GB2361523A

GB2361523A - Superconducting magnet apparatus

Info

Publication number: GB2361523A
Application number: GB0110697A
Authority: GB
Inventors: Yasutsugu Morii
Original assignee: Toshiba Corp
Current assignee: Toshiba Corp
Priority date: 1998-03-31
Filing date: 1999-03-24
Publication date: 2001-10-24
Anticipated expiration: 2019-03-24
Also published as: GB2361523B; GB0110697D0

Abstract

A superconducting magnet apparatus comprises a superconducting coil (1) for generating a magnetic field, a radiation shield (2) that surrounds the superconducting coil, a refrigerator (7) for cooling the superconducting coil, and a cryostat (13) provided inside the radiation shield to store a coolant cooled by the refrigerator. The cryostat is thermally connected to the superconducting coil by a cooling pipe (19). Coolant, eg liquid helium, may also be stored during operation of the superconducting coil (1) in a storage tank (14) located outside the vacuum vessel (4), or in a reservoir (27<U>a</U>, Fig 4) located inside the radiation shield (2).

Description

2361523

TITLE OF THE INVENTION

SUPERCONDUCTING MAGNET APPARATUS BACKGROUND OF THE INVENTION

The present invention relates to a superconducting magnet apparatus for, e.g., a synchrotron orbital radiation device.

For cooling a superconducting coil for a superconducting magnet apparatus, immersion cooling of immersing a superconducting coil in a coolant and cooling it with the latent heat of evaporation of the coolant, and direct cooling with a refrigerator are generally used.

FIG. 1 is an example of a superconducting magnet apparatus employing immersion cooling and shows a superconducting magnet apparatus for a synchrotron orbital radiation device. The superconducting magnet apparatus shown in FIG. 1 comprises a pair of superconducting coils 1. A radiation shield 2 surrounds the superconducting coils 1, and a high temperature7 side thield 3 and a vacuum vessel 4 surround the radiation shield 2.

The superconducting coils 1 are respectively stored in coil containers 18, and a helium container 6 containing liquid helium 5 as a coolant and the coil containers 18 communicate with each other through pipes 6a. The superconducting coils 1 are immersed in the liquid helium 5 and held at a temperature of about 4 superconducting coil 1 from the outside by convection, conduction, or radiation. As described above, since the cooling capacity of one refrigerator 7 is limited, in the case of the refrigerator direct cooling type superconducting magnet apparatus, it is desired to decrease this heat invasion as much as possible.

In the conventional superconducting magnet apparatus that employs immersion cooling, as shown in FIG. 1, superconducting coils 1 are immersed in the liquid helium 5 to be cooled by its latent heat of evaporation. While this apparatus has high cooling characteristics, its liquid helium 5 is difficult to handle.

More specifically, prior to the operation, the liquid helium 5 must be reserved in the coil containers 18 that store the superconducting coils 1. This must be done by a person skilled in the art who has a necessary qualification. When the superconducting coils 1 are quenched (shift from superconduction to normal conduction) by a disturbance, they generate a very large Joule heat, and the reserved liquid helium 5 evaporates instantaneously. Generally, evapor ated helium gas is stored in an external gas back temporarily or is discharged to the atmosphere. In this manner, when the superconducting coils 1 are quenched, liquid helium 5 must be supplied to the helium container 6 again.

3 through a heat conducting member 12, and a high temperature-side stage 7b thereof is thermally connected to the radiation shield 2. The low and high temperature- side stages 7a and 7b are respectively cooled to temperatures of about 4. 2 K and 80 K. In this manner, since the refrigerator direct cooling type superconducting magnet apparatus does not use liquid helium 5, it is easy to handle and is suitable as a comparatively compact superconducting magnet apparatus.

The refrigerator 7 for holding a temperature of 4.2 K currently has a capacity of as low as about 1 W and thus cannot be used for a large superconducting magnet apparatus.

