Dong Gyu Yang

Numerical Analysis on Bifurcated Current Flow in No-Insulation Magnet

This paper reports empirical and numerical analyses on the electrical behavior of no-insulation (NI) coil. The spiral and radial current flows of NI coil in the case of the nominal operation and over-current condition are quantitatively identified using numerical analysis. Moreover, the origins of charging/discharging delays and magnetic field saturation are discussed in detail. Experimental results are also compared with the simulations to validate the numerical approach.

Effect of Winding Tension on Electrical Behaviors of a No-Insulation ReBCO Pancake Coil

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.

Charge–Discharge and Thermal–Electrical Characteristics of GdBCO Coils Wound With Various Types of Grease as an Insulation Material

This paper reports the charge-discharge and thermal-electrical characteristics of GdBCO single-pancake coils employing various types of grease, such as N-grease (N coil), Si-based grease (Si coil), and vacuum grease (V coil), as an insulation material to ameliorate the charging-discharging delay observed in no-insulation (NI) coils. The charge-discharge test results confirmed that the use of grease as an insulation material can effectively reduce leakage current because the turn-to-turn grease layers serve as an effective insulation material. In sudden discharge tests, the time constant values of coils wound with N-grease, Si-based grease, and vacuum grease were drastically lower than those of the NI coil. In the over-current test at 1.5 Ic, the maximum voltages of N and Si coils were 3.8 times lower than that of the V coil, indicating that the thermal and electrical stabilities of N and Si coils were considerably enhanced. Moreover, the heat dissipating test results indicated that the peak temperature of the Si coil was lower than those of other coils because the Si-based grease acted as the most effective heat dissipater in the event of local hot spot generation in the coil. In conclusion, among the various types of grease tested in this study, Si-based grease may be the most effective turn-to-turn insulator for the development of highly stable second-generation (2G) HTS magnets with ameliorated charging-discharging delay of NI coil.

Characteristic Resistance of No-Insulation and Partial-Insulation Coils With Nonuniform Current Distribution

This paper proposes a numerical approach to calculate the characteristic resistance (Rc) 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 (Rc) 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 (Rc) 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 (Rc) of a PI as well as an NI coil.

Study for Reducing the Screening Current-Induced Field in a 10-MHz No-Insulation Magnet Using Current Sweep Reversal Method

This paper reports the effects of the current sweep reversal (CSR) method on the screening current-induced field (SCF) in a no-insulation (NI) magnet wound with high-Tc superconducting (HTS) coated conductor (CC), as determined experimentally and analytically. To quantify the SCF in the NI magnet, the magnetic flux density (Bz) was calculated using the equivalent circuit model and compared to the Bz obtained empirically. In addition, the charging scenario for the current sweep reversal (CSR) method was modified to eliminate the effect of the charging delay observed in NI magnets on the SCF. The SCF observed for the NI magnet charged directly at 30 A was markedly reduced when the magnet was subjected to the modified charging scenario for CSR at 36 A by decreasing the “hysteresis of the SCF” to zero base line. The results demonstrated the validity of the proposed approach for the enhancement of the temporal stability and spatial homogeneity of NI-NMR magnets via SCF reduction.

Purification of Chemical Mechanical Polishing Wastewater via Superconducting High Gradient Magnetic Separation System With Optimal Coagulation Process

This study examined the purification of wastewater from the chemical mechanical polishing (CMP) process via a superconducting high gradient magnetic separation (HGMS) system. To remove silica, the main contaminant in CMP wastewater, the optimal coagulation process of silica-magnetite-ferric hydroxide aggregates was empirically determined. Filtration tests via superconducting HGMS system with optimal coagulation process were conducted with respect to the magnetic field and wastewater flow rate. The turbidity and Si concentration of the wastewater filtered at 2 T and 400 mL/min were in accordance with the grey water standard and reverse osmosis (RO) feed water requirement, which demonstrated the feasibility of the superconducting HGMS system for the purification of CMP wastewater.

The Effect of Bobbin Material on the Thermal Stability of a Conduction-Cooled HTS Racetrack Coil

The quench and recovery characteristics of two GdBCO-coated conductor racetrack coils, one wound onto a high-strength aluminum (AL 6061-T6) bobbin and the other wound onto an oxygen-free high-conductivity (OFHC) copper bobbin, were evaluated to investigate the effect of bobbin material on the thermal stability of the HTS racetrack coil. The test results show that the minimum quench energy values of the coil wound onto the OFHC copper bobbin were larger than those of the coil wound onto the AL 6061-T6 bobbin. Furthermore, in the case of the coil wound onto the OFHC copper bobbin, the voltage signals increased more slowly compared to the coil wound onto the AL bobbin, and the recovery time was much shorter after a hot spot was generated by a heater. Overall, the coil wound onto the OFHC copper bobbin exhibited better thermal stability due to its superior thermal diffusivity.

Over-Current Characteristics Influenced by Ag Stabilizer Thickness in a GdBCO Coated Conductor (CC)

In general, the stabilizer of a ReBCO-coated conductor (CC) plays a very important role in bypassing and transferring the excessive heat when quench occurs. For an optimal design, selecting the appropriate materials and stabilizer thickness is an important issue for enhancing the thermal/electrical stability of superconducting power devices. This study examined the electrical and thermal behavior of GdBCO CC tape in terms of the stabilizer thickness as well as the effects of variations of the Ag stabilizer thickness on the thermal/electrical stability through an over-current test. An electroplating technique was employed to obtain a Ag stabilizer with various thicknesses without serious degradation of the superconducting properties.

Numerical Analysis on Characteristic Resistance of No-Insulation and Partial-Insulation NbTi Solenoids

This paper presents a numerical approach for calculation of the characteristic resistance, i.e., RC, 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 RC values of 36.2 and 203 μΩ for the NI and PI solenoid coils, respectively. The RC 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 RC ratio of PI to NI coil was in reasonable agreement with the experimental value, demonstrating the validity of the proposed approach in this paper.

Study on Thermal-Quench Behaviors of GdBCO Coils Wound With Silicon Grease as an Insulation Material

This paper investigated quench initiation and propagation characteristics of grease insulation coils by conducting thermal-quench tests for two GdBCO single-pancake coils, namely a coil co-wound with silicon grease (termed as the SiG coil) and the other coil co-wound with Kapton tape (termed as the INS coil), as turn-to-turn insulation. The test results confirmed that the SiG coil exhibited better thermal and electrical stabilities when compared with the INS coil because the operating current could be bypassed in the radial direction through the turn-to-turn contacts when a local hot spot was generated. The SiG coil had superior thermal and electrical stabilities. However, a nonrecovering resistive zone could be generated because excessive Joule heat energy could be induced by radial current flow due to the existence of radial resistance that was mainly generated by silicon grease and nonsuperconducting materials including the substrate, stabilizer, and buffer layers within the high-temperature superconductor (HTS). Therefore, it is essential to consider critical Joule heat energy that is influenced by operating current and stored magnetic energy as well as the radial resistance to achieve self-protective 2G HTS coils.