G. Wattieaux

Comparative Studies of Double Dielectric Barrier Discharge and Microwave Argon Plasma Jets at Atmospheric Pressure for Biomedical Applications

Two low-temperature plasma jets using argon carrier gas at atmospheric pressure have been experimentally characterized using optical emission diagnostics. The first one is a double dielectric barrier discharge (DBD) plasma jet generated by a pulsed power supply (9 kV , 9.69 kHz, duty cycle: 1%) and the second one is a microwave (MW) induced plasma jet (2.45 GHz, 40 W ). The argon gas (4.5 purity) flowing through the quartz tube used to launch the plasma in open air is kept at 1 L/min for both plasma devices. Some thermodynamic parameters such as rotation (Trot) and excitation (Texc) temperatures have been determined as well as some plasma active species such as electron density, ultraviolet C (UVC) irradiance, and atomic oxygen concentrations. Most of these plasma parameters are spatially resolved along the plasma jet axis using the spectra of atomic lines (Ar and O) in the visible range and molecular bands (N2 and OH) in the UV range. At the tube outlet, the electron density and atomic oxygen concentration are one decade higher in the case of the dielectric barrier discharges (DBDs) plasma jet while Trot is higher in the case of the MW plasma jet. These differences are mainly due to the way of plasma generation. Indeed, the guided-ionization waves generated by the DBD setup cause higher nonequilibrium phenomena since the difference between Trot and Texc is shown to be much larger in the DBD case. Furthermore, at the tube outlet, it is shown that UVC irradiance produced by the MW plasma jet is about twice as large as that of the DBD plasma jet. However, at 1.7 cm away from the tube outlet, the differences between the two plasma setups on temperatures and active species production become less significant. For instance, the plasma gas temperature measured with a thermocouple becomes the same (320 K) showing the ability of both plasma setups to be used in biomedical applications without inducing a significant thermal effect.

Electrostatic Potential Radiated by a Pulsating Charge in a Two-Electron Temperature Plasma

Mutual impedance experiments have been developed to constrain the plasma bulk properties, such as density and temperature, of ionospheric and later space plasmas, through the electric coupling between an emitter and a receiver electric antennas. So far, the analytical modeling of such instruments has enabled to treat ionospheric plasmas, where charged particles are usually well characterized by Maxwellian electron distributions. With the growth of planetary exploration, mutual impedance experiments are or will be used to constrain space plasma bulk properties. Space plasmas are usually out of local thermodynamic equilibrium; therefore, new methods to calibrate and analyze mutual impedance experiments are now required in such non-Maxwellian plasmas. To this purpose, this work aims at modeling the electric potential generated in a two-electron temperature plasma by a pulsating point charge. A numerical method is developed for the computation of the electrostatic potential in a sum of Maxwellian plasmas. After validating the method, the results are used to build synthetic mutual impedance spectra and quantify the effect of a warm electron population on mutual impedance experiments, in order to illustrate how the method could be applied for recent and future planetary space missions, such as Rosetta, BepiColombo, and JUICE. In particular, we show how it enables to separate the densities and temperatures of two different electron populations using in situ measurements from the RPC-MIP mutual impedance experiment on board Rosetta.

Quantitative Determination of Density of Ground State Atomic Oxygen from Both TALIF and Emission Spectroscopy in Hot Air Plasma Generated by Microwave Resonant Cavity

Two experimental techniques have been used to quantify the atomic oxygen density in the case of hot air plasma generated by a microwave (MW) resonant cavity. The latter operates at a frequency of 2.45 GHz inside a cell of gas conditioning at a pressure of 600 mbar, an injected air flow of 12 L/min and an input MW power of 1 kW. The first technique is based on the standard two photon absorption laser induced fluorescence (TALIF) using xenon for calibration but applied for the first time in the present post discharge hot air plasma column having a temperature of about 4500 K near the axis of the nozzle. The second diagnostic technique is an actinometry method based on optical emission spectroscopy (OES). In this case, we compared the spectra intensities of a specific atomic oxygen line (844 nm) and the closest wavelength xenon line (823 nm). The two lines need to be collected under absolutely the same spectroscopic parameters. The xenon emission is due to the addition of a small proportion of xenon (1% Xe) of this chemically inert gas inside the air while a further small quantity of H2 (2%) is also added in the mixture in order to collect OH(A-X) and NH(A-X) spectra without noise. The latter molecular spectra are required to estimate gas and excitation temperatures. Optical emission spectroscopy measurements, at for instance the position z=12 mm on the axis plasma column that leads to a gas measured temperature equal to 3500 K, an excitation temperature of about 9500 K and an atomic oxygen density 2.09×1017±0.2×1017 cm−3. This is in very good agreement with the TALIF measurement, which is equal to 2.0×1017 cm−3.