In this superconducting magnet apparatus, the superconducting coil I is cooled to about 4.2 K by heat conduction with the low temperature - side stage 7a of the refricarator 7 through the heat conducting member 12, so that its electric resistance becomes zero to reach a so-called superconducting state. In this state, an energizing current is supplied to the super conducting coil I from an external power supply,(not shown) to generate a required magnetic field.

During ordinary operation, since the super conducting coil 1 has no electric resistance, the superconducting coil 1 does not generate heat by its.elf with Joule heat even if a current is supplied to it.

However, there is influx of heat into the 6 Therefore, the number of refrigerators 7 must be increased, or a refrigerator 7 having a large capacity must be loaded to remove heat generated by AC loss. AC loss occurs only during short-time energization/ deenergization, and such a measure is very uneconomical when considering long-term ordinary operation. When a large superconducting coil 1 is to be employed or a plurality of superconducting coils 1 are to be cooled with one refrigerator 7, as the refrigerator 7 and the superconducting coils 1 are thermally connected to each other through the heat conducting member 12, a temperature difference occurs among the respective portions of the superconducting coil 1 or among the respective superconducting coils 1 to cause quenching.

BRIEF SUMMARY OF THE INVENTION

The present invention has been made in order to solve the conventional problems described above, and has as its object to provide a superconducting magnet apparatus in which a superconducting coil need not be immersed in,a coolant and which has a high cooling capacity, can be handled easily, and is economical, thus improving the reliability.

The amount of liquid helium 5 to be used must be decreased as much as possible. However, in the case of immersion cooling, the use amount of liquid helium 5 is often determined by the size of the coil containers 18 depending on the size of the superconducting coils 1, and an optimum amount of helium liquid is not always stored. This causes a difficulty in handling and poses a problem in terms of conservation of natural resources as well.

Since the superconducting magnet apparatus employing direct cooling with a refrigerator as shown in FIG. 2 does not use liquid helium, it does not require liquid supplying operation and the like and can thus be handled easily. However, the cooling capacity of this apparatus is determined by the capacity of the mounted refrigerator 7. Generally, the superconducting coil 1 generates no heat while a constant current is supplied to it. However, during energization/ deenergization such as turning ONIOFF, heat is generated by a large AC loss. When turning ONIOFF is very slow and takes a long period of time (from;several ten minutes to 1 hour), cooling with the refrigerator can be performed. However, in a superconducting magnet apparatus that must be energized/deenergized within a short period of time (within several ten minutes), the AC loss sometimes reaches 10 times or more the heat inf lux.

Figure 3 is a sectional view of a superconducting magnet apparatus according to a first embodiment of the present invention; and Figure 4 is a sectional view of a superconducting magnet apparatus according to a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of a superconducting magnet apparatus according to the present invention will be described with reference to the accompanying drawings. FIG 3 is a sectional view of a superconducting magnet apparatus according to the first embodiment of the present invention.

Referring to FIG. 3, a superconducting coil 1 is surrounded by a radiation shield 2, and the radiation shield 2 is surrounded by vacuum vessel 4. A cryostat 13 is disposed above the superconducting coil 1 and thermally connected to it by cooling pipes 19.

A low temperature-side stage 7a of a refrigerator 7 is inserted in the container 13a of the cryostat 13, and a high temperature-side stage 7b thereof is thermally connected to the radiation shield 2. A storage tank 14 for storing a coolant gas is provided to the vacuum vessel 4. The cryostat 13 and storage tank 14 communicate with each other through a communicating pipe 15. A coolant such as liquid helium 5 condensed by the low temperature-side stage 7a of the refrigerator 7 is stored in the cryostat 13.

Current leads 16 serve to supply a current from an external power supply (not shown) to the superconducting coil 1. The superconducting coil 1 and cryostat 13 may also be provided with pre-cooling pipes (not shown). The pre-cooling pipes are connected to a supply system placed outside the vacuum vessel 4 to supply a pre heating coolant.

