Paschen tests in superconducting coils: why and how

Abstract

The Paschen breakdown is the outcome of an avalanche effect related with the ionization of a gas under electric stress. Paschen's law defines the voltage breakdown of rarefied (low pressure) gases as a function of the product of pressure and distance between electrodes showing a minimum value (typically of the order of hundreds V) below which the breakdown cannot take place. The actual field level depends on the gas, on the shape, material and surface conditions (roughness, cleanness and presence of oxide layers) of the electrodes; hence there is a big dispersion in the results available due to the referred number of parameters influencing the voltage breakdown. The design of the electrical insulation of superconducting magnets targets a safety factor of ×10 on dielectric strength over specified voltages using the intrinsic dielectric strength of the materials. A crack in the insulation or a flaw not detected by visual inspection would not necessarily lead to an electrical break during the acceptance tests in air given the high dielectric strength of air (typically values over 1 kV/mm). However, it would lead to an electrical failure during operation if Paschen conditions take place. Since the Paschen minimum occurs under vacuum, it is irrelevant for conventional High Voltage (HV) engineering. The application of superconducting technology for plasma fusion devices with a growing size and complexity of magnets has shown Paschen testing as an essential tool. During magnet operation inside the cryostat conditions, if a He leak takes place, critical pressures can be reached locally and Paschen breakdown might occur at voltages significantly lower than the designed ones. The present paper covers an existing gap in International Standards and scientific literature addressing Paschen testing of superconducting magnets and explains why it is needed and how the tests should be done in the most efficient way.