Swept frequency reflectometer for correlation studies in the TJ-I tokamak

An improved correlation reflectometer technique has been developed and implemented on TJ-I to measure the radial correlation lengths of the density turbulence. The method uses homodyne detection and is based on the slow sweeping of one oscillator while the other remains at fixed frequency. The decay of the coherence shows maxima and minima as the distance between the two probing points increases. The maxima correspond to the relevant coherence values. The correlation length for the overall turbulence as well as the correlation length for the different frequency intervals can be directly determined in a single shot. By the operation in the Q-band (33--50 GHz) extraordinary mode, the system reaches most of the plasma radius. Correlation lengths obtained at the edge are compared with those measured at deeper positions in the plasma. The proposed method can provide the values of the differential phase delay along the sweep, avoiding the effect of lost fringes even in the presence of turbulence. This opens the possibility of obtaining the information on the density profile if several channels are used