Observation and characterization of discharge mode transition in a three-electrode atmospheric pressure RF plasma system

A new 3-electrode plasma device powered by a combined nanosecond pulsed (nsp) source and a sub-breakdown pulsed radiofrequency (RF) source at atmospheric pressure in a N2/H2 (97.5%/2.5%) mixture is studied. By applying pulsed excitation sequences from both the nsp and RF sources, two different RF discharge modes are obtained depending on the actual participating electrode pair. In the RF dielectric-barrier-discharge (RF-DBD) mode, the discharge forms between the RF metallic pin electrode and the nsp electrode located behind a dielectric surface (exterior to the reactor). In the RF pin-to-pin (RF-PtP) mode, the RF discharge forms between the RF and grounded metallic pin electrodes, both located inside the reactor. The existence of the two RF discharge modes is highly dependent on the RF pulse duration as revealed using high-speed imaging along with a custom electrical measurement technique. It was observed that for longer duration RF pulses, RF-PtP mode is achieved through a transition of RF-DBD into RF-PtP mode (DBD-PtP transition). Electrical characterization revealed higher coupled power percentages for RF-PtP mode (up to 85%) over RF-DBD mode (25%–75%). However, deteriorating impedance matching of the RF-PtP mode decreased the available RF applied power thus yielding similar plasma power deposition as RF-DBD mode. Plasma characterization performed by analysing the emission of the N2 second positive system provided vibrational temperatures up to 3400 K and a thermalization trend of the rotational temperatures as RF pulse duration increased. Mechanisms behind the DBD-PtP transition are proposed under consideration of the obtained data. We attribute the observed discharge dynamics to diffusional forces acting on charged species such as ambipolar diffusional drift and diffusional charge flow.