Turbulence and geodesic acoustic mode behavioural studies in ASDEX Upgrade using doppler reflectometry

Abstract. The interaction of turbulence with stationary and oscillating zonal flows is important for understanding the behaviour of turbulent transport in magnetically confined plasmas. Doppler reflectometry has been used on the ASDEX Upgrade tokamak to measure profiles of Er, its fluctuations and density turbulence properties such as radial correlation lengths Lr and perpendicular k⊥-spectra. In L-mode the turbulence amplitude reduces predominantly at low k ⊥ values with radius; while in H-mode, in addition to the low k ⊥ drop around the pedestal- coincident with reduced Lr and enhanced Er shearing- turbulence falls at all k ⊥ values with spectral narrowing towards the core region. Turbulence driven geodesic acoustic modes (GAMs) are observed in the ohmic and L-mode plasma edge, but not in H-modes nor deep in the core. The mode frequency scales as sound speed over major radius, but with an additional inverse plasma elongation dependence in the edge. The GAM amplitude increases linear with the temperature gradient (turbulence drive) up to the L to H-mode transition, but displays a complex dependence on the magnetic configuration: shape, elongation, local q value and field-null presence etc. 1.