Abstract. An overview of the H-mode threshold power in ASDEX Upgrade which addresses the impact of the tungsten versus graphite wall, the dependences upon plasma current and density, as well as the influence of the plasma ion mass is given. Results on the H-L back transition are also presented. Dedicated L-H transition studies with electron heating at low density, which enable a complete separation of the electron and ion channels, reveal that the ion heat flux is a key parameter in the L-H transition physics mechanism through the main ion pressure gradient which is itself the main contribution to the radial electric field and the induced flow shearing at the edge. The electron channel does not play any role. The 3D magnetic field perturbations used to mitigate the ELMs are found to also influence the L-H transition and to increase the power threshold. This effect is caused by a flattening of the edge pressure gradient in the presence of the 3D fields such that the L-H transitions with and without perturbations occur at the same value of the radial electric field well, but at different heating powers.
Abstract. The installation of a new poloidal charge exchange recombination spectroscopy (CXRS) diagnostic at ASDEX Upgrade (AUG) has enabled the determination of the radial electric field, , using the radial force balance of impurity ions.has been derived from charge exchange (CX) spectra measured on different impurity species, such as He 2+ , B 5+ , C 6+ and Ne 10+ . The resulting profiles are found to be identical within the uncertainties regardless of the impurity species used, thus, demonstrating the validity of the diagnostic technique. The profile has been compared to the main ion pressure gradient term, which is found to be the dominant contribution at the plasma edge, thus, supporting that the well is created by the main ion species. The profile has been measured in different confinement regimes including L-, I-and H-mode. The depth of the well and the magnitude of the shear are correlated with the ion pressure at the pedestal top. The temporal evolution of the measured CX profiles and the resulting have been studied during an ELM cycle. At the ELM crash, the minimum is less deep resulting in a reduction of the E×B shear. Within 2 ms after the ELM crash, the edge kinetic profiles have nearly recovered and the well is observed to recover simultaneously. In high density type-I ELM mitigated H-mode plasmas, obtained via externally applied magnetic perturbations with toroidal mode number = 2, no clear effect on due to the magnetic perturbations has been observed.
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