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本文首先综述了大规模应用的超导磁体, 依赖并推动铌三锡Nb3Sn导线技术进步, 向更强磁场发展的趋势. 着重分析了超高场14 T全身MRI磁体的挑战性技术. 选择青铜Nb3Sn导线, 采用Nb3Sn线圈和NbTi线圈相结合的混合结构, 对14 T全身MRI磁体进行了电磁概念设计和热稳定性及失超保护仿真分析, 并简要阐述了14 T全身MRI磁体在应力、接头和匀场方面的关键问题. 根据分析结果认为: 1) Nb3Sn导线是14 T全身MRI磁体需要面临的首要挑战性问题 —作为最佳选择的青铜Nb3Sn导线, 其现有产品的性能指标离14 T全身MRI磁体的要求尚存在有一定的差距; 2) 14 T全身MRI磁体的失超保护涉及线圈的铜超比设计、运行电流同线圈电感的协调配置、被动保护的分段策略和主动保护的失超触发控制以及主动屏蔽结构磁体在失超过程中的逸散磁场限制等多个十分复杂的环节, 是最具挑战性的综合性技术.This paper presents a brief review of the development trend of superconducting magnets in large scale applications towards high magnetic fields, depending on and pushing the Nb3Sn wire technics' continuous improvement. The focus is on analysis of the technology challenges of 14 T whole-body superconducting magnets. Using the Bonze Nb3Sn wires and on the base of a combination design of Nb3Sn and NbTi coils, an electromagnetic conception design of a 14 T whole-body MRI magnet is presented, and the thermal stability and quench protection are analyzed by simulations. The critical issues on stress, joints as well as shimming of 14 T whole-body superconducting magnets are also discussed. According to the results, this paper believes: 1) Nb3Sn wires are of the first important issue for 14 T whole-body superconducting magnets—the Bonze Nb3Sn wire is of the best choice but the performance specifications of the current products need to be improved further to match the requirements; 2) quench protection of 14 T whole-body superconducting magnets is one of the most complicated technics that covers design of the copper to superconductor (Cu/SC) ratio, coordination of the operating current and coil inductances, subdivisions of passive protection circuits and quench triggering control of active protection, as well as the stray field limitation during the transient process.
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Keywords:
- superconducting magnets /
- MRI /
- high magnetic field /
- large scale applications
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[2] Védrine P, Yildirim A 2019 Report on Superconducting Magnet Market Study Grant Agreement 731086
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图 1 14 T全身MRI磁体线圈结构示意图 (a)无屏蔽; (b)主动屏蔽
Fig. 1. 14 T whole body MRI magnet coil configurations: (a) Unshielded; (b) actively shielded.
图 2 14 T全身MRI磁体磁场强度等值线分布 (a)无屏蔽近场; (b)主动屏蔽近场; (c)无屏蔽远场(场强单位: Gs); (d)主动屏蔽远场(场强单位: Gs)
Fig. 2. Magnetic field intensity contours of 14 T whole-body MRI magnet: (a) Unshielded near field; (b) actively shielded near field; (c) unshielded far field (Field intensity unit: Gs); (d) actively shielded far field (Field intensity unit: Gs)
图 3 14 T全身MRI磁体线圈的洛伦兹力周向应力分布 (a)无屏蔽; (b)主动屏蔽
Fig. 3. 14 T whole-body MRI magnet coil Lorentz force circumferential stress distribution: (a) Unshielded; (b) actively shielded
图 4 14 T全身MRI磁体线圈失超仿真结果 (a)线圈电压(无屏蔽); (b)线圈电压(主动屏蔽); (c)线圈电流(无屏蔽); (d)线圈电流(主动屏蔽); (e)线圈电阻(无屏蔽); (f)线圈电阻(主动屏蔽); (g)线圈热点温度(无屏蔽); (h)线圈热点温度(主动屏蔽); (i) 失超3 s后线圈温度分布(无屏蔽); (j)失超3 s后线圈温度分布(主动屏蔽)
Fig. 4. Simulation results of 14 T whole-body MRI magnet during quench: (a) Voltages of the coils (unshielded); (b) voltages of the coils (actively shielded); (c) currents of the coils (unshielded); (d) currents of the coils (actively shielded); (e) resistances of the coils (unshielded); (f) resistances of the coils (actively shielded); (g) hot spot temperatures of the coils (unshielded); (f) hot spot temperatures of the coils (actively shielded); (i) temperature distributions in the coils after 3 s of quench (unshielded); (j) temperature distributions in the coils after 3 s of quench (actively shielded)
表 1 各线圈电流密度和铜超比预设
Table 1. Current density and copper/superconductor ratio of each coil.
Nb3Sn线圈1 Nb3Sn线圈2 NbTi
线圈3NbTi
线圈4NbTi补
偿线圈NbTi屏
蔽线圈电流密度/A·mm–2 80 95 25 35 65 65 铜超比 2 2 10 10 8 8 
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表 2 无屏蔽/主动屏蔽优化设计结果对比
Table 2. Comparison of unshielded/active shielded optimization design results.
无屏蔽 主动屏蔽 线圈最大长度/m 2.99 2.98 线圈最大外径/m 1.63 3.18 Nb3Sn导线总量/m3(不含铜) 0.460 0.466 NbTi导线总量/m3(不含铜) 0.189 0.308 磁场不均匀度(ppm on 40 cm DSV) 1.1 2.4
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表 3 无屏蔽/主动屏蔽线圈磁场对比
Table 3. Comparison of unshielded/active shielded coil magnetic field.
