Method for generating linear current-field characteristics and eliminating charging delay in no-insulation superconducting magnets

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1 Superconductor Science and Technology PAPER Method for generating linear current-field characteristics and eliminating charging delay in no-insulation superconducting magnets To cite this article: Seokho Kim et al Supercond. Sci. Technol. 00 Manuscript version: Accepted Manuscript Accepted Manuscript is the version of the article accepted for publication including all changes made as a result of the peer review process, and which may also include the addition to the article by IOP Publishing of a header, an article ID, a cover sheet and/or an Accepted Manuscript watermark, but excluding any other editing, typesetting or other changes made by IOP Publishing and/or its licensors This Accepted Manuscript is IOP Publishing Ltd. During the embargo period (the month period from the publication of the Version of Record of this article), the Accepted Manuscript is fully protected by copyright and cannot be reused or reposted elsewhere. As the Version of Record of this article is going to be / has been published on a subscription basis, this Accepted Manuscript is available for reuse under a CC BY-NC-ND.0 licence after the month embargo period. After the embargo period, everyone is permitted to use copy and redistribute this article for non-commercial purposes only, provided that they adhere to all the terms of the licence Although reasonable endeavours have been taken to obtain all necessary permissions from third parties to include their copyrighted content within this article, their full citation and copyright line may not be present in this Accepted Manuscript version. Before using any content from this article, please refer to the Version of Record on IOPscience once published for full citation and copyright details, as permissions will likely be required. All third party content is fully copyright protected, unless specifically stated otherwise in the figure caption in the Version of Record. View the article online for updates and enhancements. This content was downloaded from IP address... on /0/ at 0:

2 Page of 0 AUTHOR SUBMITTED MANUSCRIPT - SUST-0.R 0 Method for generating linear current-field characteristics and eliminating charging delay in no-insulation superconducting magnets Seokho Kim, Seungyong Hahn, Kwangmin Kim and David Larbalestier Applied Superconductivity Center, National High Magnetic Field Laboratory, Florida State University E. Paul Dirac Drive, Tallahassee, FL 0, US shahn@asc.magnet.fsu.edu November Abstract. No-insulation (NI) rare-earth barium copper oxide (REBCO) magnets are promising for high field or high temperature superconducting magnets because they simplify quench protection. However, the turn-to-turn leakage current path induced by the absence of insulation introduces non-linearities into the magnetic field current characteristic and significant delay in reaching the desired field. This paper shows that active feedback control can mitigate both the non-linearity and the charging delay. To verify our approach, simulations and tests were performed with an NI REBCO magnet made of double-pancake (DP) coils. A proportional and integral (PI) feedback control of the power supply was adopted which allowed determination of the appropriate PI gains using dynamic simulations of the equivalent circuit of the NI magnet. Feedback control tests were then performed in liquid nitrogen at K. The time to reach.% of the target magnetic and to become essentially steady-state was reduced by more than 00 times from s without control to 0. s with control. The results demonstrate a potential that one of the most significant perceived disadvantages of an NI magnet can essentially be removed by active feedback control of the power supply current. Keywords: charging delay, feedback control, no-insulation, REBCO magnet. Introduction For high field superconducting magnets, rare-earth barium copper oxide REBa Cu O x (REBCO) tape is a promising option on account of its strong mechanical properties and excellent in-field current carrying capacity [ ]. The great challenge of magnet On sabbatical leave from the School of Mechanical Engineering, Changwon National University, South Korea. Author to whom any correspondence should be addressed.

