JP2005156052A - Superconductive member cooling device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent solidification of low temperature liquefied gas without using an electric heater in a device cooling low temperature liquefied gas such as liquid nitrogen to a supercooling temperature under the atmospheric pressure by a GM refrigerating machine, and cooling a superconductive member by the supercooled liquid nitrogen. <P>SOLUTION: In the superconductive member cooling device, mixed gas of adding gas (for example, nitrogen gas) of the same type as the low temperature liquefied gas (for example, liquid nitrogen) to gaseous helium is used as a working mixture for the refrigerating machine, and a temperature of the low temperature liquefied gas in a heat insulation vessel is prevented from dropping to a solidification temperature or less. Particularly, in the mixed gas, proportions of the low temperature liquefied gas to the gas of the same type are within a range of 3-30vol%. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a superconducting member cooling device for cooling and holding a superconducting member such as a superconducting transformer, a superconducting magnet, various other superconducting coils, or a superconducting cable, in particular, a high-temperature superconducting member at a low temperature by a low-temperature liquefied gas such as liquid nitrogen. It is about.

When cooling a superconducting member such as a superconducting coil, especially a superconducting member using high-temperature superconductivity, a low-temperature liquefied gas such as nitrogen, hydrogen, neon, or helium can be used as a cooling medium. In many cases, inexpensive liquid nitrogen (LN ₂ ) is used. In this case, in general, saturated liquid nitrogen at atmospheric pressure, that is, liquid nitrogen of about 77 K is used. That is, a superconducting member is accommodated in a heat insulating container substantially open to the atmosphere called a cryostat that is vacuum-insulated, and about 77 K of atmospheric pressure saturated liquid nitrogen is injected into the heat insulating container. Usually, the superconducting member is immersed in nitrogen to cool and hold the superconducting member.

  By the way, in a high-temperature superconducting member, it is known that the superconducting characteristics will be greatly improved if the temperature is slightly lowered. For example, it is known that the critical current increases several times even if it falls from 77K to 70K. Therefore, recently, various attempts have been made to cool the superconducting member using liquid nitrogen or the like having a temperature lower than the saturation temperature under atmospheric pressure (that is, the supercooling temperature under atmospheric pressure).

  As one of the methods, the present inventors obtained liquid nitrogen by cooling it to a supercooling temperature under atmospheric pressure, for example, 65K, using a small ultra-low temperature refrigerator represented by a GM refrigerator (Gifford McMahon refrigerator). Patent Document 1 proposes a superconducting member cooling device in which liquid nitrogen at a supercooling temperature under atmospheric pressure is used as a cooling medium for cooling the superconducting member.

  The superconducting member cooling device according to Patent Document 1 basically includes a cooling-side heat insulating container that contains a superconducting member and is substantially open to the atmosphere for cooling the superconducting member, and the cooling-side heat insulating container. A supply-side heat insulation container substantially open to atmospheric pressure for containing liquid nitrogen to be supplied; liquid nitrogen supply means for supplying liquid nitrogen to the supply-side heat insulation container; and in the supply-side heat insulation container A refrigerator for cooling the liquid nitrogen to the supercooling temperature under atmospheric pressure, and the liquid nitrogen cooled to the supercooling temperature under atmospheric pressure in the supply side heat insulating container in the cooling side heat insulating container A space on the liquid surface of the supply side heat insulating container and the cooling side heat insulating container is set to atmospheric pressure or a pressure higher than atmospheric pressure, and the cooling is performed by the transfer means. In the side insulation container Which is characterized in that as the order of the superconducting member is immersed in liquid nitrogen supply has been supercooled state, shows a specific example in FIG.

  In FIG. 2, the superconducting member 1 to be cooled is disposed at the bottom of the cooling side heat insulating container 3. The cooling-side heat insulating container 3 is composed of a general-purpose cryostat substantially open to the atmosphere, the outer peripheral wall portion and the bottom wall portion being a vacuum heat insulating structure 5, and the upper end being opened and closed A possible lid 7 is provided. The lid portion 7 is not vacuum-sealed with respect to the container body, and the lid portion 7 is provided with a current introduction terminal similar to a general-purpose cryostat. The inside of the cooling side heat insulating container 3 is substantially open to the atmosphere through a gap between the container main body part and a current introduction terminal. In addition, although the safety valve 19 is provided in the cover part 7, this safety valve 19 is open | released when internal pressure exceeds +10 kPa with respect to external atmospheric pressure, for example, and internal pressure is made into atmospheric pressure-atmospheric pressure +10 kPa. In the range of, i.e., atmospheric pressure or slightly higher than atmospheric pressure. The superconducting member 1 is suspended from the lid portion 7 by support members 9A and 9B.

