Session HW2: High Pressure Discharges I

Diagnostics of atmospheric-pressure N$_2$/O$_2$ plasma jet with green optical emission

We recently developed an atmospheric-pressure nitrogen plasma jet, in which we observed green optical emission when adding a small amount of oxygen. It was identified that the blue color of the plasma originated from the $^1D - ^1S$ forbidden transition of atomic oxygen at a wavelength of 557.73 nm. This forbidden transition is well known as the source of the green color of an aurora, but we believe that the observation of the forbidden optical emission from a small atmospheric-pressure plasma source is surprising and interesting. In the work, we examined the densities of various species in the plasma jet by optical emission spectroscopy and laser-induced fluorescence spectroscopy. The optical emission spectrum indicated a line emission at 557.73 nm with a broadband tail component. The broadband tail component suggests that the transition probability of the forbidden line is enhanced by the formation of the ON$_2^*$ excimer. We measured the densities of atomic oxygen and nitrogen by two-photon laser-induced fluorescence. In addition, we currently try to detect ${\rm N}_2({\rm A}^3\Sigma_u^+)$ by laser-induced fluorescence or cavity-ringdown absorption spectroscopy.   

Optical Analysis of High Pressure Microplasma Glow and Arc Discharges

This work presents an analysis of high-pressure microplasma discharges in He-Ar mixtures. The long term objective of this project is to use glow discharges as a gain media for lasing. A glow discharge is desired for such operations due to its uniform and stable nature. However, in atmospheric pressures, arc discharges are significantly more common. Arcing is much less voluminous and can be destructive to electrodes over time. Thus, discerning glow discharges from arc while also maximizing the number of metastables is critical. Determining plasma states visually is inaccurate and subject to bias. Optical Emission Spectroscopy (OES) is used to examine the argon metastable population and is explored as a way to distinguish between the different plasma states. Experiments were conducted at high pressures (740-760 torr), .5-5 percent argon content and an electrode distance of around 3 mm, in which both glow and arc were achieved and observed with OES. Certain argon wavelengths notably changed in intensity between discharges. In addition to metastable population, electron temperature and density will be found using the cross-point method for broadening of hydrogen Balmer lines [1]. [1] J. Torres et al., J. Phys. D: Appl. Phys. 36 (2003)   

Spatiotemporal behavior of OH radical in AC-excited atmospheric pressure Ar plasma jet generated in open air

Reactive species generated by atmospheric pressure plasma jets (APPJs) play important roles to achieve very interesting results on the applications in various fields such as biotechnology, medicine, surface treatment, etc. In atmosphere, the gas-phase reactions caused by APPJ produce many kinds of reactive species in the gas-phase. Moreover, in the case of pulse-modulated APPJs, the composition of the species supplied to the sample would be changed temporally. As results, the reactions make it difficult to understand the mechanisms of surface reaction due to APPJ treatment. Therefore, spatiotemporal diagnosis of the reactive species generated by APPJ is required to clarify the gas-phase reactions and understand the mechanism of surface reaction. In this study, spatiotemporal measurement of gas-phase OH radical generated by an AC excited Ar gas APPJ in open air was performed by laser induced fluorescence (LIF) spectroscopy. From results, the highest LIF signal of OH radical was observed at the front edge of the APPJ. Moreover, it was found that the localized LIF signal moved toward the direction of gas downstream with the progress of discharge. The average shift speed of the localized position was about 1.3 mm/ms.   

A Spectroscopic Study of Discharge Species Produced in a Packed Bed Dielectric Barrier Discharge Reactor

Previous studies have shown that plasma formation and propagation in packed bed appear to occur in cyclic tandem as microdischarges followed by surface ionization waves and restrikes. Because the electron energy distribution can differ depending on the nature of the discharge, excited species and radicals formed within each discharge type may differ considerably. This effort utilizes emission spectroscopy to assess species production from neutral and ionized states of N$_{\mathrm{2}}$ as well as total emission intensity, thereby providing estimates of electron temperatures and electron density. Combined with results from modeling of the PBR with a plasma-hydrodynamic simulations simulation, scaling laws will be discussed for methods to optimize PBRs for chemical selectivity. This work is expected to yield a great deal of insight into the plasma-induced chemistry prevailing inside of packed bed reactors.   

Electric fields and electron properties in non-thermal atmospheric pressure plasmas in contact with different targets

This work brings absolute values for some of the fundamental properties of a non-thermal atmospheric pressure plasma, such as the electric field, electron density and temperature, in the presence of different targets. The plasma source in question is a kHz-driven Helium plasma jet characterized by low dissipated power (below or around 1 W), low gas temperature (below 40 degC), with gas flow speed between 3 and 10 m/s. The plume of the plasma jet is in contact with different targets, ranging from low permittivity materials such as glass, to water, to metal. The electric field is determined by optical emission spectroscopy on the forbidden He lines, the charge density on one type of targets by means of the Pockels' effect, and the electron density and temperature by means of Thomson scattering. The electrical properties of the target influence the plasma in the plume of the jet, as well the plasma on the target-plasma interface. High permittivity targets feature one or several return strokes, followed by hightened electron temperatures and densities, while the low permittivity targets cause the formation of fast surface ionization waves. The behaviour of the discharge as a function of the target properties is discussed, in the context of the values for the $E$, $n_e$ and $T_e$.\\ \\In collaboration with: Marlous Hofmans, Elmar Slikboer, Olivier Guaitella, LPP CNRS, Ecole Polytechnique, UPMC, Universite Paris-Saclay; Bart Klarenaar, Richard Engelin, Technische Univeristeit   

