Atmospheric pressure plasmas: A review

Gas discharge plasmas and their applications

Laser-induced breakdown spectroscopy (LIBS) has become a very popular analytical method in the last decade in view of some of its unique features such as applicability to any type of sample, practically no sample preparation, remote sensing capability, and speed of analysis The technique has a remarkably wide applicability in many fields, and the number of applications is still growing From an analytical point of view, the quantitative aspects of LIBS may be considered its Achilles' heel, first due to the complex nature of the laser–sample interaction processes, which depend upon both the laser characteristics and the sample material properties, and second due to the plasma–particle interaction processes, which are space and time dependent Together, these may cause undesirable matrix effects Ways of alleviating these problems rely upon the description of the plasma excitation-ionization processes through the use of classical equilibrium relations and therefore on the assumption that the laser-induced 

Laser-Induced Breakdown Spectroscopy (LIBS), Part I: Review of Basic Diagnostics and Plasma–Particle Interactions: Still-Challenging Issues Within the Analytical Plasma Community

Laser scattering provides a very direct method for measuring the local densities and temperatures inside a plasma. We present new experimental results of laser scattering on an argon atmospheric pressure microwave plasma jet operating in an air environment. The plasma is very small so a high spatial resolution is required to study the effect of the penetration of air molecules into the plasma. The scattering signal has three overlapping contributions: Rayleigh scattering from heavy particles, Thomson scattering from free electrons and Raman scattering from molecules. The Rayleigh scattering signal is filtered out optically with a triple grating spectrometer. The disentanglement of the Thomson and Raman signals is done with a newly designed fitting method. With a single measurement we determine profiles of the electron temperature, electron density, gas temperature, partial air pressure and the N2/O2 ratio, with a spatial resolution of 50 µm, and including absolute calibration.

/pdf/laser-scattering-on-an-atmospheric-pressure-plasma-jet-57jh2qt1p7.pdf

Laser scattering on an atmospheric pressure plasma jet: disentangling Rayleigh, Raman and Thomson scattering

The plasma needle is a source to create a non-thermal radiofrequency plasma at atmospheric pressure. To improve the ease of working on biological samples, a flexible plasma probe was designed. In the new configuration, the needle was confined in a plastic tube through which helium flow was supplied. The new set-up was characterized by impedance measurements and emission spectroscopy. Impedance measurements were performed by means of an adjustable matching network; the results were modelled. The discharge was found to be entirely resistive; the measured voltage was in the range 140–270 Vrms and it was in excellent agreement with model results. From the resistance, the electron density was estimated to be 1017 m−3.Optical measurements showed substantial UV emission in the range 300–400 nm. Active oxygen radicals ( and ) were detected. Furthermore, the influence of helium flow speed was investigated. At low flow speeds, the density of molecular species in the plasma increased.UV emission and density of active species are important factors that determine the performance of plasma in the treatment of biological materials. Therefore, the new characterization will help us to understand and optimize the interactions of atmospheric plasma with cells and tissues.

/pdf/electrical-and-optical-characterization-of-the-plasma-needle-3mgnyf4zzw.pdf

Electrical and optical characterization of the plasma needle

The behavior of molecules in microwave-induced plasmas studied by optical emission spectroscopy. 1. Plasmas at atmospheric pressure

On the electron temperatures and densities in plasmas produced by the "torche a injection axiale"

Plasmas created by the microwave torch Torche a Injection Axiale (TIA), which are around 2 mm in diameter and 15 mm long, are investigated. In these plasmas large gradients are present so that the edge is supposed to play an important role. Using global Thomson scattering measurements, in which global refers to the fact that the size of the laser beam is approximately equal to the diameter of the plasma, the electron densities and temperatures were determined. However, these results lead to discrepancies in the particle balance: the production of free electrons is much larger than the classical losses due to recombination, convection and diffusion. Radially resolved Thomson scattering measurements show the plasma has a hollow structure. Although this enhances the losses due to diffusion, still a large discrepancy remains between production and destruction of free electrons in the argon plasmas. Probably some molecular processes are significant as well. A good candidate is the charge transfer between argon ions and nitrogen molecules, since mixing with the surrounding air has a large impact on the plasma.

/pdf/steep-plasma-gradients-studied-with-spatially-resolved-2jyjqshk7n.pdf

Steep plasma gradients studied with spatially resolved Thomson scattering measurements

Diode laser absorption experiments have been performed on atmospheric microwave induced argon plasmas produced by the `Torche a Injection Axiale' in order to study the 4s 3P2 metastable densities and deduce plasma properties. It is found that the density is maximal in the active plasma zone (~1.5×1018 m−3) and rapidly decreases in the recombining plasma zone. In strongly ionizing atmospheric plasmas the 4s population can be used to estimate the electron temperature. The obtained value (1.3 eV) is considerably lower than the temperature found with Thomson scattering in a previous study (~2.0 eV). The very strong scattering of the laser spot on the plasma in the expansion zone just above the nozzle suggests that in this region the plasma is filamentary. Above this zone lateral measurements have been Abel-inverted. Hollow profiles, like those determined for the electron density in previous studies, have not been observed for the 4s distribution. It is found that the plasma zone with high 4s density is extended further downstream with increasing gas flow whereas the introduction of typically a few per cent of CO2 already results in a significant quenching of the metastables. The width of the absorption profile varies from typically 10 GHz in the active zone, where Van der Waals broadening is dominant, to 4 GHz in the recombining zone, where Doppler broadening becomes significant. The position of the central absorption frequency is found to be dependent on among other things the position in the plasma. If this shift is attributed to the Stark effect it is obtained that the maximum electron density is within (2–4)×1021 m−3. Since this latter value is rather high, it must be concluded that the Van der Waals effect induces a shift as well. The width and shift have been used to study trends in plasma properties (i.e. gas temperature and electron density) as a function of various parameters (i.e. position in the plasma, gas flow and gas composition). The observed tendencies are in agreement with findings in previous studies.

https://iopscience.iop.org/article/10.1088/0963-0252/12/3/305/pdf

Eah Eric Timmermans

Papers

The behavior of molecules in microwave-induced plasmas studied by optical emission spectroscopy. 1. Plasmas at atmospheric pressure

On the electron temperatures and densities in plasmas produced by the "torche a injection axiale"

Steep plasma gradients studied with spatially resolved Thomson scattering measurements

Plasma characterization of an atmospheric microwave plasma torch using diode laser absorption studies of the argon 4s 3P2 state

On the atomic state densities of plasmas produced by the “torche à injection axiale”