Heterodyne O-mode reflectometer on the JT-60U tokamak

A three-channel heterodyne O-mode reflectometer system has been developed for density fluctuation measurements on the JT-60U tokamak. The system consists of one fixed channel with a frequency of 34 GHz and two selectable frequency channels with frequencies in the range of 34–40 and 48–50 GHz in order to measure the change of fluctuation amplitude, fluctuation coherence, and movement of the cutoff layer during the discharge. The reflectometer system has now become a standard diagnostic for density fluctuation measurements on JT-60U covering both core and edge plasma regions. The capabilities of the system are illustrated for three experimental cases: edge transport barriers in H-mode plasmas, internal transport barriers in reversed shear plasmas, and fast turbulence burst at the onset of multifaceted asymmetric radiation from the edge.