3 AUTHOR SUBMITTED MANUSCRIPT - SUST-0.R Page of 0 0 Elimination of charging delay in no-insulation superconducting magnets protection, however, has been one of the major impediments to widespread use of REBCO for high field magnets. First proposed in [], the no-insulation (NI) high temperature superconductor (HTS) winding technique has become regarded as providing a feasible solution for protection of direct current (DC) high field REBCO magnets, even when designed to be operated at a substantially higher current density, > 0 A/mm than that of the conventional insulated REBCO magnets, typically 0 A/mm or less [0 ]. Recently, a standalone. T mm all-rebco NI magnet was successfully constructed and energized [0]; its self-protecting feature was demonstrated in multiple quench tests without any quench detection and protection systems implemented. Yet, one of the residual concerns for future NI REBCO high field magnets is the charging delay due to the internal turn-to-turn shorts within NI coils []. An approach to mitigate charging delays is to increase the turn-to-turn contact resistance in an NI coil without sacrificing the self-protecting feature of the coil [ ]. Although successful to some extent, we propose another approach at the operational level rather than the fundamental material-based methods: active feedback control of the transport current into an NI coil. Our control algorithm is based on the wellestablished proportional and integral (PI) feedback control applied to an NI REBCO magnet that consists of a stack of double-pancake coils having a winding diameter of mm. Details of the design, construction, and operation of the feedback control system will be described, as well the results of magnet tests in liquid nitrogen at K.. Test magnet Table summarizes key parameters of our test magnet which was originally designed, constructed, and tested at the MIT Francis Bitter Magnet Laboratory [, ]. The multi-width (MW) technique, essentially a conductor grading technique, was adopted and the double pancake (DP) coils were wound with NI REBCO tapes of five different tape widths:. mm (C),. mm (C),. mm (C),. mm (C), and. mm (C). Fig. shows a to-scale drawing of the upper half of the magnet. When operated in a bath of liquid helium at. K, the magnet surpassed design field of T and reached T before it quenched at A/mm. The self-protecting feature of the magnet was demonstrated with multiple quench tests at. K as well as K []. Although the magnet was designed for use in liquid helium, control simulations and tests were conducted, for ease of test, in liquid nitrogen at K. To simulate D electromagnetic behaviors of an NI coil, a distributed network model has been used to calculate the coil s local current distribution in both azimuthal and radial directions [ ]. Yet, for our simple charging analysis, the NI magnet was modeled with a lumped circuit consisting of an inductor (L) that represents the magnet s overall inductance and a parallel resistor (R c ) that essentially sums all the turn-to-turn contact resistance values. This lumped circuit model has been reasonably successful, since early days of the NI research, in simulation of charging responses and

4 Page of 0 AUTHOR SUBMITTED MANUSCRIPT - SUST-0.R 0 Elimination of charging delay in no-insulation superconducting magnets Table : Key Parameters of the Multi-Width (MW) No-Insulation (NI) Magnet Parameter C C C C C Measured dimensions Average tape width [mm]..... Min. self-field I c at K [A] ID; average OD [mm].0; 0. Overall height [mm]. Number of DP Turn per pancake Conductor per DP [m] 0 Operational characteristics Magnet constant, α [mt/a]. Total Inductance, L [H] 0. Measured coil I c at K [A]. Measured coil I c at. K [A] Figure : Sketch of the multi-width pancake stacks above the magnet midplane (z = 0) post-quench behaviors of various NI coils. [, ]. In the model, a power supply current (I p ) divides into a spiral current (I θ ) that generates axial magnetic field, and a radial leakage current (I Rc ) through the contact resistance.. Active control of power supply current.. System transfer function Equations () and () are the circuit equations for Fig., where B is the magnet center field proportional to I θ with a magnet constant of α in (). The magnet inductance,

5 AUTHOR SUBMITTED MANUSCRIPT - SUST-0.R Page of 0 0 Elimination of charging delay in no-insulation superconducting magnets Figure : Equivalent electrical circuit model for a no-insulation magnet with a current source. L was calculated, and the contact resistance, R c was estimated to be. mω from measured magnetic fields and magnet voltages in a separate charging test, allowing the charging time constant (τ c = L/R c ) to be determined as s. L di θ(t) dt = R c I Rc (t) () I p (t) = I Rc (t) + I θ (t) () B(t) = α I θ (t) () By applying the Laplace transform to (), () and (), the overall system transfer function for the central magnetic field ( B) can be obtained with respect to the output power supply current(ĩp) in (), which is the transfer function for our control simulation... PI control B(s) Ĩ p (s) = αr c Ls + R c () The key principle of fast tracking of the target field is that the PI controller monitors the discrepancy between the reference field and the actual field and then controls the amount of the power supply current to track the reference field as long as the operation limits including power supply voltage and maximum power supply current are met. During this procedure, the azimuthal current in the coil, which generates the center field, is usually smaller than the power supply current, mainly due to the radial leak current through the turn-to-turn contacts. Figure : PI feedback control diagram for the NI magnet.

