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Design Considerations of the Superconducting Magnet

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Design Considerations of the Superconducting Magnet 1 Electromagnetic, stress and thermal analysis of the Superconducting Magnet 1 1 , 2 Yong Ren *, Xiang Gao * 1 Institute of Plasma Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, PO Box 1126, Hefei, Anhui, 230031, People's Republic of China 2University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China *E-mail: [email protected] and [email protected] Abstract—Within the framework of the National Special but also to implement a background field superconducting Project for Magnetic Confined Nuclear Fusion Energy of China, magnet for testing the CFETR CS insert and TF insert coils. the design of a superconducting magnet project as a test facility During the first stage, the main goals of the project are of the Nb3Sn coil or NbTi coil for the Chinese Fusion Engineering composed of: 1) to obtain the maximum magnetic field of Test Reactor (CFETR) has been carried out not only to estimate the relevant conductor performance but also to implement a above 12.5 T; 2) to simulate the relevant thermal-hydraulic background magnetic field for CFETR CS insert and toroidal characteristics of the CFETR CS coil; 3) to test the sensitivity field (TF) insert coils. The superconducting magnet is composed of the current sharing temperature to electromagnetic and of two parts: the inner part with Nb3Sn cable-in-conduit thermal cyclic operation. During the second stage, the conductor (CICC) and the outer part with NbTi CICC. Both superconducting magnet was used to test the CFETR CS insert parts are connected in series and powered by a single DC power and TF insert coils as a background magnetic field supply. The superconducting magnet can be cooled with supercritical helium at inlet temperature of 4.5 K. The total superconducting magnet during the manufacturing stage of the inductance and stored energy of the superconducting magnet are CFETR magnets. about 0.278 H and 436.6 MJ at an operating current of 56 kA In this paper, the magnetic field, the strain of the Nb3Sn respectively. An active quench protection circuit was adopted to cable and the stress of the jacket for the superconducting transfer the stored magnetic energy of the superconducting magnet with a detailed 2-D finite element method were magnet during a dump operation. analyzed. The temperature margin behavior of the In this paper, the design of the superconducting magnet and the main analysis results of the electromagnetic, structural and superconducting magnet was analyzed with the 1-D thermal-hydraulic performance are described. GANDALF code [34]. The quench behavior of the superconducting magnet was given with the adiabatic hot spot Keywords—Cable-in-conduit conductor (CICC), CFETR, temperature criterion and the 1-D GANDALF code. Quench, Superconducting magnet, Stress, Thermal-hydraulic behavior. II. DESIGN OF THE SUPERCONDUCTING MAGNET The superconducting magnet is designed to provide a I. INTRODUCTION maximum magnetic field of above 12.5 T. The HINESE Fusion Engineering Test Reactor (CFETR) is superconducting magnet should have enough inner space to C being designed to bridge the gaps between the ITER and provide a background magnetic field for testing the long Demo in China [1-6]. The superconducting magnet for the superconducting samples such as CFETR CS insert and TF CFETR reactor consists of a CS coil with 6 modules, 8 PF insert coils. The Nb3Sn cable-in-conduit conductor (CICC) coils, 16 TF coils and a set of correction coils [6]. The CFETR and NbTi CICC are graded for reducing the cost of the CS coil with Nb3Sn conductor will operate in pulsed mode. superconducting strands for the superconducting magnet. The The electromagnetic and thermal cyclic operation often results superconducting magnet, which has a cold bore of 1400 mm in in Nb3Sn CICC conductor performance degradation [7-19]. diameter, will produce about 12.6 T maximum magnetic field Generally, the qualification tests of the Nb3Sn CICC at 56 kA. The total self-inductance and stored energy of the conductor in electromagnetic and thermal cyclic operation superconducting magnet are about 0.278 H and 436.6 MJ were performed to evaluate the relevant Nb3Sn CICC coil respectively. Table 1 lists the main parameters of the performance [14-19]. Occasionally, the qualification tests of superconducting magnet. Figure 1 shows the cross section of a the Nb3Sn CICC conductors with a long length in relevant winding pack of the superconducting magnet. Figure 2 shows conditions of magnetic field, current density and mechanical the magnetic field distribution of the superconducting magnet. strain are of great importance for the fabrication of the large The superconducting magnet consists of two modules. The scale superconducting magnet [20-33]. Therefore, a design inner module is layer-wound winding; the outer module is activity has been started to design a superconducting magnet pancake-winding. The inner module with Nb3Sn CICC has project not only to estimate the CFETR CS coil performance eight layers; each layer has one cooling channel. The outer module with NbTi CICC has 20 pancakes with 200 turns. 