was how to properly install the refrigerator to the cryostat, and not the heat shield structure provided to prevent heat from entering from the outside. That is, in order to reduce heat intrusion from the outside and suppress the amount of evaporation of liquid helium in the liquid helium tank, it is necessary to provide a heat shield structure and sufficiently cool it. However, since the heat shield structure is cooled by thermal conduction, it is necessary to use a good thermal conductor as its material. Good thermal conductors are also usually good electrical conductors, so eddy currents can easily flow through them.By the way, NMR
- In order to obtain NMR-CT images of various surfaces of the object using a CT device, a coil called a gradient coil, which tilts the magnetic field, is activated in a pulsed manner on top of a uniform static magnetic field. I'm letting you do it. However, if the heat shield plate is formed of a good electrical conductor, an eddy current based on the pulsed magnetism will flow through the heat shield plate, and the magnetic field will be shielded by the action of the eddy current, which is disadvantageous.

[Purpose of the invention]

The present invention has been made in view of the above points, and its purpose is to provide a superconducting device using a cryostat equipped with a heat shield structure that reduces eddy current without impairing the cooling effect. There is a particular thing.

[Summary of the invention]

In order to achieve the above object, the present invention provides a liquid helium tank that houses a superconducting coil, a heat shield structure provided around the liquid helium tank to reduce heat intrusion into the liquid helium tank, and a heat shield structure provided around the liquid helium tank to reduce heat intrusion into the liquid helium tank. A helium tank consisting of a liquid nitrogen tank installed around the tank to reduce heat intrusion into the tank, an outer wall that surrounds the outside with a vacuum, and a heat exchanger and an expansion machine that generates low temperature in the space inside the outer wall. In a superconducting device comprising a refrigerator, the refrigerator cools liquid nitrogen in a liquid nitrogen tank and the heat shield structure, the heat shield structure being connected to an evaporator of the refrigerator. a heat shield part made of a material with high thermal conductivity and provided in a heat concentration part, and a heat shield plate fixed to the heat shield part and made of a material with low electrical conductivity. It is something.

[Embodiments of the invention]

An embodiment of the present invention will be described below based on the drawings, but before that, the key points of the present invention will be explained.

FIG. 3 is a perspective view showing the external appearance of the cryostat that is the basis of the present invention, and FIG. 4 is a sectional view thereof.

In these figures, 1 is an insulated vacuum vessel, 2 is a helium vessel, 3 is a nitrogen vessel, 4 is a superconducting coil, and 5 is a 20"K
Heat shield plate, 6 is 80"K heat shield plate, 7 is external claw stand, 8 is helium refrigerator, 9 is liquid nitrogen, 1
0 is liquid helium. The superconducting coil 4 is usually placed in a helium container 2;
It is immersed in liquid helium 10 inside.

Since the liquid helium 10 needs to be used at an extremely low temperature of 4.2°, it is stored in a helium container 2 within an insulated vacuum container 1 in order to reduce heat intrusion from room temperature (300°K). This helium container 2 is surrounded by a heat shield plate 6 at 800 and then by a heat shield plate 5 at 200 in this order. Here, the temperature of 80° is usually generated by liquid nitrogen, and the temperature of 20'K is generated by the refrigerator 8. Furthermore, liquid nitrogen 9 is stored in the nitrogen container 3, and the refrigerator 8 is attached to the external claw stand 7.

In a cryostat having such a structure, the heat shield plate 5 is usually cooled by thermal conduction, so a material with good thermal conductivity (Al1.Cu) is used. Since a material with good thermal conductivity is a good electrical conductor, the eddy current flowing through the heat shield plate will also be considerably large. As a result, the magnetic field generated by the gradient coil is shielded, and extra power is required to obtain the desired magnetic field strength. This also distorts the gradient magnetic field strength distribution, leading to distortion of images, which is the lifeblood of NMR-CT equipment. Therefore, in the present invention, the structure as a heat shield plate is
By using materials with good thermal conductivity in areas where heat concentration occurs and materials with poor electrical conductivity in other areas, the cooling effect of the heat shield structure is not impaired. This is designed to reduce eddy currents.

FIG. 1 is a sectional view of a main part showing an embodiment of the present invention, and FIG. 2 is a perspective view showing the configuration of a heat shield structure used in an embodiment of the present invention. In addition, in these figures, figures 3 and 4 are also used.
Components that are the same as those shown in the figures are given the same reference numerals, and their explanations will be omitted.