6 Page of 0 AUTHOR SUBMITTED MANUSCRIPT - SUST-0.R 0 Elimination of charging delay in no-insulation superconducting magnets Fig. shows our PI feedback control block diagram for the NI magnet. Simulations were performed using MATLAB Simulink, while experiments were conducted using a data acquisition system with an analog output (National Instruments SCXI- and SCXI-) controlled by the LabVIEW software. In the experiments, the control signal was transferred to the current control terminal of the power supply through an analog output system. According to (), the NI magnet is a first-order system that does not produce any control instability in the PI control. An ideal power supply with infinite current and voltage capacities may maximize the controllability, yet an actual power supply has voltage and current limits. Moreover, the maximum power supply current should be carefully determined in consideration of operation details, e.g., current-carrying capacity of REBCO tapes and extra Joule heat due to additional leakage current by an excessive power supply current. We used a Sorenson power supply (model SGI0X0C-DAA) having respective voltage and current limits of 0 V and 0 A. To prevent over-current damage, the power supply current was limited to A in consideration of the current capacity of the REBCO tapes at K in field. A Hall sensor, made by Arepoc and having a sensitivity of. mv/t at an operating current of ma, was installed at the center of the magnet to monitor the magnet center field during the feedback control.. Simulation and experimental results.. Magnet charging without feedback control To verify the charging delay without feedback control, the initial experiments were performed by linear ramping of the power supply current up to. A at a ramp rate of 0.0 A/s, which corresponds to a magnet center field of 0. T. As shown in Fig., the measured magnetic field demonstrates a significant delay with respect to the power supply current; it achieved.% of the target field (0. T) at approximately s after the power supply current reached. A. Note that our choice of.% is arbitrary, clearly insufficient in terms of the temporal stability requirement for NMR and MRI magnets... Magnet charging with feedback control Generally, a larger P-gain is preferred to reduce the control error in a first order system. Nonetheless, the maximum P-gain was set to be,000 to suppress control instability arising from time delay of the digital control loop and limited resolution of the analogto-digital converter control signal. Furthermore, the I-gain was selected to reduce the residual error within a range to avoid an overshoot response. Fig. shows simulation results of magnetic field vs. time with the feed back control. Fig. a compares the results with different PI gains when the reference signal ramping rate (db ref /dt) was set to be. mt/s. Fig. b presents those with a faster reference

7 AUTHOR SUBMITTED MANUSCRIPT - SUST-0.R Page of 0 0 Elimination of charging delay in no-insulation superconducting magnets Figure : Charging test results of the NI magnet without feedback control. The power supply current was increased to. A at a constant ramp rate of 0.0 A/s. Figure : Simulations of magnet center field vs. time for the NI magnet with feedback control: (a) various P and I gains with a reference ramping rate of. mt/s; (b) fixed P and I gains with a faster reference ramping rate of mt/s. signal ramping rate, mt/s with given P and I gains of 00 and, respectively. As shown in the figure, the errors between the reference and control values decrease as the PI gain increases. At. mt/s, the maximum power supply current was only A,

8 Page of 0 AUTHOR SUBMITTED MANUSCRIPT - SUST-0.R 0 Elimination of charging delay in no-insulation superconducting magnets Figure : Test results of the NI magnet with the feedback control; reference signal ramping rate:. mt/s; P-gain: 00; I-gain:. while it was limited to A to avoid the coil damage for the faster ramp of mt/s. Fig. shows the experimental results for the. mt/s case with respective PI gains of,000 and, which show good agreement with the simulation results in Fig. a. Both simulation and experiment demonstrate that the charging delay was reduced by more than 00 times from s without control to 0. s with control. A residual field of mt was observed in Fig. before the charging was started mainly due to the screening current in the REBCO tapes. However, the screeningcurrent-induced field (SCF) was considered as a disturbance in the control system and thus mitigated during the control, which may have been challenging otherwise without the control.. Conclusion Active current control was applied to reduce the charging delay and steady-state field instability of an NI magnet consisting a stack of double pancake coils wound with multiwidth no-insulation REBCO tapes. The results demonstrated a substantial (>00 times) reduction of the charging delay: the charging time to reach.% of the target field (0. T) was reduced from s without control to 0. s with control. Also, a

9 AUTHOR SUBMITTED MANUSCRIPT - SUST-0.R Page of 0 0 Elimination of charging delay in no-insulation superconducting magnets fully linear B(t) ramp was obtained with the control. The magnet did not experience a quench by AC loss and excessive Joule heat from leakage current mainly owing to its large stability margin in K, though this issue needs to be further investigated when the feedback control technique is applied to a magnet operating at a lower temperature, e.g., in liquid helium at. K. Acknowledgement This work was supported by the National High Magnetic Field Laboratory (which is supported by the National Science Foundation under NSF/DMR-0), by the State of Florida, and by the Changwon National University Research Fund in.

