Analysis of the H-mode density limit in the ASDEX Upgrade tokamak using bolometry

The high confinement mode (H-mode) is the operational scenario
foreseen for ITER, DEMO and future fusion power plants. At high
densities, which are favourable in order to maximize the fusion
power, a back transition from the H-mode to the low confinement
mode (L-mode) is observed. This H-mode density limit (HDL) occurs
at densities on the order of, but below, the Greenwald density. In
this thesis, the HDL is revisited in the fully tungsten walled
ASDEX Upgrade tokamak (AUG). In AUG discharges, four distinct
operational phases were identified in the approach towards the HDL.
First, there is a stable H-mode, where the plasma density increases
at steady confinement, followed by a degrading H-mode, where the
core electron density is fixed and the confinement, expressed as
the energy confinement time, reduces. In the third phase, the
breakdown of the H-mode and transition to the L-mode, the overall
electron density is fixed and the confinement decreases further,
leading, finally, to an L-mode, where the density increases again
at a steady confinement at typical L-mode values until the
disruptive Greenwald limit is reached. These four phases are
reproducible, quasi-stable plasma regimes and provide a framework
in which the HDL can be further analysed. Radiation losses and
several other mechanisms, that were proposed as explanations for
the HDL, are ruled out for the current set of AUG experiments with
tungsten walls. In addition, a threshold of the radial electric
field or of the power flux into the divertor appears to be
responsible for the final transition back to L-mode, however, it
does not determine the onset of the HDL. The observation of the
four phases is explained by the combination of two mechanisms: a
fueling limit due to an outward shift of the ionization profile and
an additional energy loss channel, which decreases the confinement.
The latter is most likely created by an increased radial convective
transport at the edge of the plasma. It is shown that the four
phases occur due to a coupling of these two mechanisms. These
observations are in line with studies made at AUG with carbon
walls, although in those discharges the energy loss was most likely
caused by the full detachment of the divertor. The density of the
HDL depends only weakly on the plasma current, unlike the Greenwald
limit, and can be increased by high heating power, again unlike the
Greenwald limit. The triangularity of the plasma has no influence
on the density of the HDL, though improves the performance of the
plasma, since the onset of the degrading H-mode phase occurs at
higher densities. It is explicitly shown that the HDL and also the
L-mode density limit are determined by edge parameters. Using
pellet fueling, centrally elevated density profiles above the
Greenwald limit can be achieved in stable H-modes at a moderate
confinement. Future tokamaks will have intrinsic density peaking.
Consequently, they will most likely operate in H-modes above the
Greenwald limit.