Transition des basses fréquences aux hautes fréquences d’une décharge à barrière diélectrique en hélium à la pression atmosphérique

Atmospheric-pressure diffuse discharges are of great interest both for their physical complexity as well as for their numerous industrial applications. A good method to generate diffuse discharges at atmospheric pressure is by means of a dielectric barrier discharge. Up to now, the low frequency range (below 300kHz) and the high-frequency range (above 3 MHz) have been vastly studied, leaving the medium frequency range (between 0.3 and 3 MHz) almost empty of experimental data. The aim of this thesis by publications is two fold. First, the discharge modes obtained in the medium frequency range are to be understood and characterized. Second, the influence of the excitation frequency on the discharge is studied over a wide spectrum (from 25 kHz to 15 MHz). This includes frequencies from the very-low-frequency range to the high-frequency range. The experimental investigation is undertaken with a dielectric barrier discharge in a well-controlled environment of helium at atmospheric pressure. In order to identify the multiple discharge modes, many diagnostics are performed. Fast imaging is used to identify discharge modes according to their time evolution. Then electrical measurements, optical emission spectroscopy and absorption spectroscopy are deployed to measure the electron density, the electron temperature and the density of atoms in a metastable state respectively. At a fix excitation frequency in the medium-frequency range, two distinct discharge modes occur. The first one is identified as the Ω mode in which ionization is at the centre of the discharge and the power density is about 0.1W/cm3. The second one is identified as the hybrid mode in which ionization mainly occurs in the sheath region and the power density can reach about 10W/cm3. When the frequency is above 1MHz, the transition from Ω mode to hybrid mode is subject to a hysteresis. This hysteresis is due to efficient trapping of ions in the gas gap which does not allow ions to participate in secondary electron emission as long as the space charge is too low to modify the applied electric field. However, when the frequency is below 1 MHz the hybrid mode can no longer be sustained continuously but rather oscillate spontaneously between the Ω and the hybrid mode. This phenomenon is due to less efficient ion trapping that cannot maintain ions in the sheath when the electric field is modified by the space charge. Furthermore, over the entire investigated frequency range, many other diffuse discharge modes are sustained. In addition to the atmospheric pressure glow discharge (in which maximum ionization occurs on the cathode side) usually observed in the low-frequency range, a Townsend-like mode is also generated at frequencies about 100kHz. In this case, the maximum ionization occurs on the anode side. From 200 to 3000kHz, the Ω and the hybrid mode are achieved. Finally, from 5 MHz, the usual RF-α mode is reached. Over the complete range of frequency investigated, the mean electron density (time and spatially averaged) increase from e14 to e17/m3 while the electron temperature ranges between 0.3 and 0.5 eV. Both electronic temperature and density of atoms in a metastable state are maximum in the hybrid mode.

Keyphrases: Dielectric barrier discharge, Atmospheric-pressure plasma, Low frequency, Medium frequency, High frequency, Electron density, Electron temperature, Atoms in a metastable state