Flashover characteristics along spacer at cryogenic temperature influenced by minute gaps between spacer and electrode

The electric insulation of the large-scale helical device (LHD) consists of the composite insulation of the liquid and spacer. It is very important from a point of superconducting device protection to elucidate surface flashover characteristics along the spacer in liquid. If a minute gap exists between an electrode and a spacer, the electric discharge in a minute gap will serve as a trigger of flashover, it means that the possibility of reducing the flashover voltage becomes high.We measured the flashover voltage of liquid nitrogen (LN2) and liquid helium (LHe) along the cylindrical small gap spacer sandwiched between parallel plane electrodes.The flashover voltage decreased with the decrease of the voltage rising rate when the voltage polarity of the electrode adjacent to the minute gap was negative, but the flashover voltage was almost constant with a decrease of the voltage rising rate when the voltage polarity of the electrode adjacent to the minute gap was positive.