This embodiment is configured as follows. The heat shield structure 60 is constructed by providing a heat shield portion 61 made of a good thermal conductor in a portion where heat concentration occurs, and connecting a heat shield plate 62 made of a poor electrical conductor to the heat shield portion 61. That is, the evaporator and heat shield plate 62 of the refrigerator 8 are made of a shield cylinder 63 made of a material with good thermal conductivity, and a heat shield plate 64 for equalizing the temperature of the good heat conductor. 6 cylinders 63 connected via a shield part 61
Since all the heat load absorbed by the refrigerator 8 is concentrated here, the temperature equalization heat shield plate 64 is made of a material with good thermal conductivity in order to minimize the temperature difference. . Further, in the heat shield plate 62, it is only necessary to conduct heat in the axial direction to the temperature equalizing heat shield plate 64.
Since the heat load is distributed in the circumferential direction, no temperature difference will occur even if the material is made of a material with a certain degree of poor thermal conductivity.

According to this embodiment, the good thermal conductivity portion where eddy currents easily flow is only a small portion compared to the entire heat shield plate 62, and the influence of eddy currents generated in that portion is small.

Therefore, the refrigerator 8
Since the cylinder 63 and the temperature equalizing heat shield plate 64 are provided, the influence of eddy currents is significantly reduced.

In particular, in the case of a superconducting device for NMR-CT, since the pulse magnetic field generated by the gradient coil becomes zero at the center in the axial direction of the coil, by installing the temperature equalizing heat shield plate 64 in this area, Eddy currents can be minimized.

The temperature-uniforming heat shield plate 64 can also be obtained by pasting a plate made of a material with good thermal conductivity only in that portion on the heat shield plate 62 made of a material with poor thermal conductivity.

Note that 70 is a pipe that guides the nitrogen gas in the nitrogen container 3 to the evaporator 81 of the refrigerator 8, and 71 is a pipe that leads the nitrogen liquid liquefied in the evaporator 81 to the nitrogen container 3.

According to this embodiment, the heat absorbed by the refrigerator is uniformly transmitted without any temperature difference because the cylindrical shield plate made of a good heat conductor is integrated with the shield cylinder.

By installing a temperature equalizing shield plate made of a good thermal conductor between the shield cylindrical part, which is connected to the evaporator of the refrigerator and where heat is concentrated, and the shield plate made of a material with poor thermal conductivity, the temperature difference can be reduced. Therefore, it is possible to obtain a heat shield structure that has a large cooling effect without any eddy current flow.

In particular, in the case of NMR-CT equipment, by making the axial center of the shield plate made of a good thermal conductor the center of the gradient coil, no pulsed magnetic field acts on this shield plate, and therefore no eddy currents occur. This can be prevented from occurring.

〔Effect of the invention〕

As described above, according to the present invention, a shield plate made of a good thermal conductor is provided in the heat concentration part, and a heat shield plate made of a poor electrical conductor is provided in other parts. A high-performance superconducting device can be obtained without generating eddy currents.

Further, according to the present invention, there is an advantage that no temperature difference occurs in the heat shield plate. Further, according to the present invention, when used in an NMR-CT apparatus, there is an effect that other control system signals etc. are not affected.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of essential parts showing one embodiment of the present invention, FIG. 2 is a perspective view of the same embodiment, FIG. 3 is a perspective view showing the external appearance of a cryostat that is the basis of the present invention, and FIG. The figure is a sectional view of the same. 1... Insulated vacuum container, 2... Helium container, 3...
・Nitrogen container, 4... Superconducting coil, 5... Shield at 20°, 6... 800K shield, 7... Stand with external power, 8... Freezer, 9... Liquid helium, 10...・
・Liquid nitrogen, 60... Heat shield structure, 61...
Heat shield portion, 63... Shield cylinder, 62... Shield plate made of poor heat conductor, 64... Shield plate made of good heat conductor.

Claims (1)

[Claims] 1. A liquid helium tank that houses a superconducting coil, a heat shield structure provided around the liquid helium tank to reduce heat intrusion into the liquid helium tank, and a heat shield structure provided around the liquid helium tank to reduce heat intrusion into the liquid helium tank. It has a liquid nitrogen tank provided around it, an outer wall that surrounds the outside with a vacuum, and a helium refrigerator consisting of a heat exchanger and an expansion machine that generates low temperature in the space inside the outer wall. In a superconducting device configured to cool liquid nitrogen in a liquid nitrogen tank and the heat shield structure, the heat shield structure is provided in a heat concentration part connected to an evaporator of the refrigerator and is a heat conductor. 1. A superconducting device comprising: a heat shield portion made of a material with high conductivity; and a heat shield plate fixed to the heat shield portion and made of a material with low electrical conductivity.