Zero-dimensional models for plasma chemistry

Abstract

In this thesis two different models for examining the chemistry in a plasma are studied. Both are zero dimensional, meaning that the configuration and transport aspects of plasmas are combined and reduced to frequencies. The first type of models are Collisional Radiative Models (CRM), in which the atomic state distribution function can be seen as being composed by contributions of a limited number of atomic states, typically the atom and ion ground state. Transitions between levels are facilitated by electrons and photons. The second type of models are Global Plasma Models (GPM), whose goal it is to predict mean values of internal plasma parameters as a function of external control parameters. Collisional Radiative Models The tasks of a CRM are threefold: calculating the ASDF, the effective conversion frequencies, and the source terms for the energy equation. In a general exploration, the derivation is described to structure the various collisional and radiative processes into simple matrix equations. Initially, a Quasi Steady State (QSS) solution is pursued, but the results are extended to a time-dependent solution. The properties of the agents facilitating the transitions (electrons and photons), are allowed to be of a transient nature. This description of aCRMis implemented in the Plasimo framework; a plasma modeling platform. The addition of a CRM in the form of a model-plugin to Plasimo are described, as well as the details for constructing and solving the CRM, both for the case of QSS and transient plasmas. The Plasimo CRM is used in two applications: a CRM of an Extreme UltraViolet (EUV) driven plasma in Ar, and a Laser Induced Fluorescence experiment in Ar plasmas. Both models are time-dependent; in the first application due to the electrons created by an EUV pulse, in the second application due to a laser pulse. The CRM of the EUV driven plasma gives insight into the time dependent spectrum of the plasma. It is shown that optical emission spectroscopy can be used to monitor the electron energy in the plasma, and thereby the sputtering action of ions. The CRM of an Ar LIF experiment helps in understanding the mechanisms that follow a perturbation imposed on the Ar system by a laser pulse. The results of the model are compared to the results of experiments on a Surfatron Induced Plasma in which the electron density and temperature are well known. Global Plasma Models A GPM is also implemented as a model plugin for Plasimo. As in the CRM plugin, the species densities are modeled, but this is extended with the modeling of the electron density and electron energy balance. The model is a collection of chemical reactions, and the external control parameter is the input power density. This model plugin is used to model a High Power Impulse Magnetron Sputtering plasma. Although the model is zero dimensional, sputtering is included in the form of a frequency of particle density entering the plasma; the same as dealing with other transport aspects. The model plugin is a valuable tool in studying the relevant processes in the creation of species.