  As will be described later, liquid nitrogen (cooling liquid nitrogen) 11 at a supercooling temperature under atmospheric pressure is supplied to the bottom of the cooling side heat insulating container 3 via a transfer tube 45 as described later, and the superconducting member 1 is liquid. Immerse in nitrogen 11. Further, the outer shape of the horizontal cross section is substantially similar to the inner peripheral shape of the horizontal cross section of the cooling side heat insulating container 3 at a position slightly below the liquid surface 11A of the liquid nitrogen 11 in the cooling side heat insulating container 3. And a heat insulating member 13 having a predetermined thickness in the vertical direction. The heat insulating member 13 is suspended from the lid portion 7 by the support members 9A and 9B described above, and the periphery of the heat insulating member 13 maintains a slight gap 14 with respect to the inner peripheral wall surface of the cooling side heat insulating container 3. It is made like so. On the other hand, in the space 15 (the space between the lid 7 and the liquid surface 11A) 15 left above the liquid surface 11A of the cooling liquid nitrogen 11 in the cooling-side heat insulating container 3, an external first nitrogen gas supply source is provided. A nitrogen gas at atmospheric pressure is supplied from 16 through a nitrogen gas supply pipe 18. Further, one end of a reflux pipe 17 to be described later is opened at a position on the lower surface side of the heat insulating member 13 in the cooling side heat insulating container 3.

  Further, a supply-side heat insulating container 21 is provided separately from the cooling-side heat insulating container 3 that is substantially open to the atmosphere as described above.

  The supply side heat insulating container 21 is substantially open to the atmosphere like the cooling side heat insulating container 3 described above, and the outer peripheral wall portion and the bottom wall portion thereof are the vacuum heat insulating structure 23, and the upper end is A lid 25 that can be opened and closed is provided. The lid 25 is not vacuum-sealed with respect to the container body, and the inside of the supply-side heat insulating container 21 is substantially open to the atmosphere through a gap between the lid 25 and the container body. It has become a state. The supply-side heat insulation container 21 is supplied with liquid nitrogen 33 from an external liquid nitrogen supply source 27 through a control valve 29 and a supply pipe 31. Then, at a position slightly lower than the liquid surface 33A of the liquid nitrogen 33 in the supply side heat insulating container 21, the outer shape of the horizontal cross section is substantially similar to the horizontal cross section shape of the supply side heat insulating container 21. None and the heat insulating member 35 having a predetermined thickness in the vertical direction is disposed in a state of being suspended from the lid portion 25 by the support members 37A and 37B. Similarly to the heat insulating member 13 in the cooling side heat insulating container 3, the heat insulating member 35 also has a slight gap 39 around the inner peripheral wall surface of the supply side heat insulating container 21.

  Further, in the supply-side heat insulation container 21, the liquid nitrogen 33 in the supply-side heat insulation container 21 is brought to a supercooling temperature lower than the temperature of the saturated liquid nitrogen under atmospheric pressure (a temperature lower than about 77K, for example, 65K). A refrigerator 41 for cooling is disposed. The refrigerator 41 has a compressor (compressor) 41A for compressing a working gas (usually pure helium gas), expands the compressed high-pressure working gas to obtain a low temperature, and cools the low temperature ( A cooling head 41B for exchanging heat with liquid nitrogen), a switching unit 41C such as a valve motor for switching the flow of the high-pressure working gas from the compressor 41A and the expanded low-pressure working gas returning from the cooling head 41B; The cylinder portion 41D is formed with a passage for reciprocating the working gas between the switching portion 41C and the cooling head 41B. The switching portion 41C is disposed on the lid portion 25 of the supply-side heat insulating container 21. The cylinder part 41D passes through the lid part 25 downward from the switching part 41C and passes through the space 47 on the liquid nitrogen liquid surface 33A in the supply side heat insulating container 21, and below it. There are immersed in liquid nitrogen, the cooling head 41B is provided in the immersion portion in its portion or in liquid nitrogen. Here, the cooling head 41B has a configuration in which a heat transfer block made of a good heat transfer material such as copper is provided on the outer surface thereof. The compressor 41A is normally disposed at a position away from the supply-side heat insulating container 21, and a high-pressure gas pipe 41E and a low-pressure gas pipe for circulating the working gas between the compressor 41A and the switching unit 41C. It is tied by 41F.

