DescriptionHigh-pressure cold plasmas create and maintain a non-equilibrium between the accelerated electrons in the electric field and the heavy species maintained at low temperature. This allows the creation of high concentrations of reactive species with a low energy cost. Many applications can benefit from these properties like surface processing, treatment of gas effluents, combustion, plasma medicine, etc. For most applications, the major objective is to generate a specific chemistry, e.g. in biomedicine, the emphasis is currently on understanding and optimizing the production of O-, H- and N-species that have a high oxidative power and play major roles in biological functions.
Due to the diversity of applications of cold plasmas, a large number of discharge regimes exist. They depend, among others, on the mode of excitation of the plasma (DC, pulsed, RF, MW, laser-induced), the geometry of the electrodes, the pressure or the nature of the gas. This presentation focuses on atmospheric pressure, capacitively coupled plasmas, generated in a noble gas with small admixtures of other gases. It will specifically report on recent works that used a combination of advanced optical diagnostics and computational modelling to establish reaction pathways in complex mixtures (He+H2O+O2) to identify independent control strategies .
Nevertheless, atmospheric pressure discharges face major challenges. Tailoring the plasma gas-phase chemistry to each specific application is one of them. It is complicated by the high collisionality of atmospheric pressure plasmas leading to fast relaxation and quenching processes. In these conditions, classic technics used at low pressure to control the electron energy distribution function are not applicable anymore. Tailoring the chemistry is therefore usually achieved by adjusting external parameters such as: the gas composition, the electric field distribution (by modifying the source design), or the applied voltage characteristic. The plasma chemistry is also highly dependent on the complexity of the system, like in the presence of a plasma-vapor-liquid interface in biomedicine, plasma-dust/nanoparticles interactions in material synthesis or gas depollution, etc.
In addition, as collisionality increases, new difficulties in plasma diagnosis arise. The development of the discharge generally becomes unstable and inhomogeneous, leading to the formation of one or several filaments, a few hundred micrometre large. The experimental study of those filaments requires to reach high spatial and temporal resolution. Laser diagnostics, among other technics, enable to reach down to fs time resolution and micrometre size at the focus point. But it can lead to intrusive levels of light intensity and special technics are being developed to overcome those issues.
Future opportunities include i) the use of new types of voltage waveforms (extreme overvoltages, sub-ns rise times, dual-frequency waveforms) ii) new diagnostics are being adapted to non-thermal plasmas (Electric-Field Second Harmonic Generation, Background Oriented Schlieren or fs-TALIF) iii) the development of kinetic models describing the details of vibrational excitation of nitrogen iv) the study of plasmas-surface interactions.
 Brisset et al. 2021 J. Phys. D: Appl. Phys. 54 285201
|Period||13 Apr 2022|
|Event title||IOP Conference on Plasma Physics|
|Degree of Recognition||National|