Fabrication of NbTi Superconducting Joints for 400-MHz ... - IEEE Xplore

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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 22, NO. 2, APRIL 2012

Fabrication of NbTi Superconducting Joints for 400-MHz NMR Application Junsheng Cheng, Jianhua Liu, Zhipeng Ni, Chunyan Cui, Shunzhong Chen, Shousen Song, Lankai Li, Yinming Dai, and Qiuliang Wang

TABLE I PARAMETERS FOR THE MAGNET

Abstract—NbTi superconducting joints (SJs) for a 400-MHz nuclear magnetic resonance (NMR) magnet system were fabricated using the superconducting solder matrix replacement in an open-air condition. A detection device for testing the resistance of SJ has been established. The results show that the overall resistance of SJs is 9.58 × 10−12 Ω under the background field of 1 T by summation of individual joint resistance. The resistance of SJs and the capability for current load should meet the demands of the NMR system. The SJs are placed inside the cylindrical vessel above the magnet. The magnetic flux inside the top of the vessel is no more than 0.3 T to assure performance of joints. As results, there is only 0.0001 ppm for homogeneity deviation caused by SJs on this NMR system, and therefore, the negative effect is negligible. Index Terms—Nuclear magnetic resonance (NMR) spectrometer, solder, superconducting joint (SJ), superconducting magnet.

I. I NTRODUCTION

A

400-MHz superconducting nuclear magnetic resonance (NMR) spectrometer has been designed, manufactured, and assembled in IEE CAS [1]. This magnet system operates with zero-vapor liquid helium, which reduce maintenance requirements and therefore the operation cost. The superconducting magnet is immersed in 4-K liquid helium and enclosed in the liquid helium vessel. In order to keep the system at required temperature, a pulse tube cryocooler (PTR) is used to minimize, or even can stop, helium refills. At the first stage, PTR is directly connected to the surrounding thermal shield, which will be cooled down to a temperature in the range of 40 to 50 K. At the second stage, PTR is relocated at the top of the liquid helium vessel to condense helium vapor back to liquid. The magnet structure consists of 17 superconducting coils in series. Each superconducting joint (SJ) connects two adjacent coils. All coils and joints work in the persistent mode, and the persistence of an NMR magnet is dependent upon the resistive losses within the coil circuit. The total resistance in the circuit is equal to the summation of wire resistance in each coil and joint [2]. The superconducting wire resistance is a result of

Manuscript received November 6, 2011; revised January 5, 2012; accepted January 12, 2012. Date of publication March 5, 2012; date of current version March 30, 2012. This paper was recommended by Associate Editor M. Parizh. This work was supported in part by the National Natural Science Foundation of China under Grant 50925726 and Grant 10755001 and in part by the Instrument Program in the Ministry of Science and Technology of China. The authors are with the Institute of Electrical Engineering, Chinese Academy of Sciences, and the Key Laboratory of Applied Superconductors, Chinese Academy of Sciences, Beijing 100190, China (e-mail: jscheng@ mail.iee.ac.cn). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TASC.2012.2185795

the n-value losses, which can be minimized by a considerable current margin in the magnet design. The contribution from joint resistance is basically a summation of resistances of all the interconnections in the circuit. To achieve satisfactory magnetic field stability in the NMR system, the joints between magnet and its switch, and also between different sections of the magnet, must be made with great care [3], [4]. Some methods for fabricating SJ between the NbTi alloy conductors have been developed, such as soldering [5], [6], cold-pressing welding, diffusion welding [7], spot welding [8], [9], ultrasonic welding [10], diffusion bonding, and solder matrix replacement [2]. Among these methods, the solder matrix replacement method has the advantage of convenience. However, the vacuum environment is required during making SJ to avoid contamination. In fact, it is difficult to place the whole magnet structure into a vacuum environment such as a glove box. An SJ fabrication method operated in an open-air environment is therefore developed for the magnet manufacture. In this paper, an SJ fabrication technology for a NbTi wire using the superconducting solder matrix replacement in an openair environment is presented. In addition, the joint resistance measurement and the joint position in the magnet are also discussed. The purpose of this research is to develop the SJs fabrication technology to meet the requirement of a 400-MHz NMR in our institution.

