JP2000150224A - Excitation control method for superconducting coil - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an excitation control method for a superconducting coil which can restrain the AC loss to the utmost. SOLUTION: This excitation control method of a superconducting coil is a method for controlling the excitation of a superconducting coil, and an exciting current of the superconducting coil is restrained to be at most 60% of a critical current of the superconducting coil. Excitation of the superconducting coil is performed by an AC current. The superconducting coil is cooled through heat conduction to a refrigerator. A superconducting wire used for the superconducting coil is constituted of an oxide superconducting wire which is covered with a metal. An AC current used for exciting current has a periodicity of 0.001-0.1 Hz.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for controlling the operation of a conduction cooling type superconducting coil of a refrigerator, and more particularly, to a method of operating a conduction cooling type superconducting coil constituting a superconducting magnet stably without causing quench. On how to do.

[0002]

2. Description of the Related Art Conventionally, coils using a normal conductor such as copper and coils using a metal-based superconductor that becomes superconducting at liquid helium temperature have been used. When attempting to generate a high magnetic field using copper, heat is generated so much that it is necessary to forcibly flow water or the like for cooling. A coil using a normal conductor such as copper has problems such as high power consumption, lack of compactness, and difficulty in maintenance.

On the other hand, superconducting coils are useful for various applications because they can generate a large magnetic field with small power. But,
When using a metal-based superconducting wire for the coil, the temperature must be extremely low
(Neighborhood) is required, and the cooling cost increases. Further, since the metal-based superconductor is used at an extremely low temperature having a small specific heat, it has poor stability and is liable to be quenched.

On the other hand, recently, techniques for performing magnetic separation, crystal pulling, and the like using an oxide high-temperature superconducting coil that can be used at a relatively high temperature have been considered. Since the oxide high-temperature superconducting coil can be used at a relatively high temperature as compared with the metal-based superconducting coil, it has been found that the oxide high-temperature superconducting coil can be used in a region having a large specific heat and has excellent stability. Oxide high-temperature superconducting coils are expected to be put to practical use in magnets that are easier to use.

[0005] Oxide high-temperature superconductors become superconducting at liquid nitrogen temperatures, but at liquid nitrogen temperatures so far the critical current density and its magnetic field properties are not very good. For this reason,
At present, oxide high-temperature superconductors are used as coils for generating a low magnetic field. On the other hand, oxide high-temperature superconducting coils may have higher performance at temperatures lower than liquid nitrogen temperature. For use at this low temperature, cooling with liquid helium is considered,
Its cost is high and handling is complicated. Therefore, an attempt has been made to cool the oxide superconducting coil to a very low temperature using a refrigerator whose operation cost is relatively low and which is easy to handle.

That is, since a coil using a metal-based superconducting wire is unstable, no AC energizing operation has been performed for a refrigerator-cooled type coil. However, a coil using an oxide superconducting wire has a high stability. Attempts are also being made to try AC energizing operation in machine-cooled coils. However, since a refrigerator cooling coil generally has a cooling capacity of only several W to several tens of W, it is desired to reduce the heat generated by the coil as much as possible.

[0007]

The oxide superconducting wire is
Because of the brittle nature of ceramics, it is difficult to make it ultrafine and multifilamentary, and it is composed of about 61 filaments contained in the metal matrix layer. In addition, the 61-filament filaments are in a state of being electrically coupled via a metal layer interposed therebetween, and electromagnetically, the entire 61-filament behaves as a single core. As a result, there is a problem that the coil made of the oxide superconducting wire generates a large AC loss and generates heat when the AC operation is performed.

In view of such problems in the prior art,
An object of the present invention is to provide a superconducting coil excitation control method capable of suppressing AC loss as much as possible.

[0009]

The superconducting coil excitation control method according to the present invention is characterized in that the exciting current of the superconducting coil is limited to 60% or less of the superconducting coil critical current.

The method of controlling excitation of a superconducting coil has a great effect of reducing AC loss, particularly when exciting by an AC current.

Further, in the excitation control method for a superconducting coil, when applied to a refrigerator-cooled type coil having a limited cooling capacity, stable excitation can be performed even under the limited cooling capacity. .

Further, this excitation control method for a superconducting coil can obtain more favorable effects in a coil using a metal-coated oxide superconducting wire that is more stable than a coil using a metal-based superconducting wire.

Furthermore, this excitation control method for a superconducting coil can be preferably applied to excitation by an alternating current having a periodicity of, for example, 0.001 to 0.1 Hz.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As an example of an embodiment of the present invention, the following example was attempted.

