Poloidally asymmetric response of turbulence to the H mode on the CCT tokamak

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Abstract
A distinct poloidal asymmetry is observed in the response of the density fluctuations at the H mode transition on the CCT tokamak. The fluctuations are located on the same density layer and are monitored by using reflectometers at four poloidal positions (θ = 0°, 90°, 180°, 270°). On the outboard midplane the fluctuations decrease significantly at the transition, while those on the inboard midplane generally show little or no reduction. The fluctuations located at the top and bottom of the tokamak exhibit an intermediate response. All poloidal positions show different time histories even for the same shot. These observations may indicate a poloidal symmetrization of the fluctuations at the H mode transition. A poloidal variation in the E × B velocity induced Doppler shift is also observed, indicating a poloidally asymmetric E × B velocity profile. The significance of these observations is discussed, and comparison is made with the transition properties on the larger neutral beam heated DIII-D tokamak