Streamers at the subnanosecond breakdown of argon and nitrogen in nonuniform electric field at both polarities

An ICCD camera was used to study plasma glow at the stage of the streamer (ionization wave) formation in the tip–plane gap with a length of 3 mm filled with argon or nitrogen at a pressure of 12.5–400 kPa. Positive and negative nanosecond voltage pulses were applied across the gap. Images of streamer were obtained at different time at its propagation along the gap. A streak-camera equipped with a spectrometer was used to measure time evolution of the radiation intensity of nitrogen molecules at a wavelength of 337.1 nm in several regions along the gap at the negative polarity. Average streamer velocity (1.8 cm/ns) was estimated from experimental data at atmospheric pressure of nitrogen. Amplitude–time characteristics of voltage, discharge current and the current of runaway electron beam behind the aluminum-foil anode with a thickness of 10 $\mu$m were measured. Reasons for a diffuse discharge under the given experimental conditions were discussed.