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Conduction-cooled Bi2Sr2Ca2Cu3Ox (Bi-2223) magnet for magnetic separation
Physica C 350 (2001) 76±82
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Conduction-cooled Bi2Sr2Ca2Cu3Ox (Bi-2223) magnet for magnetic separation H. Kumakura a,*, T. Ohara a, H. Kitaguchi a, K. Togano a, H. Wada a, H. Mukai b, K. Ohmatsu b, H. Takei b a b
National Research Institute for Metals, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan Sumitomo Electric Industries, Ltd., 1-1-3 Shimaya, Konohana-ku, Osaka 554-0024, Japan Received 24 July 2000; accepted 21 August 2000
Abstract A prototype of a conduction-cooled Bi2 Sr2 Ca2 Cu3 Ox (Bi-2223) magnet system for magnetic separation was constructed. The magnet system has a 200 mm room temperature bore and generates ®elds higher than 1 T in an 11-liter room temperature space. The magnet axis of the system was designed to be horizontal in order to attain eective magnetic separation. The magnet consisted of 42 double-pancake coils. Each double-pancake coil was fabricated by the react and wind method using 61 multi®lamentary Bi-2223/Ag composite tapes. Three Bi-2223/Ag tapes, each 66.9 m long, were bundled and wound into a double-pancake coil. A stainless steel tape was also co-wound to reinforce the coil. Eighteen Cu disks were inserted between the pancakes and thermally connected to the cryocooler in order to enhance the conduction cooling. The magnet was cooled down to 13 K using a Giord±McMahon-type cryocooler, and it was tested at various excitation speeds. In a magnetic separation system, it is important to attain rapid energizing and de-energizing of the magnet in order to obtain high separation eciency. We cyclically excited the magnet up to 1.7 T at a speed of 1.7 T/min. The temperature gradually increased with the number of excitations; however, the temperature saturated at 35±38 K, and stable operation of the magnet was obtained at these temperatures. This result indicates that the Bi-2223 magnet is promising for magnetic separation. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Bi2 Sr2 CaCu2 Ox ; Conduction cooling; Thermal stability
1. Introduction Due to the continued progress of Bi-based highTc oxide superconductors, we can expect some practical applications of conductors without liquid helium cooling. For example, the practical level of the critical current has already been obtained * Corresponding author. Tel.: +81-298-59-2327; fax: +81298-59-2301. E-mail address: [email protected] (H. Kumakura).
at 20±30 K for Bi-oxide/Ag multi®lamentary superconducting tapes and wires. Because 20±30 K is eciently obtained by a cryocooler, a Bi-based oxide superconducting magnet cooled with a cryocooler has a great potential in many technological applications. One of the interesting applications of a cryogen-free magnet is as a magnet for magnetic separators. Magnetic separation has been used for many years to collect magnetic particles such as iron ore. In addition to its application to the industrial production process,
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 0 ) 0 1 5 6 9 - 0
H. Kumakura et al. / Physica C 350 (2001) 76±82
magnetic separation can also be applied to remove biogenic and anthropogenic pollutants from aquatic systems and industrial waste from streams. Thus, magnetic separation has recently received much attention in connection with the preservation of the global environment. For ecient magnetic separation, the magnetic ®eld gradient rB should be large because the magnetic force is proportional to rB. A high gradient magnetic separator (HGMS) uses a ®nely divided ferromagnetic matrix to create a large gradient and collect even paramagnetic materials [1]. In most cases, however, conventional copper electromagnets are being used for HGMS. Hence, the generation of magnetic ®elds in a large volume is costly in terms of electric power, iron core loss, and copper losses. For example, a large electromagnet for kaolin separation may consume as much as 500 kW electric power, all of which is converted into heat. For these reasons, superconducting magnets for magnetic separation were proposed as early as 1970. In 1986, a conventional metallic superconducting magnet was installed in an HGMS for kaolin puri®cation [2]. The advantage of superconductivity is the lack of electric power dissipation at a constant ®eld. However, conventional metallic superconducting magnets usually require expensive liquid helium for cooling, which hinders the signi®cant decrease of the running cost. Thus, a cryogen-free conduction cooled Bi-based oxide superconducting magnet is now expected as an alternative to the conventional superconducting magnet for magnetic separation. Another advantage of a cryogen-free magnet is that the magnet attains much higher stability at 20±30 K due to the much larger speci®c heat of the conductors. Thus, it is possible to energize the magnet with fast current ramps, which is important for magnetic separation. The National Research Institute for Metals (NRIM) has started a research program for the development of the prototype of an oxide superconducting magnetic separation system which can be operated with a cryocooler. In previous papers, we fabricated small Bi-2223 [3,4] and Bi2 Sr2 CaCu2 Oy (Bi-2212) [5] pancake magnets and tested them at various temperatures using a Gifford±McMahon (GM)-type cryocooler in order to
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investigate the applicability of the Bi-based oxide superconductors to the magnetic separation system. The results indicate that the Bi-2223 magnet is a promising candidate for a magnetic separation system designed to be operated at temperatures of 20±40 K. Based on this result, we have recently constructed a prototype of the Bi-2223 conduction cooled magnet for a magnetic separation system. In this paper, we report the design, fabrication, and test results of the magnet system.
