Compared with the diffuse mode of helium plasma jets, a filamentary mode is normally produced in the plasma plume with less expensive inert gas, such as argon, which is undesirable because intense discharge in the filamentary mode may cause damage to fragile samples. Many efforts have been attempted to realize the diffuse mode of an argon plasma jet. In this paper, the diffuse mode is realized in an atmospheric pressure argon plasma jet with increasing biased voltage (V b) applied to a downstream plate electrode. Results show that the diffuse mode transits from the filamentary mode with the increase of V b. Waveforms of voltage, current and integrated light emission reveal that the filament originates from a positive discharge with a needle anode, while the diffuse mode results from negative discharges with a needle cathode, which consist of a Trichel-pulse phase and a slower-varying phase. By fast photography, the positive discharge is attributed to a streamer regime, whereas the negative discharges belong to a Townsend-discharge regime. From optical emission spectroscopy, electron density, electron excitation temperature, vibrational temperature, and rotational temperature are studied as functions of V b.

As a promising candidate for generating large‐scale plasma at high pressure, microhollow cathode discharge (MHCD) with a cylindrical hole has been numerically simulated at an argon pressure higher than 50 Torr by a two‐dimensional particle‐in‐cell Monte Carlo collision method. Results indicate that MHCD is operated in a confined phase at the beginning, which transits to an expanded phase and reaches a steady‐state at about 160 ns. In a steady‐state MHCD, the majority of electrons have a low energy of several eV, and some electrons have a high energy up to 480 eV. High‐energy electrons mainly appear adjacent to the cathode surface. With increasing applied voltage, there is a transition from the confined phase to the expanded phase for a steady‐state MHCD. Moreover, maximal density of charged particles increases for the expanded phase with increasing applied voltage and argon pressure or decreasing hole diameter. Besides, discharge morphology has been investigated in detail with varying operating parameters. All these results have been compared with those simulated by the fluid model.

Transition from a Townsend mode to a normal glow mode has been reported in the literature for uniform dielectric barrier discharge (DBD) at atmospheric pressure. In this paper, through a one-dimensional fluid model, more modes of uniform DBD in helium and transitions between them are found with varying rising time of a saw-tooth voltage. The results indicate that a positive discharge initiates at the positive-slope voltage phase, whose pulse duration decreases, while the peak value increases with decreasing rising time. During this process, a negative discharge initiating at the negative-slope voltage phase keeps weakening to almost zero current. The predominant positive discharge is then investigated through analyzing spatial distributions of electron density, ion density, and electric field at the peak current moment. In combination with the voltage-current curve, discharge modes of DBD are revealed to transit from a Townsend, a normal glow, to an abnormal glow with decreasing voltage rising time. These mode transitions are qualitatively explained by analyzing the gap voltage and electron density averaged in the gap just before discharge initiation. The results also suggest that by reducing the rising time or increasing voltage amplitude, DBD is prone to operate in the abnormal glow mode. Moreover, DBD in the abnormal glow mode has an increasing peak current and a decreasing pulse duration with increasing voltage amplitude. Finally, the critical voltage amplitude is given as a function of voltage rising time for the mode transitions from the Townsend to the normal glow and the normal glow to the abnormal glow.

An atmospheric pressure dielectric barrier discharge driven by an asymmetric sinusoidal voltage has been investigated numerically with a one-dimensional fluid model. Results indicate that, as a controlling parameter, the asymmetric degree (Das) of applied voltage can significantly affect discharge characteristics. With an increasing absolute value of Das [abs(Das)], the maximal current density remains almost constant at low abs(Das) and increases significantly at high abs(Das), while the phase of the discharge with the maximal current gradually approaches toward 0° of the applied voltage. Meanwhile, discharge transits from a Townsend mode to a glow mode. Moreover, the electron density (ne) increases more than three orders of magnitude during this process, hence it is viable to manipulate ne by adjusting Das. Besides, N4+ is the predominant positively charged species with abs(Das) lower than about 60%. Out of this range, He2+ takes the place of N4+, becoming the predominant positively charged species. Further investigations reveal that in the primary discharge, direct ionization takes an important role with high abs(Das), while Penning ionization becomes important with low abs(Das). Moreover, dominant reactions for electron production are clarified. These results are of great significance in the manipulation of ne for a homogeneous barrier discharge.

This paper describes the realization of homogeneous dielectric barrier discharge (DBD) in argon at atmospheric pressure. The effect of the morphology of the dielectric surface (especially the dielectric surface covered by hollow ceramic beads (99% Al2O3) with different diameters) on discharge is investigated. With different dielectrics, the argon DBD presents two discharge modes, namely, a filamentary mode and a homogeneous one. Fast photography shows that the filamentary mode operates in a streamer discharge, and the homogeneous mode operates in a Townsend discharge regime. It is found that a homogeneous discharge can be generated within a certain voltage range. The range of voltage amplitude decreases, and the breakdown voltage increases with the increase of the mean diameter of the ceramic beads. Waveforms of the total current and optical emission signal present stochastic pulses per half voltage cycle for the filamentary mode, whereas one single hump per half voltage cycle for the homogeneous mode. In the homogeneous mode, the intensity of optical emission decreases with the mean diameter of the ceramic beads. The optical emission spectrum is mainly composed of atomic lines of argon and the second positive system of molecular nitrogen. It reveals that the electron density decreases with increasing mean diameter of the ceramic beads. The vibrational temperature increases with increasing mean diameter of the ceramic beads. It is believed that a large number of microdischarges are formed and smaller ceramic beads have a larger activation surface area and more point discharge. Electrons liberated in the shallow well and electrons generated from microdischarges can increase the secondary electron emission coefficient of the cathode and provide initial electrons for discharge continuously. Therefore, the breakdown electric field is reduced, which contributes to easier generation of homogeneous discharge. It is confirmed by the simulation results.