G D Stancu

Atmospheric pressure plasma diagnostics by OES, CRDS and TALIF

Nanosecond repetitively pulsed (NRP) discharges were used to generate atmospheric pressure plasmas in air or nitrogen preheated at 1000?K. In order to understand the physico-chemical mechanisms that control the number densities of active species, in situ optical diagnostic techniques were developed. The ground state of atomic oxygen was measured by two-photon absorption laser induced fluorescence (TALIF), the density of N2(A) was measured by cavity ring down spectroscopy (CRDS) and the densities of N2(B) and N2(C) were measured by optical emission spectroscopy (OES). Temporally and spatially resolved density measurements were performed in the main operating regimes of the NRP discharge, namely the diffuse and filamentary regimes. The diagnostic techniques and associated challenges are presented and the effects of these discharges on the chemistry are discussed.

Experimental study of the hydrodynamic expansion following a nanosecond repetitively pulsed discharge in air

We report on an experimental study of the hydrodynamic expansion following a nanosecond repetitively pulsed (NRP) discharge in atmospheric pressure air preheated up to 1000 K. Single-shot schlieren images starting from 50 ns after the discharge were recorded to show the shock-wave propagation and the expansion of the heated gas channel. The temporal evolution of the gas temperature behind the shock-front is estimated from the measured shock-wave velocity by using the Rankine-Hugoniot relationships. The results show that a gas temperature increase of up to 1100 K can be observed 50 ns after the nanosecond pulse.

Argon metastables in HiPIMS: time-resolved tunable diode-laser diagnostics

Time-resolved tunable diode-laser absorption spectroscopy measurements were performed on the argon metastable (Ar-m) level 3s(2)3p(5)(P-2(3/2)degrees)4s excited at 801.478 nm, in the dense plasma r ...

Time-Resolved CRDS Measurements of the N2(A3Σu+) Density Produced by Nanosecond Discharges in Atmospheric Pressure Nitrogen and Air

Cavity ring-down spectroscopy (CRDS) is used to measure the number density of N2(A3Sigmau+) metastables produced by nanosecond repetitively pulsed discharges in nitrogen and air preheated at 1000 K and atmospheric pressure. The densities of N2(A) are inferred from the absorbance of the Q1(22) and Q3(16) lines of the (2 <-- 0) vibrational band of the first positive system (B3Pig - A3Sigmau+) of N2 at 769.945 nm. The procedure for determining the temporal evolution of the density of metastable from the measured ring down signals is presented. The maximum number densities are in the range of 10(14)-10(15) molecules cm-3 for air and nitrogen discharges, respectively. In nitrogen, the decay of the N2(A) density is shown to be a second-order process with a rate coefficient of 1.1 x 10(-9) cm3 s-1 at 1600 K with a factor of 2 uncertainty. In air, the decay is estimated to be 1 order of magnitude faster than that in nitrogen owing to quenching by atomic and molecular oxygen. Furthermore, the rotational temperature is determined by comparison of CRDS measurements and simulations of several rotational lines of the (2 <-- 0) band of the first positive system of N2 between 769.8 and 770.7 nm. The rotational and vibrational temperatures are also determined by comparison of optical emission measurements and simulations of the second positive system of N2 between 365 and 385 nm. In these CRDS measurements, we achieved a temporal resolution down to 50 ns.

Study of an H2/CH4 moderate pressure microwave plasma used for diamond deposition: modelling and IR tuneable diode laser diagnostic

Infra-red tuneable diode laser spectroscopy (IR TDLAS) has been used to detect and quantify the methyl radical and three stable carbon-containing species (CH4, C2H2 and C2H6) in a moderate pressure microwave (f = 2.45 GHz) bell-jar reactor used for diamond films deposition. A wide range of experimental conditions was investigated, with typical pressure/power required to perform diamond deposition, i.e. pressure from 2500 to 12 000 Pa and power from 600 W to 2 kW, which means gas temperatures ranging from 2200 to 3200 K, when the power density increases from 9 to 30 W cm−3. Since TDLAS is a line of sight averaged technique, the analysis of the experimental data required the use of a one-dimensional non-equilibrium transport model that provides species density and gas temperature variations along the optical beam. This model describes the plasma in terms of 28 species/131 reactions reactive flow. The thermal non-equilibrium is described by distinguishing a first energy mode for the electron and a second one for the heavy species. Parametric studies as a function of power density and methane percentage in the gas mixture are presented. The good agreement obtained between measurement and one-dimensional radial calculations allows a validation of the thermo-chemical model, which can be used as a tool to enlighten the chemistry in the spatially non-uniform H2/CH4 microwave discharge used for diamond deposition. This is especially of interest for high power density discharge conditions that remain poorly understood.

