Characteristics of Precursor-Dependent Breakdown in Helium Dielectric Barrier Discharge Jet

The generation of excited species in dielectric barrier discharge jets (DBD jets) when a dielectric substrate is present is highly time dependent and complex, sensitive to precursor addition and power. Both the DBD jets fed with pure helium (He DBD jet) and helium/methyl methacrylate (He/MMA DBD jet) show the characteristics of multiple short-duration (1-2 μs) current pulses during breakdown but contain different numbers, widths, and temporal sequence of pulses. In the He DBD jet, the short-duration pulses overlap to form a single broad current pulse, 4-16 μs long, per half-cycle, whereas with MMA, several broadened current pulses, 2-4 μs long, occur. It was observed that each short-duration current pulse corresponds with a leading ionization wave (or plasma bullet) followed by the formation of a plasma channel. However, if the time between ionization waves is small enough, the propagation of the second ionization wave is less observable. In He/MMA mixtures, the propagation of multiple ionization waves is more difficult to observe with increasing power as the time between current pulses decreases. With only helium, the time between current pulses is so short that only one leading wave can be observed. As the discharge power rises, more plasma channels with a thinner and brighter structure are formed during each voltage cycle, leading to observations of a mode change from a diffuse mode to a concentrated mode in the DBD jets.