II. BACKGROUND OF THE M AGNET The superconducting magnet with a center field of 9.4 T is required for the 400-MHz NMR system, which has a warm bore size of 54 mm and an operating current of 67.2 A. The parameters for the magnet are listed in Table I [1]. The magnet consists of 17 coils, including main coils, shield coils, and compensation coils, to achieve highly homogenous magnetic field with very little stray field. All coils will be

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CHENG et al.: FABRICATION OF NbTi SUPERCONDUCTING JOINTS FOR 400-MHz NMR APPLICATION

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TABLE II SPECIFICATIONS OF THE JOINTS

Fig. 1. Photograph of a NbTi wire joint.

connected in series by SJs with one superconducting switch in the circuit. There are ten joints in the NMR circuit, as shown in Table II. From calculation, it is noted that the maximum resistance tolerance for each joint must be less than 2.56 × 10−11 Ω to make the corresponding maximum resistance of the magnet system under 2.56 × 10−10 Ω. Five types of NbTi/Cu round conductor are used to coil the magnet, and the filamentary number of the wire is 54. The ratio of copper to noncopper is 1.35. The residual resistance rate is no less than 70. The diameters are 0.40, 0.50, 0.60, 0.75, and 0.85 mm, respectively. The NbTi/CuNi round conductor with a diameter of 0.50 mm is also used for the persistent switch, and the ratio of copper to noncopper is 1.3.

III. FABRICATION OF J OINTS The manufacturing process of superconducting joints was divided into three steps as follows. Step 1) The bared ends of the NbTi wire are immersed in molten tin at about 350 ◦ C in the air. A flowing nitrogen gas with the purity of 99.99% is maintained above the molten metal liquid to keep the solder away from contaminants and oxidation. With occasional stirring of tin in the container with wire ends and careful flexing of the bundle of exposed superconducting NbTi wires to speed dissolution and removal of the copper matrix from the enclosed filaments, the copper matrix will be dissolved after about 2 h. Then, a brush-like bundle or set of NbTi filaments coated with tin will be obtained. Step 2) The filament bundles of wire are immersed in the PbSnBi alloy superconducting solder at about 200 ◦ C and gently stirred. The tin on the filament is replaced with the superconducting solder in 20 min. Step 3) The melting solder is infused in a copper pipe with one open end. Then, the filaments are put into the pipe and immersed in the melt superconducting solder to solidify into a joint, as shown in Fig. 1. In order to minimize resistance of the joint, over 100-mm wire ends out of the joint are coiled with copper wire and then soldered together with lead–tin alloy, and consequently, the joint is strengthened by this method.

Fig. 2. Schematic of the joint resistance testing system.

IV. J OINT R ESISTANCE M EASUREMENT There are two general methods to measure joint resistance: four-point method and coil current decay method. By comparison with the former, the coil current decay method can reach higher measurement precision up to 10−13 to 10−14 Ω [4], [11]. The principle of the measurement is based on the fact that the current decay in a loop connecting with a joint is owing to the joint resistance. Considering that, in the persistent mode, an NMR magnet has a high requirement for tiny joint resistance, we choose the coil current decay method to measure joint resistance. A detection device for testing the resistance of the SJ is established in the laboratory of IEE, our institution in Beijing. Its schematic is shown in Fig. 2. It consists of: 1) a superconducting loop including a joint; 2) a superconducting transformer for inducing current in the loop; 3) a Hall sensor for measuring the loop current; 4) a magnet for producing background magnetic field up to 3 T on the joint; and 5) a heater wrapped around the loop for initializing the loop to quench, if necessary. The power supply of the magnet and transformer is a 200 Amp AMI model 12200PS-420 Digital PS System. During testing, all of the parts are mounted on a stainless steel holder and then immersed into liquid helium at 4.2 K. The circuit is charged by a transformer and operated in the persistent current mode. Then, the field decay rate is observed by the Hall

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Fig. 3. Resistances of 1# joint under varied magnetic field.

Fig. 4.

Load line for 1# joint and wire.

Fig. 5.

