JPH02238682A - Cryostat - Google Patents

Abstract

PURPOSE:To reduce intruded heat due to radiation and conduction and loss due to eddy current generated within a refrigerant container by a gradient coil by composing a part which exists between the gradient coil and a superconductive coil of two or more shielding materials consisting of materials with an improved electrical conductivity mutually coupled thermally. CONSTITUTION:A part of a thermal shield 2 which is greatly affected by a gradient coil 4, namely the part between a superconductive coil 5 and the gradient coil 4, is composed of two or more materials with improved electrical conductivity through a cylindrical electrical isolation layer. Thus, a fluctuating magnetic filed generated by the gradient coil 4 at a shielding part constituted in multiples, passing through the thermal shield 2 and reaching a refrigerant container 3, is reduced, thereby reducing loss of eddy current in the refrigerant container 3.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a cryostat for a superconducting magnet such as an MRI (magnetic resonance imaging apparatus).

(Prior Art) Conventionally, there is a cryostat as shown in FIG. 3, which is configured to maintain a superconducting magnet such as an MRI at a low temperature and reduce the amount of evaporation of its coolant as much as possible.

This cryostat has at least one thermal coil 2 between a cryogenic refrigerant container 3 housing a superconducting coil 5 and a vacuum container 7 in order to reduce heat intrusion into the refrigerant due to conduction and radiation. A method of actively cooling using a small refrigerator 1 or the like is used.

Further, the gradient coil 4 turns the uniform magnetic field generated by the superconducting coil 5 into a gradient magnetic field, and is excited in a pulsed manner in order to obtain an image signal by nuclear magnetic resonance.

(Problems to be Solved by the Invention) However, with the above structure, although the intrusion heat due to radiation and conduction can be reduced, the fluctuating magnetic field generated by the gradient coil 4 or eddy currents are generated in the refrigerant container 3.
This has the disadvantage that the loss is very large.

An object of the present invention is to provide a cryostat that can not only reduce heat intrusion due to radiation and conduction, but also reduce loss due to eddy currents generated in a refrigerant container by a gradient coil.

[Structure of the invention]

(Means for Solving the Problems) The cryostunt of the present invention has a double or multiple shield that exists between the superconducting coil and the gradient coil, which is at least a part of the thermal rolling that is strongly affected by the gradient coil. . However, it is assumed that the multiple shield parts are thermally coupled to each other.

(Function) By doing this, the multi-layered shield section reduces the amount of the fluctuating magnetic field generated by the gradient coil that penetrates the heat shield and reaches the refrigerant container, reducing eddy current loss in the refrigerant container. do.

(Example) Hereinafter, an example of the cryostat according to the present invention will be described with reference to FIG.

FIG. 1 shows the gradient coil 4 of the heat shield 2.
This is a schematic configuration diagram when the portion existing between the superconducting coil 5 and the superconducting coil 5 is doubled, and 1 in the figure is a single-stage refrigerator, or the same applies to a refrigerator with two or more stages. . In other words, the same applies to the case of two or more stages of heat rolling. Heat rolled 2 is made of copper or aluminum plate. By multiplexing the portions of the Kno-Nurs 1 and 2, leakage magnetic flux to the refrigerant container 3 can be reduced compared to conventional heat shields.

That is, when the magnetic flux generated by the gradient 1 and coil 4 passes through the portion of the heat seals 1 and 2 that is disposed on the gradient 1 and coil 4 side, an eddy current is generated that cancels the change in the magnetic flux. This vortex 7B flow causes a heat load due to surreal heat generation to be applied to the heat shield 2. at the same time,
This eddy current causes the gradient coil 4 of the heat coil 2 to
The magnetic flux passing through the side-located parts is reduced to some extent. The magnetic flux passing through the portion of the heat shield 2 further generates an eddy current in a portion of the heat shield 2 disposed on the superconducting coil 5 side, and is further weakened. The heat shield 2
The Joule heat generated in each part of the heat shield 2 is cooled in common by the cold head of the refrigerator 1 because each part of the heat shield 2 is thermally coupled. This means that the number of heat shields can be selected regardless of the number of stages of the refrigerator 1, and the number can be selected so that the magnetic flux reaching the refrigerant container 3 is below the allowable value.

(Other Embodiments) Almost the same effect can be obtained when the shield 2 shown in FIG. 1 is made into a multilayer shield with electrical insulation between the layers as shown in FIG.

〔Effect of the invention〕

As is clear from the above description, according to the present invention, MR
In a first-class taliostat, it is possible to suppress not only constant heat intrusion such as conduction and radiation, but also eddy current loss that occurs in the refrigerant container when a gradient coil is applied, and as a result, the refrigerant It is possible to provide thaliostat with low evaporation amount.

[Brief explanation of drawings]

FIG. 1 is a sectional view of a cryostat 1 according to an embodiment of the present invention, FIG. 2 is a sectional view of a main part of another embodiment, and FIG. 3 is a schematic diagram of a conventional cryostat 1. 1...Small refrigerator 2...Heat shield 3...Refrigerant container 4...Gradien 1 Coil 5...Superconducting coil 9...Insulating layer agent Patent attorney Noriyuki Ken Yudo Ken Deshimaru Procedural amendment (voluntary) 1998

Claims (1)

[Claims] In a cryostat that is equipped with a cryogenic refrigerant container that houses a superconducting coil, a heat shield and a vacuum container that are arranged so as to surround this container, and is arranged opposite to a gradient coil, at least the gradient coil and the superconductor of the heat shield are arranged. A cryostat characterized in that the portion between the coils is composed of two or more shielding materials that are thermally bonded to each other.