Chemical mechanisms inducing a dc current measured in the flowing post-discharge of an RF He-O ₂ plasma torch

The post-discharge of an RF plasma torch supplied with helium and oxygen gases is characterized by mass spectrometry, optical emission spectroscopy and electrical measurements. We have proved the existence of a dc current in the post-discharge (1-20A), attributed to the Penning ionization of atmospheric nitrogen and oxygenated species. The mechanisms ruling this dc current are investigated through experiments in which we discuss the influence of the O ₂ flow rate, the He flow rate and the distance separating the plasma torch from a material surface located downstream.