The analysis of the gas phase chemistry of a cold atmospheric plasma is a fundamental step for a more thorough understanding of the effects it can induce on target substrates. This work aims at investigating, by means of optical spectroscopic techniques, the kinetics of O ₃ , NO ₂ and NO ₃ produced by a Surface Dielectric Barrier Discharge. The phenomenon of discharge poisoning (or ozone quenching ) in static ambient air was investigated varying the electrical power density applied to the plasma source. Kinetics of the reactive oxygen and nitrogen species production were obtained by means of time resolved UV/VIS optical absorption spectroscopy and highlighted how the discharge poisoning takes place once the applied power density ovecomes a critical value of 0.11 W cm ⁻² . An ozone-enriched atmosphere (with a maximum O ₃ density around 3000–4000 ppm) is thus obtained when the source is operated below the critical power density, while a NO _x -enriched atmosphere (highest concentrations of NO ₂ around 1250 ppm and NO ₃ around 35 ppm) is obtained at higher applied power densities. Moreover, since the production of NO, one of the most important quenchers of O ₃ , is directly related to the vibrational energy of nitrogen molecules, the vibrational population of N ₂ , determined by processing emission spectra of N ₂ (B → C) band, was studied. Finally, considerations regarding both the energy cost of production of a reactive oxygen species and reactive nitrogen species atmosphere and the possibility of on-line monitoring its chemical composition, were presented in order to emphasize the potential of optical absorption spectroscopy techniques for the on-line control of plasma assisted industrial processes.

The present work aims to investigate the interaction between a plasma jet and targets with different physical properties. Electrical, morphological and fluid-dynamic characterizations were performed on a plasma jet impinging on metal, dielectric and liquid substrates by means of Intensified Charge-Coupled Device (ICCD) and high-speed Schlieren imaging techniques. The results highlight how the light emission of the discharge, its time behavior and morphology, and the plasma-induced turbulence in the flow are affected by the nature of the target. Surprisingly, the liquid target induces the formation of turbulent fronts in the gas flow similar to the metal target, although the dissipated power in the former case is lower than in the latter. On the other hand, the propagation velocity of the turbulent front is independent of the target nature and it is affected only by the working gas flow rate.

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.

Summary form only given. Atmospheric pressure plasma jets (APPJs) are widely investigated for plasma assisted medical treatments due to their versatility, granted by the diverse possible combinations of driving power supply, gas employed and source architecture. In this work, a single electrode plasma jet driven by voltage pulses with nanosecond rise time and pulse duration in the order of few tens of nanoseconds is characterized; this plasma source has been previously used for the treatment of polymer solutions to improve their electrospinnability, to increase nanoparticle dispersion and was preliminary characterized in a previous work. Here a set of diagnostic techniques is adopted to get deeper insights on the correlation between the plasma structure and the fluid-dynamic behaviour of the jet (iCCD and high-speed Schlieren imaging) and to assess the compatibility of the plasma jet for the treatment of thermo-sensible and biological materials (temperature measurements, OES and UV absolute radiometry). Depending on the operating conditions, the plasma plume presents a single front or several branched sub-fronts; comparing results from Schlieren and iCCD imaging, it is observed that branching of the plasma jet front occurs in spatial regions where the flow is turbulent. Oscillations both in light emission intensity and plume length are observed during the temporal evolution of the plasma discharge. The plasma plume spectrum, measured by OES, is characterized by N₂, N₂⁺, NO, OH spectral bands as a consequence of mixing with ambient air. Measurements of UV irradiance of the APPJ and temperature of the plasma plume, found to be compatible with biomedical applications, are also presented.