2. Magnet design To achieve high eciency of magnetic separation, a large room temperature space under the high magnetic ®eld is desired. In this program, we designed a magnet system with a room temperature bore space larger than 10 liter under a ®eld higher than 1 T. The magnet axis of the system was designed to be horizontal in order to attain eective magnetic separation, and the bore was clear through the body of the magnet. The dimensions of the room temperature bore space were 200 mm in diameter and 352 mm in length (volume: 11 liter). The magnet was assembled with pancake coils made of Bi-2223 tape conductors. Three tapes were bundled in order to obtain large current capacities. The critical current Ic and the overall critical current density Je of the tapes in a ®eld of 2 T at 20 K were 190 A and 2:1 104 A/cm2 when the ®eld was parallel to the tape surface, and 110 A and 1:2 104 A/cm2 when the ®eld was perpendicular to the tape surface, respectively. The slope of the log V±log I curve (n-value) in the zero ®eld was 20 and 35 at 20 and 4.2 K, respectively. The speci®cations of the magnet are listed in Table 1. The ®eld distribution in the 200 mm room temperature bore space was calculated from the dimensions of the magnet. Fig. 1(a) and (b) shows the axial and radial component of the ®eld within the room temperature bore space, respectively, when the ®eld at the center of the magnet is 1.7 T. In order to obtain a 10 l space under 1 T, the magnetic ®eld at the center of the magnet should be higher than 1.7 T, which corresponds to an applied current of 180 A (60 A for each tape) and
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Table 1 Speci®cations of the magnet Outer diameter Bore Length Room temperature bore Number of pancakes Total number of turns Inductance Magnetic ®eld (center)
306 mm 240 mm 352 mm 200 mm 42 3276 1.6 H 1.7 T
Conductors Dimensions Length Insulation
3:7 mmw 0:24 mmt 3 66:9 m 3/pancake Polyimide tape
a coil current density of 5420 A/cm2 . With increasing the distance along the axis from the center of the magnet, the axial component of the ®eld decreased, but 1.0 T, which is enough for the magnetic separation, was obtained at the end of the magnet (175 mm from the center). The radial component, on the other hand, increased with the distance from the center and achieved a maximum value of 0.5 T at the end of the magnet. This radial component should be taken into account for magnet design because a Bi-2223 tape conductor shows large Ic anisotropy with respect to the ®eld orientation. Fig. 2 shows the load lines of the magnet and Ic ±B curves at 20 K of the short Bi2223 tape in ®elds applied parallel and perpen-
Fig. 2. Load lines of the magnet and critical current (Ic ) vs. ®eld (B) curves at 20 K of Bi-2223 tape. Two load lines are shown for the maximum axial and radial ®eld components in the magnet. Two Ic ±B curves are shown for ®elds parallel and perpendicular to the tape surface.
dicularly to the tape surface. Two load lines are shown for the maximum axial and radial ®eld components in the magnet. It is clear that the performance of the magnet is governed by the radial ®eld component (perpendicular to the tape surface) rather than by the axial component (parallel to the tape surface). The current at the intersection of the load line (radial) and Jc ±B curve
Fig. 1. Magnetic ®eld distribution inside a room temperature volume with a diameter of 200 mm and a length of 352 mm when the ®eld at the center of the magnet is 1.7 T. The (a) axial and (b) radial components of the ®eld are shown as functions of the axial distance (Z mm) and the radial distance (r mm) from the center of the magnet.
H. Kumakura et al. / Physica C 350 (2001) 76±82
(B?tape) is 110 A. This current is much larger than the required applied current of 60 A (per tape), which generates a ®eld of 1.7 T at the center of the magnet. The highest axial and radial ®eld components experienced by the magnet are 1.82 and 1.07 T, respectively. Hoop stresses applied to the pancake coils were also calculated from the dimensions of the magnet and the current. The maximum hoop stress which the Bi-2223 conductors experienced was calculated to be 14 MPa when the applied current was 180 A. This maximum hoop stress is comparable to the critical stress of the Bi-2223 conductor, at which the Ic degradation of Bi-2223 tape starts. Hence, the bundled Bi-2223 tape conductors were reinforced with 100 lm-thick stainless steel tape. This reinforcement considerably decreased the hoop stress applied to the Bi-2223 tape conductors. 3. Magnet fabrication The magnet consisted of 42 double-pancake coils stacked on a central bore tube. Each double-pancake coil was fabricated by the react and wind method using 61-multi®lamentary Bi-2223/Ag composite tapes prepared by the oxide-powderin-tube method. Details of the Bi-2223/Ag conductors have been presented elsewhere [6]. Three Bi-2223 tapes, each 66.9 m long, were bundled and wound into a double-pancake coil with polyimide tape with a thickness of 13 lm for insulation. Thus, the Bi-2223 tape conductors used for the magnet were 8429 m in total length. Stainless steel tape with a thickness of 100 lm was also co-wound in order to enhance the mechanical strength of the coil. A glass ®ber reinforced plastic sheet with a thickness of 100 lm was inserted between two windings of each double pancake for insulation. Fig. 3 shows one of the pancake coils. The dimensions of the pancake coils were: 240 mm bore, 306±308 mm outer diameter, and 7.9±8.4 mm height. Before the magnet assembly, we measured the Ic values of all pancake coils at 77 K and zero ®eld in order to check the current carrying capacity of the coils. Ic at 77 K of the coils ranged between 57 and 66 A
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Fig. 3. A Bi-2223 double-pancake coil.
