J.J.A.M. van der Mullen

Excitation Equilibria in Plasmas; A Classification

The rich nature of the plasma state manifests itself in the large variety of studies focused on the atomic state distribution function (ASDF). Many specific calculations dedicated to various exemplary plasma situations can be found in literature. In this study we follow and continue the line of Bibermanl, Fujimoto2, and Seaton3 by trying to find a classification in these results4. The central item is to find a (analytical) relation between the ASDF and the underlying plasma properties. We confine ourselves to those situations in which the excitation kinetics is ruled by (Maxwellian) electrons in atomic plasmas. Molecular processes will not be considered.

PLASIMO, a general model: I. Applied to an argon cascaded arc plasma

A non-LTE argon cascaded arc plasma is studied and modelled with the general plasma simulation program PLASIMO. The structure of PLASIMO is flexible and transparent, so that apart from the study given in the present paper several other multicomponent stationary plasmas in a wide pressure range ( to 1 bar), from local thermal equilibrium (LTE) to non-LTE, and with different energy coupling mechanisms can be simulated as well. The modular structure is divided into three main parts: the transport part which forms the heart of the model, the plasma configuration part, and the composition part. The latter two parts define the input parameters for the transport part and are controlled by the PLASIMO user. The three parts are again divided into separate modules. The strong modularity makes PLASIMO easy to handle and easy to adjust or expand. Results of PLASIMO applied on the cascaded arc are compared with experimental data and show reasonable agreement. The influence of the boundary conditions on the simulation results is discussed.

On the atomic state distribution function in inductively coupled plasmas—II: The stage of local thermal equilibrium and its validity region

This paper, the second in the series, deals with the study of the impact of different types of equilibrium departures on the atomic state distribution function in plasmas. Global trends are studied and attention is paid to the first stage of equilibrium departure: the stage of local thermal equilibrium (LTE).

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.

A novel collisional radiative model with a numerical bottom and an analytical top

Abstract A cut-off procedure for numerical collisional radiative (CR) models is developed and is based on knowledge of the excitation flow in the collision-dominated part of an atomic system as revealed by analytical models. This new cut-off technique makes it possible to reduce the number of levels in a numerical CR-model drastically in comparison with the older, numerical cut-off technique used by Bates et al, in which the truncation level was demanded to be in Saha equilibrium. The model is applied to an ionizing and recombining argon plasma as an example of a more general technique which can also be applied to other atomic systems. The results are in good agreement with the analytical top model developed previously in our group and also with the experiments and numerical calculations of Vlcek and Pelikan.

On the atomic state distribution function in inductively coupled plasmas—I. Thermodynamic equilibrium considered on the elementary level

This is the first part of a series of articles dealing in a tutorial way with the atomic state distribution function in plasmas, especially inductively coupled plasmas. Attention is paid to the influence of the various types of equilibrium departures on this function. The deviation from its equilibrium value, i.e. the Saha-Boltzmann distribution function is traced back to the imbalance on the elementary level. The various stages of equilibrium departure are related to the dominance of specific kind of processes ordered in a hierarchy. This first paper of the series treats the state of thermodynamic equilibrium (TE). Serving as a starting position for the study of equilibrium departure, TE is presented as a collection of elementary balances, all in equilibrium.

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)

Revision of the criterion to avoid electron heating during laser aided plasma diagnostics (LAPD)

A criterion is given for the laser fluency (in J/m2) such that, when satisfied, disturbance of the plasma by the laser is avoided. This criterion accounts for laser heating of the electron gas intermediated by electron-ion (ei) and electron-atom (ea) interactions. The first heating mechanism is well known and was extensively dealt with in the past. The second is often overlooked but of importance for plasmas of low degree of ionization. It is especially important for cold atmospheric plasmas, plasmas that nowadays stand in the focus of attention. The new criterion, based on the concerted action of both ei and ea interactions is validated by Thomson scattering experiments performed on four different plasmas.

Demixing in a metal halide lamp, results from modelling

Convection and diffusion in the discharge region of a metal halide lamp is studied using a computer model built with the plasma modelling package Plasimo. A model lamp containing mercury and sodium iodide is studied. The effects of the total lamp pressure on the degree of segregation of the light emitting species are examined and compared to a simpler model with a fixed temperature profile. Significant differences are observed, justifying the use of the more complete approach.

An elemental diffusion description for LTE plasma models

A novel method to describe diffusive processes in plasmas in local thermodynamic equilibrium (LTE) was developed, based on the transport of elements instead of individual species. This method combines the elegance of the LTE description of a chemical composition with the flexibility of explicit transport for each element. A simple model of a metal halide lamp containing Hg dosed with NaI is used to illustrate the method.