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Experimental study on a Nb3Al insert coil under high magnetic field
Cryogenics 76 (2016) 29–32
Contents lists available at ScienceDirect
Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
Experimental study on a Nb3Al insert coil under high magnetic field Guang Zhu a, Yinming Dai a, Junsheng Cheng a, Kun Chang a, Jianhua Liu a, Qiuliang Wang a,⇑, Xifeng Pan b, Chao Li b a b
Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, China Western Superconductor Technology, Co. Ltd, Xi’an 710018, China
a r t i c l e
i n f o
Article history: Received 6 December 2015 Received in revised form 7 February 2016 Accepted 29 March 2016 Available online 2 April 2016 Keywords: Nb3Al Superconducting magnet Experiment High field
a b s t r a c t Nb3Al is one of the most promising superconductors to replace Nb3Sn in large scale, high field superconducting magnet. Since the complicated conductor manufacturing process, long and stable Nb3Al conductor is difficult to acquire in a commercial scale. Based on a 70 m length of Nb–Al precursor conductor, we designed and fabricated a Nb3Al coil. The coil winding, low temperature diffusion heat treatment and epoxy impregnation are described in detail. The finished Nb3Al coil is tested as an insert in a background magnet. The test is performed at the background field from 7 T to 15 T. The test results are analyzed and presented in this paper. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction A15 superconducting materials are usually used to manufacture coils in high field superconducting magnets above 10 T. For example, Nb3Sn, the most commonly used A15 superconductor, has very good critical performances under zero strain conditions, which upper critical current density could reach 3000 A/mm2 at 4.2 K and 12 T background field [1]. However, the critical current of Nb3Sn may show severe degradations under large strain [2]. In developing large scale superconducting magnets, such as high field tokamak and accelerator dipole magnet as well as high field solenoid with large diameter, substantial strains may appear due to the existence of huge electromagnetic force in the coil windings, resulting in magnet performance degradation. Applications of large-scale high field superconducting magnet require high strength, high performance superconducting materials to fabricate the coils windings. Recently, progresses in developing high strength A15 superconducting materials have been achieved, such as CuNb or Ta reinforced Nb3Sn [3–5] and Nb3Al conductors [6–8]. As one of the A15 superconductor, Nb3Al is a promising superconducting material for large scale, high field superconducting magnet. The critical temperature is 18.9 K and upper critical magnetic field is 29.5 T at liquid helium temperature. Compared with Nb3Sn superconductor, Nb3Al has a prominent feature which critical performance is insensitive to strain. It is of great significance for engineering applications of large scale, high field superconducting magnet. Therefore, Nb3Al has been thought to be a promising ⇑ Corresponding author. http://dx.doi.org/10.1016/j.cryogenics.2016.03.003 0011-2275/Ó 2016 Elsevier Ltd. All rights reserved.
candidate material for upgrading magnet in Large Hadron Collider (LHC), International Thermonuclear Experiment Reactor (ITER) and other high field applications. Fabrication of Nb3Al wire is still immature and in a laboratory level. Typical manufacturing methods of Nb–Al wire include Jelly Roll, Rod-In-Tube and Powder-In-Tube methods. The Nb3Al precursors are usually processed by Rapid Heating/Quenching and Transformation (RHQT) or Low Temperature Diffusion method [8–10]. Recently, serval experimental Nb3Al magnets were developed based on RHQT technology [11,12]. The magnet application of Nb3Al wire is limited by the fact that it is difficult to obtain long and uniform Nb3Al wires due to the complicity of wire fabrication process. Recently, Nb3Al wire samples have been successfully fabricated by Western Superconducting Technologies Co. Ltd. (WST) under the support of China National Magnetic Confinement Fusion Science Program. The purpose of developing Nb3Al superconductor is for future Tokamak magnet and other high field magnet applications. Based on the Nb3Al superconductor development, we manufactured an experimental Nb3Al coil with a short piece length of Nb3Al precursor. Magnet coil design, winding and heat treatment are described in detail. After impregnation, the coil is tested under high magnetic field to examine its performance. 2. Nb3Al conductor Nb3Al wires made by other methods [13–16] showed diverse critical performances, which include RHQT, low temperature diffusion (in Cu-matrix or Nb-matrix) and high energy ball milling
30
G. Zhu et al. / Cryogenics 76 (2016) 29–32 1000
Critical Current (A)
800
600
400 18 cores-850oC/25h 18 cores-850oC/40h 18 cores-900oC/40h
200
0
4
5
6
7
8
9
10
11
12
13
Fig. 2. Coil shape optimization in background magnet.
