Influence of discharge polarity on streamer breakdown criterion of ambient air in a non-uniform electric field

Streamer breakdown of atmospheric air with non-uniform dc electric field in a needle-to-plateelectrode configuration is studied using a semi-analytic model and experimental measurements.A high voltage (either positive or negative) is applied to a hollow needle with0.51 mm outerdiameter and0.25 mm inner diameter separated from a planar ground electrode by a gapdistance of 0.1-1.4 cm. Breakdown voltages are recorded for both positive and negativedischarge polarities. Empirical relations between the critical avalanche size for streamerbreakdown and the gap distance are proposed. Using these empirical relations, a semi-analyticmodel based on Meek's criterion for streamer breakdown is developed to accurately predict themeasured breakdown voltages. It is found that forpd > 380 Torr cm (ord > 0.5 cm at oneatmosphere) streamer breakdown of ambient air occurs at a lower applied voltage for apositively biased needle compared to that with a negatively biased needle, referred as thepolarity effect. Forpd < 380 Torr cm breakdown is attained at a lower applied voltage with anegatively biased needle compared to that with a positively biased needle, and breakdown modetransits from the polarity effect to the so called inverted polarity effect.