  In addition, a liquid feed pump 43 is disposed in the supply side heat insulating container 21 in a state of being hung from the lid portion 25. The liquid feed pump 43 is disposed such that the intake (pump outlet) is positioned below the heat insulating member 35 in the supply side heat insulating container 21 (usually near the bottom of the supply side heat insulating container 21). The outlet side of the liquid feed pump 43 is connected to a transfer tube 45, and the transfer tube 45 is guided into the cooling side heat insulating container 3 as described above. Further, the reflux pipe 17 from the cooling side heat insulating container 3 is led into the supply side heat insulating container 21, and the leading end side opening end of the reflux pipe 17 opens in the supply side heat insulating container 21.

  Further, a space 47 (a space between the lid portion 25 and the liquid surface 33A) 47 left above the liquid surface 33A of the liquid nitrogen 33 in the supply side heat insulating container 21 is supplied from an external second nitrogen gas supply source 49. Through the nitrogen gas supply pipe 51, nitrogen gas at atmospheric pressure or a pressure slightly higher than atmospheric pressure is supplied.

  Here, the liquid nitrogen supply source 27, the control valve 29, and the supply pipe 31 constitute liquid nitrogen supply means 63 for supplying liquid nitrogen to the supply side heat insulating container 21. Furthermore, the liquid feed pump 43 and the transfer tube 45 constitute transfer means 65 for transferring the liquid nitrogen cooled to the supercooled state at atmospheric pressure in the supply side heat insulating container 21 to the cooling side heat insulating container 3. On the other hand, the first nitrogen gas supply source 16 and the nitrogen gas supply pipe 18 are the first for supplying nitrogen gas at atmospheric pressure or a pressure slightly higher than atmospheric pressure to the space 15 on the liquid surface in the cooling side heat insulating container 3. The nitrogen gas supply means 67 is configured, and the second nitrogen gas supply source 49 and the nitrogen gas supply pipe 51 are at atmospheric pressure or slightly higher than atmospheric pressure in the space 47 on the liquid surface in the supply side heat insulating container 21. The second nitrogen gas supply means 69 for supplying the nitrogen gas is configured.

  The overall function of the superconducting member cooling device of Patent Document 1 shown in FIG. 2 as described above will be described below.

  The liquid nitrogen supplied from the liquid nitrogen supply source 27 of the liquid nitrogen supply means 63 to the supply-side heat insulating container 21 is about 77K. The liquid nitrogen is cooled in the supply-side heat insulating container 21 by the refrigerator 41. The head 41B is cooled under a pressure of about atmospheric pressure to atmospheric pressure + 10 kPa, and the temperature is lowered to a temperature lower than the saturated liquid nitrogen temperature (about 77K) under atmospheric pressure, for example, about 65K. Then, the liquid nitrogen 33 that has been supercooled to about 65K or slightly higher than atmospheric pressure is pumped from the vicinity of the bottom of the supply-side heat insulating container 21 by the liquid feed pump 43, and is transferred to the atmosphere via the transfer tube 45. It is led into the cooling side heat insulating container 3 which is substantially opened. The supercooled liquid nitrogen introduced into the cooling-side heat insulating container 3 is indicated by reference numeral 11 in FIG. 2 and corresponds to the cooling liquid nitrogen.

  In the cooling-side heat insulating container 3, the superconducting member 1 is cooled and held at, for example, about 67 to 70K by the liquid nitrogen 11 in a supercooled state of 65K as described above. Further, the liquid nitrogen whose temperature has risen to, for example, about 70 K or more due to heat from the superconducting member 1 in the cooling side heat insulating container 3 returns to the supply side heat insulating container 21 via the reflux pipe 17. The fluid nitrogen refluxed to the supply-side heat insulating container 21 in this manner is cooled again to about 65 to 70K by the cooling head 41B of the refrigerator 41 under atmospheric pressure or a pressure slightly higher than atmospheric pressure, as described above. Then, it is sent again to the cooling side heat insulating container 3 by the liquid feed pump 43.