10 Page of 0 AUTHOR SUBMITTED MANUSCRIPT - SUST-0.R 0 Elimination of charging delay in no-insulation superconducting magnets Reference [] L Rossi, X Hu, F Kametani, D Abraimov, A Polyanskii, J Jaroszynski, and D C Larbalestier. Sample and length-dependent variability of and.k properties in nominally identical RE coated conductors. Supercond. Sci. Technol., :0,. [] D. C. van der Laan, J. D. Weiss, P. Noyes, U. P. Trociewitz, A. Godeke, D. Abraimov, and D. C. Larbalestier. Record current density of Amm at.k and T in CORC accelerator magnet cables. Supercond. Sci. Technol., :000,. [] K. Osamura, H.-S. Shin, K.-P. Weiss, A. Nyilas, A. Nijhuis, K. Yamamoto, S. Machiya, and G. Nishijima. Mechanism for the uniaxial strain dependence of the critical current in practical REBCO tapes. Supercond. Sci. Technol., :0,. [] S. Fujita, H. Satoh, M. Daibo, Y. Iijima, M. Itoh, H. Oguro, S. Awaji, and K. Watanabe. Characteristics of REBCO Coated Conductors for T Cryogen-Free Superconducting Magnet. IEEE Trans. Appl. Supercond., :0,. [] V. Selvamanickam, M. Heydari Gharahcheshmeh, A. Xu, E. Galstyan, L. Delgado, and C. Cantoni. High critical currents in heavily doped (Gd,Y)Ba Cu O x superconductor tapes. Appl. Phys. Letts., 0:0,. [] V. Selvamanickam, M. Heydari Gharahcheshmeh, A. Xu, Y. Zhang, and E. Galstyan. Critical current density above MAcm at K, T in.µm thick heavily-doped (Gd,Y) Ba Cu O x superconductor tapes. Supercond. Sci. Technol., :00,. [] H. W. Weijers, W. D. Markiewicz, A. V. Gavrilin, A. J. Voran, Y. L. Viouchkov, S. R. Gundlach, P. D. Noyes, D. V. Abraimov, H. Bai, S. T. Hannahs, and Tim. P. Murphy. Progress in the Development and Construction of a -T Superconducting Magnet. IEEE Trans. Appl. Supercond., ():0,. [] U. P. Trociewitz, M. Dalban-Canassy, M. Hannion, D. K. Hilton, J. Jaroszynski, P. Noyers, Y. Viouchkov, H. W. Weijers, and D. C. Larbalestier.. t field generated using a layer-wound superconducting coil made of (RE)Ba cu o x (RE=rare earth) coated conductor. Applied Physics Letter, :0,. [] S. Hahn, D. K. Park, J. Bascuñán, and Y. Iwasa. HTS Pancake Coils without Turn-to-Turn Insulation. IEEE Trans. Appl. Supercond., :,. [0] S. Yoon, J. Kim, H. Lee, S. Hahn, and S.-H. Moon. T mm all-gdba CuO-x multi-width no-insulation superconducting magnet. Supercond. Sci. Technol., :0LT0,. [] J. B. Song, S. Hahn, T. Lecrevisse, J. Voccio, J. Bascuñán, and Y. Iwasa. Over-current quench test and self-protecting behavior of a T/ mm multi-width no-insulation REBCO magnet at. K. Supercond. Sci. Technol., :0,. [] S. Hahn, Y. Kim, J. Song, J. Voccio, J. Ling, J. Bascuñán, and Y. Iwasa. A -mm/-t Multi- Width No-Insulation ReBCO Magnet: Key Concept and Magnet Design. IEEE. Trans. Appl. Supercond., ():0,. [] Hongyu Bai, W. Denis Markiewicz, Jun Lu, and Hubertus W. Weijers. Thermal Conductivity Test of YBCO Coated Conductor Tape Stacks Interleaved With Insulated Stainless Steel Tapes. IEEE Trans. Appl. Supercond., ():0,. [] S. Hahn, Y. Kim, D. K. Park, K. Kim, J. Voccio, J. Bascuñán, and Y. Iwasa. No-Insulation Multi-Width Winding Technique for High Temperature Superconducting Magnet. Appl. Phys. Lett., 0:,. [] S. Hahn, D. K. Park, J. Voccio, J. Bascuñán, and Y. Iwasa. No-Insulation (NI) HTS Inserts for > GHz LTS/HTS NMR Magnets. IEEE Trans. Appl. Supercond., :,. [] K. L. Kim, S. Yoon, K. Cheon, J. Kim, H. Lee, S. Lee, D. L. Kim, and S. Hahn. 0-MHz/- mm All-REBCO Nuclear Magnetic Resonance Magnet: Magnet Design. IEEE. Trans. Appl. Supercond., :,. [] J. Bascuñán, S. Hahn, T. Lecrevisse, J. Song, D. Miyagi, and Y. Iwasa. An 00-MHz all-rebco Insert for the.-ghz LTS/HTS NMR Magnet ProgramA Progress Report. IEEE. Trans. Appl.

