The radial electric field and its associated shear in the ASDEX Upgrade tokamak

The radial electric field (E-r) and its shear (partial derivative E-r/partial derivative r) are believed to be fundamental for turbulence suppression in magnetically confined plasmas. Doppler reflectometry offers a direct method of obtaining E-r and partial derivative E-r/partial derivative r measurements. It is a type of microwave radar technique which uses the back-scatter of microwaves from a radial position in the plasma where the refractive index equals zero. In this paper, the use of Doppler reflectometry for E-r measurements is investigated. The technique is extended for partial derivative E-r/partial derivative r measurements by simultaneously probing the plasma with two microwave beams at different frequencies. E-r and partial derivative E-r/partial derivative r measurements are presented for a wide variety of plasma conditions. The measurements show that E-r and its associated shear are linked to plasma confinement. Their absolute values increase with confinement at the plasma edge. Furthermore, the measurements show dependence on the applied plasma auxiliary heating and the plasma shape. The E-r shear results suggest that negative shear is more influential than positive shear, in contradiction to several theoretical models.