Joint resistances under 1 T magnetic field.

sensor, which is located at the center of the NbTi wire ring, for several hours. The experimental data are collected using Labview software through a GPIB interface. V. R ESULTS AND D ISCUSS SJs for an NMR magnet system are usually located in the low field area or mounted on a shielding box. A joint can be considered a good one if the joint itself is not a weak point in the magnet system; that means its resistance is low enough to satisfy the decay rate. This will be discussed in two aspects: joint resistance and joint location in the magnet system as follows. A. Joint Resistance Using the joint resistance testing system, several samples of a NbTi joint made by the aforementioned method have been tested up to sufficient magnetic field. The current decay in the superconducting loop is mainly caused by joint resistance. If two current values I1 at time t1 and I2 at t2 are measured, then the joint resistance can be calculated as Rj = L

ln(I2 /I1 ) t2 − t1

(1)

where L is loop inductance, and Rj is joint resistance. Resistances of 1# joint under various magnetic fields are shown in Fig. 3. The obvious difference of the joint resistance and capability carrying current can be observed as the sample joint placed in varied magnetic field. The joint resistance under 1 T magnetic field is as low as 3.5 × 10−12 Ω. A comparison for capability carrying current between the joint and the NbTi wire under varied magnetic field is shown in Fig. 4. Although capability carrying current by the joint is much lower than that by the NbTi wire, the current load of the joint still reaches more than 160 A, which is much higher than the operation current of 67.2 A. Therefore, the capability carrying current of the joint is considered as satisfactory for the magnet. All joint resistances under 1 T magnetic field are shown in Fig. 5. The resistance value for all sample joints is in the range of 10−12 ∼ 10−13 Ω at a certain current level under 1 T magnetic field. Among the joints, 1# joint has the resistance of 3.5 × 10−12 Ω at a current load of 93.8 A, 2# is 3.9 × 10−13 Ω

at 97.7 A, 3# is 7.0 × 10−13 Ω at 90.5 A, 4# is 2.3 × 10−13 Ω at 119.9 A, 5# has 1.0 × 10−13 Ω at 85.8 A, 6# is 1.8 × 10−13 Ω at 89.5 A, 7# is 2.7 × 10−13 Ω at 102.5 A, and 8# is 4.8 × 10−13 Ω at 109.3 A. Subsequently, the overall resistance of the magnet is calculated as 9.58 × 10−12 Ω by summation of all resistances from individual joints. The results reassure that the joint resistance and the tolerant current level satisfy the requirement for the magnet operating at 85.8 A. Finally, in order to find the right location for joints, we simply need to identify regions that are at 1 T background field. B. Location of the Joints The configuration of the magnet is shown in Fig. 6, in which the axis “z” represents the vertical direction and the “r” represents radial direction of the magnet. The origin of the coordinate system is at the center of the magnet. The superconducting magnet has the height of 430 mm and the diameter of 396 mm. Its “z” value is from −215 to 215 mm. The magnet is fixed and enclosed in a sealed vessel that is filled up with liquid helium to keep the temperature low at around 4.2 K. The height of the helium vessel is 558 mm. The distance between the magnet and the bottom of the vessel is 10 mm. The height of space above the magnet for accessories is 37 mm. After that, there is still a space of 81 mm in height that may be used for the joints with a “z” value of from 252 to 333 mm. In addition, considering the reserved space for warm bore of the magnet, all joints should be located in an annular space as the region predefined for joint position shown in Fig. 6. Therefore,

CHENG et al.: FABRICATION OF NbTi SUPERCONDUCTING JOINTS FOR 400-MHz NMR APPLICATION

Fig. 6.

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Joint location in the magnet system.

Fig. 8. Inhomogeneity of the magnetic field considering the joints.

Fig. 7.

Magnetic field of joint candidate position.

the annular space for the joints has the height of 81 mm from z = 252 to 333 mm, and its radial coordinate is from 150 to 195 mm. Magnetic intensity (Bsum ) in the annular space is calculated, and the results are shown in Fig. 7. In this region, the maximum magnetic intensity (Bmax ) is found at the bottom inner circle, that is, nearest to the center of the magnet. Alternatively, the minimum magnetic intensity (Bmin ) lies on the top outer circle, that is, the farthest position from the center of the magnet. On the one hand, the joint should be placed at the top of the region to avoid the strong magnetic intensity. On the other hand, it is very important to keep the joints in good cooling condition, as the joints out of the liquid helium may cause quenches to raise temperature. Therefore, the joints should be placed as low as possible in the vessel in case the level of liquid helium drops during operation. It is noted that the maximum magnetic intensity in the annular space is less than 0.4 T. The joints have no problem to work under such magnetic field. Therefore, the joints can be placed in the lower part of the region with the “z” value from 252 to 282 mm, as the shaded part shown in Fig. 7. The magnetic intensity there is less than 0.3 T, which guarantees