To operate the superconducting magnet apparatus according to the first embodiment having this arrangement, the interior of the vacuum vessel 4 is evacuated to a high vacuum degree by a vacuum pump (not shown), and the radiation shield 2 is cooled to a predetermined temperature by the refrigerator 7. If the superconducting coil In order to achieve the above object, according to the first aspect of the present invention, there is provided a superconducting magnet apparatus comprising:

a superconducting coil for generating a magnetic field; a radiation shield surrounding said superconducting coil; a refrigerator for cooling said superconducting coil; a container provided inside said radiation shield to store a coolant which is liquefied by said refrigerator; a thermally conducting connection comprising a cooling pipe between said container and said superconducting coil, and a storage tank connected to said container for storing evaporated coolant, wherein the refrigerator liquefies the coolant in the container.

Preferably a vacuum vessel is arranged to surround the radiation shield, a storage tank is provided for the vacuum vessel to store evaporated coolant, and a communicating pipe for allowing the storage tank and the cryostat to communicate with each other, so that when the coolant gasifies in the cryostat by quenching or the like, the evaporated coolant is stored in the storage tank formed in the vacuum vessel through the communication pipe.

The storage tank may be integrally formed with the vacuum vessel.

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.

Figure 1 is a sectional view of a conventional superconducting magnet apparatus employing immersion cooling; Figure 2 is a sectional view of a conventional superconducting magnet apparatus employing direct cooling with a refrigerator; heat inf lux, and the evaporated coolant gas is therefore liquefied again in the cryostat 13.

According to this first embodiment, the cryostat 13 is provided. To cool the interior of the cryostat 13, a minimum amount of coolant necessary when the heat load exceeds the cooling capacity of the refrigerator 7 is stored in the cryostat 13, thereby cooling the superconducting coil 1 by conduction. The super conducting coil 1 can thus be cooled efficiently without immersing it in liquid helium. Thus, no coil container 18 is necessary for storing the superconducting coil 1.

As for non-steady state heat generation during energization/deenergization and the like, the heat can be removed by the latent heat of evaporation of the stored coolant. At this time, the evaporated coolant gas is temporarily stored in the storage tank 14 and liquefied again during ordinary operation. The coolant need not be supplied from the outside, and the apparatus is thus easy to handle.

In place of condensing liquid helium by the low temperature -side stage 7a of the refrigerator 7 and storing it in the cryostat 13, liquid helium in an amount corresponding to the evaporated amount may be filled in the cryostat 13 from the outside. The storage tank 14 may be formed integrally with the vacuum vessel 4. Although the liquid helium 5 is used -9 1 is a small one, it can be cooled to a predetermined temperature (e.g., 4.2 K) by only the refrigerator 7. If a 1-ton class superconducting coil 1 is used, pre-cooling takes as long as about one week.

If such a large superconducting coil 1 is used, it is pre-cooled by supplying the pre- cooling coolant to the pre-cooling pipes. For example, liquid nitrogen is supplied to the pre-cooling pipes to cool the superconducting coil 1 to 80 K, so that the pre-cooling time is shortened to about 1/3. With copper, stainless steel, or the like that generally forms the superconducting coil 1, the higher the temperature, the larger its large specific heat. Therefore, a large effect can be obtained when the superconducting coil 1 is pre-cooled to 80 K. From the pre-cooling temperature of 80 K to 4.2 K, the superconducting coil 1 is cooled by the refrigerator 7. When liquid helium 5 is supplied into the cryostat 13 from the outside through a supply pipe, the superconducting coil 1 can be pre-cooled from 80 K down to 4 K within a short period of time (about 1 hour). When pre-cooling is complete, the coolant gas stored in the storage tank 14 by continuous operation of the refrigerator 7 is condensed to be liquefied by the low temperature-side stage 7a in the cryostat 13.

When the superconducting coil 1 is energized/deenergized, an AC loss is produced, and the heat load as the sum of the AC loss and the heat influx exceeds the cooling capacity of the refrigerator 7. In this case, the liquid helium stored in the cryostat 13 evaporates to compensate for the insufficient cooling capacity of the refrigerator 7 with its latent heat of its evaporation. The coolant gas evaporated at this time is temporarily stored in the storage tank 14. In ordinary operation, the superconducting coil 1 has no electric resistance. Even when a current is supplied to the superconducting coil 1, no Joule heat is generated but only heat influx exists.

