Discharge voltage of dielectric gaps and the generalized secondary ionization coefficient γ of the dielectric cathode

The coefficient γ of PMMA (polymethyl methacrylate) is estimated with reference to partial discharges. Partial discharge inception voltage was measured in a system in atmospheric air where a PMMA plate is inserted between a metal sphere and a metal plate. A ramp voltage is applied, and the PMMA plate is the cathode. The point of initial discharge on the plate is detected by a color-powder technique. A field computation gives discharge gap length at the discharge point and voltage across the air gap. When the thickness of the PMMA plate is varied, the discharge point changes, and the discharge gap length and the voltage across the gap change accordingly. Thus, the relation between discharge voltage and air gap length for the PMMA cathode is obtained. The resulting discharge voltage for an air gap length of 0.1-0.4 mm is lower than the Paschen voltage of metal gaps by 20-27%. The coefficient γ is calculated as 7.8×10^-4-1.0×10^-1 for the PMMA cathode. These values are larger than γ for metal gaps calculated from Paschen voltage