Cold atmospheric plasma jet in humid environment

The medical application of cold atmospheric plasma jet (CAPJ) is currently a hot research topic, especially for its selectivity during cancer treatment with limited healthy cell damage. However, the plasma jet in atmospheric is extremely sensitive to the surrounding environment, such as the Relative Humidity (RH). A CAPJ was tested with the RH varied from 35{\%} to 80{\%} and the discharge voltage varied from 4500V to 7000V. Two major trends of spatial averaged electron density ($n_{e})$ and Optical Emission Spectrum (OES) intensities of the CAPJ were found. When the discharge voltage is high, the increment of RH decreases $n_{e}$ but increase OES intensities. However, when the discharge voltage is low, both of them are reduced. The phenomena imply how RH reduces the electric field of a streamer and shifts the electron energy distribution function passing through an ionization threshold. These results suggest that the concentration of reactive species in CAPJ for cancer treatment may be reduced in actual surgery comparing with its values in the laboratory due to the high RH at the vicinity of tissue. An accurate RH control is thus required for a consistent plasma jet performance.   

Optical emission spectroscopy characterization of a kHz pulsed atmospheric pressure N$_2$ microwave plasma

Non-equilibrium plasmas have the potential of producing, energy efficiently, large amounts of species while operating at low gas temperatures. Pulsed power modulation is one of the tools for minimizing gas heating processes and increasing the overall non equilibrium of the plasma. In this contribution, we present a fundamental characterization of a kHz pulsed microwave discharge by optical emission spectroscopy in pure nitrogen at atmospheric pressure. The plasma is operated at 2.45 GHz and generated inside a cylindrical cavity coupled to a coaxial resonator. The coaxial resonator is designed with a metallic pin to enhance the electric field at the bottom of the cylindrical cavity. The microwave generator (0-3 kW peak power) can be pulsed in the 500 Hz-20 kHz range. A tangential gas flow injection (5-45 L/min) is used to stabilize the plasma in the center of the reactor. Rotational and vibrational temperatures of radiatively emitting species such as N$_2(C)$, N$_2(B)$ and N$_2^+(B)$ are measured both in continuous and pulsed operation regimes. The effects of gas flow and power variation on the spatial and temporal properties of the plasma are experimentally investigated. It is shown that power pulsing strongly affects the spatial distribution of the plasma inside the resonator.   

Variability of an Atmospheric Pressure Plasma Jet for Tissue Surface-Treatment

Medical surface-treatment using non-thermal atmospheric pressure plasma jets (APPJs) relies on the production of reactive oxygen and nitrogen species (RONS) in the aqueous medium surrounding the tissue. Reactive species in the liquid are maintained by solvation from the gas-phase and produced in situ. Using an APPJ operating in a He and O$_{\mathrm{2}}$-admixture, hydrogen containing reactive oxygen species (ROS) rely on humidity in the ambient and evaporation of H$_{\mathrm{2}}$O. Reactive nitrogen species (RNS) depend on entrainment and dissociation of N$_{\mathrm{2}}$ from the air environment. Aside from operational parameters (e.g., pulsing characteristics), many uses of APPJs involve hand-held procedures which introduce variation in electric field and gas flow fields; which may in turn alter RONS production. A computational investigation was conducted to characterize RONS production and transport for a He/O$_{\mathrm{2}}$ APPJ operating in humid air interacting with a liquid layer. An initial study of pulse repetition frequency and flow rate demonstrated the consequences of convection during the inter-pulse period on RONS production. APPJ characteristics and activation of the liquid will be discussed as a function of the angle of application, which affects both plume dynamics and environmental entrainment. The production of ROS and RNS scale differently, perhaps enabling some ability to tune their relative contributions.   

Discharge physics of an helium nanopulse discharge at atmospheric pressure

A nanopulse discharge is generated in atmospheric pressure helium inside a dielectric tube (id 2 mm) using two long linear electrodes painted on diametrically opposed sides of the tube. Applying a voltage of 3 kV with a pulse width of 340 ns at a repetition rate of 1 kHz allows to sustain a weak and diffuse discharge. Time and space-resolved He(3$^3$S $\to$ 2$^3$P) (706.4 nm) light emission distribution suggests a Townsend breakdown that last about 10 ns. The helium light distribution then quickly turns into the shape of a glow discharge and vanishes in about 50 ns. During the fall of the applied voltage, a glow discharge light emission pattern is again observed but no transition through a Townsend discharge is detected. In fact, with only 340 ns between the two discharge current peaks, memory effect should strongly impact the second discharge. In order to quantify the memory effect, the electron temperature (T$_e$) is determined from a collisional-radiative model coupled with optical emission spectroscopy of He ($n=3$) lines. With the help of a camera equipped with bandpass filters (around 667 and 728 nm), the time and spatially-resolved T$_e$ is obtained . While T$_e$ rises up to 8 eV during the first discharge it increases only up to about 2.5 eV during the second one.