Method for cooling a superconducting magnet and the superconducting magnet

A method includes the steps of: bringing a refrigerator's distal end into contact with a contact of a heat transfer member to thermally connect the refrigerator via the heat transfer member to a superconducting coil to cool the superconducting coil to cryogenic temperature; after the step of bringing the refrigerator's distal end into contact with the contact of the heat transfer member, bringing the refrigerator's distal end out of contact with the contact of the heat transfer member; and after the step of bringing the refrigerator's distal end out of contact with the contact of the heat transfer member, injecting liquid helium into a helium tank.

TECHNICAL FIELD

The present invention relates to a method for cooling a superconducting magnet and the superconducting magnet.

BACKGROUND ART

Japanese Patent Laying-Open No. 2009-32758 (PTD 1) is a prior art document disclosing a configuration of a conduction-cooled superconducting magnet device with a superconducting coil less quenchable despite power failure.

The conduction-cooled superconducting magnet device described in PTD 1 includes a cryogenic refrigerator, a tank having a refrigerant therein, a superconducting coil immersed in the refrigerant, and a heat transfer means in thermal contact with both the tank and the cryogenic refrigerator for allowing thermal conduction therebetween.

When the conduction-cooled superconducting magnet device has the cryogenic refrigerator in operation, it is adapted to allow the thermal conduction between the tank and the cryogenic refrigerator via the heat transfer means to cool the tank. Once the cryogenic refrigerator has stopped from operating, an interruption means that is provided for the heat transfer means interrupts the thermal conduction between the tank and the cryogenic refrigerator to prevent the heat transfer means from letting external heat enter the tank and thus vaporize the refrigerant.

CITATION LIST

Patent Document

SUMMARY OF INVENTION

Technical Problem

While PTD 1 describes the heat transfer means in thermal contact with both the superconducting coil and the refrigerator for allowing thermal conduction therebetween and the interruption means provided for the heat transfer means to interrupt the thermal conduction between the superconducting coil and the refrigerator, the document is silent on how they are specifically configured.

Furthermore, if a heat transfer switch which is a movable member is provided in a helium tank, the heat transfer switch may be frozen and not operate, and cannot reliably interrupt thermal conduction between the superconducting coil and the refrigerator.

The present invention has been made in view of the above issue and contemplates a method for cooling a superconducting magnet and the superconducting magnet, that can reliably prevent heat intrusion through a refrigerator when the refrigerator is not in operation.

Solution to Problem

The present invention provides a method for cooling a superconducting magnet including: a helium tank provided to store liquid helium therein; a superconducting coil accommodated in the helium tank and immersed in the liquid helium; a vacuum vessel having the helium tank accommodated therein; a refrigerator detachably secured to the vacuum vessel and having a distal end in the helium tank; and a heat transfer member located in the helium tank and thermally connected to the superconducting coil in contact therewith, and having a contact allowed to contact the distal end of the refrigerator. The method for cooling the superconducting magnet includes the steps of: bringing the refrigerator's distal end into contact with the contact of the heat transfer member to thermally connect the refrigerator via the heat transfer member to the superconducting coil to cool the superconducting coil to cryogenic temperature; after the step of bringing the refrigerator's distal end into contact with the contact of the heat transfer member, bringing the refrigerator's distal end out of contact with the contact of the heat transfer member; and after the step of bringing the refrigerator's distal end out of contact with the contact of the heat transfer member, injecting the liquid helium into the helium tank.

Advantageous Effect of Invention

The present invention can thus reliably prevent heat intrusion through a refrigerator when the refrigerator is not in operation.

DESCRIPTION OF EMBODIMENTS

Hereafter, reference will be made to the drawings to describe a method for cooling a superconducting magnet and the superconducting magnet according to a first embodiment of the present invention. In describing the following embodiments, identical or corresponding components are identically denoted and will not be described repeatedly in detail.

First Embodiment

FIG. 1shows in cross section a superconducting magnet according to a first embodiment of the present invention when it has a superconducting coil cooled to cryogenic temperature.FIG. 2shows in cross section a refrigerator in the state ofFIG. 1in an enlarged view. Note thatFIG. 1does not show an expansion member. Furthermore,FIG. 2shows the expansion member unexpanded.

FIG. 3shows in cross section the superconducting magnet according to the present embodiment when the superconducting coil has been cooled by the refrigerator and liquid helium is injected.FIG. 4shows in cross section the refrigerator in the state ofFIG. 3in an enlarged view.

