Analysis of conductive target influence in plasma jet experiments through helium metastable and electric field measurement
Résumé
The use of cold atmospheric pressure plasma jets for in vivotreatments implies most of the time plasma interaction with conductive targets. The effect of conductive target contact on the discharge behavior is here studied for a grounded metallic target and compared to the free jet configuration. In
this work, realized with a Plasma Gun, we measured helium metastable HeM (23S1) concentration (by laser absorption spectroscopy) and electric field (EF) longitudinal and radial components (by electro-optic probe). Both diagnostics were temporally and spatially resolved. Mechanisms after ionization front impact on target surface have been identified. The remnant conductive ionized channel behind the ionization front electrically transiently connects the inner high voltage electrode to the target. Due to impedance mismatching between ionized channel and target, a secondary ionization front is initiated and rapidly propagates from the target surface to the inner electrode through this ionized channel. This leads to a greatly enhance HeM production inside the plasma plume and the capillary. Forward and reverse dynamics occur with further multi reflections of more or less damped ion
ization fronts between the inner electrode and the target as long as ionized channel is persisting. This phenomenon is very sensitive to parameters such as target distance and ionized channel conductivity affecting electrical coupling between these two andevidenced using positive or negative voltage polarity and nitrogen admixture. In typical operating condition for the plasma gun used in this work, it has been found that after the secondary ionization front propagation, when the ionized channel is conductive enough, a glow like discharge occurs with strong conduction current. HeM production and all species excitation, especially reactive ones, are then driven by high voltage pulse evolution. The control of forward and reverse dynamics, impacting on the production of the glow like discharge, will be useful for biomedical applications on living tissues.