A Gamero

Experimental investigation and characterization of the departure from local thermodynamic equilibrium along a surface‐wave‐sustained discharge at atmospheric pressure

Surface‐wave‐sustained discharges (SWDs) form a particular class of high frequency (HF) discharges: their HF sustaining field is provided by a traveling wave that transfers energy as it propagates along the discharge column, yielding a plasma column with an axially decreasing electron density. SWDs have proved to be ideal for investigating experimentally and theoretically both the HF field and discharge aspects of HF plasma sources at reduced gas pressure. In this article, SWDs are utilized at atmospheric pressure to gain insight into the departure from thermodynamic equilibrium (TE) of HF sustained discharges. This departure is found to increase significantly as the electron density decreases along the plasma column whereas the gas temperature and the power absorbed per electron do not vary axially. The two‐temperature plasma model provides an adequate description of this departure from TE.

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

The emission of various low-pressure microwave-induced plasmas created and sustained by a surfatron or by a Beenakker cavity has been studied after the introduction of molecular species (i.e. N2, CO2, SF6 and SO2). Only nitrogen yielded observable emission from the non-dissociated molecule (first and second positive system). Using other gases only, emission of dissociation and association products has been observed (i.e. atomic species, CN, C2, CO, OH and NH). Studies of these intensities have been performed as functions of gas composition, pressure and position in the plasma and have provided an insight into molecular processes such as dissociation and association occurring in the plasma. It is found that parameters such as pressure and gas composition play a very important role with respect to these processes. Since no unambiguous relationship between the observed emission of dissociation or association products and the injected molecules has been found, it is established that it will be difficult to use microwave plasmas at reduced pressure as analytical excitation sources for molecular gas analysis.

An easy way to determine simultaneously the electron density and temperature in high-pressure plasmas by using Stark broadening

This paper discusses the possibility of determining, at the same time, both the electron density and temperature in a discharge produced at atmospheric pressure using the Stark broadening of lines spontaneously emitted by a plasma. This direct method allows us to obtain experimental results that are in good agreement with others previously obtained for the same type of discharge. Its advantages and disadvantages compared to other direct methods of diagnostics, namely Thomson scattering, are also discussed.

Spectroscopic study of a stationary surface-wave sustained argon plasma column at atmospheric pressure

Abstract A spectroscopic study of an argon surface wave plasma column at atmospheric pressure under steady-state conditions shows that we are dealing with a two-temperature (2T) discharge whose excitation kinetics are controlled by electron collisions (electron excitation kinetic plasma). A partial local Saha equilibrium in the excitation space is reached from which the 4s and 4p levels are excluded. The absorbed high frequency (HF) power per electron for the maintenance of the discharge (the parameter θ) was found to be a magnitude increasing with the electron density, contrary to what occurs at low pressure. Modifying the work conditions, such as gas flow and HF power, we obtained plasmas that are similar in structure when the end of the plasma is used as a reference position.

Determination of bromide by low power surfatron microwave induced plasma after bromine continuous generation.

A simple continuous flow generation of volatile bromine is described for the determination of low concentrations of the elements by atmospheric-pressure argon microwave induced plasma (MIP) surfatron. Bromine is continuously generated by mixing the bromide with sulphuric acid and hypochlorite solutions. The bromine vapor is separated from the aqueous phase by a gas-liquid separator and is desiccated by passing it through concentrated sulphuric acid. The detection limit attained was 2 microg/l. and the precision was +/-0.7% (at the 80 microg/l. level). The proposed determination is very selective if oxidizing/reducing agents are absent. The procedure has been tested for bromide determination in two drug preparations. Good agreement between the experimental results and the certified values has been obtained.

On the differences between ionizing helium and argon plasmas at atmospheric pressure

In this paper the electron density and temperature of atmospheric helium and argon plasmas operated under similar experimental conditions are compared. The conditions are chosen such that both plasmas are ionizing. It is found that a helium plasma has a higher electron temperature and a lower electron density than an equi-operational argon plasma, i.e. an argon plasma that is operated at the same external conditions. This is mainly caused by the higher excitation potential of the first excited state and the lower mass of helium, respectively. Due to these differences in electron density and temperature the densities of the helium ground state and of the excited states are much larger than their corresponding Saha equilibrium values for a wide range of conditions. The consequence of this is that the spectroscopic methods, which are used to determine the electron density and temperature, have a very limited validity region in the case of helium. For argon the deviations are much smaller so that these methods can often be applied safely.