As shown inFIG. 1toFIG. 4, the present invention in the first embodiment provides a superconducting magnet100including a helium tank120provided to store liquid helium130therein, a superconducting coil110accommodated in helium tank120and immersed in liquid helium130, and a vacuum vessel150having helium tank120accommodated therein. In the present embodiment, a heat shield140is disposed between helium tank120and vacuum vessel150.

Furthermore, superconducting magnet100includes: a cylindrical portion160extending from vacuum vessel150to helium tank120to allow communication between the outside of vacuum vessel150and the interior of helium tank120; a refrigerator inserted in cylindrical portion160and detachably secured to vacuum vessel150, and having a distal end in helium tank120; and a heat transfer member180located in helium tank120and thermally connected to superconducting coil110in contact therewith. Heat transfer member180has a contact182located under cylindrical portion160and allowed to contact the distal end of the refrigerator.

Superconducting magnet100is configured, as will be described hereafter more specifically.

Superconducting coil110is made of a superconducting wire of a niobium titanium alloy wound in helium tank120on a bottom surface thereof in the form of a solenoid. Note that the superconducting wire is not limited in material to the niobium titanium alloy, and may for example be a niobium tin alloy. Superconducting magnet100has a plurality of superconducting coils110. When a current received from an external power supply (not shown) passes through superconducting coil110, a magnetic field is generated in an area in a direction indicated by an arrow10.

Helium tank120is formed of stainless steel and is generally annular in geometry in a side view. Note that helium tank120is not limited in material to stainless steel, and may be of any material having large rigidity.

As has been described above, helium tank120has a function as a spool for superconducting coil110. Superconducting coil110experiences large electromagnetic force. Accordingly, helium tank120is required to have large rigidity to be capable of securing superconducting coil110at a prescribed position against the electromagnetic force acting on superconducting coil110.

Furthermore, helium tank120has an upper portion with a piping161connected thereto for supplying helium tank120with helium. Piping161has a proximal end outside vacuum vessel150. Piping161has the proximal end with a valve162provided for opening/closing piping161.

Heat shield140is generally annular in a side view and surrounds helium tank120as seen in cross section. Heat shield140prevents helium tank120from having external heat intrusion through thermal radiation. While heat shield140is formed of aluminum, heat shield140is not limited in material thereto and may be of any material having high thermal conductivity.

Vacuum vessel150has superconducting coil110, helium tank120, and heat shield140accommodated therein. Vacuum vessel150has its interior and exterior vacuum-insulated. Vacuum vessel150in a side view is generally annular in geometry.

Helium tank120, heat shield140, and vacuum vessel150together configure a cryostat that is a structure that reduces/prevents heat intrusion into superconducting coil110. In the present embodiment when the cryostat has an internal temperature of 4 K it has heat intrusion in an amount of 0.6 W.

As has been described above, the cryostat is provided with cylindrical portion160for attaching the refrigerator. Cylindrical portion160has an upper end connected to an open end of vacuum vessel150, and a lower end connected to an open end of helium tank120.

In the present embodiment, superconducting magnet100has heat transfer member180having contact182located immediately under the lower end of cylindrical portion160. Heat transfer member180has a plurality of connections181thermally connected to a plurality of superconducting coils110, respectively, in contact therewith. Note, however, that heat transfer member180is in contact with each superconducting coil110with an insulating paper interposed and is thus electrically insulated therefrom. Heat transfer member180is formed of copper. Note that heat transfer member180is not limited in material to copper, and may be of any material having large thermal conductivity.

In the present embodiment, heat transfer member180has contact182shaped to be fittable to the distal end of the refrigerator. Specifically, contact182has a recess slightly larger in geometry than the distal end of the refrigerator. Note that contact182is not limited in geometry as described above, and may be of any geometry allowing it to contact the distal end of the refrigerator.

In the present embodiment, the refrigerator includes a refrigerator body170thereof and an extension member attached to a distal end of refrigerator body170. Refrigerator body170is a Gifford-McMahon (GM) refrigerator. Refrigerator body170has a refrigeration capacity of 1 W for a temperature of 4 K and thus has a refrigeration capacity sufficient for the amount of heat intrusion into the cryostat (i.e., 0.6 W). Note that the refrigerator is not limited in type to the GM refrigerator and may be any other type of refrigerator such as a pulse tube refrigerator.

Refrigerator body170has two cooling stages. A first cooling stage171is in contact with heat shield140. A second cooling stage172is connected to the extension member. Second cooling stage172and the extension member are cylinders having substantially equal diameters, respectively. While the extension member is formed of copper, the extension member is not limited in material thereto and may be of any material having high thermal conductivity.

