Destabilisation of TAE modes using ICRH in ASDEX Upgrade

In ASDEX Upgrade, toroidicity induced Alfven eigenmodes (TAEs) are destabilised by ICRH in conventional and advanced scenarios, at low density. Most unstable TAEs have toroidal modes numbers (n=3,4,5,6) and experiments with reversed current and magnetic field showed that the TAE propagate in the current direction. On one hand, the analysis of the unstable TAE in ASDEX Upgrade shows that the data is consistent with the results from previous studies performed in other tokamaks. In particular, the measured TAE frequency, in the range (150-200kHz), is consistent with theoretical TAE frequency calculated for the parameters of the discharges performed in these experiments. On the other hand, some interesting new features have also been observed in the ASDEX Upgrade data. TAE (n=-1) was observed, which propagates in the opposite direction to the plasma current. It was also concluded that plasma rotation is insufficient to explain the experimentally observed frequency differences between two adjacent toroidal mode numbers. The measured relative fluctuation amplitude of the TAE eigenfunction in the soft X-rays channels increases towards the plasma edge. These results are consistent with the ideal MHD calculations and show that the TAE are of a global nature and not core localised TAE modes. These results are particularly important, because the radial extent of AE is a key factor in the redistribution of the energetic ions in the presence of unstable TAE. In advanced Tokamak scenarious, no evidence of Alfven Cascades was observed in experiments with ICRH minority heating in the early phase of the discharge. The effect of Electron Cyclotron Current Drive (ECCD) on the TAE amplitude was studied by applying different levels of ECCD in similar plasma configurations, with equivalent ICRH power. It was observed that ECCD has a slight destabilizing effect on the TAE. (author)