Effect of solid nitrogen particles on partial discharge characteristics in slush nitrogen

In immersion cooling superconducting equipment, the refrigerant plays the double role of cooling and electrical insulation, but electric field concentration on the refrigerant causes partial discharge (PD), which can be a weak point in electrical insulation. Although slush nitrogen, which is a mixed refrigerant of liquid nitrogen and solid nitrogen particles, has high cooling performance; however, the effects of the introduction of solid nitrogen particles on the insulation performance has not been clarified. From the analysis of PD waveform characteristics, it was shown that the solid nitrogen particles have a certain barrier effect against discharge, but basically act as a weak point in electrical insulation. On the other hand, the breakdown voltage of slush nitrogen with high solid phase ratio is about 80% of that of liquid nitrogen. It was suggesting that the high cooling performance and sufficient insulation performance may be compatible depending on the insulation design.