Discharge extension of nonuniform long gaps in compressed air

The discharge extension of rod-to-plane gaps for a lightning impulse is investigated at very large electrode separations up to 50 cm in compressed air at pressures of 0.1 to 0.5 MN/m2. Time-resolved photographs with an imageconvertor camera and current oscillograms show that the intermittent stepwise extension leads to breakdown at above 0.2 MN/m2 in positive polarity and at above 0.3 MN/m2 in negative polarity. The pause time between steps decreases with pressure faster than in inverse proportion. Positively charged channel tips advance with continuous luminosity, while negative-polarity tips are dark between steps. The difference of breakdown voltage between square rod- and conical rod-to-plane gaps is discussed.