Applied Magnetic Field (T) Fig. 1. Critical current performance of Nb3Al short samples.
method. The Nb3Al wire used for the coil winding was made with jolly-roll method and low-temperature heat-treatment. The 6-cores, 18-cores and 36-cores wires were fabricated by stacking the mono-filamentary segments and cold-drawing, and 18-cores wire was chosen according to its performance. The detail of these wires can be found in reference [17]. In Fig. 1, the cross section of the 18-cores wire is shown, which depictures the final size of 0.74 mm drawn from a billet of 42 mm in diameter. Number of Nb–Al filaments is 18 which are embedded in an oxygen-free copper stabilizer. Critical current of short samples have been tested in liquid helium temperature and shown in this figure. It can be seen that the critical current is closely related to the thermal diffusion temperatures. The bare diameter of Nb–Al precursor is 0.74 mm and available piece length is only 70 m. Wire insulation is implemented by wrapping a layer of S2-glass glass fiber braiding which can withstand high temperature during heat treatment. After insulation, the final averaged wire diameter is 0.89 mm. 3. Nb3Al coil design and manufacture 3.1. Nb3Al coil design The experimental Nb3Al coil is designed based on a single piece length of 70 m wire provided by WST. In order to obtain a center magnetic field as high as possible in a 15 T background magnet, coil configuration is optimized at the condition of fixed coil inner diameter of 20 mm and constant winding volume. Critical performances and peak fields in the windings are involved in the optimization process. Critical currents at different magnetic fields are extrapolated from the short sample test data. The results are shown in Fig. 2, in which the abscissa a denotes a ratio of coil outer to inner radius. It can be seen that the center magnetic field B0 reaches its maximum of 0.225 T at a = 2.5, while the peak field ratio Bp/B0 increases to 1.06. Due to lack of test data above 12 T, some uncertainties may exist in the extrapolation of critical currents at high field region. This made us accept a compromised coil design, i.e. a = 2.05. The final height, inner and outer diameter of the coil winding are determined to be 53.4 mm, 20 mm and 41 mm respectively, and the bobbin’s are73.4 mm, 15 mm and 50 mm respectively. With the insulated Nb–Al precursor, 12 layers can be wound on a stainless steel bobbin and the total turns are 682. Inductance of the coil is calculated as 4.19 mH. Magnetic field constant of the coil is estimated as 133 Gs/A. Before coil winding, ground insulation is performed on the surface of winding bobbin. The insulation material is selected as Al2O3
Fig. 3. Lift: The coil design. Right: The Nb3Al coils after winding.
coatings with a thickness of about 0.2 mm. The designed coil is shown in Fig. 3. 3.2. Coil winding Coil winding is performed directly on the insulated coil bobbin. Dry winding method is adopted to wind the coil layer by layer. During winding process, materials such as cotton, polymer and glues are absolutely refrained from involvement to avoid possible carbonization at high temperatures. All kinds of organic pollution are carefully removed and cleaned. Solder materials, which are common in copper coil windings, are intentionally avoided during the whole winding process. As the glass fiber jacket of wire insulation is a flexible material, deformation occured during winding, which resulted in a little less turns than the designed number. After the coil winding, stainless steel over-banding are implemented and mechanically fixed. The finished coil is shown in Fig. 3. Actually, a total of 644 turns were wound using the 70 m Nb–Al conductor. Coil resistance is measured as 4.5 X at room temperatures and the ground insulation is larger than 20 MX. 3.3. Heat treatment The finished coil is just a winding of Nb–Al/Cu composite wire. Heat treatment is indispensable to transform the precursor into
G. Zhu et al. / Cryogenics 76 (2016) 29–32
superconducting A15 phase of Nb3Al. Nb3Al superconductor is a new kind of superconductor, there is no ready-made optimal heat treatment strategy. According to the test results of short samples, the heat treatment temperature was set to 850 °C, the heat treatment time was set to 40 h. To avoid oxidation of the conductor at high temperatures, the heat treatment was carried out in a vacuum heating furnace, in which the vacuum level was 10 4 Pa. The temperature difference in the furnace was controlled within ±2 °C to ensure the heating uniformity. To avoid stress concentration in rapid heating/cooling, the temperature change rate was controlled at 20 °C/h or less. The whole heat treatment took more than one week.
