A broadband fast frequency hopping reflectometer designed for measuring plasma turbulence has been recently installed at TJ-II. The main feature of the reflectometer is its possibility to be tuned, within a fraction of a millisecond, to any selected frequency while keeping synchronized the Local and Radiofrequency oscillators with the same stability as a fixed frequency system would do. The system includes heterodyne detection. Reflectometry measurements have permitted the characterization of the velocity shear layer that develops spontaneously in the edge of TJ-II plasmas above a certain critical density. Simultaneously, a second velocity shear layer develops at inner radial locations that moves radially inwards when the plasma density further increases. The interpretation of these experimental results has been crosschecked with results obtained using a 2-dimensional full-wave code. Changes the spectra shape linked to low order rational surfaces in the rotational transform profile are also reported. |
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