Experimental observations and simulation results regarding a pure He atmospheric pressure plasma jet (APPJ) and He + N ₂ APPJs interacting with a downstream dielectric substrate are presented in this paper. Experiments utilizing spatial-temporal imaging show that, in the case of the pure He APPJ, an annular plasma-substrate interaction pattern is formed. With the introduction of N ₂ , the plasma is more uniformly distributed on the substrate surface, appearing a solid interaction pattern. The experimental measurements indicate 0.5% N ₂ mixture is the optimal condition to achieve the most intense discharge, while the plasma-substrate contact area is slightly reduced by 6.1% in comparison to that of the pure He APPJ. A 2D self-consistent fluid model is constructed to provide insights into the role of the addition of trace of N ₂ on the discharge dynamics. The discharge morphologies predicated by the model is in principle consistent with the experimental observations. The simulation reveals that the conversion from the annular plasma-substrate interaction pattern to the solid one is attributed to the synthetic effect of the addition of N ₂ and the presentence of the substrate acting as the cathode to enhance the local electric field. In the solid interaction pattern, the Penning ionization makes a significant contribution to the surface discharge, especially in the afterglow region. The dominant positive ions ( N 2 + and N 4 + ) and the reactive oxygen and nitrogen species including O and N gain remarkable increment in the flux intensity to the central surface, which merits great application potential.

This work investigates the hydrodynamic characteristics of a coaxial double-ring electrode helium plasma jet by means of a “Z-type” Schlieren imaging system. The Schlieren images and visual optical photographs made show that a transition point from a laminar region to a turbulent region exists for gas flow without plasma when the helium flow rate exceeds a certain value. After plasma ignition, the laminar region shrinks with voltage increases, and the maximum length of the plasma plume is confined to the laminar region. The heat transfer equation and the spectral broadening of the He I 667.8 nm were used to estimate the increased gas temperature in the plasma jet, and the change in gas velocity by ionic momentum transfer was found by application of a double sphere collision model. As a result, gas heating is considered to be the dominant factor for the earlier onset of turbulence after plasma ignition, whereas the role of ion momentum transfer to neutral gas molecules is comparatively weak. The hydrodynamic behaviors of the plasma jet at the impact region for organic glass and silicon substrates are also researched. The ionization front propagates along the organic glass surface and contracts at the impact point on the silicon surface. More visible vortices are observed from Schlieren images with silicon substrates than with organic glass substrates. Possible mechanisms related to the different treatment effects are discussed.

We report a systematic numerical study concerning the effect of traces of nitrogen admixtures on the discharge pattern dynamics in an atmospheric helium dielectric barrier discharge. By gradually increasing the N ₂ concentration from 1 to 1000 ppm, the discharge mode transformed from the stationary pattern, via quasi-uniform structure, stationary pattern, into the complementary pattern. Analyses revealed that Penning ionization exerted a significant influence on the electron distribution, surface charge deposition and electric field profile over the gap and thus induced the transitions in pattern structure. With the increase of N ₂ content, the average rate of Penning ionization within the afterglow stage presented a non-monotonic variation trend, which conformed to previous results, and seed electron level of the main discharge varied in a similar way. When the discharge was ignited with a large amount of seed electrons, the electron distribution prior to the breakdown phase tended to homogenize and the level of radial electric field during the pre-ionization process was restrained, contributing to the homogenization of the discharge. Furthermore, the formation of complementary discharge pattern was believed to be resulted from the excessive reduction of average electron density between two adjacent main discharges at above 500 ppm N ₂ level. The reduced electron concentration made the volume ionization more susceptible to the surface charge distribution and led to the disruption of discharge channel succession between positive and negative half cycles.

Atmospheric pressure cold plasmas have great potential for surface functionalization, and the interaction between the plasmas and fibers is essential to understand the underlying physics. In this Letter, the penetration pattern and mechanism of an atmospheric pressure plasma jet (APPJ) interacting with different spacing fiber scaffolds were studied by both experiment and modeling. The intensified charge coupled device images showed that APPJ induced a radial surface streamer on the windward side of the scaffold and another axial forward streamer on the opposite side. Propagation distance of both the radial and axial streamers diminished as the spacing decreased. Similar trends were predicted by a 2D fluid model. The simulation results indicated that the high electrical field carried by the streamer head in APPJ was allowed to pass through the gap for large spacing scaffold, while it was blocked by high intensity charges at small spacing. Instead, one axial streamer was generated in the latter case. The physical insight on the penetration mechanism of plasma jet interacting with fiber array in this Letter may contribute to improve treatment uniformity of plasma technology.

Discharges in small size pores are important for understanding the interaction effect between discharge and porous materials such as catalysts. In this letter, the discharge behavior in a micrometer channel of a structured catalyst induced by an atmospheric pressure plasma jet (APPJ) is studied using a 2D fluid model. The results show that the APPJ cannot penetrate into a single channel with a radius of 120 μm due to the rapid diffusion loss of electrons to the walls. On the contrary, when the channel is connected to a millimeter cavity in the catalyst, discharges are formed in both the cavity and the channel. Analyses reveal that the strong electric field in the streamer head of the APPJ will propagate across the dielectric layer into the cavity and ignite an electron avalanche therein. Three discharge stages inside the channel can be identified as the negative streamer, the counter-propagating discharges, and the glow-like discharge. The dynamics and mechanisms of discharges are discussed. Our work provides physical insights into generating discharges in mesoscale catalyst pores and may further help develop a promising means for catalyst preparations.