with an average Ic of 59.8 A. These Ic values of the coils were almost equal to the Ic of a short tape, indicating that the Ic variation along the tape was fairly small. The generated ®eld of each coil was also measured by using a Hall probe. The ®eld at the center of the coil for an applied current of 60 A (20 A for each tape) was 210 G, which agreed well with the value calculated from the size of the coils. The 42 double-pancake coils were stacked along the coil axis. Stainless steel magnet bobbin was used for mechanical support. Eighteen Cu disks having the same bore and outer diameter as the coils were inserted between pancake coils. These disks were used in order to enhance the conduction cooling of the coils, as mentioned below. The tape conductors of adjacent coils were connected in a series by soldering the bundled three Bi-2223 tapes alternately to increase the contact area and, hence, reduce the heat generation at the joints. Fig. 4 shows the magnet. Both ends of the magnet were held with stainless steel ¯anges. Eighteen Cu disks were thermally connected to the second stage of the GM cryocooler via silver bars, as shown in the ®gure, and the magnet was cooled down through these Cu disks. A pair of superconducting current leads was used in order to minimize heat leaks to the magnet. We used ¯exible current leads made of Bi-2223/Ag tapes instead of rigid bulk current leads. The advantages of current leads made of oxide superconducting tapes are as follows: (1)
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Fig. 4. Conduction cooled Bi-2223 magnet system. The magnet consists of 42 double-pancake coils. The magnet was thermally connected to the second stage of a cryocooler through silver bars and 18 Cu disks inserted between the pancakes.
Fig. 5. Overview of the conduction cooled Bi-2223 magnet system.
The ¯exibility of the leads gives tolerance to the heat cycle and electromagnetic force. (2) A larger critical current density enables small cross-sections of the leads. (3) Current leads with various con®gurations can be easily fabricated. The current leads used in this system consisted of eight bundled Bi-2223 tapes with a length of 190 mm to increase the current capacity. The speci®cations of the Bi2223/Ag tapes were the same as those of the tapes used for the magnet. However, we used Ag±10 at.% Au alloy sheaths in order to reduce the heat transfer to the magnet. One end of each current lead (low-temperature end; LT end) was connected to the magnet, and the other end (high-temperature end; HT end) of the lead was thermally contacted to the ®rst stage of the cryocooler to suppress the temperature raise of the current leads. Normal conducting leads were employed between the ®rst stage of the cryocooler and room temperature. The magnet was enclosed in a radiation shield which was also thermally contacted to the ®rst stage of the cryocooler. The radiation shield consists of an Al alloy case covered with a multilayer insulation blanket. The magnet system was encased in a stainless steel vessel, and the vessel was evacuated down to 10ÿ6 Torr. Fig. 5 shows the overview of the magnet system in the vessel.
The magnet was cooled down with a two-stage GM cryocooler. The cooling power of the cryocooler was 60 W at 80 K (®rst stage) and 16.5 W at 20 K (second stage). The temperatures of the magnet, the ®rst stage, and the second stage of the cryocooler were monitored with Au±Fe/Ag thermocouples. Five thermocouples were attached to the pancake coils of nos. 1, 11, 22, 32, and 42. The magnet temperature in this section is the average of those measured by the ®ve thermocouples. Fig. 6 shows the temperatures of the magnet, the ®rst stage and the second stage of the cryocooler as a function of time. It took 56 h for the temperature of the magnet to cool down to 13 K, after which the temperature became constant. The temperature of the ®rst stage was cooled down to 40 K. Thus, the temperatures of the LT ends of the current leads that were thermally contacted to the ®rst stage were low enough to carry a large superconducting current to the magnet. At this magnet temperature of 13 K, the magnet was excited with several excitation speeds. The generated magnetic ®eld was monitored with a Hall probe set at the center of the magnet. Fig. 7 shows the temperature change of the magnet together with the generated ®eld when the applied current of the magnet increased up to 180 A at an excitation
4. Testing of the magnet
H. Kumakura et al. / Physica C 350 (2001) 76±82
Fig. 6. Cooling characteristics of the magnet. Temperatures of the magnet and ®rst and second stages are shown as a function of cooling time.
