Young Kyu Mo

Surface Discharge Characteristics of GFRP in GN 2 and LN 2 for Designing a Superconducting Coil System

Solid insulation materials used in the application of a superconducting apparatus need to be robust in terms of both mechanical and dielectric performance. The surface discharge characteristics of solid insulation materials are inferior to their penetration breakdown characteristics. Therefore, dielectric accidents in a superconducting appliance can occur as a form of surface discharge rather than as a form of penetration breakdown. In this paper, dielectric experiments on the surface discharge characteristics of glass-fiber-reinforced plastic in gaseous nitrogen (GN2) at room temperature and liquid nitrogen (LN2) at 77 K are conducted to design a high-voltage superconducting coil system with high reliability. As a result, the electric field intensity at surface discharge voltage VSD under ac and dc voltages is deduced as an empirical formula.

Creepage Discharge Characteristics Along the Interface Between Two Solid Insulation Materials in GN 2

Glass-fiber-reinforced plastic (GFRP) and MC Nylon are widely used in superconducting apparatuses, such as in superconducting fault-current limiters, due to their excellent mechanical and dielectric performance. Many superconducting apparatuses are operated in a high-voltage condition. Thus, an unexpected dielectric accident can occur with damage to a superconducting magnet. Therefore, dielectric experiments on the creepage discharge characteristics of GFRP and MC Nylon were conducted for developing a high-voltage superconducting coil system. For this paper, the creepage discharges along the surfaces of solid insulators in gaseous nitrogen (GN2), and along the interface between two pieces of solid insulation in contact, were experimented on and analyzed, with respect to ac and dc voltages, at 300 K, under the absolute pressure of 0.1 MPa.

Discharge Voltage Effects on High-

Breakdown voltage can occur in a superconducting magnet system under high electric field conditions caused by inductive voltage, high-voltage operation, quench resistance, and keen edge of high-TC superconducting (HTS) tapes. Discharge voltage on a superconducting coil can degrade the critical current and the index number of HTS tapes. In this paper, HTS tape experiments are conducted to confirm the degradation characteristics of HTS tapes under dc breakdown voltage. In real systems, winding tension is applied in magnet systems. Therefore, tensile stresses were applied in the HTS tape when testing the degradation by breakdown voltage. The tension was controlled by using the bolt torque of a fabricated sample. In addition, degradation characteristics of BSCCO and YBCO tapes under dc breakdown voltage were compared. Our results revealed that HTS tape could affect breakdown voltage and tensile stress.