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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.
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>
High-temperature superconductors for NMR/MRI magnets:opportunities and challenges
Iwasa, Yukikazu,Bascunan, Juan,Hahn, Seungyong,Yao, Weijun The Korea Institute of Applied Superconductivity a 2009 초전도와 저온공학 Vol.11 No.2
The unique features of HTS offer opportunities and challenges to a number of applications. In this paper we focus on NMR and MRI magnets, illustrating them with the NMR/MRI magnets that we are currently and will shortly be engaged: a 1.3 GHz NMR magnet, an "annulus" magnet, and an $MgB_2$whole-body MRI magnet. The opportunities with HTS include: 1) high fields (e.g., 1.3 GHz magnet); 2) compactness (annulus magnet); and 3) enhanced stability despite liquid-helium-free operation ($MgB_2$whole-body MRI magnet). The challenges include: 1) a large screening current field detrimental to spatial field homogeneity (e.g., 1.3 GHz magnet); 2) uniformity of critical current density (annulus magnet); and 3) superconducting joints ($MgB_2$magnet).
Young-Gyun Kim,Seungyong Hahn,Kwang Lok Kim,Oh Jun Kwon,Haigun Lee IEEE 2012 IEEE transactions on applied superconductivity Vol.22 No.3
<P>This paper reports a study of two HTS racetrack coils wound with GdBCO coated conductors, one without turn-to-turn insulation and the other with Nomex insulation. The coils were characterized by charge-discharge, over-current, and sudden discharge tests. Thermal and electrical characteristics of the GdBCO racetrack coil without insulation outperformed the insulated one. The test results confirmed that although the coil without insulation has a slow charging/discharging time constant, the no-insulation winding technique may enable a compact and protection-free racetrack-typed coil with enhanced mechanical integrity as well as better thermal and electrical stabilities.</P>
Dong Gyu Yang,Seungyong Hahn,Youngjae Kim,Yoon Hyuck Choi,Hyun-Jin Shin,Haigun Lee Institute of Electrical and Electronics Engineers 2015 IEEE transactions on applied superconductivity Vol.25 No.3
<P>This paper presents a numerical approach for calculation of the characteristic resistance, i.e., R<SUB>C</SUB>, of no-insulation (NI) and partial-insulation (PI) low-temperature superconductor solenoid coils, based on a contact resistance matrix model implementing “Hertzian contact.” The proposed method was applied to both NI and PI NbTi solenoid coils, for which test results have been reported. To consider the Hertzian contact effect, the stresses within the coil were quantitatively identified using a force balance equation. The stresses calculated in the axial and radial directions were 104 and 16 MPa, respectively, which consequently enabled estimation of the R<SUB>C</SUB> values of 36.2 and 203 μΩ for the NI and PI solenoid coils, respectively. The R<SUB>C</SUB> ratio of PI to NI coil was 5.6, implying that the estimated charge/discharge delay time of the NI coil was 5.6 times larger than that of the PI coil. The simulated R<SUB>C</SUB> ratio of PI to NI coil was in reasonable agreement with the experimental value, demonstrating the validity of the proposed approach in this paper.</P>
Dong Gyu Yang,Seungyong Hahn,Youngjae Kim,Kwang Lok Kim,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 proposes a numerical approach to calculate the characteristic resistance (R<SUB>c</SUB>) of partial-insulation (PI) and no-insulation (NI) high-temperature superconductor pancake coils with the nonuniform current path in such coils taken into consideration. Recently, an analytic approach has been proposed to estimate (R<SUB>c</SUB>) of an NI coil, where the coil current is assumed to be “uniform” over the entire coil. This model, however, is not effective to explain the increase of (R<SUB>c</SUB>) when a coil is modified from NI to PI. In this paper, we first introduce our numerical approach based on a finite element analysis. Then, the charging characteristics of selected PI and NI coils that we had previously reported are analyzed by the proposed approach. Reasonable agreement between the measured and calculated data validates the proposed approach to estimate (R<SUB>c</SUB>) of a PI as well as an NI coil.</P>
Jung-Bin Song,Seungyong Hahn,Youngjae Kim,Voccio, John,Jiayin Ling,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 experimental and analytical results of electromagnetic forces between no-insulation (NI) and insulation (INS) coils under time-varying conditions for NI coils that may be used in wind power generator. Three test pancake coils, one NI and the others INS, of the identical winding i.d., bore size, and number of turns, are wound with GdBCO coated tape. To determine an electric circuit model of the NI coil, the coil was charged-and-discharged at different ramping rates. The data were used to compute a characteristic resistance of a parallel inductor/resistor circuit model. In force measurement, two sets of pancake pairs were tested, NI-INS and INS-INS, the later for comparison. The coils were tested in a bath of liquid nitrogen at 77 K. The dynamic force response of the NI-INS pair was compared with that of INS-INS. The analysis based on the circuit model was tested with the experimental results. Analysis agrees well with experiment, validating our technique to analyze the time-varying forces between the NI-INS pair.</P>