Fig. 7. Excitation of the magnet up to 1.7 T at a speed of 1.7 T/ min. The temperatures of both ends of the current lead are also shown in the ®gure. HT end: high-temperature end. LT end: low-temperature end.
speed of 1.7 T/min. The temperatures of both ends of the current lead, the HT end and the LT end, are also shown in the ®gure. The temperature of the magnet rapidly increased to 26 K during the excitation. However, the temperature started to decrease as soon as the ®eld reached the maximum value, as shown in the ®gure, and the temperature went down to 15 K after 30 min. This indicates that the conduction cooling mechanism of this magnet system works well, and that the heat generated in the magnet immediately transferred to the second stage of the cryocooler. The generated
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®eld for the current of 180 A was 1.7 T, which was equal to the designed ®eld. After the 30 min excitation, the current decreased to zero at the same speed. A similar rapid increase of temperature was also observed. The large temperature increase during the energizing and de-energizing indicates that the energy loss in the Bi-2223 conductors is large under a changing current (AC loss) and that the heat generation is too fast to be transferred to the cryocooler. Such a large AC loss of Bi-2223 conductors is primarily due to the coupling (bridging) of the Bi-2223 ®laments in the Ag sheath. Heat generation at the stationary state of the magnet was estimated from the applied current and the voltage of the magnet. At a stable operation with a current of 180 A, the total voltage of the magnet was 3.56 mV, including the voltage drops at the junctions between pancake coils. Thus, the total heat generation in the magnet was calculated to be 0.64 W. This heat generation is much smaller than the cooling power of the cryocooler. The voltage measurements of several pancake coils at the stationary state suggest that most of the heat generation comes from the normal conducting junctions. This heat generation was so small that the temperature of the magnet decreased as soon as the applied current became constant, as shown in Fig. 7. Then, we repeated the excitation of the magnet up to 1.7 T at a speed of 1.7 T/min and observed the changes in temperature. The interval between each energizing and de-energizing was ®xed to be 3.5 min. Fig. 8 shows the temperatures of the magnet and the ®rst and second stages of the cryocooler as a function of the number of excitation cycles. A rapid increase and decrease of the magnet temperature could be observed again when energizing and de-energizing the magnet. The temperatures of the magnet and cryocooler gradually increased with increasing the number of excitations, but all these temperatures saturated at the excitation cycles of 20, and no increase was observed for a larger number of cycles. The saturated temperature of the magnet, 35±38 K, was rather high; however, stable operation of the magnet was obtained at these temperatures due to a sucient temperature margin of the Bi-2223
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H. Kumakura et al. / Physica C 350 (2001) 76±82
net increased while energizing the magnet; however, the temperature started to decrease as soon as the ®eld became constant. This indicates that the cooling mechanism works well. 3. At the stationary state of the magnet, the total voltage of the magnet was as low as 3±4 mV, indicating that the heat generation in the magnet is negligibly small compared to the cooling capacity of the GM cryocooler. 4. The temperature of the magnet increased up to 35±38 K when repeating the excitation. However, no quenching was observed, indicating that the magnet was stable against the temperature increase. Fig. 8. Temperature changes of the magnet and ®rst and second stages of the GM cryocooler when the excitation of the magnet is repeated.
conductors. This result indicates that the conduction cooled Bi-2223 magnet is thermally stable against the repetition of excitation and is thus promising as a magnet for use in a magnetic separation system.
These results indicate that a conduction cooled Bi-2223 magnet is useful for applications that require a rapid increase and/or decrease of magnetic ®elds. However, the increase of the magnet temperature during rapid excitation is rather large due to the large AC loss of the Bi-2223 conductors. Reduction of AC loss is desired in order to suppress the temperature rise and attain higher excitation speeds.
5. Conclusions
References
We constructed a prototype of a conduction cooled Bi-2223 magnet for a magnetic separation system and tested the magnet at various excitation speeds.We obtained the following conclusions:
[1] J.A. Oberteuer, IEEE Trans. Mag. MAG-9 (1973) 303. [2] J.A. Selvaggi, P.V. Arend, J. Colwell, J. Adv. Cryogenic Engng. 33 (1988) 53. [3] H. Kumakura, H. Kitaguchi, K. Togano, H. Wada, K. Ohkura, M. Ueyama, K. Hayashi, K. Sato, Cryogenics 38 (1998) 639. [4] H. Kumakura, H. Kitaguchi, K. Togano, H. Wada, K. Ohkura, M. Ueyama, K. Hayashi, K. Sato, Adv. Supercond. XII (2000) 745. [5] H. Kumakura, H. Kitaguchi, K. Togano, T. Hasegawa, Y. Hikichi, Cryogenics 38 (1998) 163. [6] M. Ueyama, K. Ohkura, S. Kobayashi, K. Muranaka, T. Kaneko, T. Hikata, K. Hayashi, K. Sato, Adv. Supercond. VII (1995) 847.
