Electromagnetic characteristics of NI-REBCO coils according to contact surface conditions controlled by the oxidation state and roughness of the REBCO conductor

Abstract

Although no-insulation (NI) rare-earth barium copper oxide (REBCO) coils offer technical advantages, such as high-energy density, mechanical strength, and self-stabilizing performance, sometimes their applications are limited owing to current leaks through turn-to-turns inside the coils, which are caused by their very low turn-to-turn contact resistance. This results in temporal magnetic field delays and limits their applicability in fields requiring fast response performance in magnetic field control. These shortcomings can be addressed by increasing the turn-to-turn contact resistance of the NI coils. A NI winding method with REBCO conductor variations of surface-oxidation and roughness was proposed in this study; this method improved the magnetic field response by reducing the temporal delays of the magnetic field via the suppression of the turn-to-turn bypass current. This was achieved by increasing the turn-to-turn contact resistance within the NI coils through adjustments to the oxidation and roughness of the surface of a copper stabilizer layer, which was coated on the outermost part of the REBCO conductor by electroplating to ensure electrical and mechanical protection. In this study, the steady- and transient-state characteristics of NI coils were experimentally analyzed according to changes in the oxidation and roughness levels of the surface of the REBCO conductor. To verify the effectiveness of the proposed winding approach, basic tests were conducted in a liquid nitrogen cooling environment by constructing different types of sample coils using REBCO conductors, which were thermally oxidized at different temperatures in a convection oven and scratched using sandpaper. Further, the decay time constant and the time delay of the center magnetic field in the sample coils were measured in sudden current discharging and current charging/discharging tests, respectively. In addition, overcurrent tests using currents exceeding the critical current were conducted to analyze the thermal stability of the sample coils. In conclusion, it was experimentally verified that the time delays of the NI coils were reduced as the surface oxidation and roughness of the REBCO conductor increased. Furthermore, when the oxidizing and scratching approaches were employed, the contact resistance values of the NI coils (3.48 µΩ·cm2), which were not subjected to any surface treatment, increased to 155.01 and 4.43 µΩ·cm2, which were oxidized at 230 °C and sanded using P400-grit sandpaper, respectively. For oxidizing approach, heat treat temperatures at 200 °C – 230 °C will be suitable for increasing the contact resistance of NI coils without inserting any resistive material, thus resulting in a enhancing the magnetic response performance of NI coils.