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.

Limit-cycle oscillations (LCOs) close to the power threshold of L-to H-mode transitions are investigated in plasmas of ASDEX Upgrade. During this phase, referred to as I-phase, a strong magnetic activity in the poloidal magnetic fieldḂ θ with an up-down asymmetry is found. In some cases, the regular LCOs during I-phase transition smoothly into a phase with intermittent bursts which have similar properties to type-III edge localised modes (ELMs). Indications of precursors during the intermittent phase as well as in the regular LCO phase point to a common nature of the I-phase and type-III ELMs. The LCO frequency measured in a set of discharges with different plasma currents and magnetic fields scales as f ∼ (B 1/2 t I 3/2 p )/(nT ).

Abstract.Ion temperature gradient (ITG) and trapped electron modes (TEM) are two important micro-instabilities in the plasma core region of fusion devices (r/a ≤ 0.9). They usually coexist in the same range of spatial scale (around 0.1 < k ⊥ ρ i < 1), which makes their discrimination difficult. To investigate them, one can perform gyrokinetic simulations, transport analysis and phase velocity estimations. In Tore Supra, the identification of trapped electron modes (TEM) is made possible due to measured frequency fluctuation spectra. Indeed, turbulent spectra generally expected to be broad-band can become narrow in case of TEM turbulence, inducing "quasi-coherent" (QC) modes named QC-TEM. Therefore the analysis of frequency fluctuation spectra becomes a possible tool to differentiate TEM from ITG. We have found indications that the TEM can have a QC signature by comparing frequency fluctuation spectra from reflectometry measurements, gyrokinetic simulations and synthetic diagnostic results. Then the scope of the analysis of QC-TEM are discussed and an application is shown, namely transitions between TEM turbulence and MHD fluctuations.

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