31
voltage signal during magnet energizing. The Nb3Al coil was integrated into the clear bore of background magnet and positioned at the magnet center, as shown in Fig. 4. All the signal data were recorded automatically by an acquisition computer. In the experiment, totally 13 tests were conducted under background field from 7 T to 15 T. Quenching currents are recorded at different background field. The relationship between the quenching currents and background field is plotted in Fig. 5. It can be seen from the figure that the quenching current decreases with the
3.4. Vacuum pressure impregnation In order to ensure the solidity of coil windings, CTD 101 K epoxy resin was used to impregnate the Nb3Al coil. This kind of resin was specifically designed for superconducting magnet impregnation. The insulation property and mechanical strength are believed to be satisfactory at 4.2 K. Besides, its room temperature performance and handling characteristics are also very good. The pot-life is more than 20 h; the initial viscosity is less than 100 CP at 60 °C. The flood-filling VPI technics was used since the experimental coil is relatively small. The epoxy impregnation is carried out in a sealed vessel which can be vacuum pumped. The coil was put into the vessel together with a heater. When the vacuum dropped to 100 Pa or less, the formulated epoxy resin was poured into the vessel. During impregnation, the temperature in the vessel was controlled at 60 °C. After impregnation, the epoxy resin pre-cure was conducted at 80 °C, which made the epoxy resin turn to gel state. Then the coil was taken out and cured at 110 °C.
Fig. 5. Quenching currents of the Nb3Al coil under background field.
4. Coil test The test of Nb3Al coil was conducted in a background magnet. A Hall probe was fixed at the center of the Nb3Al coil to indicate the magnetic field induction. Voltage taps were set to record the
Fig. 4. The Nb3Al coil experimental setup.
Fig. 6. The excitation curve (upper) and quenching process (bottom) of the Nb3Al coil under 15 T.
32
G. Zhu et al. / Cryogenics 76 (2016) 29–32
increasing of background magnetic field. At 15 T background field, the quenching current is just 3.4 A which generates an extra magnetic field of 448 Gs in the center of the Nb3Al coil. Quenching currents of the coil are similar in tendency to the critical performance of short sample, but in a lower level. This phenomenon can be explained by the break of Nb3Al cores in long piece. In short Nb3Al wire sample there should be 18 cores inside. However, according to the results of the performance of the experimental coil, there are only about 2–3 cores in the conduction state, and the rest of cores are broken. The break most probably happens during the preparation of the long-wire. Thus, there is a nearly linear relationship between the performances of the coil and the short example. Besides, the heat treatment strategy on a coil should be different from that on a short sample. Anyway, there still need improvements on the heat treatment strategy of Nb3Al coils. The Nb3Al coil excitation process under 15 T has been extracted from acquisition data of Hall sensor and coil voltage, which is plotted in Fig. 6a. It can be seen that coil voltage increases slowly after a quench is detected. The exaggerated quenching process is plotted in Fig. 6b. If we take 10 4 V/m as the quenching criterion, the n value of the Nb3Al coil is 11. 5. Conclusion A Nb3Al coil is designed and fabricated based on a short piece conductor fabricated by the jelly-roll Nb–Al precursor and low temperature diffusion process. The height, inner and outer diameter of the coil is 73.4 mm, 15 mm and 50 mm respectively. Total of 644 turns are wound on a stainless steel bobbin. The magnetic constant of the coil is 133 Gs/A. The coil fabrication process is described and discussed in detail. Heat treatment strategy is practiced in the fabricated coil. The Nb3Al coil is tested as an insert in a background magnet. The test results show that the Nb3Al coil could keep superconductivity at a background field more than 15 T. In order to improve the performance of Nb3Al magnet, high performance wire winding should be further improved and more reasonable heat treatment strategies should be developed in the future. Acknowledgement This work is supported by National Magnetic Confinement Fusion Science Program (Grant No. 2011GB112002).
