Two counter-propagating He plasma plumes and ignition of a third plasma plume without external applied voltage

Abstract

In this work, the interaction of the two counter-propagating helium plasma plume inside a dielectric tube is investigated. When voltages applied to the two electrodes are the same, a dark zone always occurs no matter if the electrodes are symmetric or not. The length of the dark zone decreases from 6 mm to 2 mm when the voltages applied to the two electrodes are increased from 5.5 kV to 7.5 kV. Then with the increase of the applied voltage up to 9 kV, the length of the dark zone becomes stable and approaches to a const value of 1.8 mm. On the other hand, when the voltages applied to the two electrodes are different, instead of a dark zone, a third plasma plume is generated between the upstream and downstream plasma plumes. Detailed studies found that the third plasma plume is ignited more than 300 ns after the voltage is off, it is the difference of the local space charges of the heads of the downstream plasma and the upstream plasma induces the local electric field, which ignites the plasma rather than the external applied voltages.