Quantitative detection of methyl radicals in non-equilibrium plasmas: a comparative study

Tuneable infrared diode laser absorption spectroscopy at 16.5??m and broadband ultraviolet absorption spectroscopy at 216?nm have both been used to measure the ground state concentrations of the methyl radical in two different types of non-equilibrium microwave plasmas (f = 2.45?GHz): (i) H2?Ar plasmas of a planar reactor with small admixtures of methane or methanol, at a pressure of 1.5?mbar, and (ii) H2?CH4 plasmas of a bell jar reactor, at pressures of 25 and 32?mbar under flowing conditions. For the first time, two different optical techniques have been directly compared to verify the available data about absorption cross sections and line strengths of the methyl radical. It was found that application of the CH3 absorption cross section of the transition at 216?nm, reported by Davidson et al (1995 J. Quant. Spectrosc. Radiat. Transfer 53 581) and of the line strength of the Q(8,8) line of the ?2 fundamental band near 16.44??m, given by Wormhoudt et al (1989 Chem. Phys. Lett. 156 47), leads to satisfactory agreement.

Photocatalyst activation in a pulsed low pressure discharge

The effect of combining plasma and photocatalyst for Volatile Organic Compounds (VOC) removal was investigated in a pulsed low-pressure dc discharge. The photocatalyst was TiO2 while the VOC was acetylene (1000 ppm) diluted in dry air. The temporal evolution of C2H2 concentration was measured by Tunable Diode Laser Absorption Spectroscopy (TDLAS) in the mid-infrared region during the plasma pulse (one second). The contribution of external ultraviolet radiation and plasma exposure were quantified, both with and without a photocatalyst. The synergetic effect was clearly demonstrated.

Review on VUV to MIR absorption spectroscopy of atmospheric pressure plasma jets

Absorption spectroscopy (AS) represents a reliable method for the characterization of cold atmospheric pressure plasma jets. The method?s simplicity stands out in comparison to competing diagnostic techniques. AS is an in situ, non-invasive technique giving absolute densities, free of calibration procedures, which other diagnostics, such as laser-induced fluorescence or optical emission spectroscopy, have to rely on. Ground state densities can be determined without the knowledge of the influence of collisional quenching. Therefore, absolute densities determined by absorption spectroscopy can be taken as calibration for other methods. In this paper, fundamentals of absorption spectroscopy are presented as an entrance to the topic. In the second part of the manuscript, a review of AS performed on cold atmospheric pressure plasma jets, as they are used e.g. in the field of plasma medicine, is presented. The focus is set on special techniques overcoming not only the drawback of spectrally overlapping absorbing species, but also the line-of-sight densities that AS usually provides or the necessity of sufficiently long absorption lengths. Where references are not available for measurements on cold atmospheric pressure plasma jets, other plasma sources including low-pressure plasmas are taken as an example to give suggestions for possible approaches. The final part is a table summarizing examples of absorption spectroscopic measurements on cold atmospheric pressure plasma jets. With this, the paper provides a ?best practice? guideline and gives a compendium of works by groups performing absorption spectroscopy on cold atmospheric pressure plasma jets.

Images of a Nanosecond Repetitively Pulsed Glow Discharge Between Two Point Electrodes in Air at 300 K and at Atmospheric Pressure

For many applications of atmospheric pressure plasmas, a crucial issue is to obtain glow discharges at 300 K. We have generated such plasmas with a nanosecond repetitively pulsed method. We present experimental and simulated optical emission images of the dynamics of the formation of the glow regime at the early stages of its development.

Argon metastables in HiPIMS: validation of the ionization region model by direct comparison to time resolved tunable diode-laser diagnostics

The volume plasma interactions of high power impulse magnetron sputtering (HiPIMS) discharges operated with a Ti target is analyzed in detail by combining time-resolved diagnostics with modeling of ...