References [1] Parrell Jeffrey A, Field Michael B, Zhang Youzhu, Hong Seung. Advances in Nb3Sn strand for fusion and particle accelerator applications. IEEE Trans Appl Supercond 2005;15:1200–4. [2] Godeke A. A reviewof the properties of Nb3Sn and their variation with A15 composition, morphology and strain state. Supercond Sci Technol 2006;19:68–80. [3] Kaneko T, Seto T, Nanbu T, Murase S, Shimamoto S, Awaji S, et al. Stability of Nb3Sn wires with CuNb reinforcing stabilizer on cryocooled superconducting magnet. IEEE Trans Appl Supercond 2000;10:1235–8. [4] Iwaki Genzo, Nishijima Gen, Takahashi Masaya, Katagiri Kazumune, Watanabe Kazuo. Development of high strength Nb3Sn wires with Ta-reinforced filaments. IEEE Trans Appl Supercond 2006;16:1261–4. [5] Awaji Satoshi, Watanabe Kazuo, Oguro Hidetoshi, Hanai Satoshi, Miyazaki Hiroshi, Takahashi Masahico, et al. New 25 T cryogen-free superconducting magnet project at Tohoku University. IEEE Trans Appl Supercond 2014;24:4302005. [6] Ayai N, Yamada Y, Mikumo A, Ito M, Ohmatsu K, Sato K, et al. Development of Nb3Al superconductors for ITER. IEEE Trans Appl Supercond 1999;19:2688–91. [7] Takeuchi Takao. Nb3Al conductors for high-field applications. Supercond Sci Technol 2000;13:101–19. [8] Takeuchi T, Kikuchi A, Banno N, Kitaguchi H, Iijima Y, Tagawa K, et al. Status and perspective of the Nb3Al development. Cryogenics 2008;48:371–80. [9] Kumakura H, Kitaguchi H, Matsumoto A, Yamada H, Hirakawa M, Tachikawa K. Fabrication of A15-type superconducting tape conductors by applying the ex situ powder-in-tube method. Supercond Sci Technol 2005;18:147–51. [10] Iijima Yasuo, Kikuchi Akihiro, Banno Nobuya. Optimization of operating conditions for a RHQT (rapid-heating, quenching and transformation) processed Nb3Al superconductors. J Jpn Inst Met 2007;71:952–8. [11] Takeuchi T, Kitaguchi H, Banno N, Iijima Y, Kikuchi A, Tagawa K, et al. Fabrication and operation of a RHQT Nb3Al insert coil generating 4.5 T at 4.2 K in 15 T back-up field. IEEE Trans Appl Supercond 2007;17:2684–7. [12] Xu Qingjin, Sasaki Kenichi, Nakamoto Tatsushi, Terashima Akio, Tsuchiya Kiyosumi, Yamamoto Akira, et al. Design of a high field Nb3Al common coil magnet. IEEE Trans Appl Supercond 2010;20:176–9. [13] Liu Z, Li Pingyuan, Cui Yajing, Pan Xifeng, Yan Guo. Preparation of Nb3Al superconductor by powder metallurgy and effect of mechanical alloying on the phase formation. J Mod Transp 2014;22(1):55–8. [14] Chen Y, Liu Z, Li P, Zhang X, Yang S. Preparation of Nb3Al by high-energy ball milling and superconductivity. J Phys: Conf Ser 2014;507(1):012006. [15] Pan X, Yan G, Cui L, Chen C, Bai Z. Fabrication and superconducting properties of rod-in-tube multifilamentary Nb3Al wire with rapid heating and quenching heat-treatment. IEEE Trans Appl Supercond 2015;25:1–4. [16] Pan X, Yan G, Qi M, Cui L, Chen Y. Fabrication of Nb3Al superconducting wires by utilizing the mechanically alloyed Nb(Al)ss supersaturated solid-solution with low-temperature annealing. Physica C 2014;502:14–9. [17] Cui L, Yan G, Pan X. Fabrication and superconducting properties of a simplestructured jelly-roll Nb3Al wire with low-temperature heat-treatment. Physica C 2015;513:24–8.
