Compact superconducting magnet system free from liquid helium

Abstract: In a superconducting magnet system which has a superconducting coil member, a cryocooler for cooling the superconducting coil member into the superconducting state, and a current lead for supplying an electric current to the superconducting coil member, the current lead is made of a high-temperature superconducting material and is also cooled by the cryocooler into the superconducting state with the current lead kept in contact with the cryocooler.

Description: The present invention relates to a superconducting magnet system which is for use to generate an intense magnetic field in various systems, such as a linear motorcar, a beam accelerator, and in the measurement of magnetized material characteristics. In a conventional superconducting magnet system of the type described, coils of magnetic systems are put into a superconducting state by immersing the coil in liquid helium to cool the coils to an extremely low temperature. However, use of liquid helium renders running cost high and handling difficult in the conventional superconducting magnet system. This is because the liquid helium is expensive, volatile, and difficult to handle. Further, the conventional superconducting magnet system inevitably becomes bulky in structure, since it needs a liquid helium tank, and a liquid helium transfer tube. Recently, in order to eliminate such disadvantages of the conventional superconducting magnet system, a superconducting magnet system which may be free from liquid helium has been proposed by the inventors of the present invention in Japanese Patent Prepublication No. 258103/1992. The superconducting magnet system mentioned in the above-referenced application comprises a cryocooler which has a cooling stage, a superconducting coil which contacts the cooling stage, and current leads for supplying an electric current to the superconducting coil. The cooling stage is kept at a predetermined cooling temperature. The superconducting coil is cooled down to the predetermined cooling temperature by the cryocooler. The cryocooler may have an additional cooling stage. In the superconducting magnet system described in the above-mentioned application, no consideration has been made about the current leads which are used for supplying the electric current to the superconducting coil. In this connection, such current leads are formed by a normal conductive material. However, when the current leads are formed by a normal conductive material, it has been found that Joule's heat is inevitably generated from the current leads during supply of the electric current to the superconducting coil. The Joule's heat is propagated into the superconducting coil and deteriorates the efficiency of cooling. As a result, a heavy load is imposed on the cryocooler on cooling the superconducting coil. In order to solve the above-mentioned problem, the current leads may be formed by a high-temperature superconducting material. According to this structure, Joule's heat may not be generated from the current leads while the current leads are kept at a superconducting state. However, it becomes necessary to cool the current leads by another cryocooler which is exclusively used therefor. Consequently, the superconducting magnet system inevitably becomes bulky in size and complicated in structure. The superconducting magnet system mentioned in the above-referenced application has other disadvantages. Namely, it takes a long time to cool the superconducting coil from room temperature to the above-mentioned superconducting state at an extremely low temperature lower than about 77K. In addition, the distribution of the temperature is not uniform in the superconducting coil in the superconducting state. EP-A2-0 350 268 discloses a high-temperature ceramic superconductor for use as a cryogenic current lead in a superconducting magnet. A two stage cryocooler sleeve is provided having a second stage heat exchanger system capable of achieving lower temperatures than the first stage heat exchanger. A current lead comprising a ceramic superconductor has a critical temperature greater than the operating temperature of the first stage. The lead is tapered. Its broader end is thermally coupled to the first stage heat exchanger and its narrow end is coupled to the second stage heat exchanger. It is an object of this invention to provide a superconducting magnet system with superconducting current leads, which are not easily destroyed by thermal stress or an external force even when the superconducting current leads are substantially made of oxide ceramics. The object is solved by the features of the claims. Fig. 1 is a partial vertical sectional view of a superconducting magnet system according to a first embodiment of this invention; Fig. 2 is an elongated sectional view of a high-temperature superconducting current lead in the superconducting magnet system illustrated in Fig. 1; Fig. 3 is an elongated sectional view of a high-temperature superconducting current lead of a superconducting magnet system according to a