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Exploring new spatial and temporal limits
Towards "nano" scales and the complex surfaces
One of the major trends in the cold plasma community over the last 20 years has been the increase in studies focusing on high-pressure plasmas (>100 mbar), with notable applications such as plasma-assisted combustion, air treatment, CO2 valorization, biomedical or agricultural applications, and nanomaterial synthesis. The strong electric field gradients, charge density, and gas composition, which evolve on timescales typically in the nanosecond range in these discharges, indeed require original methods to understand the mechanisms. The team studies three main topics related to these plasma sources: (a) the dynamics of nanosecond discharges, (b) the interaction of filamentary discharges with complex dielectric surfaces (catalysts, semiconductors, biological tissues, etc.), and (c) the interaction with a liquid phase.
Nanosecond discharges, characterized by high voltage obtained on characteristic timescales of the order of magnitude of the inverse of the electron-neutral collision frequency, hold a special place in gas discharge physics. These discharges are generated with voltage pulses up to a few hundred kilovolts and rise times on the order of a few nanoseconds. They offer the possibility of maintaining, for tens of nanoseconds, high electric fields and electron densities, at pressures ranging from mbar to atmospheric pressure, allowing for effective dissociation through excited molecules, which triggers highly non-equilibrium chemistry. They have proven to be excellent model systems for plasma kinetics in high electric fields and for advanced plasma diagnostics. From a more application-oriented perspective, they allow the simultaneous triggering of weak shock waves on the microsecond scale for aerodynamics (’actuators’), the uniform production of active species for medical applications, or the controlled production of heat and radicals at pressures up to 30 bars for plasma-assisted combustion. These discharges can also be used for material processing and nanoparticle production.
The interaction of nanosecond discharges, or more conventional filamentary discharges (dielectric barrier or continuous atmospheric pressure discharges), is studied to understand the effect of plasma on target surfaces. The interaction with complex surfaces is closely linked to the development of new in-situ surface diagnostics with time resolution. For example, the development of Mueller polarimetry under plasma exposure has allowed the determination of surface electric fields, first on model surfaces and then on biological tissues. The plasma/catalyst coupling has been studied via in-situ infrared absorption in transmission, both for indoor air treatment and CO2 recycling. The development of a Raman setup also allows the study of plasma-catalyst interaction, as well as the interaction of nanosecond discharges with semiconductors, or the plasma/liquid interface with continuous or nanosecond discharges.
