V-band frequency hopping microwave reflectometer in LHD.

In order to measure the internal structure of fluctuation, the broadband frequency tunable system, which has the ability of fast and stable hopping operation, is applied in the Large Helical Device. One of the important issues of density fluctuation measurements using this reflectometer is the study of energetic particle driven magnetohydrodynamics instability. During one plasma discharge, the launching frequency changes from one frequency to another frequency, which this operation is called as frequency hopping, and the cutoff position can be scanned in the wide area. As a hopping source, a synthesizer is used because it has a quite stable and low phase noise. The frequency component of the source output is multiplied to V-band (50-75 GHz) region for plasma measurements in extraordinary mode polarization. Also this system has a heterodyne detection with single side band frequency modulation for sensitive phase and amplitude measurement. We can obtain the radial profile of Alfvén eigenmodelike oscillation in a neutral beam injected plasma.