Investigation on the streamer propagation in atmospheric pressure helium plasma jet by the capacitive probe

In this letter,the streamer propagation in the atmospheric pressure helium plasma jet with afloating electrode nozzle driven by the kHz AC power supply is investigated.The current signal induced by the space charges and the mean propagation velocity of the guided ionization waves are measured by the capacitive probe method in the discharge region.The space charges in the guided ionization waves are found to increase with the applied voltage,which enhances both the electric field near the streamer head and the propagation velocity.The applicability of the streamer mechanism to the propagation of the guided ionization waves is validated by this electrical diagnostic method.

A computational modeling study on the helium atmospheric pressure plasma needle discharge

A two-dimensional coupled model of neutral gas flow and plasma dynamics is employed to investigate the streamer dynamics in a helium plasma needle at atmospheric pressure. A parametric study of the streamer propagation as a function of needle tip curvature radius and helium gas flow rate is presented. The key chemical reactions at the He/air mixing layer which drive the streamer propagation are the direct ionization via collision with electrons, the Penning effect being not so crucial. With increasing the gas flow rate from 0.2 standard liter per minute（SLM） to 0.8 SLM, however, the emissions resulting from reactive oxygen and nitrogen species change from a solid circle to a hollow profile and the average streamer propagation velocity decreases. Air impurities（backdiffusion from ambient air） in the helium jet result in a significant increase in the streamer propagation velocity. Besides, with decreasing the tip curvature radiusfrom 200 μm to 100 μm,the electron avalanche process around the near-tip region is more pronounced. However, the spatially resolved plasma parameters distributions（electron, helium metastables, ground state atomic oxygen, etc.） remain almost the same, except that around the near-tip region where their peak values are more than doubled.

THE CRITERION FOR PROPAGATION OF THE POSITIVE STREAMER AND ITS APPLICATION

Through analysing the upward leader from a tall structure under storm condition,it is concluded,in this paper,that the intense electric field in a limited,small region provides the necessary condition for the natural lightning discharge to occur.Such a “local intense field” could make it possible that a weak streamer grows and continues to travel in ambient field of only several tens of kilovolts per meter,which otherwise,could propagate only in the ambient field of several hundreds of kilovolts per meter. Based on the positive streamer theory and energy-balance limitation,we have deduced a criterion for de- tcrmining whether the positive streamer propagates continuously or not. Using this criterion,we derive the striking distance for tall structures,and the electric environment for ar- tificially triggering lightning by rocket launching with grounding wire.Both results agree with observations.