Modeling and characterization of a micro-pulsed plasma coupled with radiofrequency at atmospheric pressure

The prospect of achieving a diffused plasma in a glow regime under atmospheric pressure conditions has consistently drawn the attention of researchers. This interest comes from the goal of improving the reproducibility of processes and of facilitating control over temperature, density, and charge flux. Nevertheless, plasma propagation at atmospheric pressure poses challenges as a result of a highly collisional regime, which accelerates and localizes energy exchanges. Consequently, propagation is typically guided by fluctuations, particularly thermal instabilities. These instabilities are responsible for the filament-like behavior of plasma propagation, the unpredictable movement of plasma channels, and their branching leading to the arc transition. A possibility to solve this issue, obtaining a diffused discharge at atmospheric pressure is offered by the combination of an radio frequency (RF) excitation with an additional high voltage electric field. A recent example is the use of a micro pulse power supply coupled with a RF generator. Aim of the thesis is the electrical and optical characterization of this discharge with a new nanopulse power supply and the modeling of the time-dependent development of the discharge. It has already been observed that for a dual sinusoidally excitation as result of the coupling the RF plasma exhibits a hybrid mode with a transition from the $\Omega$ to the $\gamma$-mode depending on the relative polarization of the electrodes. In another perspective, the LF voltage induces an ions drift to the cathode, reducing the plasma density in the bulk. In this condition, the $\Omega$ excitation is reduced in the bulk and to the anode. Currently, on the cathode side, alternatively adding or subtracting the RF voltage, a pulsed emission of the secondary electrode occurs and, consequently, triggers the $\gamma$ ionization mechanism when both polarizations are negative. Therefore, there is a competition between the RF parameters, which control plasma creation in the discharge bulk, and the LF voltage, which controls the drift of ions, that is, their losses. At the same time, ignition of the $\gamma$-mode is gated by satisfying a self-sustainment criterion. The effect of the frequencies coupling was observed in a jet in open air and 2D modeled. However, the plasma propagation was chaotic due to the charges on the surfaces. The substitution of the LF with a $\mu$s-pulse is expected to increase the stability. In addition to the experimental description of the discharge by electrical characterization, spectroscopy and the optical emission time resolved, the thesis work will include an extended model with COMSOL which tries to include all the different metastable levels and lumps higher energy states.