Vibrational kinetics in repetitively pulsed atmospheric pressure nitrogen discharges: average-power-dependent switching behaviour.

Characterisation of the vibrational kinetics in nitrogen-based plasmas at atmospheric pressure is crucial for understanding the wider plasma chemistry, which is important for a variety of biomedical, agricultural and chemical processing applications. In this study, a 0-dimensional plasma chemical-kinetics model has been used to investigate vibrational kinetics in repetitively pulsed, atmospheric pressure plasmas operating in pure nitrogen, under application-relevant conditions (average plasma powers of 0.23-4.50 W, frequencies of 1-10 kHz, and peak pulse powers of 23-450 W). Simulations predict that vibrationally excited state production is dominated by electron-impact processes at lower average plasma powers. When the average plasma power increases beyond a certain limit, due to increased pulse frequency or peak pulse power, there is a switch in behaviour, and production of vibrationally excited states becomes dominated by vibrational energy transfer processes (vibration-vibration (V-V) and vibration-translation (V-T) reactions). At this point, the population of vibrational levels up to v ⩽ 40 increases significantly, as a result of V-V reactions causing vibrational up-pumping. At average plasma powers close to where the switching behaviour occurs, there is potential to control the energy efficiency of vibrational state production, as small increases in energy deposition result in large increases in vibrational state densities. Subsequent pathways analysis reveals that energy in the vibrational states can also influence the wider reaction chemistry through vibrational-electronic (V-E) linking reactions (N + N 2 ( 40 ⩽ v ⩽ 45 ) → N ( 2 D ) + N 2 ( A ) and N + N 2 ( 39 ⩽ v ⩽ 45 ) → N + N 2 ( a ' ) ), which result in increased Penning ionisation and an increased average electron density. Overall, this study investigates the potential for delineating the processes by which electronically and vibrationally excited species are produced in nitrogen plasmas. Therefore, potential routes by which nitrogen-containing plasma sources could be tailored, both in terms of chemical composition and energy efficiency, are highlighted.

Sensitivity Analysis in Plasma Chemistry: Application to Oxygen Cold Plasmas and the LoKI Simulation Tool.

The number of numerical models and their complexity are increasing rapidly. This not only accompanies with a bigger impact in various fields but also with a growing opacity. In the case of a deterministic model with hundreds of inputs, it can become difficult, while crucial, to identify which mechanisms are the most influential on the results. Screening sensitivity analysis methods, such as the Morris method, were developed to establish the ranking of inputs' influence on a specified output. The advantages of these methods lie in their easy adaptation to different model formulations or input sets and in providing accurate rankings at relatively low computational costs. In this work, a sensitivity analysis based on the Morris method is applied to a cold oxygen plasma model. The robustness of the method and some of its enhancements are tested, and the rankings obtained are compared with other existing ones for similar conditions. Some significant results are nonintuitive, which highlights the interest of using well-designed sensitivity analysis to study, control, and optimize complex systems.

Causes of plasma column contraction in surface-wave-driven discharges in argon at atmospheric pressure.

In this work we compute the main features of a surface-wave-driven plasma in argon at atmospheric pressure in view of a better understanding of the contraction phenomenon. We include the detailed chemical kinetics dynamics of Ar and solve the mass conservation equations of the relevant neutral excited and charged species. The gas temperature radial profile is calculated by means of the thermal diffusion equation. The electric field radial profile is calculated directly from the numerical solution of the Maxwell equations assuming the surface wave to be propagating in the TM_{00} mode. The problem is considered to be radially symmetrical, the axial variations are neglected, and the equations are solved in a self-consistent fashion. We probe the model results considering three scenarios: (i) the electron energy distribution function (EEDF) is calculated by means of the Boltzmann equation; (ii) the EEDF is considered to be Maxwellian; (iii) the dissociative recombination is excluded from the chemical kinetics dynamics, but the nonequilibrium EEDF is preserved. From this analysis, the dissociative recombination is shown to be the leading mechanism in the constriction of surface-wave plasmas. The results are compared with mass spectrometry measurements of the radial density profile of the ions Ar^{+} and Ar_{2}^{+}. An explanation is proposed for the trends seen by Thomson scattering diagnostics that shows a substantial increase of electron temperature towards the plasma borders where the electron density is small.