Broadband reflectometry for the density profile and fluctuation measurements in the JT-60 tokamak

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Abstract
A broadband reflectometric system in O‐mode operation has been developed for the density profile determination, MHD and turbulent fluctuation measurement, and evaluation of the particle diffusion coefficient in the JT‐60 tokamak. For the profile determination, full Ka‐Q band frequencies of BWOs have been swept in 750 μs, in order to minimize the effect of doppler shift due to the fluctuation of reflection layers. The heterodyne fixed‐frequency reflectometer has unveiled its effectiveness to probe MHD activities, diagnosing the m=1 tearing mode oscillations. It has also enabled the observation of the dramatic suppression of edge plasma density fluctuations at the L‐ to H‐mode transition in the LHCD limiter plasma. Furthermore, the propagation delay of density pulses has been observed by different fixed‐frequency channels during a series of sawteeth, from which the particle diffusion coefficient was evaluated, with a newly proposed method applicable to fully diffusive plasmas.