second embodiment of this invention; Fig. 4 is a partial horizontal sectional view of a high-temperature superconducting current lead illustrated in Fig. 3, which is seen in the direction of VI line. Fig. 5 is a partial vertical sectional view of a superconducting magnet system according to a further embodiment of this invention; Fig. 6 is an elongated sectional view of a cylindrical magnetic shield according to a modification of the superconducting magnet system illustrated in Fig. 5; Fig. 7 is a partial vertical sectional view of a superconducting magnet system according to a further embodiment of this invention; Fig. 8 is a graphical representation for use in describing a relationship between an external magnetic field and an internal magnetic field in the high-temperature superconducting magnetic shield of the superconducting magnet system illustrated in Fig. 7, which is cooled down to 4.2K; Fig. 9 is a graphical representation for use in describing a relationship between an external magnetic field and an internal magnetic field in the high-temperature superconducting magnetic shield of the superconducting magnet system illustrated in Fig. 7 which is cooled down to 77K. Referring to Fig. 1, a superconducting magnet system 100 comprises a cryocooler 102, a first cooling stage 102A, and a second cooling stage 102B. The first cooling stage 102A is cooled down to a first predetermined temperature of, for example, 77K while the second cooling stage 102B is cooled down to a second predetermined temperature between 4K and 10K lower than the first predetermined temperature. The superconducting magnet system 100 further comprises a superconducting coil member 104, a pair of current leads 106, and a thermal shielding plate 107. The superconducting coil member 104 is brought into contact with the second cooling stage 102B and thereby cooled down to the second predetermined temperature. Each of the pair of the current leads 106 supplies an electric current to the superconducting coil member 104 and has first and second ends 106A and 106B directed downwards and upwards of Fig. 1. Each current lead 106 is brought into contact with both the first cooling stage 102A and the second cooling stage 1023 at the first and the second ends 106A and 106B, respectively. The thermal shielding plate 107 is kept in contact with the first cooling stage 102A and prevents the superconducting coil member 104 and the current leads 106 from being subjected to heat. The first and the second cooling stages 102A and 102B, the superconducting coil member 104, the current leads 106, and the thermal shielding plate 107 are contained in a cryostat 108. It is to be noted in the illustrated example that each of the current leads 106 is formed by a high-temperature superconducting material of, for example, a Bi-based oxide. The superconducting coil member 104 substantially consists of a coil bobbin 110 and a superconducting wire 112 wound around the coil bobbin 110. The superconducting wire 112 is covered by a copper block 114 which is effective to cool the superconducting wire 112. The coil bobbin 110 and the copper block 114 are brought into contact with and fixed to the second cooling stage 102B. With this structure, the superconducting wire 112 can be efficiently cooled down to the second predetermined temperature, namely, a very low temperature, between 4K and 10K. The current leads 106 are connected to an external power supply 116 through a current lead terminal 118 and a current lead wire 120 which may have normal conductivity. The first end 106A of each current lead 106 is thermally coupled to the first cooling stage 102A while the second end 106B of each current lead 106 is thermally coupled to the second cooling stage 102B. In the above-mentioned superconducting magnet system according to the first embodiment of this invention, each current lead 106 is composed of the high-temperature superconducting material, as mentioned before, and is therefore put into a superconducting state when it is cooled down to the first predetermined temperature, namely, 77K together with the first cooling stage 102A. In this event, Joule's heat is not generated from the current leads 106 and the superconducting coil member 104, even when an electric current is caused to flow through the current leads 106. This is because both the current leads 106 are put into the superconducting state together with the superconducting coil member 104. Referring now to Fig. 2, description is made about a structure for fixing the current leads 106 to both the first and the second cooling stages 102A and 102B which may be located on high and low temperature sides of the superconducting magnet system, respectively. In the illustrated example, each current lead 106 comprises a current lead bulk 120, a first electrode 122 located on the high temperature side, and a second electrode 124 placed on the low temperature side. The current lead bulk 120 is made of a high-temperature oxide superconducting material which is put into the superconducting state, when cooled down to about 70K or so. The high temperature