Preliminary spectroscopic experiments with helium microwave induced plasma produced in air by use of a new structure: the axial injection torch

Abstract In this experimental study we have used spectroscopic methods to characterize helium plasma obtained by means of a novel waveguide-fed microwave plasma torch at atmospheric pressure, the axial injection torch. This device produces a “plasma flame” by coupling high frequency (HF) power at 2.45 GHz to the discharge. Various flame parameters (namely the electron density number and the electron and gas temperatures) have been determined by using spectroscopic diagnostic techniques that provided an estimate in terms of the helium flow rate, absorbed HF power and axial position in the experiments. These preliminary results suggest some departure from local thermodynamic equilibrium (LTE) and seem to indicate the utility of the discharge as an excitation source for emission spectroscopy. Comparison with other microwave torches already described in the literature is made in terms of the electron density and the electron and gas temperature.

A novel method to determine the electron temperature and density from the absolute intensity of line and continuum emission: application to atmospheric microwave induced Ar plasmas

An absolute intensity measurement (AIM) technique is presented that combines the absolute measurements of the line and the continuum emitted by strongly ionizing argon plasmas. AIM is an iterative combination of the absolute line intensity–collisional radiative model (ALI–CRM) and the absolute continuum intensity (ACI) method. The basis of ALI–CRM is that the excitation temperature T13 determined by the method of ALI is transformed into the electron temperature Te using a CRM. This gives Te as a weak function of electron density ne. The ACI method is based on the absolute value of the continuum radiation and determines the electron density in a way that depends on Te. The iterative combination gives ne and Te. As a case study the AIM method is applied to plasmas created by torche a injection axiale (TIA) at atmospheric pressure and fixed frequency at 2.45 GHz. The standard operating settings are a gas flow of 1 slm and a power of 800 W; the measurements have been performed at a position of 1 mm above the nozzle. With AIM we found an electron temperature of 1.2 eV and electron density values around 1021 m−3. There is not much dependence of these values on the plasma control parameters (power and gas flow). From the error analysis we can conclude that the determination of Te is within 7% and thus rather accurate but comparison with other studies shows strong deviations. The ne determination comes with an error of 40% but is in reasonable agreement with other experimental results.

Atmospheric microwave-induced plasmas in Ar/H2 mixtures studied with a combination of passive and active spectroscopic methods

Several active and passive diagnostic methods have been used to study atmospheric microwave-induced plasmas created by a surfatron operating at a frequency of 2.45 GHz and with power values between 57 and 88 W. By comparing the results with each other, insight is obtained into essential plasma quantities, their radial distributions and the reliability of the diagnostic methods. Two laser techniques have been used, namely Thomson scattering for the determination of the electron density, ne, and temperature, Te, and Rayleigh scattering for the determination of the heavy particle temperature, Tg. In combination, three passive spectroscopic techniques are applied, the line broadening of the Hβ line to determine ne, and two methods of absolute intensity measurements to obtain ne and Te. The active techniques provide spatial resolution in small plasmas with sizes in the order of 0.5 mm. The results of ne measured with three different methods show good agreement, independent of the plasma settings. The Te values obtained with two techniques are in good agreement for the condition of a pure argon plasma, but they show deviations when H2 is introduced. The introduction of a small amount (0.3%) of H2 into an argon plasma induces contraction, reduces ne, increases Te, enhances the departure from equilibrium and leads to conditions that are close to those found in cool atmospheric plasmas.

Experimental investigation of the electron energy distribution function (EEDF) by Thomson scattering and optical emission spectroscopy

The electron temperature of an argon surface wave discharge generated by a surfatron plasma at intermediate pressures is measured by optical emission spectroscopy (OES) and Thomson scattering (TS). The OES method, namely absolute line intensity (ALI) measurements gives an electron temperature which is found to be (more or less) constant along the plasma column. TS, on the other hand, shows a different behaviour; the electron temperature is not constant but rises in the direction of the wave propagation. In the pressure range of this study, it is theoretically known that deviations from Maxwell equilibrium are expected towards the end of the plasma column. In this paper, we propose a combination of methods to probe the electron energy distribution function (EEDF) in this relatively high-pressure regime. The ALI method combined with a collisional–radiative model allows one to measure the effective (Maxwellian) creation temperature of the plasma while TS measures the mean electron energy of the EEDF. The differences between the two temperature methods can be explained by the changes in the form of the EEDF along the plasma column. A strong correlation is found with decreasing ionization degree for different pressures. Numerical calculations of the EEDF with a Boltzmann solver are used to investigate the departure from a Maxwellian EEDF. The relatively higher electron temperature found by TS compared with the ALI measurements is finally quantitatively correlated with the departure from a Maxwellian EEDF with a depleted tail. (Some figures may appear in colour only in the online journal)