Homogeneous Dielectric Barrier Discharge in He/N2 Mixtures Driven by Unipolar Sub-microsecond Square Pulses

To shift the electron energy distribution function to high values in non-equilibrium plasmas, sub-microsecond high-voltage pulses have been used. Higher electron energies lead to an enhanced degree of ionization and excitation. In this paper, nanoseconds high-voltage pulses are used to drive a dielectric barrier discharge, one of the widely used methods to generate non-equilibrium, atmospheric pressure plasmas. To study the ignition process of the discharge and its structure, a high-speed ICCD camera was used. Using a 5 ns exposure time, photos showed that there are two separate discharges, referred to in this paper as the primary discharge and the secondary discharge. The first discharge starts from the bulk of the gap and the whole gap reaches its maximum emission intensity within 20 ns. Before the secondary discharge starts, there are weak emission layers next to both electrodes. The secondary discharge develops from the anode and expands towards the cathode, with the marked presence of a dark space adjacent to a weakly luminous cathode layer. Since the agents responsible for the efficient plasma reactivity are the ultraviolet (UV) photons and the chemically reactive species, the UV radiation and reactive species generation efficiency are also studied. The emission spectra shows that most of the UV is emitted by excited NO radicals, where the oxygen atoms come from residual trace amounts of air. In addition to NO and NO
₂
, excited N
₂
, N
₂

⁺
, OH, and He are also present in the plasma