side of the current lead bulk 120 is brazed by solder to one end of the first electrode 122 that is not fixedly supported and which therefore as a free end on the high temperature side. The low temperature side of the current lead bulk 120 is brazed by solder to the second electrode 124. On the high temperature side, the first electrode 122 is connected to the current lead wire 123 of normal conductivity and is also connected to the first cooling stage 102A by way of a heat anchor copper wire 126, a copper plate 128, and an insulator 130 which may be formed, for example, by a plate of aluminum nitride. On the low temperature side, the second electrode 124 is not only connected to the second cooling stage 1023 by way of an insulator 131 which may be formed, for example, by a plate of aluminum nitride but also fixed thereto by a bolt to form a fixed end. The second electrode 124 is also electrically connected to the superconducting wire 112 of the superconducting coil member 104 (Fig. 1). With this structure, it is possible to prevent a thermal stress imposed on the current lead 106 because the current lead 106 is fixed nowhere and provides the free end on the high temperature side. Besides, the low temperature side of the current lead 106 is cooled down to the second predetermined temperature, such as 4K to 10K by conduction cooling and kept at such an extremely low temperature, since the current lead 106 is in close contact with the second cooling stage 102B which is cooled down to the second predetermined temperature, As mentioned before, the current lead 106 forms the free end on the high temperature side and is not directly connected to the first cooling stage 102A of the cryocooler 102. As a result, the current lead 106 is cooled down to the first predetermined temperature of about 70K on the high temperature side, because the current lead 106 is in thermal contact with the first cooling stage 102A through the above-mentioned heat anchor copper wire 126. As mentioned above, in the first embodiment of the present Invention, electric power or electric current is supplied to the current lead 106 on condition that the current lead bulk 120 is kept below the first predetermined temperature and put in the superconducting state. This means that the current lead 106 has an extremely low electric resistance. Therefore, a very low load is imposed on the cryocooler 102 in cooling the superconducting coil member 104 in comparison with the conventional superconducting magnet system mentioned in the preamble of the instant specification. As a result, it becomes unnecessary to use a plurality of cryocoolers. Furthermore, the superconducting magnet system illustrated in Figs. 1 and 2, as a whole, becomes compact in structure. Referring to Figs. 3 and 4, description will proceed to a superconducting magnet system according to a second embodiment of this invention. The superconducting magnet system according to the second embodiment has a structure similar to that of the first embodiment except that the current lead 106 and electrodes in contact with the current lead 106 are somewhat different from those illustrated in Fig. 2. As illustrated in Figs. 3 and 4, the electrodes depicted at 132 and 134 are located on the high and the low temperature sides, respectively. Each of the electrodes 132 and 134 is similar in structure to each other, as illustrated in Fig. 4. As shown in Fig. 4, each of the electrodes 132 and 134 is formed by a flexible material and defines a pair of circles therein. The current lead 106 is formed by a superconductive material and has first and second end portions placed on the high and the low temperature sides, respectively. The first end portion of the current lead 106 is inserted into one of the two circles of the flexible circular electrode 132 and fixed thereto by solder, while the second end portion of the current lead 106 is inserted into the corresponding one of the two circles of the electrode 134 and fixed thereto by solder. On the high temperature side, a first connection electrode 122 is inserted into the other one of the two circles of the electrode 132, while a second connection electrode 124 is inserted on the low temperature side into the other one of the two circles of the electrode 134. Each of the electrodes 132 and 134 is fastened by a bolt 136. Besides, each electrode 132 and 134 is made of a thin copper plate shaped into the configuration illustrated in Fig. 4. With this structure, the current lead 106 is free from a thermal stress, since both the first and the second end portions of the current lead 106 form free ends, as illustrated in Figs. 3 and 4. Referring now to Fig. 5, description will proceed to a superconducting magnet system according to a further embodiment of this invention. As illustrated in Fig. 5, the superconducting magnet system has cylindrical magnetic shields 160 each of which surrounds each current lead bulk 120, respectively. The magnetic shields 160 are made of a superconductive material, such as an oxide high temperature superconductive material. Alternatively, the magnetic shields 