11 AUTHOR SUBMITTED MANUSCRIPT - SUST-0.R Page 0 of 0 0 Elimination of charging delay in no-insulation superconducting magnets 0 Supercond., :,. [] X. Wang, S. Hahn, Y. Kim, J. Bascuñán, J. Voccio, H. Lee, and Y. Iwasa. Turn-to-turn contact characteristics for equivalent circuit model of no-insulation ReBCO pancake coil. Supercond. Sci. Technol., :00,. [] Y. H. Choi, S. Hahn, J. B. Song, D. G. Yang, and H. G. Lee. Partial Insulation of GdBCO Single Pancake Coils for Protectio-free HTS Power Applications. Supercond. Sci. Technol., :0,. [] J. Kim, S. Yoon, K. Cheon, K. H. Shin, S. Hahn, D. L. Kim, S. Lee, H. Lee, and S.-H. Moon. Effect of Resistive Metal Cladding of HTS Tape on the Characteristic of No-Insulation Coil. IEEE. Trans. Appl. Supercond., :0,. [] S.-G. Kim, Y. H. Choi, D. G. Yang, Y.-G. Kim, and H. Lee. Charge-Discharge and Thermal- Electrical Characteristics of GdBCO Coils Wound With Various Types of Grease as an Insulation Material. Supercond. Sci. Technol., :000,. [] T. S. Lee, Y. J. Hwang, J. Lee, W. S. Lee, J. Kim, S. H. Song, M. C. Ahn, and T. K. Ko. The effects of co-wound Kapton, stainless steel and copper, in comparison with no insulation, on the time constant and stability of GdBCO pancake coils. Supercond. Sci. Technol., :0,. [] W. D. Markiewicz, J. J. Jaroszynski, D. V. Abraimov, R. E. Joyner, and A. Khan. Quench analysis of pancake wound REBCO coils with low resistance between turns. Supercond. Sci. Technol., :000,. [] Y Wang, Wan Kan Chan, and Justin Schwartz. Self-protection mechanisms in no-insulation (RE)Ba Cu O x high temperature superconductor pancake coils. Supercond. Sci. Technol., :000,. [] Aika Ikeda, Takahiro Oki, Tao Wang, Atsushi Ishiyama, Katsutoshi Monma, So Noguchi, Tomonori Watanabe, and Shigeo Nagaya. Transient Behaviors of No-Insulation REBCO Pancake Coil During Local Normal-State Transition. IEEE Trans. Appl. Supercond., ():0,. [] Y. Yanagisawa, K. Sato, K. Yanagisawa, H. Nakagome, X. Jin, M. Takahashi, and H. Maeda. Basic mechanism of self-healing from thermal runaway for uninsulated REBCO pancake coils. Physica C, :,. [] S. Hahn, Y. Kim, J. Ling, J. Voccio, D. K. Park, J. Bascuñán, H.-J. Shin, H. Lee, and Y. Iwasa. Noinsulation Coil Under Time-Varying Condition: Magnetic Coupling with External Coil. IEEE Trans. Appl. Supercond., :0,. [] J. Song, S. Hahn, Y. Kim, J. Voccio, J. Ling, J. Bascuñán, H. Lee, and Y. Iwasa. HTS Wind Power Generator: Electromagnetic Force Between No-Insulation and Insulation Coils Under Time-Varying Conditions. IEEE. Trans. Appl. Supercond., ():00,. [] Y. H. Choi, S. Hahn, H.-J. Shin, D.-H. Kang, and H. Lee. Experimental and Analytical Studies on Electromagnetic Behaviors of the GdBCO Racetrack Coils in a Time-Varying Magnetic Field. IEEE. Trans. Appl. Supercond., ():0,. [] K. R. Bhattarai, K. Kim, S. Kim, S. Lee, and S. Hahn. Quench Analysis of a Multi-Width No-Insulation T mm REBCO Magnet. submitted for publication at IEEE Trans. Appl. Supercond.,.