Subject:Characterisation of continuous and fast ionisation discharges by femtosecond TALIF
Understanding the development and the kinetics of low-temperature plasmas and revealing their potential for applications requires the precise knowledge of the spatial and temporal distribution of plasma parameters. Nowadays the production mechanisms of nitrogen (N) atomic radicals in high pressure plasmas are still under study. This internship focuses primarily on the characterization of N production in continuous microwave discharges and in fast ionisation nanosecond pulsed discharges that can be employed for plasma-assisted combustion and for applications based on N production (e.g. N plays a fundamental role in nitrogen fixation for fertilizer synthesis, nitric oxide production or combustion, which are topics under investigation at the EM2C laboratory).
In situ N density can be measured by femtosecond (fs) Two-photon Absorption Laser Induced Fluorescence (TALIF) technique . This advanced laser diagnostic enables to reach spatial resolution down to a few tens of μm and an extreme temporal resolution, which is limited by the laser pulse duration, namely ~100 fs. TALIF is based on the simultaneous absorption by a ground state atom of two photons, which combined energy is chosen to correspond to the energy gap between the ground state and an excited state of the atom. The excited state species will decay by different processes, including spontaneous radiative decay to a lower energy state. By monitoring this emission and with the help of a proper calibration, it is possible to directly relate the fluorescence signal intensity to the density of the atom in the ground state.
Performing the fs-TALIF diagnostic, the student will first study the N distribution in a continuous discharge jet generated by microwaves exhibiting strong spatial gradients (on the scale of a few μm) . He/she may then characterise a fast ionisation nanosecond pulsed discharge by performing time-resolved measurements, that represents an additional challenge for the implementation of the diagnostic . Note that the specific microwave discharge and the fast ionisation waves have demonstrated very interesting features. The microwave discharges are known to produce very large density of key radicals (e.g. 70 % dissociation of O2). The high voltage nanosecond discharge develops as a fast ionisation wave, with strong ionisation degree (up to 1% locally), high reduced electric fields (150-300 Td), while remaining strongly out of equilibrium, with gas temperature close to ambient temperature. Additionally, the plasma properties are nearly homogenous over large volume even at atmospheric pressure. These features are ideal to study N kinetics at high pressure. This internship project can be continued in the frame of doctoral studies.
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