BROAD-BAND HOMODYNE REFLECTOMETER DEVOTED TO DENSITY FLUCTUATION MEASUREMENTS

A microwave reflectometer has been used in the TJ-I tomakak (R=0.30 m, a=0.10 m, IpIp< 50 kA, Bt < 1.5 T) to monitor density fluctuations in the plasma bulk. Experiments were made in the range 33-50 GHz, x mode. The system uses a single antenna for launching and receiving the microwave beam. Since broadband operation usually requires homodyne detection, different techniques were used to overcome the limitations due to this detection scheme. Radial profiles of the density turbulence were taken and the behavior of the spectra analyzed. A slow frequency sweeping has been performed to provide a method to normalize the signals reflected from different radial positions. In addition this technique can be used to obtain information on the absolute value of the phase oscillation amplitude. Spectra of the edge density fluctuations obtained by reflectometry and by Langmuir probes are compared. By the use of two reflectometers in the same antenna, radial correlation experiments can be performed. Some discussions on the ability of homodyne reflectometry to determine coherence lengths are also presented. A slow sweeping of the frequency is proposed to overcome the lack on the information given by the homodyne system.