1 EXW/P7-20 Dynamics of fast ions during sawtooth oscillations in the TEXTOR toka- mak measured by collective Thomson scattering S. K. Nielsen 1), M. Salewski 1), H. Bindslev 1), A. Buerger 2), V. Furtula 1), M. Kantor 2) 3) 4), S. B. Korsholm 1), H. R. Koslowski 3), A. Kramer-Flecken 3), F. Leipold 1), F. Meo 1), P. K. Michelsen 1), D. Moseev 1), J. W. Oosterbeek 3), M. Stejner 1), E. Westerhof 2) and the TEXTOR team 1) Association Euratom - Risø National Laboratory for Sustainable Energy, Technical Univer- sity of Denmark, DK-4000 Roskilde, Denmark 2) FOM-Institute for Plasma Physics Rijnhuizen, Association EURATOM-FOM Trilateral Eu- regio Cluster, The Netherlands 3) Institute for Energy Research-Plasma Physics, Forschungszentrum J¨ ulich GmbH, Associa- tion EURATOM-FZJ, Trilateral Euregio Cluster, 52425 J¨ ulich, Germany 4) Ioffe Institute, RAS, Saint Petersburg 194021, Russia e-mail contact of main author: skni@risoe.dtu.dk Abstract. Experimental investigations of sawteeth interaction with fast ions measured by collective Thomson scattering (CTS) on TEXTOR are presented. Time-resolved measurements of localised 1D fast-ion distribution functions allow us to study fast-ion dynamics during several sawtooth cycles. Saw- tooth oscillations interact strongly with the fast ion population in a wide range of plasma parameters. The phase space density for large velocities along the resolved direction oscillates out of phase with the sawtooth oscillation during hydrogen neutral beam injection. These oscillations are interpreted to originate from fast hydrogen ions with energies close to the full injection energy. At lower energies the fast ions in the plasma centre are strongly redistributed. The redistribution of fast ions from deuterium neutral beam injection in the plasma centre is dependent on the resolved direction. We find no evidence of inverted sawteeth outside the inversion radius in the fast ion distribution function. 1. Introduction Fast-ion confinement in tokamak plasmas is essential for operation of fusion power plants. Ig- nition in a tokamak reactor relies on good confinement of the fusion born alpha particles which need to transfer their energy to the bulk plasma before they leave the plasma. Expulsion of fast ions from the plasma would result in a lower efficiency and may also damage the tokamak wall. A number of plasma instabilities have been found to interact with fast ions [1–4]. Amongst these instabilities is the sawtooth instability. Sawteeth are periodic oscillations in electron pressure which occur in tokamaks when the safety factor, q, is below unity in the plasma centre [5]. In a typical sawtooth cycle the electron tem- perature and density, and hence the electron pressure, slowly build up and then suddenly drop at an event called the sawtooth crash. Large sawtooth crashes have been reported to trigger neo- classical tearing modes [6] which degrade the plasma confinement and may lead to a disruption of the plasma. The trigger mechanism for the sawtooth crash is believed to be a kink mode which rapidly grows in the last few milliseconds before the crash. The kink mode is driven unstable when two conditions are fulfilled: (1) the local magnetic shear in the vicinity of the q = 1 surface is above a critical value, and (2) the change in ideal potential energy of the kink mode, δ W = δ W t + δ W f , is below zero [7]. Here δ W t and δ W f are the bulk plasma and the fast particle contributions,