With a bare pin-to-nozzle electrode configuration, an ac-driven non-thermal air plasma jet with a length of several centimeters is generated. Depending on the gas flow rate, the propagation speed of these plasma columns is in the range of 100–350 m/s, which is close to the gas flow velocity. This indicates that the propagation of the plasma jet is determined by the transport of long lifetime species by gas flow instead of the electron-impact excitation and ionization of molecules under a high electric field. In comparison to the N2, O2, and Ar plasma jets, the length of the air plasma jet is much shorter than that of the N2 plasma jet but longer than that of the O2 and Ar plasma jets. A simple kinetic model reveals that the N2 (A3Σ) metastable plays a crucial role in sustaining the length of the N2 and air plasma jets, whose lifetime is strongly dependent on the quenching effect of O2, O, and NO as well as the compensation from the three-body recombination of nitrogen atoms. Based on the flow-dependent mechanism of the plasma jet, a visually uniform air plasma brush with a width of 2.8 cm and a length of 1.6 cm and a visually uniform N2 plasma brush with a width of 4 cm and a length of 4.4 cm have been demonstrated.

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A diffuse He gas-confined barrier discharge insulated by an Ar gas layer instead of a solid dielectric is reported for the first time. It is unexpected to observe that the diffuse Ar plasma attached to the electrode is generated along with the He plasma. The Ar/He/Ar plasma layers with diffuse appearance are visually separated by dark space and thus form the stratified glow. The presence of the stratified mode is largely dependent on the applied voltage, the Ar flow rate and the diameter of the helium gas flow. As the diameter of the helium gas flow decreases from 2.5 mm to 0.9 mm, the discharge mode transits from a stratified glow to filamentary with the amplitude of the discharge current increasing from 0.28 mA to 3.8 A. High-speed photographs of the stratified glow show that the plasma is ignited at the He/Ar gaseous interface, and then expands uniformly towards both the He and Ar gas layers. After the plasma front in He gas layer is quenched at the opposite gaseous interface, the plasma volume starts expanding towards the periphery of the electrode, similar to the dielectric barrier glow discharge.

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With ambient air instead of solid dielectric operating as the insulating layer, a diffuse helium/air gas-confined barrier discharge with a coaxial jet configuration is demonstrated for the first time. The effects of the helium gas flow rate, the diameter, and the vertical position of the helium gas flow on the breakdown characteristics of the diffuse mode are investigated. As the applied voltage increases, a diffuse plasma layer is first ignited within the helium gas column followed by a typical filamentary discharge bridging the whole gap. With the replacement of ambient air by N2 or O2 gas, the diffuse mode can be achieved with relatively lower breakdown voltage in the case of N2 gas while it is not observable in the case of O2 gas. The dynamics of the diffuse discharge show that the plasma front crosses the helium gas column vertically at an average velocity of ∼104 m/s, and then splits into two horizontally counter-propagating plasma fronts with the dark channel left behind.