Abstract
Before a complete set of impurity control techniques were implemented in the Doublet III tokamak, two distinct discharge types were obtained, with nearly equal probability at high power densities with the same external control parameters. In the normal discharge (type S), sawtooth activity develops quickly and no impurity peaking on axis occurs thereafter. The other type of discharge (type O) initiates with more plasma/wall interaction, proceeds through the current-ramp phase with no central MHD activity, and impurity density profiles are found to be peaked on axis. Later in the current flat-top phase, type-O discharges experience the sudden onset of a large oscillation with m = 1, n = 1, and a concurrent decrease of central impurity densities. The (sinusoidal) oscillation eventually evolves into sawteeth, and type-O discharges attain a final state identical to type-S discharges. Measurements of absolute impurity densities show a direct correlation of impurity confinement with the presence or absence of tearing-mode activity, which suggests that the impurity transport is modified by the influence of the m = 1 magnetic island.
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