In the present embodiment, two extension members different in length are selectively used. Specifically, when superconducting coil110is cooled to cryogenic temperature, a long extension member190shown inFIG. 2is used, and once superconducting coil110has been cooled by the refrigerator, a short extension member192shown inFIG. 4is used.

Long extension member190has a length L1and short extension member192has a length L2, and length L1is larger than length L2. Long extension member190has a heater191incorporated therein, and short extension member192a heater193incorporated therein.

As shown inFIG. 2andFIG. 4, the refrigerator attached in cylindrical portion160has refrigerator body170with the distal end positioned in helium tank120and spaced from contact182of heat transfer member180.

As shown inFIG. 2, refrigerator body170having the distal end with long extension member190attached thereto configures a long refrigerator having a length allowing the long refrigerator to have a distal end thereof in contact with contact182of heat transfer member180.

As shown inFIG. 4, refrigerator body170having the distal end with short extension member192attached thereto configures a short refrigerator having a length allowing the short refrigerator to have a distal end thereof out of contact with contact182of heat transfer member180.

In the present embodiment, the refrigerator has the distal end with a surface having an expansion member199attached thereto. Expansion member199expands in response to the refrigerator having the distal end fitted in contact182of heat transfer member180and thus fills a space between contact182and the distal end.

In the present embodiment, expansion member199is a wire formed of indium. Specifically, the wire of indium is wound on an end of extension member190that serves as the distal end of the refrigerator.

Note that expansion member199is not limited in material to indium and may be lead or a similar material providing large expansion and having large thermal conductivity. Furthermore, expansion member199is geometrically not limited to wire, and it may be a sheet.

Superconducting magnet100thus configured is cooled in a method, as will be described hereafter. Superconducting magnet100is cooled in two states: superconducting coil110is initially cooled from a room temperature to a cryogenic temperature of about 4 K (hereinafter also referred to as initial cooling), and thereafter, superconducting coil110is cooled to be held at cryogenic temperature (hereinafter also referred to as steady cooling).

FIG. 5is a flow chart of a method for cooling the superconducting magnet according to the present embodiment. As shown inFIGS. 1, 2 and 5, in the present embodiment, superconducting magnet100is cooled in the method, as follows: in the initially cooling, the refrigerator has the distal end brought into contact with contact182of heat transfer member180and the refrigerator is thus thermally connected via heat transfer member180to superconducting coil110to thus cool superconducting coil110to cryogenic temperature (S100).

Specifically, as shown inFIG. 1andFIG. 2, in the initially cooling, the above described long refrigerator is inserted into cylindrical portion160and secured to vacuum vessel150. A gasket168is provided between the long refrigerator and vacuum vessel150for vacuum. Gasket168for vacuum prevents helium tank120from receiving air externally flowing thereinto.

When the long refrigerator's distal end is fitted to contact182of heat transfer member180, expansion member199is squashed between long extension member190and contact182and thus expands therebetween. As a result, expansion member199fills a space between the long refrigerator's distal end and contact182of heat transfer member180to allow them to be in thermally close contact with each other.

The refrigerator body170thus has second cooling stage172thermally connected to heat transfer member180via heat transfer member180and expansion member199. In that condition, vacuum vessel150is vacuumed and helium tank120is filled with helium gas, and the refrigerator is then started to operate.

The initially cooling is completed once superconducting coil110has been cooled to cryogenic temperature via the refrigerator's distal end through heat transfer member180. Once the initial cooling has been completed, the initial cooling is shifted to the steady cooling. In shifting to the steady cooling, initially, helium tank120is internally filled with one atmosphere of helium gas and the long refrigerator is subsequently removed from vacuum vessel150.

Then, as shown inFIG. 3andFIG. 4, long extension member190is replaced with short extension member192and short extension member192is attached to refrigerator body170to configure the short refrigerator. The short refrigerator is inserted into cylindrical portion160and secured to vacuum vessel150. In doing so, gasket168for vacuum is replaced with a gasket169for internal pressure, and gasket169for internal pressure is disposed between the short refrigerator and vacuum vessel150. Gasket169for internal pressure prevents helium tank120from having its internal helium gas flowing out thereof.

The short refrigerator secured to vacuum vessel150has the distal end out of contact with contact182of heat transfer member180and thus spaced therefrom. Thus after the initial cooling (S100) the refrigerator has the distal end out of contact with contact182of heat transfer member180(S110). This thermally disconnects the refrigerator from heat transfer member180.

