A Sola

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 the Excitation Temperature in a Nonthermodynamic-Equilibrium High-Pressure Helium Microwave Plasma Torch

This paper analyzes the influence of the departure from thermodynamic equilibrium on three spectroscopic methods for determining the excitation temperature: the classical Boltzmann plot method, the method of absolute intensity measurements, and the method of the ratio of atomic to ionic lines for one element. Use of these methods is illustrated with experiments conducted in a helium microwave plasma in nonthermodynamic equilibrium produced by an axial injection torch at atmospheric pressure. The excitation temperature was determined at a variable microwave power and different positions along the flame. The results are discussed in terms of the atomic-state distribution function (ASDF) calculated for the plasma.

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.

Atomic emission spectroscopy for the on-line monitoring of incineration processes

A diagnostic measurement system based on atomic emission spectroscopy has been developed for the purpose of on-line monitoring of hazardous elements in industrial combustion gases. The aim was to construct a setup with a high durability for rough and variable experimental conditions, e.g. a strongly fluctuating gas composition, a high gas temperature and the presence of fly ash and corrosive effluents. Since the setup is primarily intended for the analysis of combustion gases with extremely high concentrations of pollutants, not much effort has been made to achieve low detection limits. It was found that an inductively coupled argon plasma was too sensitive to molecular gas introduction. Therefore, a microwave induced plasma torch, compromising both the demands of a high durability and an effective evaporation and excitation of the analyte was used as excitation source. The analysis system has been installed at an industrial hazardous waste incinerator and successfully tested on combustion gases present above the incineration process. Abundant elements as zinc, lead and sodium could be easily monitored.

Effective recombination coefficients in argon surface-wave-produced plasma

The authors present experimental data for the effective recombination coefficient in a pulsed surface-wave-produced plasma. To determine these data they studied the relaxation of electron density to equilibrium. A 2.45 GHz Surfatron was used in order to couple the HF energy, and three different Pyrex cylindrical tubes of internal radius a=1.5, 2.5 and 4.5 mm, with a corresponding external radius b=a+1.5, were employed. The gas used in the discharge was argon, in the 0.5-12 Torr pressure range. They obtained the total losses by ambipolar diffusion and recombination, divided by pressure, as a function of the quotient between the effective surface wave electric field and gas pressure. At the lower end of the pa range diffusion losses dominate but recombination becomes more important for higher values of pa.