J Jeroen Jonkers

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.

On the electron temperatures and densities in plasmas produced by the “torche à injection axiale”

Abstract The electron temperature and the electron density of plasmas created by the “Torche a Injection Axiale” (TIA) are determined using Thomson scattering. In the plasma with helium as the main gas, temperatures of around 25 000 K and densities of between 0.64 and 5.1 × 1020m−3 are found. In an argon plasma the electron temperature is lower and the electron density is higher: 17 000 K and around 1021 m−3 respectively. From these results it can be established that the ionisation rates of both plasmas are much larger than the recombination rates, which means that the plasmas are far from Saha equilibrium. However, deviations from a Maxwell electron energy distribution function, as reported for the “Microwave Plasma Torch” (MPT), are not found in the TIA. The excitation and ionisation power of the TIA appears to be stronger than that of the MPT.

THE EXCITATION TEMPERATURE IN (HELIUM) PLASMAS

Measuring excitation temperatures is a widely used way to characterise plasmas. In this paper we show that in most (atmospheric) helium plasmas this temperature is hardly related to any plasma quantity. It will appear that due to the strong non-equilibrium character of these plasmas the excitation temperature is lower than the electron temperature. The reason for the fact that, in particular, helium plasmas are far from equilibrium is the easy ambipolar diffusion of helium ions. Together with the relatively high electron temperature (necessary to sustain a helium plasma) this results in an excitation temperature, which strongly depends on the ionisation energies of the levels used to determine this temperature, rather than on the electron temperature. Since due to the specific atomic structure of helium only a limited amount of excited levels can be observed, always more or less the same excitation temperature is found.

CRModel : A general collisional radiative modeling code

This paper describes CRModel, a general collisional radiative modeling code written in C++. It uses a flexible plain text input file to describe all relevant aspects of the considered plasma region, including collisional cross sections and radiative decay probabilities. This allows the model to be used for a wide range of atomic or ionic plasma species. The types of results that the model generates, also specified in the input file, include atomic state densities and ionization and recombination coefficients. CRModel was developed on Linux/UNIX workstations, but can also be used on the PC (i.e. Windows) platform.

Steep plasma gradients studied with spatially resolved Thomson scattering measurements

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.

Absorption measurements on a low-pressure, inductively coupled, argon - mercury discharge for lighting purposes: 1. The gas temperature and argon metastable states density

The gas temperature and the absolute density of the argon level in an 80 W inductively coupled low-pressure argon - mercury plasma are determined for three different argon filling pressures. This is done by measuring the line profile of the transition in argon, using a tuneable laser diode. Since the width of this argon line is found to depend on the kinetic heavy particle temperature only, radial profiles of the gas temperature can be obtained. It turns out that the maximum gas temperature in this discharge (550 - 810 K depending on the filling pressure) is significantly higher than that in a common tubular fluorescent lamp. From the radial distribution of the argon 4s density it can be concluded that the maxima of the electron density and of the electron temperature are situated close to the coil. It is also found that the position of these maxima depends on the argon filling pressure.

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

Abstract The atomic state densities of helium and argon plasmas produced by the microwave driven plasma torch called the “torche a injection axiale” are presented. They are obtained by absolute line intensity measurements of the excited states and by applying the ideal gas law to the ground state. It will be shown that the atomic state distribution function (ASDF) does not obey the Saha-Boltzmann law: the ASDF cannot be described by one temperature. From the shape of the ASDF it can be concluded that the plasma is ionising. By extrapolating the measured state densities towards the ionisation limit, a minimum value of the electron density can be determined.

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.

A collisional radiative model for mercury in high-current discharges

A collisional radiative model is presented for mercury discharges with electron temperatures between 0.75-2 eV and electron densities between 1018-1020 m-3. Such plasma parameters are encountered in a number of modern light sources, such as mercury-operated induction lamps and the compact fluorescent lamp. The analytical top model has been used, which allows the majority of the non-equilibrium levels to be taken into account implicitly. As a result, indirect ionization processes involving highly excited atomic mercury states are taken into account in spite of the relatively low number of levels (19) which has been considered. The influence of higher atomic mercury levels on the ionization rate coefficient has been carefully analysed, and has been found to contribute significantly. Furthermore, a consistent means of quantifying the production of radiation by the plasma will be presented by introducing the specific effective emissivities. These enable one to express the total radiated power in terms of the densities of the transport-dominated states and the electron density and temperature. These coefficients, as well as the net coefficients of ionization and recombination, will be presented and discussed, enabling their usage in plasma transport models.

Excitation balances and transport properties in atmospheric microwave-induced plasmas studied by power interruption experiments

Atmospheric microwave-induced argon plasmas with and without analyte injection have been exposed to power interruption experiments in order to study transport processes and to reveal dominant excitation balances. From the time-dependent behaviour of line intensities due to electron cooling and quenching during the power interruption, it is found that electron loss channels, such as diffusion, convection and the dissociative recombination of molecular ions, are much larger than for inductively coupled plasmas. It is found that in the ionizing part of the plasma electron dominated mechanisms are responsible for the population of radiative levels. Significant changes in the responses to power interruption are observed when small amounts of molecular compounds are injected (>0.5%), probably due to a decrease of the electron density. Furthermore, it is found that in the recombination zone downstream in the plasma an electron-independent excitation mechanism, probably thermal excitation, is responsible for the population of radiative levels of analytes with relatively low excitation energies. From the downstream propagation of a disturbance created in the ionizing part of the plasma the local axial gas velocity has been determined. In the analyte excitation zone of the plasma typical velocities are around 25 m s-1, whereas in the recombining zone velocities of 12-18 m s-1 are obtained.