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Development of a Cryogen-Free Compact 3 T Superconducting Magnet for an Electromagnetic Property Measurement System

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Development of a Cryogen-Free Compact 3 T Superconducting Magnet for an Electromagnetic Property Measurement System applied sciences Article Development of a Cryogen-Free Compact 3 T Superconducting Magnet for an Electromagnetic Property Measurement System Jae Young Jang 1, Myung Su Kim 1, Young Jin Hwang 2 , Seunghyun Song 1, Yojong Choi 1 and Yeon Suk Choi 1,* 1 Korea Basic Science Institute, Daejeon 34133, Korea; [email protected] (J.Y.J.); [email protected] (M.S.K.); [email protected] (S.S.); [email protected] (Y.C.) 2 Department of Electrical and Electronic Engineering, Korea Maritime and Ocean University, Busan 49112, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-42-865-3913 Abstract: A cryogen-free portable 3 T high-temperature superconducting magnet for an electro- magnetic property measurement system has been developed to serve as a user facility at the Korea Basic Science Institute. The metallic insulation method was adopted to reduce the charging delay without sacrificing the self-protecting feature. A genetic-algorithm-aided optimized design was carried out to minimize the superconducting tape consumption while satisfying several design con- straints. After the design, the compact high-temperature superconducting magnet composed of eight double-pancake coil modules was wound with high-temperature superconducting tape and stainless steel tape, and integrated with a two-stage cryo-cooler. The 3 T magnet was successfully cooled to approximately 20 K with a cryo-cooler and reached the target field of 3 T without any problems. Long-term measurements and a range of other tests were also implemented to verity the performance Citation: Jang, J.Y.; Kim, M.S.; of the magnet. Test results demonstrated the feasibility of a cryogen-free portable high-temperature Hwang, Y.J.; Song, S.; Choi, Y.; Choi, superconducting magnet system for electromagnetic property measurement experiments. Y.S. Development of a Cryogen-Free Compact 3 T Superconducting Keywords: cryogen-free; portable; high-temperature superconducting magnet; optimized design; Magnet for an Electromagnetic electromagnetic property measurement system Property Measurement System. Appl. Sci. 2021, 11, 3074. https://doi.org/ 10.3390/app11073074 1. Introduction Academic Editor: Luigi La Spada As superconducting wires allow the flow of very high current density levels compared to copper wires, superconducting magnets generate a high magnetic field with compact Received: 26 February 2021 and light windings. There are many industrial and scientific devices employing supercon- Accepted: 26 March 2021 Published: 30 March 2021 ducting magnets to generate a high magnetic field, and several devices have been commer- cialized given their high performance and quality levels [1–5]. Superconducting magnets Publisher’s Note: MDPI stays neutral are classified as low-temperature superconducting (LTS) magnets wound with lower tem- with regard to jurisdictional claims in perature superconducting wires, such as NbTi and Nb3Sn [6,7], and high-temperature published maps and institutional affil- superconducting (HTS) magnets fabricated with high-temperature superconducting tapes iations. such as Rare-earth barium copper oxide (REBCO) and Bismuth strontium calcium copper oxide (BSCCO) [8,9]. LTS wires, which were commercialized much earlier than HTS tapes, are much less costly than HTS tapes but require liquid helium or massive and expensive cryo-coolers to keep the magnet temperature under 4.2 K due to the low critical temper- ature required (< 10 K). Recently commercialized HTS tapes, on the other hand, have a Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. high critical temperature (> 90 K) and do not need liquid helium to cool the magnets [8]. This article is an open access article This means that it is possible to operate a superconducting magnet with an inexpensive distributed under the terms and and compact liquid-helium-free system. Therefore, the handling and operation of the HTS conditions of the Creative Commons magnet system would be very safe and comfortable for both users and vendors [10]. For Attribution (CC BY) license (https:// these reasons, research on HTS magnets has been active recently. Many research institutes creativecommons.org/licenses/by/ adopt or consider the use of HTS tapes to develop nuclear magnetic resonance (NMR) 4.0/). systems that exceed 1 GHz [11–15]. Magnetic resonance imaging (MRI) researchers have Appl. Sci. 2021, 11, 3074. https://doi.org/10.3390/app11073074 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 3074 2 of 15 also been developing liquid-helium-free HTS MRI systems to manufacture robust, safe and compact MRI system [16–20]. HTS magnets have also been applied to dipole magnets in relation to research on accelerators [21–24] and rotating machines for power generation and ship propulsion [25–28]. In addition, many studies