1. The magnet has a 200 mm room temperature bore and generates ®elds higher than 1 T in an 11-liter room temperature space. 2. Cu disks inserted between the pancake coils were eective to increase the cooling channels and enhance the heat transfer from the magnet to the cryocooler. The temperature of the mag-
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Conduction-cooled Bi2Sr2Ca2Cu3Ox (Bi-2223) magnet for magnetic separation H. Kumakura a,*, T. Ohara a, H. Kitaguchi a, K. Togano a, H. Wada a, H. Mukai b, K. Ohmatsu b, H. Takei b a b
National Research Institute for Metals, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan Sumitomo Electric Industries, Ltd., 1-1-3 Shimaya, Konohana-ku, Osaka 554-0024, Japan Received 24 July 2000; accepted 21 August 2000
Abstract A prototype of a conduction-cooled Bi2 Sr2 Ca2 Cu3 Ox (Bi-2223) magnet system for magnetic separation was constructed. The magnet system has a 200 mm room temperature bore and generates ®elds higher than 1 T in an 11-liter room temperature space. The magnet axis of the system was designed to be horizontal in order to attain eective magnetic separation. The magnet consisted of 42 double-pancake coils. Each double-pancake coil was fabricated by the react and wind method using 61 multi®lamentary Bi-2223/Ag composite tapes. Three Bi-2223/Ag tapes, each 66.9 m long, were bundled and wound into a double-pancake coil. A stainless steel tape was also co-wound to reinforce the coil. Eighteen Cu disks were inserted between the pancakes and thermally connected to the cryocooler in order to enhance the conduction cooling. The magnet was cooled down to 13 K using a Giord±McMahon-type cryocooler, and it was tested at various excitation speeds. In a magnetic separation system, it is important to attain rapid energizing and de-energizing of the magnet in order to obtain high separation eciency. We cyclically excited the magnet up to 1.7 T at a speed of 1.7 T/min. The temperature gradually increased with the number of excitations; however, the temperature saturated at 35±38 K, and stable operation of the magnet was obtained at these temperatures. This result indicates that the Bi-2223 magnet is promising for magnetic separation. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Bi2 Sr2 CaCu2 Ox ; Conduction cooling; Thermal stability
1. Introduction Due to the continued progress of Bi-based highTc oxide superconductors, we can expect some practical applications of conductors without liquid helium cooling. For example, the practical level of the critical current has already been obtained * Corresponding author. Tel.: +81-298-59-2327; fax: +81298-59-2301. E-mail address: [email protected] (H. Kumakura).
at 20±30 K for Bi-oxide/Ag multi®lamentary superconducting tapes and wires. Because 20±30 K is eciently obtained by a cryocooler, a Bi-based oxide superconducting magnet cooled with a cryocooler has a great potential in many technological applications. One of the interesting applications of a cryogen-free magnet is as a magnet for magnetic separators. Magnetic separation has been used for many years to collect magnetic particles such as iron ore. In addition to its application to the industrial production process,
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 0 ) 0 1 5 6 9 - 0
H. Kumakura et al. / Physica C 350 (2001) 76±82
magnetic separation can also be applied to remove biogenic and anthropogenic pollutants from aquatic systems and industrial waste from streams. Thus, magnetic separation has recently received much attention in connection with the preservation of the global environment. For ecient magnetic separation, the magnetic ®eld gradient rB should be large because the magnetic force is proportional to rB. A high gradient magnetic separator (HGMS) uses a ®nely divided ferromagnetic matrix to create a large gradient and collect even paramagnetic materials [1]. In most cases, however, conventional copper electromagnets are being used for HGMS. Hence, the generation of magnetic ®elds in a large volume is costly in terms of electric power, iron core loss, and copper losses. For example, a large electromagnet for kaolin separation may consume as much as 500 kW electric power, all of which is converted into heat. For these reasons, superconducting magnets for magnetic separation were proposed as early as 1970. In 1986, a conventional metallic superconducting magnet was installed in an HGMS for kaolin puri®cation [2]. The advantage of superconductivity is the lack of electric power dissipation at a constant ®eld. However, conventional metallic superconducting magnets usually require expensive liquid helium for cooling, which hinders the signi®cant decrease of the running cost. Thus, a cryogen-free conduction cooled Bi-based oxide superconducting magnet is now expected as an alternative to the conventional superconducting magnet for magnetic separation. Another advantage of a cryogen-free magnet is that the magnet attains much higher stability at 20±30 K due to the much larger speci®c heat of the conductors. Thus, it is possible to energize the magnet with fast current ramps, which is important for magnetic separation. The National Research Institute for Metals (NRIM) has started a research program for the development of the prototype of an oxide superconducting magnetic separation system which can be operated with a cryocooler. In previous papers, we fabricated small Bi-2223 [3,4] and Bi2 Sr2 CaCu2 Oy (Bi-2212) [5] pancake magnets and tested them at various temperatures using a Gifford±McMahon (GM)-type cryocooler in order to
77
investigate the applicability of the Bi-based oxide superconductors to the magnetic separation system. The results indicate that the Bi-2223 magnet is a promising candidate for a magnetic separation system designed to be operated at temperatures of 20±40 K. Based on this result, we have recently constructed a prototype of the Bi-2223 conduction cooled magnet for a magnetic separation system. In this paper, we report the design, fabrication, and test results of the magnet system.
