Abstract
The ASDEX Upgrade programme is directed towards physics
input to critical elements of the ITER design and the
preparation of ITER operation, as well as addressing
physics issues for a future DEMO design. After the
finalization of the tungsten coating of the plasma facing
components, the re-availability of all
flywheel-generators allowed high-power operation with up
to 20 MW heating power at Ip up to 1.2 MA. Implementation
of alternative ECRH schemes (140 GHz O2- and X3-mode)
facilitated central heating above ne = 1.2 * 1020 m-3 and
low q95 operation at Bt = 1.8 T. Central O2-mode heating
was successfully used in high P/R discharges with 20 MW
total heating power and divertor load control with
nitrogen seeding. Improved energy confinement is obtained
with nitrogen seeding both for type-I and type-III ELMy
conditions. The main contributor is increased plasma
temperature, no significant changes in the density
profile have been observed. This behaviour may be
explained by higher pedestal temperatures caused by ion
dilution in combination with a pressure limited pedestal
and hollow nitrogen profiles. Core particle transport
simulations with gyrokinetic calculations have been
benchmarked by dedicated discharges using variations of
the ECRH deposition location. The reaction of normalized
electron density gradients to variations of temperature
gradients and the Te/Ti ratio could be well reproduced.
Doppler reflectometry studies at the L-H transition
allowed the disentanglement of the interplay between the
oscillatory geodesic acoustic modes, turbulent
fluctuations and the mean equilibrium E * B flow in the
edge negative Er well region just inside the separatrix.
Improved pedestal diagnostics revealed also a refined
picture of the pedestal transport in the fully developed
H-mode type-I ELM cycle. Impurity ion transport turned
out to be neoclassical in between ELMs. Electron and
energy transport remain anomalous, but exhibit different
recovery time scales after an ELM. After recovery of the
pre-ELM profiles, strong fluctuations develop in the
gradients of ne and Te. The occurrence of the next ELM
cannot be explained by the local current diffusion time
scale, since this turns out to be too short. Fast ion
losses induced by shear Alfvén eigenmodes have been
investigated by time-resolved energy and pitch angle
measurements. This allowed the separation of the
convective and diffusive loss mechanisms
Original language | English |
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Article number | 094012 |
Journal | Nuclear Fusion |
Volume | 51 |
Issue number | 9 |
DOIs | |
Publication status | Published - 2011 |
MoE publication type | A1 Journal article-refereed |