无屏蔽 主动屏蔽 中心磁场/T 14 14 Nb3Sn线圈1最大磁密/T 14.76 14.66 Nb3Sn线圈2最大磁密/T 10.67 10.02 NbTi线圈3最大磁密/T 8.28 7.40 NbTi线圈4最大磁密/T 7.54 6.28 NbTi补偿线圈最大磁密/T 6.78 6.01 NbTi屏蔽线圈最大磁密/T — 4.36 5 Gs线(径向m × 轴向m) 21.2 × 26.6 11.8 × 14.8 线圈总能量/MJ 260 280
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表 4 无屏蔽/主动屏蔽线圈最大洛伦兹力对比
Table 4. Comparison of unshielded/active shielded coil maximum Lorentz force.
无屏蔽 主动屏蔽 最大周向应力/MPa 645 651 补偿线圈轴向力/t, 压强/MPa –2540, –78.6 –1560, –43.3 屏蔽线圈轴向力/t, 压强/MPa — 1310, 20.3
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表 5 无屏蔽/主动屏蔽线圈电感及导线长度对比
Table 5. Comparison of unshielded/active shielded coil inductances and wire lengths.
无屏蔽 主动屏蔽 运行电流/A 244 248.8 总电感/H 8746 10287 Nb3Sn导线总长/km 315.6 320.4 NbTi导线总长/km 331.8 591.7
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表 6 无屏蔽/主动屏蔽线圈最小失超能量对比
Table 6. Comparison of unshielded/active shielded coil minimum quench energy.
无屏蔽 主动屏蔽 Nb3Sn线圈1最小失超能量/mJ 34 42 Nb3Sn线圈2最小失超能量/mJ 68 78 NbTi线圈3最小失超能量/mJ 32 68 NbTi线圈4最小失超能量/mJ 36 104 NbTi补偿线圈最小失超能量/mJ 14 18 NbTi屏蔽线圈最小失超能量/mJ — 32
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[1] Seidel P 2015 Applied Superconductivity: Handbook on Devices and Applications (Wiley-VCH) pp448−580
[2] Védrine P, Yildirim A 2019 Report on Superconducting Magnet Market Study Grant Agreement 731086
[3] Field M B, Zhang Y, Miao H, Gerace M, Parrell J A 2014 IEEE T. Appl. Supercon. 24 6001105
Google Scholar
[4] https://www.iter.org/proj/inafewlines[2020-11-30]
[5] Bordini B 2019 International Conference on Magnet Technology MT-26 (Canada: Vancouver)
[6] Barzi E, Zlobin A V 2016 IEEE T. Nucl. Sci. 63 783
Google Scholar
[7] Nishijima G, Matsumoto S, Hashi K, Ohki S, Goto A, Noguchi T, Iguchi S, Yanagisawa Y, Takahashi M, Maeda H, Miki T, Saito K, Tanaka R, Shimizu T 2016 IEEE T. Appl. Supercon. 26 4303007
Google Scholar
[8] Iwasa Y, Bascuñán J, Hahn S, Voccio J, Kim Y, Lécrevisse T, Song J, Kajikawa K 2015 IEEE T. Appl. Supercon. 25 4301205
[9] Cosmus T C, Parizh M 2011 IEEE T. Appl. Supercon. 21 2104
Google Scholar
[10] Lvovsky Y, Stautner E W, Zhang T 2013 Supercon. Sci. Tech. 26 093001
Google Scholar
[11] Warner Rory 2016 Supercon. Sci. Tech. 29 094006
Google Scholar
[12] Quettier L, Aubert G, Belorgey J, et al. 2020 IEEE T. Appl. Supercon. 30 4401705
[13] Liang J, Jiang X, Li H, Wei X 2009 IEEE T. Appl. Supercon. 19 1282
Google Scholar
[14] Iwaki G, Nishijima G, Takahashi M, Katagiri K, Watanabe K 2006 IEEE T. Appl. Supercon. 16 1261
Google Scholar
[15] Oguro H, Awaji S, Watanabe K, Sugimoto M, Tsubouchi H 2013 Supercon. Sci. Tech. 26 094002
Google Scholar
[16] Chen J, Jiang X 2013 IEEE T. Appl. Supercon. 23 4701104
Google Scholar
[17] Chen J, Jiang X 2012 IEEE T. Appl. Supercon. 22 4903104
Google Scholar
[18] Vedrine P, Aubert G, Beaudet F, Belorgey J, Berriaud C, Bredy P, Donati A, Dubois O, Gilgrass G, Juster F P, Meuris C, Molinie F, Nunio F, Payn A, Schild T, Scola L, Sinanna A 2010 IEEE T. Appl. Supercon. 20 696
Google Scholar
[19] 蒋晓华, 韩朔 1991 电工技术学报 0 12
Google Scholar
[20] https://www.jastec-inc.com/e_products_wire/list.html [2020-11-30]
[21] Sugimoto M, Katayama K, Takagi A, Shimizu H, Tsubouchi H, Awaji S, Oguro H 2018 IEEE T. Appl. Supercon. 28 6000105
Google Scholar
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