We report on a detailed numerical study of the two-phase operation of a surface Alternating Current (AC) Dielectric Barrier Discharge (DBD) actuator. We showcase that when the quasi-periodic regime has been established, residual volume and surface charges play an important role on the discharge evolution strongly coupling the positive and negative phases. It is shown that the quasi-neutral streamer discharge found on the positive phase serves as both a positive and negative charge generator and acts as a virtual anode. As the streamer is not attached to the dielectric surface, most of the surface charging occurs during its after-burn (relaxation) phase. The positive surface charge leads to an distant zone of high electric field and thus ion drift but also interacts majorly with the negative discharge phase. During the latter, microdischarges form near the active electrode and an intense cathode layer feeds with charges the discharge volume. Each microdischarge is followed by a plasma layer formation attached to the dielectric layer expanding further at each repetition until it occupies a volume linked to the streamer elongation length and positively charged surface portion. The strong coupling between the positive and negative phases along with the strong impact of the streamer discharge on both suggest implications that have been ignored so far in terms of EHD force production and its spatiotemporal distribution.

The development of microwave plasma streamers at 110 GHz in atmospheric pressure air is numerically investigated taking into account the intense gas heating and its effects on the plasma formation and dynamics. The simulations are based on an implicit finite difference time domain formulation of Maxwell's equations coupled with a simple plasma fluid model and a real gas Euler equation solver. The numerical results show how the formation of a shock wave due to the large microwave power absorbed by the plasma and converted into gas heating strongly modifies the streamer elongation and dynamics. A microwave streamer filament stretches along its axis because of ionization-diffusion mechanisms in the enhanced electric field at the streamer tips. The change in the gas density distribution associated with the formation of shock wave due to gas heating strongly modifies the ionization and diffusion mechanisms and tends to limit the on-axis microwave streamer elongation by enhancing resonance effects. The simulations suggest that gas heating effects also play an important role in the observed bending or branching of microwave streamers after they have reached a critical length.

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