Velocity shear layer measurements by reflectometry in TJ-II plasmas

A broadband fast frequency hopping reflectometer designed for measuring plasma turbulence is in operation at TJ-II. The main feature of the reflectometer is its possibility to be tuned to any selected frequency within a fraction of a millisecond. This property enables us to probe several plasma layers within a short time interval during the discharge, permitting the characterization of the radial distribution of plasma fluctuations. The system uses a standard antennas arrangement designed to probe the plasma perpendicularly. Although the present reflectometer arrangement was not meant to measure the perpendicular rotation velocity, the system allows us to extract important information about the velocity shear layer, namely its spatial localization. Reflectometry measurements showing the so-called phase run-away have permitted the spatial localization of the velocity shear layer that develops spontaneously in TJ-II plasmas at a certain critical density. A comparison with results obtained using Langmuir probes helps in interpretation of the results. Besides, the interpretation of the experimental results has been crosschecked with results obtained using a two-dimensional full-wave code.