Velocity modulations in view of the elliptical approach at Wendelstein 7-X

Abstract

The estimation of the poloidal velocity of the turbulence and the poloidal mean flow velocity are important quantities for the study of sheared flows on turbulence and transport. The estimation depends on the underlying model of the turbulence. Beside the propagation time of the turbulence, its decay with the fading time must be considered. For the description of the propagation, the elliptical approach is applied, which takes into account the propagation and fading time of the turbulence. The model has been applied successfully in experimental fluid dynamics and is confirmed by direct numerical simulations, also. In this paper, the elliptical approach is applied in the analysis of density fluctuations, measured by poloidal correlation reflectometry at two different fusion devices, TEXTOR and W7-X. For the latter, it is demonstrated that the elliptical approach is necessary for a correct description of the turbulence propagation. In addition, the velocity modulations are investigated, which in principle can be either generated by an oscillation of the propagation time of density fluctuations and/or an oscillation of the fading of the turbulence. An example for low frequency velocity oscillations in W7-X will be given in the paper, showing a relation between turbulence properties and small oscillations on the measured diamagnetic plasma energy.