Effects of Voltage on Dielectric Breakdown Characteristics in Gaps Partially Immersed in Liquid Nitrogen

It is well known that abnormally high voltages with various voltage waveforms occur during a quench and cause the electrical insulation in superconducting magnet coils to deteriorate. No data have been reported for the effects of voltage waveforms on dielectric breakdown characteristics in non-uniform field gaps partially immersed in liquid coolant. In the present study, effects of voltage on dielectric breakdown characteristics in non-uniform field gaps, partially immersed in liquid nitrogen coolant (PI condition), are examined with ac and impulse voltages. Furthermore, the dielectric breakdown characteristics of non-uniform gaps completely immersed in liquid nitrogen or wholly in cryogenic nitrogen gas are also investigated and compared with those under the PI condition for ac and impulse voltages. The obtained results are compared with data already reported for dc voltages. It was found that the Sumoto effect appears for ac voltage, but does not appear for impulse voltage. Breakdown characteristics of nonuniform field gaps under the PI condition are strongly influenced by the appearance of the Sumoto effect which depends on the voltage waveforms.