Doppler reflectometry for the investigation of propagating density perturbations

Doppler reflectometry is characterized by a finite tilt angle of theprobing microwave beam with respect to the normal onto the cutoffsurface. According to the Bragg condition the diagnostic selects densityperturbations with wave number K⊥ in the reflecting layer. Fromthe Doppler shift of the returning microwave the propagation velocity ofthese perturbations v⊥ can be obtained directly. The signalintensity contains information about the perturbation amplitude. Thediagnostic potential of Doppler reflectometry is demonstrated bothnumerically by the use of two-dimensional full-wave codes andexperimentally by an antenna system with variable tilt angle installedat the W7-AS stellarator. During stationary plasma conditions themeasured profile of the propagation velocity v⊥(r) is dominatedby the E×B velocity of the plasma, which is obtained frompassive spectroscopy. Transient states of the plasma can be followedwith a temporal resolution of less than 50 µs. Thus, Dopplerreflectometry allows us to investigate the interdependence of shearedflow and turbulence on that timescale.