  Here, nitrogen gas at atmospheric pressure or slightly higher than atmospheric pressure is introduced into the space 15 above the liquid surface 11A of the cooling liquid nitrogen 11 in the cooling-side heat insulating container 3 through the nitrogen gas supply pipe 18. . Therefore, the space 15 on the liquid surface of the cooling side heat insulating container 3 is filled with nitrogen gas at atmospheric pressure or slightly higher than atmospheric pressure. Therefore, the pressure in the cooling side heat insulating container 3 is maintained at atmospheric pressure or a pressure higher than atmospheric pressure, and air is prevented from being drawn in from outside through the sealing portion of the lid portion 7 or the current introduction terminal portion. Is done.

  Further, since the heat insulating member 13 is disposed under the liquid level of the cooling liquid nitrogen 11 in the cooling side heat insulating container 3, the liquid level of the cooling liquid nitrogen 11 (about 77K because it is a gas-liquid interface) and its A thermal gradient can be applied to the lower side of the heat insulating member 13, particularly to the bottom of the cooling side heat insulating container where the superconducting member 1 is located. Further, the presence of the heat insulating member 13 prevents convective stirring between the bottom surface near the liquid surface 11A. As a result, the cooling liquid nitrogen 11 at the bottom where the superconducting member 1 is located can be maintained in a supercooled state at a low temperature of about 65K. Thus, since the superconducting member 1 is surrounded by the low-temperature liquid nitrogen 11 in a supercooled state of, for example, 65 to 70K, even if the superconducting member 1 generates heat during operation of the superconducting member 1, the surrounding liquid nitrogen remains. There is a margin of about 10K before the vaporization temperature under atmospheric pressure (about 77K) or higher, so that the heat generation of the superconducting member 1 immediately vaporizes the surrounding liquid nitrogen and generates gas bubbles. It can be effectively prevented.

  Note that nitrogen gas at atmospheric pressure or slightly higher than atmospheric pressure is introduced into the space 47 above the liquid surface 33A of the liquid nitrogen 33 in the supply-side heat insulating container 21 through the nitrogen gas supply pipe 51, and the nitrogen It will be filled with gas. Therefore, the pressure in the supply side heat insulating container 21 is maintained at atmospheric pressure or a pressure slightly higher than atmospheric pressure, and air is prevented from being drawn in from the outside through the sealing portion of the lid portion 25 and the like.

  Further, similarly to the cooling side heat insulating container 3, a heat insulating member 35 is also disposed below the liquid nitrogen 33 level in the supply side heat insulating container 21, and therefore the liquid surface of the liquid nitrogen 33 (because it is a gas-liquid interface). A thermal gradient can be given between about 77K) and the lower side of the heat insulating member 35, in particular, near the cooling head 41B of the refrigerator 41. Further, the presence of the heat insulating member 35 prevents convective stirring between the vicinity of the liquid surface 33 </ b> A and a portion below the heat insulating member 35. As a result, the liquid nitrogen 33 in the vicinity of the inlet of the liquid feed pump 43 is maintained in a low-temperature supercooled state of about 65 to 70K, and the low-temperature supercooled liquid nitrogen of about 65 to 70K is cooled. It can be fed into the side heat insulating container 3.

  In such a superconducting member cooling device shown in Patent Document 1, the superconducting member can be cooled to a lower temperature more reliably than when saturated liquid nitrogen at atmospheric pressure of about 77 K is used as a cooling medium. Therefore, the performance of the superconducting member can be improved. Moreover, in this case, the space on the liquid surface of the supercooled cooling liquid nitrogen in the cooling-side heat insulating container is filled with nitrogen gas at atmospheric pressure or a pressure slightly higher than atmospheric pressure, so that a large amount of moisture is contained from the outside. It is possible to reduce the possibility that air at atmospheric pressure is sucked into the inside, so that strictness is not required for the sealing of the lid portion of the cooling side heat insulating container and the current introduction terminal, and further, the superconducting member is immersed. Since the liquid nitrogen for cooling is in a supercooled state as described above, even if the superconducting member generates heat during the operation of the superconducting member, there is a temperature margin for the liquid nitrogen to reach the vaporization temperature around the heat generating portion. Therefore, gas bubbles are not generated immediately around the heat generating part, and therefore there is an advantage that the gas bubbles are less likely to reduce the insulation of the superconducting member or the cooling efficiency. Since the liquid nitrogen in the supply-side heat insulation container is cooled to the supercooling temperature under atmospheric pressure by the refrigerator and circulated, the operation is stopped to supply liquid nitrogen to the supply-side heat insulation container. Since it is not necessary, it can be operated continuously for a long time, and there are various advantages such as the trouble of replenishing liquid nitrogen and the trouble of stopping and resuming operation are unnecessary.