Thereafter, operating the refrigerator is resumed and valve162is opened to inject liquid helium130through piping161into helium tank120(S120). Liquid helium130is injected into helium tank120until the former is stored in the latter to attain a prescribed amount as measured with a level indicator (not shown). Once injecting liquid helium130has been completed, valve162is closed.

Thus after the initial cooling has been shifted to the steady cooling, helium volatilized in helium tank120is cooled by the refrigerator and thus again liquefied. As a consequence, liquid helium130continues to cool superconducting coil110and thus holds it at cryogenic temperature.

Note that, as has been discussed above, in the steady cooling, the cryostat has heat intrusion in an amount of 0.6 W, whereas the refrigerator's refrigeration capacity is 1 W and thus has an excess of 0.4 W. When the refrigerator has an excessive refrigeration capacity continuously, helium tank120has its internal helium gas liquefied more than necessary and thus has an internal pressure lower than one atmosphere. This is unpreferable as it would help external air to enter helium tank120. Accordingly, in the present embodiment, heater193of short extension member192is powered with a power of 0.4 W to maintain a pressure in helium tank120constantly.

Thus in the present embodiment superconducting magnet100is cooled in a method such that before liquid helium130is injected into helium tank120the refrigerator has the distal end out of contact with contact182of heat transfer member180so that if in the steady cooling the refrigerator is stopped superconducting coil110can nonetheless be steadily prevented from having heat intrusion via the refrigerator.

Hereafter will be described a method for cooling a superconducting magnet and the superconducting magnet according to a second embodiment of the present invention. Note that the superconducting magnet of the present embodiment is different from superconducting magnet100of the first embodiment only in how the refrigerator is configured, and accordingly, the remainder in configuration of the superconducting magnet of the present embodiment will not be described.

Second Embodiment

FIG. 6shows in cross section a refrigerator in a superconducting magnet according to the second embodiment of the present invention in an enlarged view when the superconducting magnet has a superconducting coil cooled to cryogenic temperature.FIG. 7shows in cross section the refrigerator in the superconducting magnet according to the present embodiment in an enlarged view when the superconducting coil has been cooled by the refrigerator and liquid helium is injected.

In the present embodiment, the extension member is not used, and two refrigerator bodies different in length and refrigeration capacity are selectively used. More specifically, the refrigerator is implemented as a long refrigerator170aand a short refrigerator170bused selectively, long refrigerator170ahaving a larger refrigeration capacity than short refrigerator170b.

Long refrigerator170ahas a first cooling stage171aand a second cooling stage172a. Short refrigerator170bhas a first cooling stage171band a second cooling stage172b.

Long refrigerator170ahas a refrigeration capacity of 1.5 W for a temperature of 4 K, and short refrigerator170bhas a refrigeration capacity of 1 W for the temperature of 4 K. Furthermore, long refrigerator170aand short refrigerator170bare each configured to have an output adjustably.

In vacuum vessel150, long refrigerator170ahas a length L3and short refrigerator170bhas a length L4, length L3being larger than length L4. As shown inFIG. 6, long refrigerator170ahas a length allowing long refrigerator170ato have a distal end thereof in contact with contact182of heat transfer member180. As shown inFIG. 7, short refrigerator170bhas a length allowing short refrigerator170bto have a distal end thereof out of contact with contact182of heat transfer member180.

In the present embodiment, expansion member199is wound on the distal end of long refrigerator170a. Expansion member199expands in response to long refrigerator170ahaving the distal end fitted in contact182of heat transfer member180and thus fills a space between contact182and the distal end of long refrigerator170a.

Thus the initial cooling can be done with long refrigerator170aof a large refrigeration capacity to cool superconducting coil110in a reduced initial cooling time. Furthermore, the steady cooling can be done with short refrigerator170bof a relatively small refrigeration capacity to reduce a cost of superconducting magnet100shipped after the initial cooling.

Furthermore, long refrigerator170aand short refrigerator170bthat are each configured to have an output adjustably, allow the steady cooling to be done without using a heater and instead by adjusting the output of short refrigerator170bto maintain the pressure in helium tank120constantly.

It should be understood that the embodiments disclosed herein are illustrative and non-restrictive in any respect. Accordingly the scope of the present invention is not construed only through the above embodiments; rather, it is defined by the claims. Furthermore, it also encompasses any modifications within the scope and meaning equivalent to the terms of the claims.

REFERENCE SIGNS LIST