(EX/4-1Rb) Control of MHD Instabilities by ECCD: ASDEX Upgrade Results and Implications for ITER

H. Zohm1), G. Gantenbein2), F. Leuterer1), A. Manini1), M. Maraschek1), Q. Yu1)
1) Max-Planck-Institute fuer Plasmaphysik, Garching, Germany
2) Forschungszentrum Karlsruhe, IHM, Karlsruhe, Germany

Abstract.  MHD instabilities limit the operational space of tokamaks; their control is therefore of great interest for present day and future tokamaks, such as ITER. Based on recent success in control of sawteeth and Neoclassical Tearing Modes (NTMs), ECCD is foreseen as an MHD control tool in ITER. Hence, it is important to expand our physics base of MHD control by ECCD and to verify the control strategies proposed for ITER in present day experiments. In this paper, we report on recent experiments in ASDEX Upgrade in this area. Experiments on sawtooth tailoring of plasmas with dominant NBI heating have been performed with broad and narrow deposition of ECCD, indicating a better control with the narrower deposition profile. Using Bt-ramps, it was possible to map out precisely the different deposition regions suited for either stabilization or destabilization of sawteeth. The effects of co and ctr-ECCD as opposed to pure heating have clearly been documented. In addition, ICRH is used to study the fast particle stabilisation of sawteeth, exploring the possibility to approach the ITER situation where alpha-particles are supposed to have a large stabilising effect on sawteeth. In the area of NTM control, experiments with optimised deposition, maximising the figure of merit ηNTM = jECCD/jbs, verify the relevance of this figure of merit, which has been adopted by ITER for the optimisation of the NTM stabilization system and have increased significantly the βN range in which (2,1) NTMs can be completely stabilized in ASDEX Upgrade. Another crucial point for ITER is the question if injection has to be phased with the O-point of the island or can be just DC. Experiments in ASDEX Upgrade with an artificially broadened deposition profile mimic the situation in ITER, where it is expected that the marginal island size will be significantly smaller than the deposition. These experiments, conducted at Wmarg/d = 0.5, whereas previous experiments had Wmarg/d = 1 and ITER is expected to have Wmarg/d = 0.1-0.5, indicate an advantage of modulated injection, also in the FIR regime of NTMs at higher βN. The experiments are accompanied by modelling using a fully nonlinear resistive MHD code as well as an analysis based on the modified Rutherford equation in order to enable the prediction for ITER. Finally, we will also discuss the next steps undertaken at ASDEX Upgrade in this area.

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