Japanese Patent No. 2859250

  As described above, the superconducting member cooling apparatus using the refrigerator as disclosed in Patent Document 1 has various advantages, but the present inventors have repeatedly conducted experiments and examinations for practical use, and have yet to do so. It turns out that there are points to be improved.

  That is, in a superconducting member cooling device using such a refrigerator, a heat load due to heat generation from the superconducting member or external heat intrusion, particularly an extremely large heat load due to abnormal heat generation from the superconducting member or excessive heat intrusion is applied. Even in such a case, in order to reliably cool and maintain the superconducting member at a predetermined low temperature, it is necessary to use a refrigerator having a cooling capacity sufficiently larger than that required during normal operation. In other words, it is necessary to use a refrigerator having an excessive cooling capacity with respect to the cooling capacity required during normal operation.

  However, when a refrigerator having an excessive cooling capacity is used in this way, during normal operation, that is, when an abnormal heat load is not applied, the liquid nitrogen in the supply-side heat insulating container into which the cooling head of the refrigerator is inserted Is excessively cooled, the liquid nitrogen becomes lower than its solidification temperature (about 63K), and solidification of liquid nitrogen may start near the cooling head.

  If solidification of liquid nitrogen is started in this way, natural convection of liquid nitrogen in the supply-side heat insulation container is less likely to occur, and as a result, heat transfer from the cooling head to the entire liquid nitrogen in the supply-side heat insulation container is prevented. If the liquid nitrogen at a position away from the cooling head is hindered and the temperature rises without being sufficiently cooled, and the liquid nitrogen that has risen in temperature is fed to the cooling side insulation container, the superconductivity There is a possibility that the temperature of the member may rise without being sufficiently cooled. Also, the solidified mass of liquid nitrogen generated in the supply-side heat insulation container is clogged in the liquid feed pump, so that the liquid nitrogen cooled in the supply-side heat insulation container cannot be smoothly sent into the cooling-side heat insulation container, and the temperature of the superconducting member May rise. Further, depending on the case, the solidification of liquid nitrogen may apply stress to the superconducting coil constituting the superconducting member, and the coil may be loosened or broken. Here, in the apparatus shown in FIG. 2, since the superconducting member is disposed in the cooling side heat insulating container different from the supply side heat insulating container, stress due to solidification of liquid nitrogen is directly applied to the superconducting member cooling member. Although it is rare, it is conceivable that the superconducting member is directly immersed in liquid nitrogen in a heat insulating container provided with a refrigerator as will be described later. May directly add to the superconducting member.

  In the superconducting member cooling device that cools liquid nitrogen using a refrigerator as described above, various problems arise when liquid nitrogen is solidified. As a general measure for avoiding such problems, an electric heater is installed in the cooling head of the refrigerator, the temperature of the cooling head is detected by a temperature sensor, and the temperature of the cooling head becomes too low. In such a case, it is conceivable to control the temperature of the cooling head so as not to be lowered below the solidification temperature of liquid nitrogen by operating the electric heater to raise the temperature of the cooling head surface.

  However, when implementing a method using such an electric heater, it is necessary to arrange a temperature sensor directly on or near the cooling head, and a device for controlling the heater output using a signal from the temperature sensor is required. For example, the cost of the entire cooling device must be high. Further, even when an electric heater is used as described above, if the control mechanism does not operate normally, the response is delayed, or the temperature sensor malfunctions or malfunctions, the temperature of the cooling head becomes 63K or less. It cannot be said that the solidification of liquid nitrogen is not started at all.

  In the above description, the case where liquid nitrogen is used as a cooling medium for cooling the superconducting member has been described as an example. However, other low-temperature liquefied gases such as liquid neon and liquid hydrogen are used as the cooling medium. It is clear that a similar problem occurs when the refrigeration unit is cooled to a supercooling temperature under atmospheric pressure using a refrigerator. In other words, when the low-temperature liquefied gas is cooled to a supercooling temperature under atmospheric pressure by cooling with a refrigerator, there is always a risk of cooling to below the solidification temperature and solidifying, and an electric heater is used as a solution. However, there was always the fear as mentioned above.