160 may be made of a metallic superconductive material, such as NbTi and the like. Thus, the current lead bulk 120 is surrounded by the cylindrical magnetic shield 160 of superconductivity. It is therefore effective to favorably and considerably reduce an external magnetic field imposed on the current lead bulk 120. As a result, it can be prevented that a leakage flux from the superconducting coil member 104 deteriorates a critical current of the current lead bulk 120, even when the current lead bulk 120 is made of an oxide high temperature superconducting material. Besides, the cylindrical magnetic shield 160 can be cooled down to an extremely low temperature of, for example, not higher than 5K by the contact with the second cooling stage 102B. With this structure, the cylindrical magnetic shield 160 can be kept at a temperature lower than a critical temperature of the superconductive material (for example, 9.8K in a case of NbTi). The cylindrical magnetic shields 160 illustrated in Fig. 5 may be modified in Fig. 6. As illustrated in Fig. 6, the cylindrical magnetic shields 160' (one of which is not shown) extend from the first cooling stage 102A to surround each current lead bulk 120. The cylindrical magnetic shields 160' are made of a high-temperature superconducting material. The cylindrical magnetic shields 160' can be cooled down to the low temperature of, for example, 77K by the contact with the first cooling stage 102A. Referring to Figs. 7, 8, and 9, description will proceed to a superconducting magnet system according to a further embodiment of this invention. The superconducting magnet system according to this embodiment has a structure similar to that of the embodiment mentioned before except for the followings. Similar portions are designated by like reference numerals. As illustrated in Fig. 7, the superconducting magnet system comprises a cryocooler 102, a first cooling stage 102A of a first predetermined temperature and a second cooling stage 1023 of a second predetermined temperature lower than the first predetermined temperature. Like in Fig. 1, a superconducting coil member 104 is brought into contact with the second cooling stage 102B to thereby be cooled to the second predetermined temperature lower than the first predetermined temperature by the cryocooler 102. In addition, a pair of current leads 206 are included in the illustrated example to supply an electric current to the superconducting coil member 104 and is electromagnetically shielded by a pair of magnetic shield portions 208. Each of the magnetic shield portions 208 is composed of a high-temperature superconducting material and surrounds each of the current leads 206. As shown in Fig. 7, the current leads 206 are kept in contact with the second cooling stage 102B. Each magnetic shield portion 208 is fixed to an insulating member 210 on the low temperature side. In this embodiment, the magnetic shield portions 208 are cooled to an extremely low temperature by thermal conduction, since each magnetic shield portion 208 is brought into contact with the second cooling stage 102B. Consequently, the magnetic shield portions 208 protect the current leads 206 from the external magnetic field. In the interim, each of the magnetic shield portions 208 may be composed of a usual superconducting material other than the above-mentioned high-temperature superconducting material. Thus, according to the example illustrated in Fig. 7, both the usual and the high-temperature superconducting materials can be used as a material of the magnetic shield portions 208, since the magnetic shield portions 208 can be cooled not only down to the low temperature of, for example, 77K but also down to the extremely low temperature of, for example, not higher than 5K by the contact with the second cooling stage 102B. Preferably, the magnetic shield portions 208 should be composed of the high-temperature superconducting material, since the magnetic shield portions 208 of such a material can provide an excellent shield effect, compared with the magnetic shield portions 208 of the usual superconducting material, as mentioned below. Referring now to Figs. 8 and 9, description is made about magnetic shield characteristics of the magnetic shield portions 208. As shown in Fig. 8, the magnetic shield portions 208 can succeed in shielding the external magnetic field completely at the point of 0,091 T, when cooled to 4.2K. On the other hand, as shown in Fig. 9, the magnetic shield portions 208 can shield the external magnetic field completely at the point of 0.016 T, when cooled to 77K. Thus, when cooled to 4.2K, the magnetic shield portions 208 provide a shield effect equal to six times that of 77K. Each magnetic shield portion 208 may be composed of an oxide high-temperature superconducting material and a heat-conductive metal. The heat-conductive metal may be selected from a group consisting of copper, silver, and aluminum. The inventive current leads may not be always kept in contact with the cooling stage. On the other hand, more than two pairs of the current leads may also be employed.

