ASDEX Upgrade Team
(EX/2-1) Study of Turbulence and Radial Electric Field Transitions in ASDEX Upgrade using Doppler Reflectometry
1) Max-Planck-Institut für Plasmaphysik, D-85748 Garching bei München, Germany
2) Institut für Plasmaforschung, Pfaffenwaldring 31, D-70569 Stuttgart, Germany
Abstract. The radial electric field (Er) is a crucial factor in the performance of magnetically confined plasmas and on density turbulence. On ASDEX Upgrade Doppler reflectometry has been developed for direct measurement of Er profiles, its radial shear and its fluctuations. In the plasma edge the Er radial profile shows the narrow negative well coinciding with the steep pedestal pressure gradient whose depth scales with the plasma confinement: from typically - 5 kV/m in ohmic and L-mode conditions to - 30 kV/m in H-modes, to over - 50 kV/m in improved H-modes. The structure of the edge Er profile is notably robust, but the core Er of non-NBI heated discharges when increasing collisionality reverses from positive to negative with a transition in the dominant turbulence from TEM to ITG. Coherent Er fluctuations with geodesic acoustic mode (GAM) behaviour are observed in the plasma edge, coinciding with region of high plasma vorticity and Er shearing. GAMs are not detected in the core or in H-mode. The mode has the expected frequency scaling of sound speed over major radius but with additional dependency on plasma elongation and q.
Full paper and slides available (PDF)