  The present invention has been made against the background described above. In a superconducting member cooling apparatus in which a low-temperature liquefied gas is cooled to a supercooling temperature under atmospheric pressure by a refrigerator and used as a cooling medium for the superconducting member. Without using a heater, it is possible to reliably and stably prevent a situation in which the low-temperature liquefied gas is cooled below its solidification temperature and start to solidify, and the apparatus is also low in cost. An object of the present invention is to provide a superconducting member cooling apparatus.

  In order to solve the above-described problems, in the superconducting member cooling device of the present invention, basically, as the working gas of the refrigerator, mixing is performed by adding the same kind of gas as the low-temperature liquefied gas of the superconducting member cooling medium to helium gas. Gas was used. That is, pure helium gas is generally used as the working gas of the GM refrigerator, but if a small amount of the same kind of gas as the low-temperature liquefied gas of the cooling medium for cooling the superconducting member is added and mixed with this, the refrigerator A unique phenomenon occurs in which the cooling capacity of the gas rapidly decreases near the solidification temperature of the low-temperature liquefied gas. Therefore, by utilizing such a phenomenon, the temperature of the low-temperature liquefied gas cooled by the refrigerator in the heat insulating container is prevented from dropping below its solidification temperature.

  Specifically, according to the first aspect of the present invention, the low-temperature liquefied gas is accommodated in the heat insulation container, and the cooling head of the refrigerator is inserted into the heat insulation container so that the low-temperature liquefied gas in the heat insulation container is brought into atmospheric pressure In the superconducting member cooling apparatus that is cooled to the supercooling temperature at the subcooling temperature, and the superconducting member is cooled by the low-temperature liquefied gas at the supercooling temperature under atmospheric pressure, helium gas is used as the working gas of the refrigerator. Using a mixed gas to which the same kind of gas as the low-temperature liquefied gas is added, the low-temperature liquefied gas in the heat insulating container is prevented from being lowered below its solidification temperature.

  The invention according to claim 2 is the superconducting member cooling device according to claim 1, wherein a ratio of the same kind of gas as the low-temperature liquefied gas is set in the range of 3 to 30 vol% in the mixed gas. Is.

  According to the superconducting member cooling device and the control method therefor according to the present invention, when the low-temperature liquefied gas is cooled to the supercooling temperature under the atmospheric pressure by the refrigerator, the low-temperature liquefied gas is solidified without using an electric heater. This can be reliably and stably prevented, and the reliability of the entire apparatus can be increased as compared with the case where an electric heater is used, and the apparatus cost can be reduced.

  In the embodiment of the superconducting member cooling device of the present invention, the external configuration of the device may be the same as that of the conventional superconducting member cooling device shown in FIG. 2, and therefore the external configuration of the device will be described. In the following description, reference is made to the reference numerals for the external device configuration shown in FIG.

  A cooling head 41B of a refrigerator 41 represented by a GM refrigerator is inserted into the supply-side heat insulation container 21 and immersed in liquid nitrogen 33 as a low-temperature liquefied gas in the supply-side heat insulation container 21. The cooling head 41B cools the liquid nitrogen 33 to a supercooling temperature under atmospheric pressure, for example, 65K. In this embodiment, the “insulated container” in claim 1 corresponds to the supply-side insulated container 21 in this embodiment.

  In the refrigerator represented by the GM refrigerator, the working gas is compressed by the compressor 41A, reaches the inside of the cooling head 41B via the switching portion 41C via the high-pressure gas pipe 41E, and further flows in the cooling portion 41B. A circulation flow is formed such that the flow returns from the inside to the low-pressure gas pipe 41F via the switching part 41C, and cooling capacity is obtained by Simon expansion of the working gas in the meantime. In particular, in the superconducting member cooling device of the present invention, when liquid nitrogen 33 is used as the low-temperature liquefied gas, the same kind of gas as the low-temperature liquefied gas with respect to helium gas as the working gas of the refrigerator 41 (therefore, in this example) Nitrogen gas is added and mixed.

  Here, as a working gas of a small cryogenic refrigerator represented by a GM refrigerator, pure helium gas is usually used conventionally.