Claims: A superconducting magnet system (100) comprising: a cryocooler (102) which has a cooling stage (102B) cooled down to a predetermined temperature; a superconducting coil member (104) which is kept in contact with said cooling stage (102B) to thereby be cooled to said predetermined temperature by said cryocooler (102); and a pair of current leads (106;206) of a high-temperature superconducting ceramic material each having first (106A) and second (106B) end portions for supplying an electric current to said superconducting coil member (104); characterized in that at least one of said first and second end portions is not mechanically fixed but left as a free end. A system as claimed in Claim 1, wherein said current lead (106;206) is kept in thermal contact with said cooling stage (102B). A system as claimed in Claim 1 or 2,wherein said current lead (106;206) is surrounded by a magnetic shield (160;160';208). A system as claimed in claim 3, wherein said magnetic shield (160;160';208) is formed by a superconductive material. A system as claimed in Claim 3, wherein said magnetic shield (160;160';208) is formed by a high temperature superconducting material. A system as claimed in Claim 3, wherein said magnetic shield (160;160';208) is composed of an oxide high-temperature superconducting material and a thermal conductive metal. A system as claimed in Claim 6, wherein said thermal conductive metal is selected from a group consisting of copper, silver, and aluminum. A system as claimed in any one of Claims 1 to 7, wherein said cryocooler (102) further comprises at least one additional cooling stage (102A) cooled down to an additional temperature higher than said predetermined temperature, said first end portion (106A) being kept in thermal contact with said additional cooling stage (102A) while said second end portion is kept in thermal contact with said cooling stage (102B). A system as claimed in any one of Claims 1 to 8, wherein said first end portion (106A) is loosely supported. A system as claimed in any one of Claims 1 to 8, wherein both said first (106A) and said second (106B) end portions are loosely supported. A system as claimed in any one of Claims 1 to 10, further comprising an electrode (124) and a flexible circular electrode (134), said electrode (124) being positioned between said current lead (106;206) and said cooling stage, said flexible circular electrode (134) being interposed between said electrode (124) and said current lead (106;206). A system as claimed in any one of Claims 3 to 11, wherein said magnetic shield (160;160';208) is located between said cooling stage (102B) and said additional cooling stage (102A). A system as claimed in any one of Claims 3 to 12, wherein said magnetic shield (160;208) is kept in thermal contact with said cooling stage (102B). A system as claimed in any one of Claims 3 to 12, wherein said magnetic shield (160') is extended from said additional cooling stage (102A). A system as claimed in Claim 13, wherein said magnetic shield (208) is fixed to an insulating member (210) in contact with said cooling stage (102B). A system as claimed in any one of Claims 1 to 4, wherein said cryocooler (102) further comprises at least one additional cooling stage (102A) cooled down to an additional temperature higher than said predetermined temperature.

Inventors: HASEBE TSUGINORI, HATA FUMIAKI, JIKIHARA KAZUNORI, KUNG CHONG CHIN, SAKURABA JUNJI, WATANABE KAZUO, YAMADA YUTAKA, HASEBE, TSUGINORI, HATA, FUMIAKI, JIKIHARA, KAZUNORI, KUNG, CHONG CHIN, SAKURABA, JUNJI, WATANABE, KAZUO, YAMADA, YUTAKA
Applicants: SUMITOMO HEAVY INDUSTRIES, SUMITOMO HEAVY INDUSTRIES, LTD
Application year: 1993
Main classification: H01F6
IPC codes: H01F6