Contents lists available at ScienceDirect
Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
Experimental study on a Nb3Al insert coil under high magnetic field Guang Zhu a, Yinming Dai a, Junsheng Cheng a, Kun Chang a, Jianhua Liu a, Qiuliang Wang a,⇑, Xifeng Pan b, Chao Li b a b
Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, China Western Superconductor Technology, Co. Ltd, Xi’an 710018, China
a r t i c l e
i n f o
Article history: Received 6 December 2015 Received in revised form 7 February 2016 Accepted 29 March 2016 Available online 2 April 2016 Keywords: Nb3Al Superconducting magnet Experiment High field
a b s t r a c t Nb3Al is one of the most promising superconductors to replace Nb3Sn in large scale, high field superconducting magnet. Since the complicated conductor manufacturing process, long and stable Nb3Al conductor is difficult to acquire in a commercial scale. Based on a 70 m length of Nb–Al precursor conductor, we designed and fabricated a Nb3Al coil. The coil winding, low temperature diffusion heat treatment and epoxy impregnation are described in detail. The finished Nb3Al coil is tested as an insert in a background magnet. The test is performed at the background field from 7 T to 15 T. The test results are analyzed and presented in this paper. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction A15 superconducting materials are usually used to manufacture coils in high field superconducting magnets above 10 T. For example, Nb3Sn, the most commonly used A15 superconductor, has very good critical performances under zero strain conditions, which upper critical current density could reach 3000 A/mm2 at 4.2 K and 12 T background field [1]. However, the critical current of Nb3Sn may show severe degradations under large strain [2]. In developing large scale superconducting magnets, such as high field tokamak and accelerator dipole magnet as well as high field solenoid with large diameter, substantial strains may appear due to the existence of huge electromagnetic force in the coil windings, resulting in magnet performance degradation. Applications of large-scale high field superconducting magnet require high strength, high performance superconducting materials to fabricate the coils windings. Recently, progresses in developing high strength A15 superconducting materials have been achieved, such as CuNb or Ta reinforced Nb3Sn [3–5] and Nb3Al conductors [6–8]. As one of the A15 superconductor, Nb3Al is a promising superconducting material for large scale, high field superconducting magnet. The critical temperature is 18.9 K and upper critical magnetic field is 29.5 T at liquid helium temperature. Compared with Nb3Sn superconductor, Nb3Al has a prominent feature which critical performance is insensitive to strain. It is of great significance for engineering applications of large scale, high field superconducting magnet. Therefore, Nb3Al has been thought to be a promising ⇑ Corresponding author. http://dx.doi.org/10.1016/j.cryogenics.2016.03.003 0011-2275/Ó 2016 Elsevier Ltd. All rights reserved.
candidate material for upgrading magnet in Large Hadron Collider (LHC), International Thermonuclear Experiment Reactor (ITER) and other high field applications. Fabrication of Nb3Al wire is still immature and in a laboratory level. Typical manufacturing methods of Nb–Al wire include Jelly Roll, Rod-In-Tube and Powder-In-Tube methods. The Nb3Al precursors are usually processed by Rapid Heating/Quenching and Transformation (RHQT) or Low Temperature Diffusion method [8–10]. Recently, serval experimental Nb3Al magnets were developed based on RHQT technology [11,12]. The magnet application of Nb3Al wire is limited by the fact that it is difficult to obtain long and uniform Nb3Al wires due to the complicity of wire fabrication process. Recently, Nb3Al wire samples have been successfully fabricated by Western Superconducting Technologies Co. Ltd. (WST) under the support of China National Magnetic Confinement Fusion Science Program. The purpose of developing Nb3Al superconductor is for future Tokamak magnet and other high field magnet applications. Based on the Nb3Al superconductor development, we manufactured an experimental Nb3Al coil with a short piece length of Nb3Al precursor. Magnet coil design, winding and heat treatment are described in detail. After impregnation, the coil is tested under high magnetic field to examine its performance. 2. Nb3Al conductor Nb3Al wires made by other methods [13–16] showed diverse critical performances, which include RHQT, low temperature diffusion (in Cu-matrix or Nb-matrix) and high energy ball milling
30
G. Zhu et al. / Cryogenics 76 (2016) 29–32 1000
Critical Current (A)
800
600
400 18 cores-850oC/25h 18 cores-850oC/40h 18 cores-900oC/40h
200
0
4
5
6
7
8
9
10
11
12
13
Fig. 2. Coil shape optimization in background magnet.
