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 (GN 2 ) at room temperature and liquid nitrogen (LN 2 ) 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 $V_{\mathrm{SD}}$ under ac and dc voltages is deduced as an empirical formula.