  However, in a GM refrigerator, when working gas in which an appropriate amount of other gas is added to and mixed with helium gas is used, the cooling capacity (refrigerator output) of the GM refrigerator rapidly increases near the solidification temperature of the added gas. It has already been reported in the literature ("Experimental study on pulse tube refrigeration with helium and nitrogen mixture", ZH Gan, G.B. Chen) that the output is reduced to almost zero at the solidification temperature of the additive gas. G. Thummes, C. Heiden, Cryogenics 40 (2000) 333-339). The report in this document was made in the laboratory from an academic interest until the end, and is not intended for industrial use. However, if the above phenomenon is used effectively, the cooling of the refrigerator 41 is performed. It is possible to reliably control the temperature of the head 41B so that it does not become lower than the solidification temperature of liquid nitrogen.

  That is, in the above document, when nitrogen gas is mixed in helium gas at various ratios in the range of 3 to 25 vol% as the working gas of the GM refrigerator, as shown in FIG. It has been shown that there is a phenomenon in which the temperature rapidly decreases near the solidification temperature (63 K) of liquid nitrogen and becomes almost zero at the solidification temperature.

  And when the present inventors performed the same experiment, it was confirmed that the same phenomenon as the above-mentioned literature occurs. Also, when a gas other than nitrogen gas, such as hydrogen gas or neon gas, is added to the helium gas, the output of the refrigerator suddenly drops near the solidification temperature of the additive gas, and the output of the refrigerator at the solidification temperature of the additive gas. It has been confirmed that there is a phenomenon that becomes almost zero.

  Therefore, in the superconducting member cooling device shown in FIG. 2, if a mixed gas obtained by adding an appropriate amount of nitrogen gas to helium gas is used as the working gas of the refrigerator 41, the surface of the cooling head 41 </ b> B of the refrigerator 41 is caused by the above phenomenon. Or it can prevent reliably that the temperature of the vicinity falls to the solidification temperature of liquid nitrogen. That is, if the temperature of the cooling head 41B decreases to about 63K, which is the solidification temperature of liquid nitrogen, the cooling capacity of the refrigerator 41 rapidly decreases as it approaches that temperature, and the freezing capacity becomes almost zero at the solidification temperature. Therefore, it is possible to reliably prevent the cooling head 41B from reaching the solidification temperature of liquid nitrogen and starting solidification of liquid nitrogen in the vicinity thereof.

  Here, in the example of FIG. 2, liquid nitrogen is used as a cooling medium (low-temperature liquefied gas) for cooling and holding the superconducting member, and as a gas to be mixed / added to helium gas as the working gas of the refrigerator. Nitrogen gas is used, but in short, a low-temperature liquefied gas as a cooling medium for cooling the superconducting member (cooled to the supercooling temperature under atmospheric pressure in the supply side heat insulating container 31 by the refrigerator 41) The same kind of gas as the target low-temperature liquefied gas) may be added to the helium gas, and the mixed gas may be used as a working gas for the refrigerator. For example, when liquid neon is used as the low-temperature liquefied gas, it is sufficient to use helium gas with neon gas added as the working gas for the refrigerator. When the low-temperature liquefied gas is liquid hydrogen, the working gas for the refrigerator is used. What added helium gas to hydrogen gas may be used.

  Further, as described above, the addition ratio of the gas mixed and added to the helium gas as the working gas of the refrigerator (the same kind of gas as the low-temperature liquefied gas) usually occupies about 3 vol% to 30 vol% with the entire mixed gas being 100%. It is preferable to blend as described above. If the ratio of the additive gas is less than 3 vol%, it is difficult to obtain a phenomenon in which the cooling capacity rapidly decreases near the solidification temperature of the additive gas. On the other hand, if it exceeds 30 vol%, the cooling capacity of the refrigerator tends to decrease overall. And become inefficient.

  In the example of FIG. 2, the cooling-side heat insulating container 3 is provided separately from the supply-side heat insulating container 21, and the superconducting member 1 to be cooled is disposed in the cooling-side heat insulating container 3 and sent from the supply-side heat insulating container 21. The liquid pump 43 transfers liquid nitrogen at a supercooling temperature under atmospheric pressure to the cooling-side heat insulating container 3 to cool and hold the superconducting member 1 in the cooling-side heat insulating container 3. The cooling side heat insulating container 3 may be omitted, and the superconducting member 1 may be immersed in the liquid nitrogen in the supply side heat insulating container 21 to directly cool and hold the superconducting member 1. As the working gas 41, the same mixed gas as described above can be used.

