Experimental study on kinetic instabilities in electron cyclotron resonance heated plasma

Abstract

Electron Cyclotron Resonance Ion Sources (ECRISs) are used to produce Highly Charged Ions (HCI). Ions are created in a magnetised plasma sustained by the resonant interaction of electron gyrofrequency with an externally applied Radio Frequency (RF) electromagnetic wave. It has been established that the ECRIS plasmas exhibit kinetic plasma instabilities, limiting the parameter space available for optimising the extracted beam currents. The thesis is devoted to gaining insights into the mechanisms underlying the formation of these kinetic instabilities on various ECR ion sources. The thesis is divided into six chapters. The first two chapters deal with the relevance of the work in the context of various scientific fields and the background theoretical knowledge required for understanding the processes involved in electron cyclotron resonance plasma production, along with the background on kinetic instabilities. The third chapter is devoted to the technical aspects of ECR ion sources, using one of the ion sources utilised in the experiments as a representative example, as well as an in-depth discussion of the diagnostic methods employed during the course of the dedicated experimental campaigns. One of the goals of this work is to refine the understanding of the physical mechanisms determining the complex relation between the ECRIS magnetic field parameters and the transition from stable to unstable operation regime. As a result, a computational tool was developed to efficiently calculate the threedimensional magnetic field structure in ECRIS and extract the relevant magnetic field parameters from the closed iso-magnetic field surface where the electron’s gyrofrequency equals the frequency of an externally applied electromagnetic wave. Chapter four details the development of this computational tool. Additionally, the same chapter has a comprehensive description of the signal processing code used to conduct statistical analysis on a huge amount of data collected during one of the experimental campaigns devoted to the temporal study of instability-induced electromagnetic emissions. The fifth chapter summarises all experimental campaigns and the major findings obtained. Several of the general characteristics of the ECRIS magnetic field were determined using magnetic field computations in five different ion sources. A relationship between the energy of hot electrons and the average magnetic field gradient parallel to the magnetic field lines on the resonance surface has been established through experiments conducted with superconducting SECRAL-II ion source at IMP-CAS, China. The effect of magnetic field configuration on managing the hot electron population and consequently controlling the instability was established through studies conducted at GANIL, France, utilising the GTS ion source. The ECRIS at JYFL, Finland was used to conduct an experimental campaign which established a relationship between plasma bremsstrahlung and the energy distribution of electrons lost from confinement, as well as the effect of kinetic instabilities on the shape of energy distribution of lost electrons. Another experimental campaign with JYFL 14 GHz ECR ion source revealed a temporal relation between pulsated emission of electromagnetic wave during instability with the strength of the emitted signal by undertaking a comprehensive statistical analysis of the collected data. The experiment also revealed a grouping (temporally) of electromagnetic emissions due to instability which were previously observed in space and magnetospheric plasma. The final chapter provides a comprehensive discussion of all the experiments performed along with the scope of future research in this field. Keywords: ECR ion source, plasma physics, plasma instabilities, magnetically confined plasmas, plasma bremsstrahlung, electron energy distribution.