of HTS magnet applications are underway in many other fields. The Korea Basic Science Institute (KBSI) also commenced a research project to develop a conduction-cooled HTS magnet for an electromagnetic property measurement system (EMPS) in 2020. The goal of the project is to develop a liquid-helium-free 3 T/25 mm REBCO magnet for various physical property measurements and optical experiments. The operating temperature of the magnet will exceed 20 K, and a cost-effective small cryo- cooler can sufficiently cool the magnet. Because the total system volume including the HTS magnet, cryostat, and cryo-cooler is smaller than those of other LTS magnet systems or a conventional electromagnet, users can transport our system easily and safely. The low magnetic field error is another merit of our system. In general, the center magnetic field at room-temperature (RT) bore is approximately calculated by the pre-acquired magnet constant (center field per the operating current). The center field, however, can vary slightly under identical operating conditions due to many reasons, such as the characteristics of the magnetic components of the cryostat, the screening current used, and others [29–33]. To compensate for these types of field errors, we installed a cryogenic Hall sensor at the top of the REBCO magnet. The ratio of the center field and the magnetic field measured by the Hall sensor is pre-calculated, and users can determine the center field value easily with the Hall sensor. A drawing of the 3 T HTS EMPS system is shown in Figure1. The system mainly consists of a cryostat, a REBCO magnet, a Gifford-McMahon (GM) 20 K cryo- cooler [34], thermal shields, HTS current leads, and thermal links for conduction-cooling, among other components. Figure 1. Drawing of the 3 T HTS electromagnetic property measurement system (EMPS). The 3 T REBCO magnet was designed using a genetic algorithm, often used in relation to the optimization theories, to minimize the consumption of the HTS tape [35,36]. To protect the magnet from the quench phenomenon without a charging delay, the well-known approach, known as ‘metallic insulation’ (MI), is applied to the magnet [37–39]. The 3 T REBCO magnet was fabricated based on the design result and integrated with a 20 K cryo-cooler and a cryostat. After the construction step, the 3 T magnet was cooled to the op- erating temperature of 20 K with a two-stage cryo-cooler. The magnet successfully charged to the target field and long-term charging tests were repeated to verify the performance Appl. Sci. 2021, 11, 3074 3 of 15 capabilities of the magnet. After a few physical property measurement tests, the 3 T magnet system will be operated as a new EMPS user facility. 2. HTS Magnet Design with Genetic Algorithm SuNAM REBCO tapes (4.1 mm width and 0.14 mm thickness) were used to fabricate the 3 T HTS magnet. The superconducting layer of the tape is located between the 20 um copper stabilizer and the stainless steel substrate layer as shown in reference [40]. The minimum critical current of the tape is about 200 A @ 77 K self-field. As mentioned in the introduction, MI winding method was applied to the magnet for the quench protection with charging delay reduction [37–39]. Figure2 shows the quench-protection mechanism of the MI technique. There are three types of winding methods for a REBCO magnet: (1) insulation winding, as shown in Figure2a; (2) no-insulation winding, as shown in Figure2b [ 41,42]; and (3) metallic insulation winding, as shown in Figure2c [ 37–39]. For the insulation type of winding, an insulation layer with a material such as Kapton is co-wound with the HTS tape to electrically insulate each HTS tape. Accordingly, the turn-to-turn contact resistance of the magnet is very high and the magnet current does not bypass when a fault occurs. As the normal zone propagation velocity of HTS tape is very slow, the generated heat is accumulated at that hot spot and the magnet is easily damaged. On the other hand, the no-insulation type of winding has very low contact resistance, and the magnet current is very easily bypassed through the turn-to-turn contact when a fault occurs. This technique protects the HTS magnet, but it leads to a charging delay when charging the magnet [38,43]. The MI is a compromise technique between protection and charging delay of a HTS magnet. Given that the turn-to-turn contact resistance of the MI is not exceedingly high, the charging delay is reduced without sacrificing the self-protection capability. Many studies demonstrate protecting performance by the MI technique [37–39]. Figure 2. Quench-protection mechanism of the (a) insulation winding, (b) no-insulation winding, and (c) metallic- insulation winding. As the target RT bore is approximately 1 inch (25 mm), the inner radius of the RT bore cylinder is 12.5 mm. Considering the thickness of the RT bore thermal shield and multi-layer insulation (MLI) between the RT bore and HTS magnet bobbin for conduction cooling, the required distance between the RT bore cylinder and bobbin is about 12.5 mm.
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