Microwave reflectometry in TJ-II

Microwave reflectometry is commonly used to measure electron density profiles and fluctuations. In TJ-II a broadband fast frequency hopping system has been recently installed for measuring plasma turbulence while an Amplitude Modulation reflectometry system provides information on the density profile. The first system allows probing several plasma layers within a short time interval during the discharge, permitting the characterization of the radial distribution of plasma fluctuations. The second system provides the time evolution of the density profile what allows the radial localization of the fluctuation measurements. In addition, we have developed a two- dimensional full-wave numerical code to help in the interpretation of the experimental data and to study the viability and performance of different reflectometry techniques. Experimental measurements have permitted the characterization of the shear layer in the perpendicular rotation velocity of the fluctuations. These observations have been crosschecked with results obtained using the two-dimensional full-wave code. Both experimental and numerical results demonstrate the capability of the reflectometer to measure the velocity shear layer with a high spatial resolution. The experiments also show a modifications in the reflectometry signals spectra associated with the presence of a low order rational surface in the rotational transform profile within the radial range covered by the reflectometer. These experimental results can be interpreted in terms of a localized increase in the perpendicular rotation velocity of the fluctuations. Finally we present some numerical studies that have been performed to study the viability of the Doppler reflectometry technique to determine the perpendicular rotation velocity in TJ-II. (author)