Electrical breakdown characteristics of superconducting magnet system in sub-cooled liquid nitrogen

Dielectric characteristics of gaseous helium (GHe) injected into the cooling system to make sub-cooled nitrogen condition with constant pressure is found to be extraordinarily weak in dielectric strength [1]. In high voltage superconducting machines using the sub-cooled nitrogen cooling system, the current lead part in GHe environment could be electrical weak points rather than the superconducting magnet part in liquid nitrogen (LN2). In order to enhance dielectric characteristics of current lead part, three methods are suggested in this paper. The first one is enclosing the current leads with solid dielectric material and the second is exposing the current leads to high vacuum, and the last is topping up the magnet system with dielectric materials such as Glass Bubbles (GB) which is usually used as heat insulator. In addition, the extinction time of bubbles caused by the electrical breakdown is measured to determine the suitable pressure of sub-cooled nitrogen system. It is found that the characteristics of bubble extinction in LN2 are drastically enhanced over 1 bar.