At this time, the cooling capacity of the refrigerator 7 exceeds the 12 thermal contact with the superconducting coil 1 by adhesion or the like, so that the cooling pipes 19 can follow deformation of the superconducting coil 1.

A second embodiment of the present invention will now be described. FIG. 4 is a sectional view of a superconducting magnet apparatus according to the fifth embodiment of the present invention. In the second embodiment, a coolant cooled by a refrigerator 7 is circulated through a cryogenic pipe 27 provided in thermal contact with a superconducting coil 1 directly or indirectly, thereby cooling the superconducting coil 1.

Referring to FIG. 4, in the superconducting magnet apparatus, the superconducting coil 1 is surrounded by a radiation shield 2, which is, in turn surrounded by a vacuum vessel 4. A refrigerating/liquefying machine 28 is constituted by the refrigerator 7 and a compressor 29. The cryogenic pipe 27 connected to the refrigerating/liquefying machine 28 is mounted in thermal contact with the superconducting coil 1.

To operate this superconducting magnet apparatus, the interior of the vacuum vessel 4 is evacuated to a high vacuum degree by a vacuum pump (not shown), and the radiation shield 2 and superconducting coil 1 are cooled to a predetermined temperature by the refrigerating/liquefying machine 28. When pre-cooling is completed, liquid helium 5 is liquefied and reserved in the cryogenic pipe 27 by continuous operatioNof the refrigerating.

When the superconducting coil 1 is energized/deenergized, heat is generated by an AC loss- The heat load as the sum of the AC loss and the heat influx exceeds the cooling capacity of the refrigerating/liquefying machine 28. In this case, the liquid helium 5 stored in the cryogenic pipe 27 evaporates to compensate for the insufficient cooling capacity of the refrigerating/liquefying machine 28 with its latent heat of its evaporation. The coolant gas evaporated at this time is temporarily stored in the compressor 29 constituting the refrigerating/liquefying machine 28.

as the coolant in this embodiment, in the case of a high-temperature super-conducting magnet apparatus or the like, liquid nitrogen may be used as the coolant. Although the low temperature-side stage 7a is inserted in the cryostat 13, it need not be inserted if the low temperature-side stage 7a is thermally connected to the cryostat 13 directly or 5 indirectly.

The cooling pipes 19 for circulating the liquid helium stored in the cryostat 13 are provided in thermal contact with the superconducting coil 1, thereby cooling the superconducting coil 1.

Heat influx into the superconducting coil 1 or heat generated by AC loss is transferred to liquid helium 5 through the pipe walls of the cooling pipes 19. During heat transfer, the liquid helium 5 evaporates to absorb the generated heat with the latent heat of evaporation. The evaporated helium 5 is returned to the cryostat 13 and liquefied again to flow through the cooling pipes 19, so as to cool the superconducting coil 1.

In this embodiment, since the superconducting coil 1 is cooled by the latent heat of evaporation of the liquid helium 5 flowing through the cooling pipes 19, no temperature difference occurs in the cooling pipes 19, and the temperature of the cooling pipes 19 is always maintained at 4.2 K, which is the temperature of liquid helium. Hence, when compared to conduction cooling using the heat conducting member 12, any temperature increase of the superconducting coil 1 can be decreased very small, and the superconducting coil 1 can be operated stably.

To connect the cooling pipes 19 to the superconducting coil 1, the cooling pipes 19 are formed into winding pipes having flexed portions on their ends in the axial direction of the superconducting coil 1. As a result, when the superconducting coil 1 deforms by the electromagnetic force, the flexed portions of the cooling pipes 19 can move free from the superconducting coil 1 while only their linear portions stay in 1 4 formed in part of the cryogenic pipe 27, so that the amount of coolant stored in the cryogenic pipe 27 can be increased. Even when non-steady state heat generation occurs during energization/deenergization or the like, the superconducting coil 1 can be operated stably. Furthermore, the cryogenic pipe 27 need not be directly attached to the superconducting coil 1, but may be attached to a cooli ng member that is in thermal contact with the superconducting coil 1, to indirectly cool the superconducting coil 1.

