Doppler reflectometry selects electron density perturbations with a finite wave vector K⊥ by a line of sight which is non-perpendicular with respect to the reflecting layer. This provides, simultaneously, a local measurement of their propagation velocity v⊥(K⊥) and of changes in their fluctuation amplitude . In the multichannel Doppler reflectometer at the Stellarator W7-AS the antenna system and signal detection are optimized for maximum temporal resolution in order to study transport bifurcations such as the transitions between L- and H-mode. For the conditions in W7-AS the quantities v⊥ and can be measured with a temporal resolution of less than 10 µs as shown for the example of an H to L back-transition. The dual antenna system installed allows us to supplement Doppler reflectometry with a simultaneous time-of-flight measurement, which is the complementary method to determine v⊥.
Study of Doppler reflectometry capability to determine the perpendicular velocity and the k-spectrum of the density fluctuations using a 2D full-wave code