2. Magnet design To achieve high eciency of magnetic separation, a large room temperature space under the high magnetic ®eld is desired. In this program, we designed a magnet system with a room temperature bore space larger than 10 liter under a ®eld higher than 1 T. The magnet axis of the system was designed to be horizontal in order to attain eective magnetic separation, and the bore was clear through the body of the magnet. The dimensions of the room temperature bore space were 200 mm in diameter and 352 mm in length (volume: 11 liter). The magnet was assembled with pancake coils made of Bi-2223 tape conductors. Three tapes were bundled in order to obtain large current capacities. The critical current Ic and the overall critical current density Je of the tapes in a ®eld of 2 T at 20 K were 190 A and 2:1 104 A/cm2 when the ®eld was parallel to the tape surface, and 110 A and 1:2 104 A/cm2 when the ®eld was perpendicular to the tape surface, respectively. The slope of the log V±log I curve (n-value) in the zero ®eld was 20 and 35 at 20 and 4.2 K, respectively. The speci®cations of the magnet are listed in Table 1. The ®eld distribution in the 200 mm room temperature bore space was calculated from the dimensions of the magnet. Fig. 1(a) and (b) shows the axial and radial component of the ®eld within the room temperature bore space, respectively, when the ®eld at the center of the magnet is 1.7 T. In order to obtain a 10 l space under 1 T, the magnetic ®eld at the center of the magnet should be higher than 1.7 T, which corresponds to an applied current of 180 A (60 A for each tape) and
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H. Kumakura et al. / Physica C 350 (2001) 76±82
Table 1 Speci®cations of the magnet Outer diameter Bore Length Room temperature bore Number of pancakes Total number of turns Inductance Magnetic ®eld (center)
306 mm 240 mm 352 mm 200 mm 42 3276 1.6 H 1.7 T
Conductors Dimensions Length Insulation
3:7 mmw 0:24 mmt 3 66:9 m 3/pancake Polyimide tape
a coil current density of 5420 A/cm2 . With increasing the distance along the axis from the center of the magnet, the axial component of the ®eld decreased, but 1.0 T, which is enough for the magnetic separation, was obtained at the end of the magnet (175 mm from the center). The radial component, on the other hand, increased with the distance from the center and achieved a maximum value of 0.5 T at the end of the magnet. This radial component should be taken into account for magnet design because a Bi-2223 tape conductor shows large Ic anisotropy with respect to the ®eld orientation. Fig. 2 shows the load lines of the magnet and Ic ±B curves at 20 K of the short Bi2223 tape in ®elds applied parallel and perpen-
Fig. 2. Load lines of the magnet and critical current (Ic ) vs. ®eld (B) curves at 20 K of Bi-2223 tape. Two load lines are shown for the maximum axial and radial ®eld components in the magnet. Two Ic ±B curves are shown for ®elds parallel and perpendicular to the tape surface.
dicularly to the tape surface. Two load lines are shown for the maximum axial and radial ®eld components in the magnet. It is clear that the performance of the magnet is governed by the radial ®eld component (perpendicular to the tape surface) rather than by the axial component (parallel to the tape surface). The current at the intersection of the load line (radial) and Jc ±B curve
Fig. 1. Magnetic ®eld distribution inside a room temperature volume with a diameter of 200 mm and a length of 352 mm when the ®eld at the center of the magnet is 1.7 T. The (a) axial and (b) radial components of the ®eld are shown as functions of the axial distance (Z mm) and the radial distance (r mm) from the center of the magnet.
H. Kumakura et al. / Physica C 350 (2001) 76±82
(B?tape) is 110 A. This current is much larger than the required applied current of 60 A (per tape), which generates a ®eld of 1.7 T at the center of the magnet. The highest axial and radial ®eld components experienced by the magnet are 1.82 and 1.07 T, respectively. Hoop stresses applied to the pancake coils were also calculated from the dimensions of the magnet and the current. The maximum hoop stress which the Bi-2223 conductors experienced was calculated to be 14 MPa when the applied current was 180 A. This maximum hoop stress is comparable to the critical stress of the Bi-2223 conductor, at which the Ic degradation of Bi-2223 tape starts. Hence, the bundled Bi-2223 tape conductors were reinforced with 100 lm-thick stainless steel tape. This reinforcement considerably decreased the hoop stress applied to the Bi-2223 tape conductors. 3. Magnet fabrication The magnet consisted of 42 double-pancake coils stacked on a central bore tube. Each double-pancake coil was fabricated by the react and wind method using 61-multi®lamentary Bi-2223/Ag composite tapes prepared by the oxide-powderin-tube method. Details of the Bi-2223/Ag conductors have been presented elsewhere [6]. Three Bi-2223 tapes, each 66.9 m long, were bundled and wound into a double-pancake coil with polyimide tape with a thickness of 13 lm for insulation. Thus, the Bi-2223 tape conductors used for the magnet were 8429 m in total length. Stainless steel tape with a thickness of 100 lm was also co-wound in order to enhance the mechanical strength of the coil. A glass ®ber reinforced plastic sheet with a thickness of 100 lm was inserted between two windings of each double pancake for insulation. Fig. 3 shows one of the pancake coils. The dimensions of the pancake coils were: 240 mm bore, 306±308 mm outer diameter, and 7.9±8.4 mm height. Before the magnet assembly, we measured the Ic values of all pancake coils at 77 K and zero ®eld in order to check the current carrying capacity of the coils. Ic at 77 K of the coils ranged between 57 and 66 A
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Fig. 3. A Bi-2223 double-pancake coil.
