A new diagnostic for ASDEX Upgrade edge ion temperatures by lithium-beam charge exchange recombination spectroscopy

This thesis work investigates the measurement of ion temperatures
at the edge of a magnetically confined plasma used for fusion
research at the ASDEX Upgrade tokamak operated by
Max-Planck-Institut für Plasmaphysik in Garching. The tokamak is
the most advanced concept in toroidal magnetic confinement fusion.
The H-mode plasma regime, default scenario of the next step
experiment ITER, is characterized by an edge transport barrier,
which is not yet fully explained by theory. Experimentally measured
edge ion temperature profiles will help to test and develop models
for these barriers. Transport theory on a basic level is introduced
as background and motivation for the new diagnostic. The standard
model for an edge plasma instability named "edge localized mode"
(ELM) observed in H-mode is described. The implementation of a new
diagnostic for ion temperature measurements with high spatial
resolution in the plasma edge region, its commissioning and the
validation of the measurements comprises the main part of this
work. The emission of line radiation induced by charge exchange
processes between lithium atoms injected by a beam source and fully
ionized impurities (of C and He) is observed with a detection
system consisting of spectrometers and fast cameras. Due to the
narrow beam (1 cm) and closely staggered optical fibers (6 mm),
unprecedented spatial resolution of edge ion temperatures in all
major plasma regimes of the ASDEX Upgrade tokamak was achieved. The
spectral width of the line radiation (He II at 468.5 nm and C VI at
529.0 nm) contains information about the local ion temperature from
thermal Doppler-broadening, which is the dominant broadening
mechanism for these lines. The charge-exchange contribution to the
total line radiation locally generated by the lithium is determined
by gating the beam. Fitting a Gaussian model function to the local
line radiation results in absolute line widths which can be
directly converted into a temperature. The equilibration of
impurities with the main plasma is fast enough that the assumption
of nearly identical temperatures as the main plasma is justified.
Corrections for systematic line broadening effects from collisional
mixing and Zeeman broadening are incorporated by model calculations
using existing routines for the involved atomic physics. Time
resolution of the diagnostic is still not suffcient to resolve ELM
events, but measuring between ELMs is possible if their frequency
is low. L-mode plasmas with and without additional heating can be
reliably diagnosed with a time resolution depending on the lithium
beam intensity and plasma density, in best cases down to 100 ms. It
was shown that diagnostic He puffing can be used to enhance the
signal-to-noise ratio. Results from L-mode plasmas with electron
heating show that ion temperatures can be significantly different
from electron temperatures at the edge. For the verification of the
new ion temperatures, comparison with data from already established
diagnostics was done. In neutral beam heated L-mode and various
H-mode plasmas the ion temperatures agree with those from a similar
diagnostic measuring in the core using heating beams where both
diagnostics overlap. They can be combined to form a complete ion
temperature profile over the whole plasma radius. In a first
application, transport coefficients have been determined by
interpretative modeling for an ohmic plasma. In summary, a new
method for measuring ion temperatures in the edge of a magnetically
confined fusion plasma has been established. The results provide an
important input to further understanding of transport in these
plasmas.