Applied Magnetic Field (T) Fig. 1. Critical current performance of Nb3Al short samples.
method. The Nb3Al wire used for the coil winding was made with jolly-roll method and low-temperature heat-treatment. The 6-cores, 18-cores and 36-cores wires were fabricated by stacking the mono-filamentary segments and cold-drawing, and 18-cores wire was chosen according to its performance. The detail of these wires can be found in reference [17]. In Fig. 1, the cross section of the 18-cores wire is shown, which depictures the final size of 0.74 mm drawn from a billet of 42 mm in diameter. Number of Nb–Al filaments is 18 which are embedded in an oxygen-free copper stabilizer. Critical current of short samples have been tested in liquid helium temperature and shown in this figure. It can be seen that the critical current is closely related to the thermal diffusion temperatures. The bare diameter of Nb–Al precursor is 0.74 mm and available piece length is only 70 m. Wire insulation is implemented by wrapping a layer of S2-glass glass fiber braiding which can withstand high temperature during heat treatment. After insulation, the final averaged wire diameter is 0.89 mm. 3. Nb3Al coil design and manufacture 3.1. Nb3Al coil design The experimental Nb3Al coil is designed based on a single piece length of 70 m wire provided by WST. In order to obtain a center magnetic field as high as possible in a 15 T background magnet, coil configuration is optimized at the condition of fixed coil inner diameter of 20 mm and constant winding volume. Critical performances and peak fields in the windings are involved in the optimization process. Critical currents at different magnetic fields are extrapolated from the short sample test data. The results are shown in Fig. 2, in which the abscissa a denotes a ratio of coil outer to inner radius. It can be seen that the center magnetic field B0 reaches its maximum of 0.225 T at a = 2.5, while the peak field ratio Bp/B0 increases to 1.06. Due to lack of test data above 12 T, some uncertainties may exist in the extrapolation of critical currents at high field region. This made us accept a compromised coil design, i.e. a = 2.05. The final height, inner and outer diameter of the coil winding are determined to be 53.4 mm, 20 mm and 41 mm respectively, and the bobbin’s are73.4 mm, 15 mm and 50 mm respectively. With the insulated Nb–Al precursor, 12 layers can be wound on a stainless steel bobbin and the total turns are 682. Inductance of the coil is calculated as 4.19 mH. Magnetic field constant of the coil is estimated as 133 Gs/A. Before coil winding, ground insulation is performed on the surface of winding bobbin. The insulation material is selected as Al2O3
Fig. 3. Lift: The coil design. Right: The Nb3Al coils after winding.
coatings with a thickness of about 0.2 mm. The designed coil is shown in Fig. 3. 3.2. Coil winding Coil winding is performed directly on the insulated coil bobbin. Dry winding method is adopted to wind the coil layer by layer. During winding process, materials such as cotton, polymer and glues are absolutely refrained from involvement to avoid possible carbonization at high temperatures. All kinds of organic pollution are carefully removed and cleaned. Solder materials, which are common in copper coil windings, are intentionally avoided during the whole winding process. As the glass fiber jacket of wire insulation is a flexible material, deformation occured during winding, which resulted in a little less turns than the designed number. After the coil winding, stainless steel over-banding are implemented and mechanically fixed. The finished coil is shown in Fig. 3. Actually, a total of 644 turns were wound using the 70 m Nb–Al conductor. Coil resistance is measured as 4.5 X at room temperatures and the ground insulation is larger than 20 MX. 3.3. Heat treatment The finished coil is just a winding of Nb–Al/Cu composite wire. Heat treatment is indispensable to transform the precursor into
G. Zhu et al. / Cryogenics 76 (2016) 29–32
superconducting A15 phase of Nb3Al. Nb3Al superconductor is a new kind of superconductor, there is no ready-made optimal heat treatment strategy. According to the test results of short samples, the heat treatment temperature was set to 850 °C, the heat treatment time was set to 40 h. To avoid oxidation of the conductor at high temperatures, the heat treatment was carried out in a vacuum heating furnace, in which the vacuum level was 10 4 Pa. The temperature difference in the furnace was controlled within ±2 °C to ensure the heating uniformity. To avoid stress concentration in rapid heating/cooling, the temperature change rate was controlled at 20 °C/h or less. The whole heat treatment took more than one week.