good performance of the joints. All joints will be fixed into special slots of the fiber-reinforced plastic plate, as shown in Fig. 6. Owing to isotropy of the joints, there is no restriction in joint orientation. In the design of this 400-MHz NMR magnet system, the homogeneity of the magnetic field produced by the superconducting coils is defined as 24.4436 ppm over the 25-mm diameter spherical volume (DSV), considering the inevitable construction errors as well as the surrounding ferromagnetic materials [12]. However, in the final magnet assembly with shimming coils, the homogeneity of the magnet achieved is about 0.2 ppm, among which the contribution from fitting joints is only 0.0001 ppm, according to our analysis. It is obvious that the effect of joints on the homogeneity of the magnetic field is insignificant and may be simply compensated by subsequently shimming. The inhomogeneity (in ppm) of the magnetic field due to the joint effect is shown in Fig. 8.

VI. C ONCLUSION The NbTi SJs for the 400-MHz NMR magnet are fabricated using the method of the superconducting solder matrix replacement in an open-air condition. The testing results by the coil current decay method show that the overall resistance of the magnet is 9.58 × 10−12 Ω under the background field of 1 T by summation of resistances from individual joints. The joint resistance and the tolerant current load of 85.8 A satisfy the requirement for the magnet operation. The joints are placed inside the cylinder above the magnet. The magnetic intensity inside the cylinder is no more than 0.3 T, which assures the performance of joints. The calculated inhomogeneity is about 0.0001 ppm produced by the joints. It is proved that the joint position almost has no negative effect on the homogeneity of magnetic field.

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R EFERENCES [1] Q. L. Wang, B. Z. Zhao, S. S. Song, J. S. Cheng, Y. Li, Y. Lei, Y. Dai, S. Chen, H. Wang, H. Wang, X. Hu, C. Cui, H. Liu, Z. Dong, C. Wang, Z. Ni, H. Huang, H. Zhang, L. Yan, and J. Wang, “High magnetic field superconducting magnet for 400 MHz nuclear magnetic resonance spectrometer,” IEEE Trans. Appl. Supercond., vol. 21, no. 3, pp. 2072–2075, Jun. 2011. [2] C. A. Swenson and W. D. Markiewicz, “Persistent joint development for high field NMR,” IEEE Trans. Appl. Supercond., vol. 9, no. 2, pp. 185– 188, Jun. 1999. [3] P. McIntyre, Y. Wu, G. Liang, and C. R. Meitzler, “Study of Nb3 Sn superconducting joints for very high magnetic field NMR spectrometers,” IEEE Trans. Appl. Supercond., vol. 5, no. 2, pp. 238–241, Jun. 1995. [4] T. Fukuzaki, H. Maeda, S. Matsumoto, S. Yokoyama, and T. Kiyoshi, “Study of joint resistance in Nb3 Al-NbTi superconducting joint for high field NMR,” IEEE Trans. Appl. Supercond., vol. 17, no. 2, pp. 1435–1437, Jun. 2007. [5] K. Seo, S. Nishijima, K. Katagiri, and T. Okada, “Evaluation of solders for superconducting magnetic shield,” IEEE Trans. Magn., vol. 27, no. 2, pp. 1877–1880, Mar. 1991. [6] R. F. Thornton, “Superconducting joint for superconducting wires and coils,” U.S. Patent 4 584 547, Apr. 22, 1986. [7] H. M. Wen, L. Z. Lin, and S. Han, “Joint resistance measurement using current-comparator for superconducting wires in high magnetic field,” IEEE Trans. Magn., vol. 28, no. 1, pp. 834–836, Jan. 1992. [8] S. Phillip, J. V. Porto, and J. M. Parpia, “Two methods of fabricating reliable superconducting joints with multifilamentary Nb–Ti superconducting wire,” J. Low Temp. Phys., vol. 101, no. 3/4, pp. 581–585, Nov. 1995. [9] J. E. C. Williams, S. Pourrahimi, Y. Iwasa, and L. J. Neuringer, “600 MHz spectrometer magnet,” IEEE Trans. Magn., vol. 25, no. 2, pp. 1767–1770, Mar. 1989. [10] J. W. Hafstrom, D. H. Killpatrick, R. C. Niemann, J. R. Purcell, and H. Thresh, “Joining NbTi superconductors by ultrasonic welding,” IEEE Trans. Magn., vol. MAG-13, no. 1, pp. 94–96, Jan. 1977. [11] Y. Iwasa, “Superconducting joint between multifilamentary wires 2. Joint evaluation technique,” Cryogenics, vol. 16, no. 4, pp. 217–219, Apr. 1976. [12] Z. P. Ni, L. K. Li, G. L. Hu, C. Wen, X. Hu, F. Liu, and Q. Wang, “Design of superconducting shim coils for a 400 MHz NMR using nonlinear optimization algorithm,” IEEE Trans. Appl. Supercond., to be published.

