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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 n(K). In the multichannel Doppler reflectometer at the Stellarator W7-AS the antenna system and signal detection are optimised 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 n can be measured with a temporal resolution of less than 10mu s 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.

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Authors: HIRSCH M, Max-Planck-Institut fur Plasmaphysik, EURATOM Association, Teilinstitut Greifswald, Greifswald (DE);HOLZHAUER E, Institut fur Plasmaforschung, Universitat Stuttgart (DE)
Bibliographic Reference: An article published in: Plasma Physics and controlled Fusion 46 (2004) pp. 593-609
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