Local edge parameters on the ASDEX Upgrade tokamak are investigated at the L-mode to H-mode transition, during phases with various types of edge-localized modes (ELMs), and at the density limit. A scaling law for the boundary electron temperature,e,b B 0.8 t I 0.5 p , is found which describes the H-mode threshold for deuterium-puffed discharges with favourable ion ∇B-drift direction. The region of stable operation is bounded by type I ELMs near the ideal ballooning limit and by a minimum temperature necessary to avoid thermal instability of the plasma edge. Stationary operation with type III ELMs imposes an upper limit on the edge temperature. Within the entire range of boundary densities investigated (n e,b 8 × 10 19 m −3 ), both L-mode and H-mode are found to be accessible. During type I ELMy H-mode, a relation of global confinement with the edge pressure gradient is found which is connected with a loss of the favourable density dependence predicted by the ITER-92P and ITER-93H ELMy H-mode scalings. At high density, better confinement is achieved in H-modes with an edge pressure gradient below the ideal ballooning limit, e.g. during type III ELMy H-mode with impurity-seeded radiation.

The so-called 'dithering cycles', i.e. periodic GH-L transitions, are Studied in the ASDEX Upgrade tokamak. The cycle repetition frequency Is .. 2 !dk; high-resolution ECE measuremenk show that T, changes during the cycle consistent with the assumption of the periodic build-up and destruction of the edge transport barrier characteristic of the Bmode. A study of the conditions under which the cycles occur reveals that with the unfavourable V B drift direction, the cycles are suppressed. These observations are interpreted in a schematic bifurcation diag", we find that the width of the bifurcation varies with the revenal of the V B drift.

The spatial distribution of density in a fusion experiment is of significant importance as it enters in numerous analyses and contributes to the fusion performance. The reconstruction of the density profile is therefore commonly done in offline data analysis. In this article, we present an algorithm which allows for density profile reconstruction from the data of the submillimeter interferometer and the magnetic equilibrium in real-time. We compare the obtained results to the profiles yielded by a numerically more complex offline algorithm. Furthermore, we present recent ASDEX Upgrade experiments in which we used the real-time density profile for active feedback control of the shape of the density profile.

Characteristics of the various types of edge localized modes (ELMs) observed during the H-mode of the ASDEX Upgrade tokamak are presented. Magnetic high-mode-number ELM precursor activity is found for type I ELMs and type III ELMs, the former only during neutral beam injection in the counter direction. The effect of ELMs on electron temperature and density profiles is discussed.