Doppler reflectometry for the investigation of propagating density perturbations

In a Doppler reflectometry experiment the line of sight is chosen to be nonperpendicular with respect to the reflecting layer. This type of diagnostic is able to probe density perturbations with the wave number in the reflecting layer selected by the tilt angle. The propagation velocity of the perturbations is calculated from the Doppler shift of the returning microwave signal whereas its intensity contains information about the perturbation amplitude. The diagnostic capability of Doppler reflectometry is demonstrated at the W7-AS stellarator using an antenna system with variable tilt angle. The measured values of the propagation velocity of density perturbations and its radial dependence are in good agreement with the E×B velocity of the plasma obtained from spectroscopy. Transient states can be followed with a temporal resolution less than 50 μs. Therefore Doppler reflectometry allows one to test the interdependence of sheared flow and turbulence on that timescale.