Time Evolution of the Spatial Structure of the Radial Electric Field in the Tohoku University Heliac

概要

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Radial electric fields are closely related to the confinement of plasma. Electrode biasing experiments were conducted to actively control the radial electric field by ramping up/down the electrode current in the Tohoku University Heliac (TU-Heliac). An emissive probe array consisting of three filaments was designed as a new type of measurement equipment. Using this probe, time evolutions of the spatial structure of the radial electric field were measured in the biasing experiments. The radial electric field tended to extend from the inner region to the outer region, while ramping up the electrode current. In the plasma inner region, the radial electric field was maintained for a longer time than in the outer region, while ramping down the electrode current. The radial electric field, poloidal flow of plasma, and fluctuation level of the ion saturation current changed appreciably in the period when the electrode voltage showed negative resistance against the electrode current. Therefore, the period when the plasma shows negative resistance agreed with the transition region to the improved confinement mode.