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(TH/P3-16) Peculiarities of Destabilization of Alfvén Modes by Energetic Ions in Stellarators

V.V. Lutsenko¹⁾, Ya.I. Kolesnichenko¹⁾, Yu.V. Yakovenko¹⁾, O.P. Fesenyuk¹⁾, A. Weller²⁾, A. Werner²⁾, H. Wobig^2)
 
1) Institute for Nuclear Research, Kyiv, Ukraine
²⁾ Max-Planck-Institut fur Plasmaphysik, IPP-EURATOM Association, Garching, Germany

Abstract.  A theory of Alfvén instabilities caused by the energetic ions in helical devices is developed, and specific examples relevant to Wendelstein-line stellarators (W7-AS, W7-X, a Helias reactor) are considered. In particular, it is shown that in stellarators there exist specific ``non-axisymmetric'' resonances, which may strongly affect the instabilities, especially, in W7-X and the Helias reactor, which are characterized by the rotational transform about unity. These resonances are associated with non-axisymmetric components of the magnetic field strength. Destabilization of Alfvén eigenmodes located in specific gaps of helical devices is studied. The effect of plasma rotation on these instabilities is analysed. The Doppler shift of the eigenmodes characterized by several coupled wave numbers is calculated. Instabilities of Energetic Particle Modes (EPM) are discussed. It is shown that the presence of ``non-axisymmetric'' resonances and additional gaps in the Alfvén continuum of stellarators result in a number of new characteristic frequencies of the EPM modes. An explanation of the temporal evolution of MHD (Alfvénic) activity observed in neutral beam experiments on W7-AS is suggested.

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