A bundle of three Bi2223 silver-coated bismuth-based superconducting wires (3.6 ± 0.4 mm × 0.23 ± 0.02 mm) is combined with a polyimide tape having a thickness of about 13 μm and a SUS tape having a thickness of about 0.1 mm. Co-wound, inner diameter 80m
m, a double pancake coil having an outer diameter of about 300 mm and a height of about 8 mm. The silver ratio of the superconducting wire used was 2.4.
And its critical current was 35-45 A (77 K). Eight layers of the produced double pancake coils were laminated and jointed. Between the double pancake coils,
It was insulated with a 0.1 mm thick FRP sheet.

As shown in FIG. 2, a copper cooling plate 32 (a part of which is omitted for clarity of the drawing) is inserted between each double pancake coil 31, and each cooling plate 32 is made of copper. It was joined to the heat conducting rod 33. With the double pancake coil 31 laminated between the pair of FRP plates 34, a high temperature superconducting coil structure 30 was obtained.

The obtained high-temperature superconducting coil was mounted on a refrigerator as shown in FIG. The first stage 41a and the second stage 41b, which are the cooling stages of the refrigerator 41,
It is housed in a heat insulating container 42. 2nd stage 41b
, A copper plate 43 is fixed. The high-temperature superconducting coil 30 is connected to the second stage 4 of the refrigerator 41 via the copper plate 43.
1b. The current lead 44 made of an oxide high-temperature superconducting wire is provided from the high-temperature superconducting coil 30 to the temperature anchor portion of the first stage 41a. The current leads 44 effectively suppress heat penetration. Further, a current lead 45 made of copper was used from the temperature anchor portion of the first stage 41a to room temperature. The high-temperature superconducting coil 30 is covered with a heat shield plate 46 for blocking invasion of radiant heat. The inside of the heat insulating container 42 is evacuated. The coil packing ratio in the superconducting coil 30 was 75%.

[0018] In such a refrigerator-cooled coil, 0 A is applied at various frequencies of 0.001 to 0.1 Hz.
The excitation and demagnetization were repeated by alternating currents of various triangular waves having various peak currents, and the coil heat generated during each of the alternating current excitation was measured. The result is shown in the graph of FIG. In the graph of FIG. 1, the horizontal axis represents the peak current value (A) of the AC current, and the vertical axis represents the heat generation per AC cycle in joules (J). The solid curve in the graph represents the actual measured value, and the dashed curve represents the calculated expected value.

During the actual measurement of the graph of FIG. 1, the temperature of the coil was maintained at about 20K. Further, the coil heat generation per unit time (J / sec) increases as the frequency of the alternating current increases, and is proportional to the frequency. This means that the heat generation of the coil is mainly based on the AC hysteresis loss. On the other hand, an expected value represented by a broken line in the graph represents a result calculated using a so-called slab approximation. This slab approximation method is, for example, Wi
lson, SUPERCONDUCTING MAGNETS, Copyright 1983, published by CLAREDON PRESS, pp. 162-163.

As is clear from the graph of FIG. 1, the hysteresis loss increases in proportion to the peak current value of the alternating current in the predicted result of the calculation, whereas the measured peak current value is about 150 A in the actual measurement result. It can be seen that the heat generation increases rapidly when the value exceeds. The critical current of the coil is about 240 A, and the generated magnetic field at this time is about 3 T for the coil center field, about 3.5 T for the maximum magnetic field parallel to the tape surface, and about 3.5 T for the maximum magnetic field perpendicular to the tape surface. 1.5T.

From the above, when exciting the superconducting coil, by controlling the exciting current to about 60% or less of the critical current of the coil, it is possible to prevent a sharp increase in the heat generation of the coil and to achieve stable excitation. Can be performed.

[0022]

As described above, according to the present invention, it is possible to provide a method of controlling excitation of a superconducting coil in which AC loss is suppressed as much as possible.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between an excitation current of a superconducting coil measured in one embodiment of the present invention and heat generation.

FIG. 2 is a schematic diagram showing the structure of a superconducting coil used in the measurement of the graph of FIG.

FIG. 3 is a schematic diagram showing a connection structure between a refrigerator and a superconducting coil used in the measurement of the graph of FIG.

[Explanation of symbols]

 Reference Signs List 30 superconducting coil 31 double pancake coil 41 refrigerator 41a first cooling stage 41b second cooling stage

Claims (5)

[Claims] 1. A method for controlling excitation of a superconducting coil, wherein the excitation current of the superconducting coil is limited to 60% or less of a critical current of the superconducting coil. 2. The method for controlling excitation of a superconducting coil according to claim 1, wherein the excitation of the superconducting coil is performed by an alternating current. 3. The excitation control method for a superconducting coil according to claim 1, wherein the superconducting coil is cooled by heat conduction to a refrigerator. 4. The superconducting coil excitation control method according to claim 1, wherein the superconducting wire used for the superconducting coil is made of a metal-coated oxide superconducting wire. 5. The excitation control method for a superconducting coil according to claim 2, wherein the alternating current used as the excitation current has a periodicity of 0.001 to 0.1 Hz. .