Doppler reflectometry for the investigation of propagating density perturbations

Doppler reflectometry is characterized by a finite tilt angle of the probing microwave beam with respect to the normal onto the cutoff surface. According to the Bragg condition the diagnostic selects density perturbations with wave number K-perpendicular to in the reflecting layer. From the Doppler shift of the returning microwave the propagation velocity of these perturbations nu(perpendicular to) can be obtained directly. The signal intensity contains information about the perturbation amplitude. The diagnostic potential of Doppler reflectometry is demonstrated both numerically by the use of two-dimensional full-wave codes and experimentally by an antenna system with variable tilt angle installed at the W7-AS stellarator. During stationary plasma conditions the measured profile of the propagation velocity nu(perpendicular to)(r) is dominated by the E x B velocity of the plasma, which is obtained from passive spectroscopy. Transient states of the plasma can be followed with a temporal resolution of less than 50 mus. Thus, Doppler reflectometry allows us to investigate the interdependence of sheared flow and turbulence on that timescale.