Radial correlation length measurements on ASDEX Upgrade using correlation Doppler reflectometry

The technique of correlation Doppler reflectometry for providing radial correlation length Lr measurements is explored in this paper. Experimental Lr measurements are obtained using the recently installed dual channel Doppler reflectometer system on ASDEX Upgrade. The experimental measurements agree well with theory and with Lr measured on other fusion devices using different diagnostic techniques. A strong link between Lr and plasma confinement could be observed. From the L- to the H-mode, an increase in the absolute value of Er shear was detected at the same plasma edge region where a decrease in Lr was measured. This observation is in agreement with theoretical models which predict that an increase in the absolute shear suppresses turbulent fluctuations in the plasma, leading to a reduction in Lr. Furthermore, Lr decreases from the plasma core to the edge and decreases with increasing plasma triangularity δ. The experimental results have been extensively modelled using a 2-dimensional finite difference time domain code. The simulations confirm that Doppler reflectometry provides robust radial correlation lengths of the turbulence with high resolution and suggests that Lr is independent of the turbulence wavenumber k⊥ and its fluctuation level.