Charge transfer equilibrium in surface barrier discharge: continuous current and negative ion-driven ionisation wave

Abstract

Sensitive measurements of very weak electrical currents in sinusoidally driven surface barrier discharge in atmospheric air are reported for discharge arrangements with and without liquid covering the top electrode. The transferred charge of the low-power discharge is carefully quantified and it is shown that after the complete voltage period the total charge balance is approaching zero. This rather obvious result is mechanically assumed in the literature, yet it is experimentally not understood in detail. We show that continuous and pulsed micro-ampere currents measured during the negative polarity are responsible for the gradual renewal of the charge transfer equilibrium abruptly distorted by strong electrical pulses caused by positive streamers during the opposite half-period. Using synchronised ICCD imaging we reveal that weak continuous current is caused by slowly expanding surface discharges stemming from previously established cathode spots. The expansion is caused by a surface ionisation wave. Based on the experimental evidence, supported by the theoretical results found in literature, we conclude that an expanding negative ion cloud is responsible for the electric field enhancement at its forefront which is sufficient to ionise the surrounding air.