Abstract
The aim of nuclear fusion energy research is to demonstrate the feasibility of nuclear fusion reactors as a future energy source. The
tokamak is the most advanced fusion machine to date, and is most likely
the first system to be converted into a reactor.
An important subject of nuclear fusion research is the
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study of the
equilibrium and stability of a plasma with respect to large scale
displacements. In a tokamak, several instabilities can occur. A class of
edge instabilities that occur in the high confinement regime, H-mode,
have been called Edge Localised Modes (ELMs). ELMs are relaxation
oscillations that cause quasiperiodic energy and particle losses out of
the confined plasma into the scrape-off layer. These losses are of
concern for future burning fusion plasmas, such as ITER, due to the
large transient heat loads expected on plasma facing components in
contact with the scrape-off layer. These heat loads may reduce the
target lifetime below tolerable levels.
Although the existence of ELMs has been known for many years, their
physics is not well understood yet. Much effort has been spent
world-wide in an attempt to improve the understanding of these
instabilities. A review of the present state of ELM research is given.
Empirically, at least three types of ELMs have been identified, which
are normally classified as type-I, type-II and type-III ELMs.
From the point of view of plasma stability, research has increasingly
focussed on the role of certain MHD instabilities, namely (finite-n)
ballooning and kink (peeling) modes, as well as coupled ballooning-kink
modes, leading to the proposition of a theoretical model called the
peeling-ballooning cycle. This thesis presents new insight into ELMs
obtained from the analysis of experimental data in the JET tokamak, and
compares the observations with present theoretical ELM models.
Low frequency coherent type-I ELM precursor modes have been identified.
Their properties are studied in detail. Precursors with low toroidal
mode numbers are known to be external kink instabilities, while
experimental findings and their comparison with stability calculations
suggest that the precursor modes with higher toroidal mode numbers are
not pure external kinks but coupled ballooning-kink modes. In spite of
their regular occurrence, there is no evidence that the precursor mode
growth rate accelerates rapidly before the ELM. This is regarded as an
indication that external kink modes or coupled peeling-ballooning modes
do not trigger the ELMs, which would contradict present theoretical ELM
models.
Another type of instability are the Washboard (WB) modes, a very common
edge instability regularly observed in the H-mode regime in JET. Since
their discovery in the late 90s, rather little attention has been paid
to them, either theoretically or experimentally. So far they have not
been regarded as an ELM-relevant instability. However, evidence for
their involvement in the pedestal and ELM dynamics is presented.
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