Analysis of fast ion induced instabilities in tokamak plasmas

TL;DR

This paper investigates the non-linear evolution of energetic particle-driven instabilities, specifically BAEs and EGAMs, in tokamak plasmas, developing a new amplitude reconstruction method and analyzing mode structure changes during plasma ramp-up.

Contribution

It introduces an advanced amplitude reconstruction technique using short time Fourier transform for analyzing transient mode structures in tokamak plasmas.

Findings

Radial eigenfunction changes are minimal for downward chirping BAEs.

EGAMs show a shrinkage of mode structure during upward chirping.

Results are consistent with theoretical predictions.

Abstract

In magnetic confinement fusion devices like tokamaks, it is crucial to confine the high energy fusion-born helium nuclei ($\alpha$-particles) to maintain the energy equilibrium of the plasma. However, energetic ions can excite various instabilities which can lead to their enhanced radial transport. Consequently, these instabilities may degrade the heating efficiency and they can also cause harmful power loads on the plasma-facing components of the device. Therefore, the understanding of these modes is a key issue regarding future burning plasma experiments. One of the main open questions concerning energetic particle (EP) driven instabilities is the non-linear evolution of the mode structure. In this thesis, I present my results on the investigation of $\beta$-induced Alfv\'{e}n eigenmodes (BAEs) and EP-driven geodesic acoustic modes (EGAMs) observed in the ramp-up phase of off-axis NBI heated plasmas in the ASDEX Upgrade tokamak. These modes were well visible on several line-of-sights (LOSs) of the soft X-ray cameras which made it possible to analyse the time evolution of their spatial structure. In order to investigate the radial structure, the mode amplitude has to be determined on different LOSs. I developed an advanced amplitude reconstruction method which can handle the rapidly changing mode frequency and the low signal-to-noise ratio. This method is based on short time Fourier transform which is widely applied in the thesis, because it is ideal to investigate the time evolution of transient wave-like phenomena. The radial structure analysis showed that in case of the observed downward chirping BAEs the changes in the radial eigenfunction were smaller than the uncertainty of the measurement, while in case of rapidly upward chirping EGAMs the analysis shows shrinkage of the mode structure. These experimental results are shown to be consistent with the corresponding theory.