31
voltage signal during magnet energizing. The Nb3Al coil was integrated into the clear bore of background magnet and positioned at the magnet center, as shown in Fig. 4. All the signal data were recorded automatically by an acquisition computer. In the experiment, totally 13 tests were conducted under background field from 7 T to 15 T. Quenching currents are recorded at different background field. The relationship between the quenching currents and background field is plotted in Fig. 5. It can be seen from the figure that the quenching current decreases with the
3.4. Vacuum pressure impregnation In order to ensure the solidity of coil windings, CTD 101 K epoxy resin was used to impregnate the Nb3Al coil. This kind of resin was specifically designed for superconducting magnet impregnation. The insulation property and mechanical strength are believed to be satisfactory at 4.2 K. Besides, its room temperature performance and handling characteristics are also very good. The pot-life is more than 20 h; the initial viscosity is less than 100 CP at 60 °C. The flood-filling VPI technics was used since the experimental coil is relatively small. The epoxy impregnation is carried out in a sealed vessel which can be vacuum pumped. The coil was put into the vessel together with a heater. When the vacuum dropped to 100 Pa or less, the formulated epoxy resin was poured into the vessel. During impregnation, the temperature in the vessel was controlled at 60 °C. After impregnation, the epoxy resin pre-cure was conducted at 80 °C, which made the epoxy resin turn to gel state. Then the coil was taken out and cured at 110 °C.
Fig. 5. Quenching currents of the Nb3Al coil under background field.
4. Coil test The test of Nb3Al coil was conducted in a background magnet. A Hall probe was fixed at the center of the Nb3Al coil to indicate the magnetic field induction. Voltage taps were set to record the
Fig. 4. The Nb3Al coil experimental setup.
Fig. 6. The excitation curve (upper) and quenching process (bottom) of the Nb3Al coil under 15 T.
32
G. Zhu et al. / Cryogenics 76 (2016) 29–32
increasing of background magnetic field. At 15 T background field, the quenching current is just 3.4 A which generates an extra magnetic field of 448 Gs in the center of the Nb3Al coil. Quenching currents of the coil are similar in tendency to the critical performance of short sample, but in a lower level. This phenomenon can be explained by the break of Nb3Al cores in long piece. In short Nb3Al wire sample there should be 18 cores inside. However, according to the results of the performance of the experimental coil, there are only about 2–3 cores in the conduction state, and the rest of cores are broken. The break most probably happens during the preparation of the long-wire. Thus, there is a nearly linear relationship between the performances of the coil and the short example. Besides, the heat treatment strategy on a coil should be different from that on a short sample. Anyway, there still need improvements on the heat treatment strategy of Nb3Al coils. The Nb3Al coil excitation process under 15 T has been extracted from acquisition data of Hall sensor and coil voltage, which is plotted in Fig. 6a. It can be seen that coil voltage increases slowly after a quench is detected. The exaggerated quenching process is plotted in Fig. 6b. If we take 10 4 V/m as the quenching criterion, the n value of the Nb3Al coil is 11. 5. Conclusion A Nb3Al coil is designed and fabricated based on a short piece conductor fabricated by the jelly-roll Nb–Al precursor and low temperature diffusion process. The height, inner and outer diameter of the coil is 73.4 mm, 15 mm and 50 mm respectively. Total of 644 turns are wound on a stainless steel bobbin. The magnetic constant of the coil is 133 Gs/A. The coil fabrication process is described and discussed in detail. Heat treatment strategy is practiced in the fabricated coil. The Nb3Al coil is tested as an insert in a background magnet. The test results show that the Nb3Al coil could keep superconductivity at a background field more than 15 T. In order to improve the performance of Nb3Al magnet, high performance wire winding should be further improved and more reasonable heat treatment strategies should be developed in the future. Acknowledgement This work is supported by National Magnetic Confinement Fusion Science Program (Grant No. 2011GB112002).
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