As has been described above, according to the present invention, a superconducting coil can be cooled ,efficiently without immersing it in a coolant. Even if energization/deenergization is performed often or the energization/deenergization time is short to generate"a large amount of heat by an AC loss, the superconducting coil can be operated stably by minimizing an increase in its temperature. As a result, an easy-to-handle superconductiing magnet apparatus having a high cooling capacity and high reliability can be provided.

More specifically, a minimum amount of coolant required for cooling is stored in a cryostat, and the superconducting coil is conduct ion-coo led through' a heat conducting member. The superconducting coil can be cooled efficiently without immersing it in liquid 13 - In ordinary operation, the superconducting coil 1 has no electric resistance. Even when a current is supplied to the superconducting coil 1, no Joule heat is generated but only heat influx exists. At this time, the cooling capacity of the refrigerating/liquefying machine 28 exceeds the heat influx, and the evaporated 5 coolant gas is therefore liquefied again to be reserved in the cryogenic pipe 27.

According to this second embodiment, a minimum amount of coolant necessary for cooling is stored in the cryogenic pipe 27 to cool the superconducting coil 1. The superconducting coil 1 can thus be cooled efficiently without immersing it in liquid helium. No coil container 18 is necessary for storing the superconducting coil 1. As for non-steady state heat generation during energization/deenergization and the like, the heat can be removed by the latent heat of evaporation of the stored coolant. At this time, the evaporated coolant gas is liquefied again by the refrigerating/liquefying machine 28. The coolant need not be supplied from the outside, and the apparatus is thus easy to handle.

Since the superconducting coil 1 is cooled by heat transfer and heat of evaporation of the liquid helium flowing through the cryogenic pipe 27, as compared to conduction cooling using a heat conducting member, a temperature increase in the superconducting coil 1 can be minimized. As a result, the superconducting coil 1 can be operated stably. Only the cryogenic pipe 27 need be connected to the superconducting coil-1 and no other heat conducting member is necessary, simplifying the structure.

As shown in FIG. 4, a coolant reservoir 27a having a diameter greatly larger (having a larger volume per unit length) than that of the cryogenic pipe 27 is 16 pipe, the amount of coolant stored in the coolant pipe can be increased. Even if non-steady state heat generation occurs during energization/deenergization or the like, the superconducting coil can be operated stably.

Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made Within the scope of the appended claims.

higher the temperature, the larger its specific heat.

If the superconducting coil is pre-cooled by inexpensive liquid nitrogen having a large heat removing capacity from 300 K to 80 K, the pre-cooling time can be shortened greatly.

According to the present invention, an external gas storage tank is not needed. No space is necessary to install pipes through which such a gas storage tank and the superconducting magnet communicate with each other, so that the apparatus can be placed compactly.

According to the present invention, the weight and manufacturing cost can be decreased. If the cylindrical portion of the vacuum vessel is formed as a double-wall container, the plate thickness can be decreased, and an increase in outer diameter of the vacuum vessel can be minimized, thereby forming a large-capacity storage tank.

Since the superconducting coil is cooled by heat transfer of liquid helium flowing through the coolant pipe, as compared'to conduction cooling employing a heat conducting member, a temperature increase of the superconducting coil can be minimized. As a result, the superconducting coil can be operated stably. Only a cryogenic pipe need be provided to the super conducting coil, and no other heat conducting member is required, simplifying the structure.

When a liquid reservoir is provided to the coolant -17

Claims

1. A superconducting magnet apparatus comprising:

2. Apparatus according to claim 1, wherein said storage tank is integrated with a vacuum vessel that surrounds said radiation shield.

3. A superconducting magnet apparatus substantially as herein described with reference to figure 3 or figure 4 of the accompanying drawings.

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