Electron heating, time evolution of bremsstrahlung and ion beam current in electron cyclotron resonance ion sources

Abstract

This thesis is a study of Electron Cyclotron Resonance Ion Source (ECRIS) plasmas and their properties. The focus has been on time evolution studies of bremsstrahlung emission, ion beam current production and numerical studies of electron heating in ECRIS plasmas. The time scales for reaching steady state bremsstrahlung production at electron energies greater than 30 keV is shown to be on the order of several hundreds of milliseconds. The ion beam currents of different elements are shown to reach steady state before the bremsstrahlung production. It is also demonstrated that by tuning the RF pulse patterns, the so-called preglow transient ion beam currents can be utilized while the amount of bremsstrahlung radiation can be significantly reduced. In numerical work it has been found, for example, that the number of high energy electrons decreases when the microwave power absorption has been modeled. The thesis is divided in the following way: Chapter 1 is a short introduction to the ECR ion source field including motivation to electron heating studies; Chapter 2 contains basic properties of plasma physics that are needed to understand the behaviour of plasmas; Chapter 3 introduces the reader to ECR ion sources, operational principles and mechanisms dictating the production of highly-charged ions; Chapter 4 concentrates on electron heating via microwaves, describing limitations in the heating processes. The model for the electromagnetic (EM) wave absorption and the description of the simulation code developed in this thesis work are presented. In addition, properties of some of the existing electron heating / ECRIS codes are discussed. Chapter 5 deals with the bremsstrahlung production, discusses some of the previous bremsstrahlung measurements and presents the experimental apparatus. Chapter 6 contains the summary of the numerical and experimental results, and Chapter 7 concludes the thesis.