Abstract
Density fluctuations are analyzed in high volume average beta and high core density discharges in the Large Helical Device (LHD) using a 2D phase contrast imaging system and far infra-red interferometer. Both these regimes share similarly high beta gradients and evidence of pressure driven MHD modes is presented. In high volume average beta plasmas, both large and ion-gyro scale density fluctuation levels increase with beta and, in the edge, compare favorably with growth rate of resistive interchange modes, showing additional dependence on density at fixed β. In high core density plasmas with internal diffusion barrier, intermittent fluctuation bursts around mid radius are observed which are triggered when the normalized density gradient exceeds a certain threshold. The intermittent character is stronger for outward shifted plasmas and there appears to be a fluctuation suppression mechanism, possibly related to temperature gradient.
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