Junsheng Cheng was born in Henan province, China, in 1976. He received the B.S. and M.S. degrees from Chang’an University, Xi’an, China, in 1999 and 2002, respectively, and the Ph.D. degree from the University of Science and Technology Beijing, Beijing, China, in 2006. He is currently working with the Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing. His current research interests focus on superconducting magnet technology, including Nb3 Sn coils, superconducting joints, and superconducting materials for magnetic resonance imaging; NMR system; and high-field science equipment.

Jianhua Liu was born in Hebei province, China, in 1981. He received the Master’s degree from Tianjin University, Tianjin, China, in 2003 and the Ph.D. degree from the Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing, China, in 2011. His major research interest includes low-temperature superconducting suspension technology and hybrid suspension technology of electrostatic and superconducting.

Zhipeng Ni was born in Chaohu, China, in 1981. He received the B.S. degree from Anhui Normal University, Wuhu, China, in 2003 and the M.S. degree from the Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, China, in 2005. He is currently working toward the Ph.D. degree in engineering with the Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing, China. His current research interests include applied superconducting technology in highly homogeneous magnets, including design of superconducting main coils, superconducting shim coils, and room-temperature shim coils, and also passive shimming for a magnetic resonance imaging or NMR system.

Chunyan Cui was born in Shandong, China, in 1979. She received the B.S. degree from Ocean University of China, Qingdao, China, in 2002, the M.S. degree from Beijing Institute of Technology, Beijing, China, in 2005, and the Ph.D. degree from the Institute of Electrical Engineering, Chinese Academy of Sciences (CAS), Beijing, in 2009. She is currently working with the Institute of Electrical Engineering, CAS. Her current research interests focus on applied superconducting technology in highly precise instrument, including superconducting suspension, torque analysis, and data acquisition and processing.

Shunzhong Chen was born in 1982. He received the B.S. degree from Beijing Jiaotong University, Beijing, China. He is currently working with the Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing. His current research interests focus on superconducting magnet temperature measurement and quench protection.

Shousen Song was born in Liaoning province, China, in 1965. He received the B.S. degree from Harbin Institute of Technology, Harbin, China, in 1968. He is currently working with the Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing, China. His current research interests focus on superconducting magnet fabrication technology.

Lankai Li was born in Shandong Province, China, in 1982. He received the B.S. degree from Jiamusi University, Jiamusi, China, in 2005 and the M.S. degree from Harbin Institute of Technology, Harbin, China, in 2007. He is currently working toward the Ph.D. degree in engineering with the Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing, China.

Yinming Dai was born in Shandong province, China, in 1965. He received the B.S. degree from Sichuan University, Sichuan, China, in 1986 and the M.S. degree from the Institute of Electrical Engineering, Chinese Academy of Sciences (CAS), Beijing, China, in 1989. He is currently working as Professor with the Institute of Electrical Engineering, CAS. His current research interests focus on superconducting magnet technology, including electromagnetic system design and optimization calculation.

Qiuliang Wang was born in Hubei, China, in 1965. He received the B.S. degree from Hubei University, Hubei, in 1986, the M.S. degree from the Institute of Plasma Physics, Chinese Academy of Sciences (CAS), Hefei, China, in 1991, and the Ph.D. degree from the Institute of Electrical Engineering, Chinese Academy of Sciences (CAS), Beijing, China, in 1994. He is currently a Professor with the Institute of Electrical Engineering, CAS, Beijing. His current research interests include massive application of applied superconductivity technology in space and medicine, high-field science equipment, cryogenic engineering, electromagnetic field, and technology of materials fabrication.