Numerical Simulations of Double Layers in Plasmas

Abstract

In this thesis, a numerical study of double layers in plasma is presented. Double layers are structures consisting of two oppositely charged space charge layers, creating a finite change in electrostatic potential over the double layer. The motivation for the topic is found in the general interest for magnetospheric physics, where double layers are known to accelerate particles into the ionosphere, creating the aurora, as seen at higher latitudes. In the first part of this thesis, theories and observations of double layers as a plasma physics phenomena are studied. Two double layer related phenomena, the Buneman instability and adiabatic cooling, are presented and their expected influence on double layers given. Then, through the introduction of BGK-solutions and the Water-bag model, a one dimensional, time stationary, electrostatic model of double layers is formed. In the second part of this thesis, the theories discussed in the first part are imple- mented into a one dimensional numerical model of double layers. The numerical model emphasizes strong (i.e. Buneman regime) double layers, and their existence is investi- gated for a large number of different plasma and boundary conditions. The results of the numerical model can confirm important observations made by previ- ous numerical and experimental studies, like the scaling law, Bohm criterion and presence of two-stream (i.e. Buneman) instabilities. The present numerical model improves on these studies by introducing simulations with varying combinations of boundary con- ditions, and a wider selection of plasma conditions for which double layers have been simulated. For the numerical studies of this thesis, a one dimensional Particle-in-Cell code was designed and developed. A thorough description of the program is given in the main part of this thesis, and the source code is shown in the appendix.