Abstract
In insulation design of superconducting coils, breakdown characteristics of helium gas are important because liquid helium, which is often used for oolant in superconducting coils, is vaporized by quenching of the superconductor or leak to vacuum. On the other hand, shortening of insulation clearance due to metal chips or solder whiskers can cause of accidental arc or short circuit in the coils. In this study, breakdown characteristics of helium gas at room temperature between 0.3μm and 10mm of gap spacing are investigated under equivalent condition of superconducting coil quenching. Apparent breakdown voltage in the left of Paschen minimum becomes lower than the value, which is predicted by the Paschen curve, depending on misalignment and surface roughness of electrodes. This deviation from Paschen curve is more remarkable at higher pressure. In extremely short gap typically below 1μm, breakdown voltage can be lower than Paschen minimum voltage. This seems to be caused by the high electric field at the cathode surface and resultant field emission.
