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The structure, fluid-dynamic behavior, temperature, and radiation emission of a cold atmospheric pressure plasma jet driven by high-voltage pulses with rise time and duration of a few nanoseconds have been investigated. Intensified charge-coupled device (iCCD) imaging revealed that the discharge starts when voltage values of 5-10 kV are reached on the rising front of the applied voltage pulse; the discharge then propagates downstream the source outlet with a velocity around 10 7 -10 8 cm/s. Light emission was observed to increase and decrease periodically and repetitively during discharge propagation. The structure of the plasma plume presents a single front or either several branched subfronts, depending on the operating conditions; merging results of investigations by means of Schlieren and iCCD imaging suggests that branching of the discharge front occurs in spatial regions where the flow is turbulent. By means of optical emission spectroscopy, discharge emission was observed in the ultraviolet-visible (UV-VIS) spectral range (N 2 , N + 2 , OH, and NO emission bands); total UV irradiance was lower than 1 µW/cm 2 even at short distances from the device outlet (<15 mm). Plasma plume temperature does not exceed 45 °C for all the tested operating conditions and values close to ambient temperature were measured around 10 mm downstream the source outlet.

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.3D static and time-dependent modelling of a dc transferred arc twin torch systemTo cite this version: Abstract. The transferred arc plasma torch device consists of two electrodes generating a plasma arc sustained by means of an electric current flowing through the body of the discharge. Modeling works investigating of transferred electric arc discharges generated between two suspended metallic electrodes, in the so called twin torch configuration, are scarce. The discharge generated by this particular plasma source configuration is characterized by a complex shape and fluid dynamics and needs a 3D description in order to be realistically predicted. The extended discharge length that goes from the tungsten pencil cathode to the flat copper anode without any particular confinement wall and the fluid dynamics and magnetic forces acting on the arc may induce an unsteady behavior. In order to capture the dynamic behavior of a twin torch discharge, a 3D time dependent plasma arc model has been developed using a customized commercial code FLUENT form in both Local Thermodynamic Equilibrium (LTE) and non-LTE. A two temperature (2T) model has been developed taking into account only the thermal non-equilibrium effects in argon plasma. The main differences between LTE and 2T models results concern the increased extension of the horizontal section of the discharge and the predicted reduced (of about 60-80V) voltage drop between the electrodes when using a 2T model.

Although the majority of atmospheric pressure plasma jet (APPJ) applications involve the interaction between the plasma and a surface, up to now the number of published papers focusing on this subject is limited, even though the nature of the target may strongly influence the plasma characteristics, the discharge structure, the generated reactive species, and consequently, the overall process. Under this framework, we investigated an APPJ impinging on a liquid surface and the effects of changing the stand-off distance, the applied peak voltage, and the pulse repetition frequency, looking at them as variable parameters often used to optimize plasma surface processes. Intensified charge-coupled device (iCCD) and Schlieren acquisitions suggest a key effect of gap width and peak voltage on the discharge morphology, velocity of the ionization front, and effluent fluid-dynamic behavior. The presence of a grounded liquid substrate enhances the electric field downstream of the source outlet: the smaller the gap the faster the ionization wave and the shorter the time for it to reach the surface. Consequently, a small gap favors the charging of the surface capacitance and the formation of surface ionization waves over the liquid target. Schlieren acquisitions highlight the formation of a transient turbulent structure propagating downstream of the gas flow, starting hundreds of microseconds after the initiation of the plasma discharge. The achieved results support the hypothesis that the formation of the turbulence is caused by a heating effect of the high-voltage electrode on the He gas flow. Another observed effect is the variation of the dimple caused by the He flow on the liquid surface as a consequence of the turbulence generated by the plasma discharge. The results presented here confirm how the gas dynamics and the discharge behavior are strongly affected by the presence of the liquid substrate and by its position with respect to the APPJ.

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