Doppler reflectometry for the investigation of propagating density perturbations

In a Doppler reflectometry experiment the line of sight is chosen to be non perpendicular with respect to the reflecting layer. This type of diagnostic is able to probe density perturbations with the wave number in the reflect ing layer selected by the tilt angle. The propagation velocity of the pertu rbations is calculated from the Doppler shift of the returning microwave si gnal whereas its intensity contains information about the perturbation ampl itude. The diagnostic capability of Doppler reflectometry is demonstrated a t the W7-AS stellarator using an antenna system with variable tilt angle. T he measured values of the propagation velocity of density perturbations and its radial dependence are in good agreement with the ExB velocity of the p lasma obtained from spectroscopy. Transient states can be followed with a t emporal resolution less than 50 mus. Therefore Doppler reflectometry allows one to test the interdependence of sheared flow and turbulence on that tim escale. (C) 2001 American Institute of Physics.