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A new cryogen, a mixture of solid argon and a small amount of liquid nitrogen, was studied to improve thermal contact between the solid cryogen and high-temperature superconducting tapes. A feasibility study of the reference design codes and the thermal requirements was conducted to optimize the design of the mixed cryogen cooling system. Experimental results showed that thermal and electrical stabilities of YBCO coated conductor tapes were significantly improved when they were impregnated with the mixed cryogen. Dry-out problems were overcome by the mixed cryogen because the small amount of liquid nitrogen improved thermal contact between the solid cryogen and superconducting tape.

Quench tests of a GdBCO racetrack pancake coil were conducted in liquid nitrogen at 77 K. Minimum quench energy and two-dimensional normal zone propagation velocities of the coil are discussed. Longitudinal normal zone propagation velocities in the curved section were almost twice those in the straight portion due to stress effects, showing that protection of the straight portion is of greater importance than protection of the curved portion when using racetrack-type, GdBCO pancake coils in large-scale rotating machines.

Quench protection is one of the most important issues for magnet applications using 2 G HTS tapes because of their non-self-protecting properties. Although many researchers have examined the quench/recovery characteristics of HTS magnets to establish the required stability, new information on the quench/recovery characteristics of the recently commercialized HTS magnet with GdBCO CC is needed. In this study, the quench/recovery characteristics of Zr-doped (Gd,Y)BCO CC pancake coils that were insulated with various materials, such as copper and Kapton tapes, were examined in a liquid nitrogen (LN2) bath. The minimum quench energy (MQE) and normal zone propagation (NZP) velocity of the Zr-doped (Gd,Y)BCO CC pancake coils are discussed in terms of the quench/recovery test results.

Here we report the effect of an external magnetic ripple field on the electromagnetic characteristics of GdBCO racetrack coils being operated with a constant DC current. Two types of GdBCO racetrack coils, one wound without turn-to-turn insulation (NI) and the other wound with Kapton tape (INS), were examined under external ripple fields generated by a permanent magnet mounted on a rotor, which was driven by a separate AC motor. The voltage fluctuations and magnetic field variations were measured with respect to the external ripple field intensity (B ERF), rotating speed, and the operating condition. When the INS and NI coils were exposed to an external ripple field (herein, I op = 80 A, B ERF = 2 mT, and 5 rpm), a voltage fluctuation occurred because a time-varying magnetic field interacted with an electric circuit creating an electromotive force. The peak-to-peak voltage (V pp = 0.29 mV) of the NI coil was ~1.86 times lower than that (0.54 mV) of the INS coil, because the voltage response of the NI coil lagged behind dB/dt due to the existence of turn-to-turn contact. Furthermore, the V pp of the INS coil increased with increasing B ERF and rotating speed, while those of the NI coil were barely affected due to the delay of electromagnetic induction. In excessive current and ripple field conditions (I op = 1.125 I c, B ERF = 8 mT, and 50 rpm) the INS coil eventually quenched while the NI coil did not, implying that the electromagnetic stability of the NI coil in excessive time-varying field conditions was superior to that of the INS coil.

This paper presents the details of a recent study on the removal of the screening-current-induced field (SCIF) in a pancake-type non-insulated high-temperature superconductor coil (NI coil). To determine the SCIF in the NI coil, the magnetic flux density (B z ) was calculated using the equivalent circuit model of the coil and compared to the B z obtained empirically. The experimental results indicate that the SCIF elimination in the NI coil was enhanced upon increasing the amplitude and frequency of the AC current being supplied to the background coil. Moreover, the SCIF in the NI coil was successfully removed by applying the appropriate external AC magnetic field intensity. This is because the magnetization direction of the SCIF changed completely from radial to spiral, a phenomenon termed the 'vortex shaking effect.' Overall, this study confirmed that the SCIF in a pancake-type NI coil can be effectively removed by exposing the coil to an external AC magnetic field.

This paper reports the transient characteristics of two types of magnetically coupled GdBCO racetrack pancake coils: (1)?a pair of insulated coils (INS?INS coils) and (2)?a pair of non-insulated coils (NI?NI coils). In order to investigate the electromagnetic behavior of the INS?INS and NI?NI coils, charging/sudden discharging tests were performed with respect to the distance between primary and secondary coils of the coil pairs. The charging/sudden discharging test results show that the amount of induced current and measured axial magnetic field decreased with increasing distance between the primary and secondary coils. Additionally, charge and discharge delays were clearly observed in the NI?NI coils. The simulated results show good agreement with the experimental results, which validates the proposed approach involving an equivalent circuit model.

