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Seungyong Hahn,Seok Beom Kim,Min Cheol Ahn,Voccio, John,Bascunan, Juan,Iwasa, Yukikazu IEEE 2010 IEEE transactions on applied superconductivity Vol.20 No.3
<P>This paper presents experimental and analytical results of trapped field characteristics of a stack of square YBCO thin film plates for compact NMR magnets. Each YBCO plate, 40 mm × 40 mm × 0.08 mm, has a 25-mm diameter hole at its center. A total of 500 stacked plates were used to build a 40-mm long magnet. Its trapped field, in a bath of liquid nitrogen, was measured for spatial field distribution and temporal stability. Comparison of measured and analytical results is presented: the effects on trapped field characteristics of the unsaturated nickel substrate and the non-uniform current distribution in the YBCO plate are discussed.</P>
Current overshoot operation of a REBCO magnet to mitigate SCF
Lee, Changhyung,Hahn, Seungyong,Bang, Jeseok,Cho, Jeonwook,Kim, Seokho The Korean Society of Superconductivity and Cryoge 2018 한국초전도저온공학회논문지 Vol.20 No.4
Due to large in-field current carrying capacity and strong mechanical strength, a REBCO wire has been regarded as a viable high temperature superconductor (HTS) option for high field MRI and > 1 GHz (>23.5 T) NMR magnets. However, a REBCO magnet is well known to have an inherent problem of field inhomogeneity, so-called 'Screening Current induced magnetic Field (SCF)'. Recently, 'field shaking' and 'current overshoot operation' techniques have been successfully demonstrated to mitigate the SCF and enhance the field homogeneity by experiments. To investigate the effectiveness of current overshooting operation technique, a numerical simulation is conducted for a test REBCO magnet composed of a stack of double pancake coils using '2D edge-element magnetic field formulation' combined with 'domain homogenization' scheme. The simulation result demonstrates that an appropriate amount of current overshoot can negate the SCF. To verify the simulation results, current overshoot experiments are conducted for the REBCO magnet in liquid nitrogen. Experimental results also demonstrate the possible application of current overshoot technique to mitigate the SCF and enhance the field homogeneity.
Dynamic Response of No-Insulation and Partial-Insulation Coils for HTS Wind Power Generator
Jung-Bin Song,Seungyong Hahn,Youngjae Kim,Miyagi, Daisuke,Voccio, John,Bascunan, Juan,Haigun Lee,Iwasa, Yukikazu Institute of Electrical and Electronics Engineers 2015 IEEE transactions on applied superconductivity Vol.25 No.3
<P>In this paper, we present results, experimental and numerical, of the electromagnetic interaction forces between pairs of racetrack coils under time-varying conditions. Three turn-to-turn insulation designs were applied to wind three racetrack coils with GdBCO coated conductor: 1) no insulation (NI); 2) partial insulation (PI) of a polyimide layer every eight turns; and 3) insulation (INS) of a polyimide layer between each, i.e., NI, PI, and INS racetracks. Two racetrack pairs, namely, NI-INS and PI-INS, were tested for their interaction forces, measured with load cell under current-ramping conditions in a bath of liquid nitrogen at 77 K. Good experimental and simulation results validate our equivalent circuit model to compute interaction forces of PI-INS racetrack pair. Overcurrent test of NI and PI coils, where each racetrack coil was charged above critical current (I<SUB>c</SUB>), was also performed to compare coil stability. This result implies that, although the PI winding technique improves the dynamic response, stability will be somewhat compromised.</P>
Yoon Hyuck Choi,Seungyong Hahn,Hyun-Jin Shin,Dong-Hyung Kang,Haigun Lee Institute of Electrical and Electronics Engineers 2015 IEEE transactions on applied superconductivity Vol.25 No.3
<P>The electromagnetic responses of two GdBCO racetrack coils, one wound without turn-to-turn insulation (NI) and the other wound with Kapton tape (INS), were examined in a time-varying magnetic field generated by a permanent magnet mounted on a rotor, which was driven by a separate motor. The currents induced by the electromotive forces were measured with respect to the rotating speed, number of turns, and the distance (d) between the permanent magnet and the coil. In the case of the NI coil, delay of the induced current was observed in a time-varying magnetic field (5 rpm, d = 10 mm) due to anisotropy of the current path, suggesting that delay of the induced current will affect the electromagnetic induction by dB/dt. Consequently, the maximum induced current (0.35 A) of the NI coil was 8.2 times lower than that of the INS coil (2.88 A). Moreover, the induced currents in the INS coil increased with increasing rotating speed and number of turns, as well as with decreasing d, while those of the NI coil were barely affected due to the anisotropic current path. The experimental results showed good agreement with the simulated ones, which validates the proposed approaches involving an NI equivalent circuit model.</P>
