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.

The terahertz detection performance and operating regimes of graphene plasmonic field-effect transistors (FETs) were investigated by a hydrodynamic model. Continuous wave detection simulations showed that the graphene response sensitivity is similar to that of other materials including Si, InGaAs, GaN, and diamond-based FETs. However, the pulse detection results indicated a very short response time, which favors rapid/high-sensitively detection. The analysis on the mobility dependence of the response time revealed the same detection regimes as the traditional semiconductor materials, i.e., the non-resonant (collision dominated) regime, the resonant ballistic regime, and the viscous regime. When the kinematic viscosity (ν) is above a certain critical viscosity value, ν_NR, the plasmonic FETs always operates in the viscous non-resonant regime, regardless of channel length (L). In this regime, the response time rises monotonically with the increase of L. When ν < ν_NR, the plasmonic resonance can be reached in a certain range of L (i.e., the resonant window). Within this window, the carrier transport is ballistic. For a sufficiently short channel, the graphene devices would always operate in the non-resonant regime, regardless of the field-effect mobility, corresponding to another viscous regime. The above work mapped the operating regimes of graphene plasmonic FETs and demonstrated the significance of the viscous effects for the graphene plasmonic detection. These results could be used for the extraction of the temperature dependences of viscosity in graphene.

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.

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.

A systematic investigation on the dynamics and evolution mechanisms of multiple-current-pulse (MCP) behavior in homogeneous dielectric barrier discharge (HDBD) is carried out via fluid modelling. Inspecting the simulation results, two typical discharge regimes, namely the MCP-Townsend regime and MCP-glow regime, are found prevailing in MCP discharges, each with distinctive electrical and dynamic properties. Moreover, the evolution of MCP behavior with external parameters altering are illustrated and explicitly discussed. It is revealed that the discharge undergoes some different stages as external parameters vary, and the discharge in each stage follows a series of distinctive pattern in morphological characteristics and evolution trends. Among those stages, the pulse number per half cycle is perceived to observe non-monotonic variations with applied voltage amplitude (V_am) and gap width (d_g) increasing, and a merging effect among pulses, mainly induced by the enhanced contribution of sinusoidal component to the total current, is considered responsible for such phenomenon. The variation of incipient discharge peak phase (Φ_pm) is dominated by the value of V_am as well as the proportion of total applied voltage that drops across the gas gap. Moreover, an abnormal, dramatic elevation in J_pm with d_g increasing is observed, which could be evinced by the strengthened glow discharge structure and therefore enhanced space charge effect.

A systematic investigation regarding the influence of nitrogen impurities on the evolution of multiple-current-pulse (MCP) behavior in a homogeneous dielectric-barrier discharge is carried out via fluid modelling. By gradually up-regulating the N ₂ content from 0.1 ppm to 10 000 ppm, the discharge evolves from MCP mode into a single-pulse mode, and finally returns to MCP mode. Analyses reveal that Penning ionization plays an important role in regulating the MCP performance. Under 10 ppm nitrogen content, the direct electron impact ionization predominates the total ionization process while Penning ionization is negligible. A relatively low pre-ionization level and seed electron level of incipient pulse ( n _e0 ) in this stage induce a strong incipient current pulse as well as MCP. As N ₂ content increases, the pre-ionization level and n _e0 are enhanced due to the elevation of Penning ionization rate, leading to the smoothing of gap voltage profile and the annihilation of MCP at 10 ppm. Above 100 ppm, Penning ionization becomes the dominant ionization channel. As N ₂ content exceeds 300 ppm, the MCP reappears, which is believed to be resulted from a slump in metastable density owing to the excessive dissipation by Penning ionization. Beyond 1000 ppm, the strengthening of glow discharge structure at the incipient current peak phase contributes to the further increase of J _pm as well as pulse number.