with an average Ic of 59.8 A. These Ic values of the coils were almost equal to the Ic of a short tape, indicating that the Ic variation along the tape was fairly small. The generated ®eld of each coil was also measured by using a Hall probe. The ®eld at the center of the coil for an applied current of 60 A (20 A for each tape) was 210 G, which agreed well with the value calculated from the size of the coils. The 42 double-pancake coils were stacked along the coil axis. Stainless steel magnet bobbin was used for mechanical support. Eighteen Cu disks having the same bore and outer diameter as the coils were inserted between pancake coils. These disks were used in order to enhance the conduction cooling of the coils, as mentioned below. The tape conductors of adjacent coils were connected in a series by soldering the bundled three Bi-2223 tapes alternately to increase the contact area and, hence, reduce the heat generation at the joints. Fig. 4 shows the magnet. Both ends of the magnet were held with stainless steel ¯anges. Eighteen Cu disks were thermally connected to the second stage of the GM cryocooler via silver bars, as shown in the ®gure, and the magnet was cooled down through these Cu disks. A pair of superconducting current leads was used in order to minimize heat leaks to the magnet. We used ¯exible current leads made of Bi-2223/Ag tapes instead of rigid bulk current leads. The advantages of current leads made of oxide superconducting tapes are as follows: (1)
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Fig. 4. Conduction cooled Bi-2223 magnet system. The magnet consists of 42 double-pancake coils. The magnet was thermally connected to the second stage of a cryocooler through silver bars and 18 Cu disks inserted between the pancakes.
Fig. 5. Overview of the conduction cooled Bi-2223 magnet system.
The ¯exibility of the leads gives tolerance to the heat cycle and electromagnetic force. (2) A larger critical current density enables small cross-sections of the leads. (3) Current leads with various con®gurations can be easily fabricated. The current leads used in this system consisted of eight bundled Bi-2223 tapes with a length of 190 mm to increase the current capacity. The speci®cations of the Bi2223/Ag tapes were the same as those of the tapes used for the magnet. However, we used Ag±10 at.% Au alloy sheaths in order to reduce the heat transfer to the magnet. One end of each current lead (low-temperature end; LT end) was connected to the magnet, and the other end (high-temperature end; HT end) of the lead was thermally contacted to the ®rst stage of the cryocooler to suppress the temperature raise of the current leads. Normal conducting leads were employed between the ®rst stage of the cryocooler and room temperature. The magnet was enclosed in a radiation shield which was also thermally contacted to the ®rst stage of the cryocooler. The radiation shield consists of an Al alloy case covered with a multilayer insulation blanket. The magnet system was encased in a stainless steel vessel, and the vessel was evacuated down to 10ÿ6 Torr. Fig. 5 shows the overview of the magnet system in the vessel.
The magnet was cooled down with a two-stage GM cryocooler. The cooling power of the cryocooler was 60 W at 80 K (®rst stage) and 16.5 W at 20 K (second stage). The temperatures of the magnet, the ®rst stage, and the second stage of the cryocooler were monitored with Au±Fe/Ag thermocouples. Five thermocouples were attached to the pancake coils of nos. 1, 11, 22, 32, and 42. The magnet temperature in this section is the average of those measured by the ®ve thermocouples. Fig. 6 shows the temperatures of the magnet, the ®rst stage and the second stage of the cryocooler as a function of time. It took 56 h for the temperature of the magnet to cool down to 13 K, after which the temperature became constant. The temperature of the ®rst stage was cooled down to 40 K. Thus, the temperatures of the LT ends of the current leads that were thermally contacted to the ®rst stage were low enough to carry a large superconducting current to the magnet. At this magnet temperature of 13 K, the magnet was excited with several excitation speeds. The generated magnetic ®eld was monitored with a Hall probe set at the center of the magnet. Fig. 7 shows the temperature change of the magnet together with the generated ®eld when the applied current of the magnet increased up to 180 A at an excitation
4. Testing of the magnet
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Fig. 6. Cooling characteristics of the magnet. Temperatures of the magnet and ®rst and second stages are shown as a function of cooling time.
