Density fluctuations in I-mode discharges in ASDEX Upgrade are studied. The I-mode specific weakly coherent mode (WCM) appears at the transition from L to I-mode. The WCM but also the turbulence in general are strongly modulated by a low frequency mode which can be related to the geodesic acoustic mode (GAM). The GAM induces an energy transfer away from the central WCM frequency, indicating an underlying instability responsible for the WCM. During the I-mode magnetic fluctuations close to the WCM frequency are intensified, which can be assigned to the geodesic Alfvénic oscillation. The geodesic Alfvénic oscillation is present already in L-mode, does not follow changes of frequency of the WCM, therefore it is not responsible for the WCM.

Forced magnetic reconnection is a topic of common interest in astrophysics, space science and magnetic fusion research. The tearing mode formation process after sawtooth crashes implies the existence of this type of magnetic reconnection and is investigated in great detail in the ASDEX Upgrade tokamak. The sawtooth crash provides a fast relaxation of the core plasma temperature and can trigger a tearing mode at a neighbouring resonant surface. It is demonstrated for the first time that the sawtooth crash leads to a dominantly ideal kink mode formation at the resonant surface immediately after the sawtooth crash. Local measurements show that this kink mode transforms into a tearing mode on a much longer timescale Magnetic reconnection is one of the fundamental processes in magnetized plasmas and central to the understanding of the conversion of magnetic into thermal and kinetic energy in astrophysics, space science and magnetic confinement research. Such plasmas are often nearly collisionless, and the reconnection rates are thus much shorter than predicted by resistive MHD theory. The main difference between astrophysical and magnetic confinement plasmas is the existence of a strong magnetic guide field in the latter case [1]. This paper deals with forced magnetic reconnection in magnetic fusion plasmas. In tokamak plasmas, large sawtooth crashes typically produce fast relaxations of the core plasma density and temperature and provide the drive for magnetic reconnection at the neighbouring resonant surfaces with safety factor q=m/n, where m and n are integer numbers and represent the poloidal and toroidal mode numbers, respectively. Magnetic reconnection rearranges the magnetic topology at the resonant surface. It can start either from noise perturbations if the gradient of the plasma current at the resonant surface provides the drive for a tearing mode [2], or it requires an external drive at the start. The second situation is more common for neoclassical tearing modes (NTMs), which require a seed island to grow. The drive for the tearing mode is usually provided by background MHD instabilities (sawteeth, ELMs, etc.) [3,4,5]. When the critical island width is reached, the pressure profile becomes flat within the island, and the neoclassical drive takes over. Small islands are not able to provide significant pressure flattening and might be driven by other mechanisms [6,7]. In this paper, only the mechanism of the seed island formation by strong internal drive due to sawteeth is investigated in detail. This type of tearing mode formation is considered to be one of the most dangerous for future fusion reactors like ITER [8], because large sawteeth provide the strongest internal magnetic perturbations compared to other possible triggers and are able to trigger the mode already at very small normalized pressure values [3,4]. Previous observations from different tokamaks, for example from JET [9] or TCV [10], report large island widths directly after the crash based on analysis of magnetic and SXR measurements. Such ...

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The transitions from type-I to small edge localized modes (ELMs) and back are studied by the electron cyclotron emission imaging (ECEI) diagnostic on ASDEX Upgrade (AUG). ECEI measurements show that the average poloidal velocity of temperature fluctuations of both type-I ELM onsets and small ELMs is the same and is close to 5-6 km/s. Radially, the temperature fluctuations are distributed in the same narrow region of 2 cm in between 0.975 ρ pol 1.025 with associated poloidal mode numbers m = 96 ± 18 and toroidal mode numbers n = 16 ± 4. The observed fluctuations related to both type-I ELMs and small ELMs vary over the transition simultaneously, however, showing slightly different behaviour. The similarities between type-I ELMs and small ELMs observed on AUG suggest that they both have the same nature and evolve together. In the transition phase a temperature fluctuation mode ('inter-ELM mode') appears, which becomes continuous in the mitigated ELM phase and might cause the ELM mitigation. The mode characteristics (velocities, frequencies and wave-numbers) obtained in the analysis can be further used for the direct comparison in various code simulations.