Effect of Winding Tension on Electrical Behaviors of a No-Insulation ReBCO Pancake Coil
Kwang Lok Kim,Seungyong Hahn,Youngjae Kim,Dong Gyu Yang,Jung-Bin Song,Bascunan, Juan,Haigun Lee,Iwasa, Yukikazu Institute of Electrical and Electronics Engineers 2014 IEEE transactions on applied superconductivity Vol.24 No.3
<P>This paper presents a study on the effects of winding tension on the characteristic resistance of a no-insulation (NI) coil. Two ReBCO NI test pancake coils, having the same winding i.d. (60 mm), o.d. (67.6 mm), and number of turns (60), were sequentially prepared in a way that the first test coil was wound with a winding tension of 12-N, tested, and then rewound with a new winding tension of 20-N for the same tests. In each test, the test coil was energized at a target current, the power supply was “suddenly” disconnected, and then the temporal decay of the coil center field was measured, from which the time constant of the test coil and the consequent characteristic resistance were obtained. To check the reproducibility of experimental data, each test was repeated four times and each time the test coil was unwound and rewound with a given winding tension. The experimental results were analyzed with equivalent circuit analyses. Correlation between the winding tension and the characteristic resistance was discussed in detail.</P>
리니어 프로그래밍을 이용한 NMR 마그넷의 수동 자장보정 방법
이상진(Sangjin Lee),한승용(Seungyong Hahn),심기덕(Kideok Sim) 대한전기학회 2010 전기학회논문지 Vol.59 No.6
Shimming is an important technique in development of nuclear magnetic resonance (NMR) magnets where image resolution is highly dependent on magnetic field homogeneity. Classically, shimming may be categorized into two types: 1) active shimming that incorporates with extra coils and precise tuning of their currents; and 2) passive shimming that incorporates with pieces of steel placed in a bore of a main magnet and their uniform magnetization under homogeneous external fields. Additional magnetic fields, produced by the coils and/or the steel sheets, compensate original magnetic field from the main magnet in such a way that the total field becomes more homogeneous. In this paper, we developed a passive shimming method based on linear programming optimization. Linear programming is well known to be highly efficient to find a global minimum in various linear problems. We firstly confirmed the linearity of magnetization of ferromagnetic pieces under a presence of external magnetic fields. Then, we adopted the linear programming to find optimized allocation of the steel pieces in the inner bore of a main magnet to improve field homogeneity.
"Leak Current" correction for critical current measurement of no-insulation HTS coil
Song, Jung-Bin,Hahn, Seungyong The Korea Institute of Applied Superconductivity a 2017 한국초전도저온공학회논문지 Vol.19 No.2
Discrepancy between a power supply current and an actual "spiral" coil current makes the conventional 4-probe measurement of a critical current ($I_c$) of a no-insulation (NI) high temperature superconductor (HTS) coil inaccurate and time-consuming. This paper presents a fast and accurate approach for $I_c$ measurement of NI HTS coils. With an NI HTS coil energized at a constant ramping rate, a complete analytic expression for the spiral coil current was obtained from a first-order partial differential equation that derived from an equivalent circuit model of the NI coil. From the analytic solution, both spiral coil current and radial leak current can be obtained simultaneously, which enables fast and accurate measurement of the NI coil $I_c$. To verify the proposed approach, an NI double-pancake (DP) coil, wound with GdBCO tapes of $6mm{\times}0.1mm$, was constructed and its $I_c$ was repeatedly measured with various ramping rates in a bath of liquid nitrogen at 77 K. The measured results agreed well with the calculated ones, which validates the proposed approach to measure $I_c$ of an NI HTS coil.