Fig. 7. Excitation of the magnet up to 1.7 T at a speed of 1.7 T/ min. The temperatures of both ends of the current lead are also shown in the ®gure. HT end: high-temperature end. LT end: low-temperature end.
speed of 1.7 T/min. The temperatures of both ends of the current lead, the HT end and the LT end, are also shown in the ®gure. The temperature of the magnet rapidly increased to 26 K during the excitation. However, the temperature started to decrease as soon as the ®eld reached the maximum value, as shown in the ®gure, and the temperature went down to 15 K after 30 min. This indicates that the conduction cooling mechanism of this magnet system works well, and that the heat generated in the magnet immediately transferred to the second stage of the cryocooler. The generated
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®eld for the current of 180 A was 1.7 T, which was equal to the designed ®eld. After the 30 min excitation, the current decreased to zero at the same speed. A similar rapid increase of temperature was also observed. The large temperature increase during the energizing and de-energizing indicates that the energy loss in the Bi-2223 conductors is large under a changing current (AC loss) and that the heat generation is too fast to be transferred to the cryocooler. Such a large AC loss of Bi-2223 conductors is primarily due to the coupling (bridging) of the Bi-2223 ®laments in the Ag sheath. Heat generation at the stationary state of the magnet was estimated from the applied current and the voltage of the magnet. At a stable operation with a current of 180 A, the total voltage of the magnet was 3.56 mV, including the voltage drops at the junctions between pancake coils. Thus, the total heat generation in the magnet was calculated to be 0.64 W. This heat generation is much smaller than the cooling power of the cryocooler. The voltage measurements of several pancake coils at the stationary state suggest that most of the heat generation comes from the normal conducting junctions. This heat generation was so small that the temperature of the magnet decreased as soon as the applied current became constant, as shown in Fig. 7. Then, we repeated the excitation of the magnet up to 1.7 T at a speed of 1.7 T/min and observed the changes in temperature. The interval between each energizing and de-energizing was ®xed to be 3.5 min. Fig. 8 shows the temperatures of the magnet and the ®rst and second stages of the cryocooler as a function of the number of excitation cycles. A rapid increase and decrease of the magnet temperature could be observed again when energizing and de-energizing the magnet. The temperatures of the magnet and cryocooler gradually increased with increasing the number of excitations, but all these temperatures saturated at the excitation cycles of 20, and no increase was observed for a larger number of cycles. The saturated temperature of the magnet, 35±38 K, was rather high; however, stable operation of the magnet was obtained at these temperatures due to a sucient temperature margin of the Bi-2223
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net increased while energizing the magnet; however, the temperature started to decrease as soon as the ®eld became constant. This indicates that the cooling mechanism works well. 3. At the stationary state of the magnet, the total voltage of the magnet was as low as 3±4 mV, indicating that the heat generation in the magnet is negligibly small compared to the cooling capacity of the GM cryocooler. 4. The temperature of the magnet increased up to 35±38 K when repeating the excitation. However, no quenching was observed, indicating that the magnet was stable against the temperature increase. Fig. 8. Temperature changes of the magnet and ®rst and second stages of the GM cryocooler when the excitation of the magnet is repeated.
conductors. This result indicates that the conduction cooled Bi-2223 magnet is thermally stable against the repetition of excitation and is thus promising as a magnet for use in a magnetic separation system.
These results indicate that a conduction cooled Bi-2223 magnet is useful for applications that require a rapid increase and/or decrease of magnetic ®elds. However, the increase of the magnet temperature during rapid excitation is rather large due to the large AC loss of the Bi-2223 conductors. Reduction of AC loss is desired in order to suppress the temperature rise and attain higher excitation speeds.
5. Conclusions
References
We constructed a prototype of a conduction cooled Bi-2223 magnet for a magnetic separation system and tested the magnet at various excitation speeds.We obtained the following conclusions:
[1] J.A. Oberteuer, IEEE Trans. Mag. MAG-9 (1973) 303. [2] J.A. Selvaggi, P.V. Arend, J. Colwell, J. Adv. Cryogenic Engng. 33 (1988) 53. [3] H. Kumakura, H. Kitaguchi, K. Togano, H. Wada, K. Ohkura, M. Ueyama, K. Hayashi, K. Sato, Cryogenics 38 (1998) 639. [4] H. Kumakura, H. Kitaguchi, K. Togano, H. Wada, K. Ohkura, M. Ueyama, K. Hayashi, K. Sato, Adv. Supercond. XII (2000) 745. [5] H. Kumakura, H. Kitaguchi, K. Togano, T. Hasegawa, Y. Hikichi, Cryogenics 38 (1998) 163. [6] M. Ueyama, K. Ohkura, S. Kobayashi, K. Muranaka, T. Kaneko, T. Hikata, K. Hayashi, K. Sato, Adv. Supercond. VII (1995) 847.
1. The magnet has a 200 mm room temperature bore and generates ®elds higher than 1 T in an 11-liter room temperature space. 2. Cu disks inserted between the pancake coils were eective to increase the cooling channels and enhance the heat transfer from the magnet to the cryocooler. The temperature of the mag-