Back and forth directed plasma bullets in a helium atmospheric pressure needle-to-plane discharge with oxygen admixtures

A sinusoidally driven needle-to-plane discharge in flowing helium at atmospheric pressure was investigated by means of electrical, optical and spectroscopic (VUV, UV and OES) measurements. 'Bullet-like' behaviour of the discharge was observed and investigated with special interest towards velocities and size of the 'forward bullet'. The influence of the discharge gap and oxygen admixture on the discharge properties was analysed. The focus of this paper is on experimental results, showing discharge development within six phases. Among these, four types of bullets are observed with respect to the direction of the gas flow and applied voltage polarity. Temporally resolved photography shows the formation of an atmospheric pressure glow discharge subsequent to the propagation of the 'forward bullet'. For the greatest gap of 15 mm, the plasma activity was restricted towards the positive voltage polarity. The discharge development under the oxygen admixture showed a delayed current pulse for the negative voltage slope, with a steep rising flank. We conclude that the main movement of the bullets in our setup does not depend on the gas flow but on the electrical field direction.