Experimental studies and modelling of high radiation and high density plasmas in the ASDEX Upgrade tokamak

Fusion plasmas contain impurities, either intrinsic originating
from the wall, or injected willfully with the aim of reducing power
loads on machine components by converting heat flux into radiation.
The understanding and the prediction of the effects of these
impurities and their radiation on plasma performances is crucial in
order to retain good confinement. In addition, it is important to
understand the impact of pellet injection on plasma performance
since this technique allows higher core densities which are
required to maximise the fusion power. This thesis contributes to
these efforts through both experimental investigations and
modelling. Experiments were conducted at ASDEX Upgrade which has a
full-W wall. Impurity seeding was applied to H-modes by injecting
nitrogen and also medium-Z impurities such as Kr and Ar to assess
the impact of both edge and central radiation on confinement. A
database of about 25 discharges has been collected and analysed. A
wide range of plasma parameters was achieved up to ITER relevant
values such as high Greenwald and high radiation fractions.
Transport analyses taking into account the radiation distribution
reveal that edge localised radiation losses do not significantly
impact confinement as long as the H-mode pedestal is sustained. N
seeding induces higher pedestal pressure which is propagated to the
core via profile stiffness. Central radiation must be limited and
controlled to avoid confinement degradation. This requires reliable
control of the impurity concentration but also possibilities to act
on the ELM frequency which must be kept high enough to avoid an
irreversible impurity accumulation in the centre and the consequent
radiation collapse. The key role of the ELM frequency is confirmed
also by the analysis of N+He discharges. Non-coronal effects affect
the radiation of low-Z impurities at the plasma edge. Due to the
radial transport, the steep temperature gradients and the ELM flush
out, a local equilibrium cannot be establish an the radiation
increases in this region. To account for these effects, an
empirical non-coronal model was developed which takes the impurity
residence time at the pedestal into account. The validity of this
assumption was verified by modelling the evolution of the
impurities and radiation for ASDEX Upgrade H-modes with nitrogen
seeding by coupling the ASTRA transport code with STRAHL. The
time-dependent simulations include impurity radiation due to
nitrogen and tungsten and the transport effects induced at the edge
by the ELMs. The modelling results have been validated against the
experimental data. The modelled radiation profiles show a very good
agreement with the measured ones over both radius and time. In
particular, the strong enhancement of the nitrogen radiation caused
by non-coronal effects through the ELM-induced transport is well
reproduced. The radiation properties of tungsten are very weakly
influenced by non-coronal effects due to the faster equilibration.
W radiation, which is highly dependent on the Elm frequency,
strongly increases when this is decreased, due to the lack of
sufficiently strong flush out of this impurity. This is in
agreement with the experimental observations and indicates that
maintaining high ELM frequency is essential for the stability and
performance of the discharges. Analyses of the high density
scenario with pellets indicate that several processes take place
when pellets are injected into the plasma. In particular, due to
their cooling effect, the temperature drops as soon as pellets are
injected. This is compensated by an increase in density. These
processes occur mainly at the edge and are propagated to the core
via stiffness. This explains why the confinement stays
approximately constant during the whole discharge. Both experiments
and transport calculations reveal that the energy confinement time
is independent of the density indicating that the currently used
scaling is not valid in this regime. The results of this thesis
will contribute towards an extension of the confinement scaling
which is currently being undertaken.