2 56.3 2 56.3 1400 ? 37.3 1151 1705 55.8 55.8 1 ? 37.7 2360.8 Figure 1. Cross section of the winding pack of the superconducting magnet. There are 10 cooling channels for the outer module in total; Table 2. Main parameters of the Nb3Sn and NbTi conductors [3, 4] each cooling channel consists of 2 pancakes. The CFETR CS HF LF Superconductor Nb3Sn NbTi type conductor was used as the inner module for the Cable pattern (2sc + 1Cu) x 3 x 3sc x 4 x 4 x superconducting magnet, while the ITER PF6 type conductor 4 x6 x 6 5 x 6 with NbTi strand was used as the outer module [6, 35]. Central spiral mm ID 8, OD 10 10 x 12 Petal wrap mm 0.05mm thick, 0.05 mm thick, The round-in-square type jackets are used for both 70% cover 50% cover conductors. The 316LN stainless steel and 316L stainless steel Cable wrap 0.08 mm thick, 0.10 mm thick, jackets are adopted for the Nb3Sn and NbTi CICC conductors. 40% overlap 40% overlap The jacket for the Nb Sn CICC conductor is 56.3 mm wide Cr coated strand mm 0.81 0.73 3 diameter and has a circular hole of 37.3 mm. The jacket for the NbTi Cu/Non-Cu for SC 1.0 1.6 CICC conductor is 55.8 mm wide and has a circular hole of Void fraction 0.30 0.343 37.7 mm [35]. Both conductors are wrapped with 1 mm thick Cable diameter mm 37.3 37.7 Conductor dimension mm 56.3×56.3 53.8×53.8 insulation. The thicknesses of the ineterlayer insulation for the Jacket Circle in square Circle in square inner module and of the interturn insulation of the outer 316LN 316L module are 1.5 mm. The ground insulation is 8 mm. Table 2 lists the specification of the Nb3Sn and NbTi conductors [3, 4]. Unlike the distributed barrier Nb3Sn internal tin (IT) superconducting strand used for the hybrid magnet superconducting outsert magnet, the single barrier Nb3Sn superconducting strand, which has the highly critical current density and low AC losses during ramping, was adopted for the Nb3Sn coil of the superconducting magnet [36-38]. The Nb3Sn strand, with a diameter of 0.81 mm, is Cr plated to reduce the coupling loss of the superconducting cable and to prevent the diffusion reaction bonding of the strands during the reaction heat treatment process [39-48]. The NbTi strand is Ni plated to prevent gradual oxidation [49]. Table 3 lists the specification of the Nb3Sn and NbTi strands. Table 1. Design parameters of the superconducting magnet. Figure 2. Magnetic field distribution of the superconducting magnet at 56 kA Superconductor Nb3Sn NbTi Jacket 316LN 316L (Unit: T). Inner diameter mm 1.4000 2.3808 Outer diameter mm 2.3608 3.5288 Table 3. Specification of the Nb3Sn and NbTi strands used for the Height mm 1.7050 1.1510 superconducting magnet. Turn insulation mm 1.0 1.0 Superconductor Nb3Sn NbTi Layer/pancake insulation mm 2.0 1.0 Minimum piece length m 1000 1000 Layer/Pancake 8 20 Strand diameter mm 0.81 0.73 Turns per layer or pancake 30 10 Twist pitch mm 15 ± 2 15 ± 2 Current kA 56 Strand coating μm 2 (Cr) 2 (Ni) Inductance H 0.278 Cu/Non-Cu 1 1.6 Stored energy MJ 436.6 Critical current at 4.2 K A >250@12 T >[email protected] T Maximum field T 12.59 5.455 n value >20 >20 Residual resistivity >100 >100 3 ratio to analyze the stress/strain distribution of the superconducting Hysteresis loss per mJ/cm3 <[email protected] K <[email protected] K over magnet under static operation mode with full design current strand unit volume over a ± 3 T cycle a ± 1.5 T cycle Effective filament μm <30 <8 applied. The cables of the superconducting magnet are diameter assumed to be fully bonded to the inner surface of the jacket [36, 51]. The calculation are based on the combinations of the following loading conditions: the thermal contraction during cool-down, the helium pressure and the Lorentz force [36, 51]. Figure 3 shows the operating strain of the Nb3Sn cable for the superconducting magnet caused by the magnetic loading. It is shown that the maximum operating strain of the Nb3Sn cable from the magnetic loading is about 0.1531%. The stress of the jacket was calculated based on the three loading conditions mentioned above. The helium pressure inside the 316LN jacket and 316L jacket during normal operations was assumed to be 0.55 MPa.
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