Jjam Joost van der Mullen

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.

Spatially resolved gas temperature measurements by Rayleigh scattering in a microwave discharge

Rayleigh scattering is used in order to deduce the gas temperature from neutral density measurements inside a microwave discharge in argon. Rayleigh scattering is a powerful non-intrusive method which provides direct heavy particle density measurements with a very high spatial resolution. The gas pressure ranges from 1 to 40 torr and the microwave power ranges from 35 to 900 W. We show that temperature gradient is very sharp at the edge of the plasma column: Tg drops to nearly room temperature within 3 cm. In contrast, Tg is nearly constant in the bulk of the plasma region. The gas temperature is determined over a large range of power and pressure conditions. We show that the gas temperature in the centre of the tube, ranging from 300 to 2500 K, is a linear function of the averaged power density per unit of length. Analytical calculations are in good agreement with experimental results.

A multi-domain boundary-relaxation technique for the calculation of the electromagnetic field in ferrite-core inductive plasmas

A technique is discussed for calculating the electromagnetic field in two-dimensional inductive plasmas with an arbitrary number of magnetic materials and load coils. The method is a generalization of the boundary-relaxation technique for systems with an arbitrary number of conducting regions, and extends its applicability to systems which contain magnetically active materials. Each material is defined on a distinct ortho-curvilinear numerical structured mesh. This facilitates the coupling of the electromagnetic model with the calculation of material transport, in which typically only one of these grids is involved. The method allows an exact and straightforward formulation of the boundary conditions and has the additional advantage that only current-carrying regions of space need to be discretized. The technique has been verified by running the model for a number of test cases for which analytical solutions exist, and later applied to a geometry which is characteristic for plasma induction lamps. A selection of these results will be demonstrated and discussed.

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.

General treatment of the interplay between fluid and radiative transport phenomena in symmetric plasmas: the sulphur lamp as a case study

A general ray-trace method for calculating the effects of radiative transfer in a control volume (CV) fluid code is presented. The method makes use of the structured CV grid of the fluid code, and is suited for geometries with a point or axis of symmetry. In particular, the specific equations for spherical and cylindrical (without z dependence) configurations are developed. The application of this method to local thermal equilibrium (LTE) and non-LTE plasma models is discussed. Various opportunities for sacrificing precision for calculation speed are pointed out. As a case study, the effects of radiative transfer in a sulphur lamp are calculated. Since an LTE description of the molecular radiation yields a computed spectrum that differs significantly from a measured one, the possibility of a non-LTE vibrational distribution of the radiating S2-B state is investigated. The results indicate that the vibrational populations may be inversed.

A microwave plasma model for a PCVD setup

A numerical model is constructed using the PLASIMO toolkit to simulate a microwave configuration which is similar to that which is in use for optical glass fibre production. The simulations offer the flow patterns and the electromagnetic (EM) energy incoupling of a two-temperature argon plasma. The Yee algorithm was used for the EM module, whereas the Semi implicit method for pressure linked equations algorithm was used to calculate the pressure and velocity field. It is found that at 400 W coupling of 2.46 GHz EM radiation in 1000 Pa argon results in a plasma that is not in local thermodynamic equilibrium, in the sense that Te/Th ≥ 4 and the ionization degree is smaller than that predicted by the Saha equations. The model results are subjected to various sanity checks.

Axial mercury segregation in direct current operated low-pressure argon–mercury gas discharge: Part II. Model

Due to cataphoresis, axial segregation of mercury will occur when the gas discharge of a fluorescent lamp is operated by means of a direct current. A consequence of this is a non-uniform axial luminance distribution along the lamp. To determine the degree of axial mercury segregation experimentally, axial luminance distributions have been measured which are converted into axial mercury vapour pressure distributions by an appropriate calibration method. The mercury segregation has been investigated for variations in lamp tube radius (3.6–4.8 mm), argon buffer gas pressure (200–600 Pa) and lamp current (100–250 mA) at mercury vapour pressures set at the anode in the range from 0.2 to 9.0 Pa. From the experiments it has been concluded that the mercury vapour pressure gradient at any axial position for a certain lamp tube diameter, argon pressure and lamp current depends on the local mercury vapour pressure. This observation is in contrast to assumptions made in earlier modelling publications in which one mercury vapour pressure gradient is used for all axial positions. By applying a full factorial design, an empirical relation of the mercury segregation is found for any set of parameters inside the investigated parameter ranges.

Semiclassical and quantum-mechanical descriptions of S2 molecular radiation

A semiclassical theory to calculate diatomic molecular radiation emission and absorption coefficients is presented in some detail. The theory is applied to the S2 {B} 3Σu-→X 3Σg- transition and the results are compared with a quantum-mechanical calculation. We show that disregarding fine structure, the semiclassical results compare very well with the average results of the quantum-mechanical theory. We conclude that the semiclassical theory is recommendable when fine structure is not important since it requires less detailed data about the molecular states and transition and its results can be computed faster.

Disturbed Bilateral Relations: a guide for plasma characterization and global models

The study of the competition between equilibrium disturbing and equilibrium restoring mechanisms, reveals that the various equilibrium departures as found in different plasmas, have much in common. They can be seen as disturbances of bilateral relation; relations effectuated by forward and corresponding backward processes in the sense of detailed balancing. The deviations from the equilibrium form of the atomic state distribution function and the electron energy distribution function of atomic plasmas, which are the result of the escape of photons and electron-ion pairs, can be given in a simple equation in which the escape per balance time plays a leading role. The same idea can be used to construct a characterisation method that relates external control parameters to average values of internal plasma properties such as the electron density, electron temperature and the gas temperature. The global discharge model presented here includes gas heating and is applicable to atomic plasmas for which convection can be neglected.

Collisional radiative models with multiple transport-sensitive levels - application to high electron density mercury discharges

In this paper some of the basic concepts in collisional radiative modelling of plasmas will be generalized. A mathematical framework is presented which is suitable for systems with an arbitrary number of transport sensitive and quasi-steady or local chemistry states. The mathematical formulation presented here leads to straightforward extensions of quantities which have been previously introduced for systems in which only the atom and ion ground states were dealt with explicitly. These are the net coefficients of ionization and recombination, the effective specific emission coefficients, and the relative overpopulation coefficients. For a given set of transport-sensitive densities these quantities can be used to calculate the particle and radiation source terms and the atomic state distribution function. Furthermore the use of matrix-vector calculus has led to concise, insightfull, yet general expressions. And although some explicit references will be made to plasmas which are governed primarily by processes involving electrons, most of the theory presented here can be carried over to other systems without great difficulty. As an example, a collisional radiative model for mercury will be presented for discharges with electron temperatures between 0.75 eV and 2 eV and electron densities between 1018 m-3 and 1020 m-3. In the current model six transport-sensitive levels have been assumed. Another 13 excited mercury states are taken into account implicitly; ladderlike excitation and ionization will be shown to be of major importance for discharges in this parameter range. The model allows the incorporation of heavy-particle reactions and a full treatment of the transfer of resonant radiation.