[Back]

A. Kallenbach, J. Schweinzer, M. Willensdorfer, E. Wolfrum et al.:
"Overview of ASDEX Upgrade results";
Nuclear Fusion, 51 (2011), 0940121 - 09401211.

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 20MW 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 heatingwas successfully used in high P/R discharges with 20MWtotal 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 Alfv´en eigenmodes
have been investigated by time-resolved energy and pitch angle measurements. This allowed the separation of the
convective and diffusive loss mechanisms.