In this study, empirical and numerical analyses on the screening current-induced field (SCIF) characteristics of a non-insulated (NI) GdBCO-coated conductor single-pancake coil were conducted. Charge and discharge tests were carried out to induce an SCIF in the NI test coil. The SCIF in the NI coil was identified by comparison between the measured and calculated magnetic fields. The experimental results exhibit an increase of the SCIF with increasing an operating current, as well as a logarithmic decay of the SCIF as a function of time. Moreover, the SCIF was not affected by the slow discharge of the NI coil. The estimation of the SCIF based on the critical-state model in a thin film using the finite element method was in fairly good agreement with the experimental results.

This paper reports experimental and analytical studies on the capture efficiency of a superconducting high-gradient magnetic separation (HGMS) system, employing ferromagnetic filters with various wire diameters and packing fractions. The magnetization force was inversely proportional to wire diameter because the magnetic field gradient increased with decreasing wire diameter. In addition, the capture efficiency of the HGMS system was increased with decreasing wire diameter and increasing packing fraction. The experimental results from the filtration tests via the superconducting HGMS system were in reasonably good agreement with the capture efficiency using the magnetization force obtained by magnetic field analysis.

This paper presents recent progress in the fabrication and test of a 3 T/60 mm room temperature bore high- Tc superconducting high gradient magnetic separator (HGMS) using a solid nitrogen (SN2) cooling system. The aim of the study was to develop an inexpensive, compact, and stable superconducting HGMS for the purification of chemical-mechanical-planarization wastewater in the semiconductor industry. The HGMS's magnet, which was cooled by SN2, was comprised of an assembly of 18 GdBCO double pancake coils, each of which was wound without turn-to-turn insulation. Cooling test results showed that the thermal stability of the high-Tc superconducting-HGMS magnet with SN2 was enhanced due to the large heat capacity of SN2. Moreover, in the filtration tests, the suspended solids value of filtrated wastewater at 2 T in a flow rate of 400 ml/min decreased to 1.6 mg/L, resulting in a 99.6% reduction in SS value compared to raw wastewater.

The electrical and thermal properties of a mixed cryogen-cooled 2G HTS coil with respect to liquid cryogen amount were examined to investigate the influence of the solid/liquid ratio on the thermal/electrical stabilities of the GdBCO coil. Thermal contact between the GdBCO coil and solid nitrogen was considerably improved as the amount of liquid neon (LNe) increased. Minimum quench energy values of the GdBCO coil were saturated as the amount of liquid neon increased, which suggested that the excessive LNe was not effective for enhancing the thermal stability of the GdBCO coil. The temperature traces of the GdBCO coil during the quench/recovery tests confirmed that the thermal stability of the coil was enhanced with the aid of the LNe.

This study examined the effects of external pressures on the thermal and electrical stabilities of stacked GdBCO CCs through quench tests. The minimum quench energies and two-dimensional normal zone propagation velocities of stacked GdBCO CCs connected electrically in series under various external pressures were examined. The quench test results showed that the minimum quench energy increased with increasing the external pressure. Longitudinal normal zone propagation velocities were greater than transverse normal zone propagation velocities. Moreover, normal zone propagation velocities increased with increasing applied pressure implying that applied pressure resulted in increased layer-to-layer thermal contact that allowed hot spots to dissipate more easily.

This paper presents quench test results, of both experiment and simulation, of a racetrack pancake coil wound with a GdBCO-coated conductor tape in a solid nitrogen (SN2) cooling system at 58 K. The longitudinal normal zone propagation velocities in the curved section were 2 times faster than those in the straight section. Therefore, the straight section should be protected more than the curved section when using racetrack-type, GdBCO pancake coils in large-scale rotating machines. The simulated results showed good agreement with the experimentally observed normal zone initiation times.

The partial insulation winding was examined to ameliorate the slow charge–discharge shown by coils wound without insulation. Single pancake coils of GdBCO coated conductor were wound without insulation, with kapton tape every five turns, and with the full use of kapton tape. They were characterized by charge–discharge, sudden discharge, and over-current testing. The improved charging and discharging and high thermal and electrical stabilities of the partially insulated coil demonstrate its potential for use in HTS power applications.