  In the example of FIG. 2, nitrogen gas having a pressure slightly higher than atmospheric pressure or slightly higher than atmospheric pressure is introduced into each space on the liquid surface of the supply side heat insulating container 21 and the cooling side heat insulating container 3 to pressurize it. A gas having a lower condensation temperature than nitrogen gas, for example, neon gas, hydrogen gas, or helium gas may be introduced into the space above each liquid surface and pressurized to atmospheric pressure or a pressure slightly higher than atmospheric pressure.

  2 shows a state in which the entire cooling head 41B of the refrigerator 41 is immersed in the liquid nitrogen 33. In some cases, only the lower part of the cooling head 41B is immersed in the liquid nitrogen 33, and the upper part is The cooling head may be arranged so that the whole is located in the space above the liquid level without exposing it to the space above the liquid level or immersing the cooling head 41B in the liquid nitrogen 33. Can omit the heat insulating members 35 and 13 below the liquid surface.

  In the superconducting member cooling device shown in FIG. 2, a supply-side heat insulating container 21 having a capacity of 160 l is used, and 67 l of liquid nitrogen 33 of 67K is injected into the supply-side heat insulating container 21, and the space above the liquid surface is filled The gauge pressure was increased to 7 kPa with nitrogen gas. On the other hand, as the refrigerator 41, a GM refrigerator having a refrigeration output of 140 W when the cooling head 41B is 64K and a working gas of the refrigerator with 10 vol% nitrogen gas added to pure helium gas is used. It was.

  The operation experiment of operating the GM refrigerator 41 with such an apparatus to cool the liquid nitrogen 33 in the supply-side heat insulating container 21 is performed five times every 24 hours for a total of 120 hours, and the temperature of the lower surface of the cooling head 41B is detected as a sensor. As a result of measurement, it was confirmed that the surface temperature of the cooling head 41B was constantly kept at 64 to 65K, and the phenomenon that liquid nitrogen solidified did not occur.

  Similar experiments were performed by changing the ratio of nitrogen gas in the working gas of the refrigerator 41 (mixed gas of helium and nitrogen gas) to 5% or 25%. In either case, the temperature of the cooling head 41B was steady. In particular, it was found to be kept at 64 to 65K.

  On the other hand, when the same experiment was performed by changing the ratio of nitrogen gas in the working gas of the refrigerator 41 to 1%, the surface temperature of the cooling head 41B may be 63K or less, and solidification of liquid nitrogen may be started. It was confirmed.

  Further, a similar experiment was performed by changing the ratio of nitrogen gas in the working gas of the refrigerator 41 to 50%. When the temperature of the cooling head 41B was 70K, the working gas was 100% pure helium as compared with the refrigerator. It was confirmed that the refrigerating capacity was reduced to about 1/3 and sufficient refrigerating capacity could not be secured.

It is a diagram which shows the relationship between the temperature of the cooling head of a refrigerator, and the output of a refrigerator when nitrogen gas is mixed and added in various ratios with respect to helium gas as working gas of GM refrigerator. It is a block diagram which shows an example of the conventional superconducting member cooling device.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Superconducting member 21 Supply side heat insulation container as heat insulation container 33 Liquid nitrogen as low temperature liquefied gas 41 Refrigerator 41A Compressor 41B Cooling head

Claims (2)

While storing the low-temperature liquefied gas in the heat insulation container, the cooling head of the refrigerator is inserted into the heat insulation container to cool the low-temperature liquefied gas in the heat insulation container to the supercooling temperature under atmospheric pressure, and the atmospheric pressure. In the superconducting member cooling device that cools the superconducting member with a low-temperature liquefied gas having a supercooling temperature below,
As a working gas of the refrigerator, a mixed gas in which the same kind of gas as the low-temperature liquefied gas is added to helium gas so that the low-temperature liquefied gas in the heat insulation container does not drop below its solidification temperature. A superconducting member cooling device. The superconducting member cooling device according to claim 1,
The superconducting member cooling device, wherein the mixed gas has a ratio of the same type of gas as the low-temperature liquefied gas in a range of 3 to 30 vol%.

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