Density, potential, and magnetic field fluctuation measurements during MHD activity in the TEXT tokamak

Abstract

A heavy ion beam probe (HIBP) has been used to measure signals associated with density, potential, and magnetic fluctuations during sawtooth and strong m=2 activity in TEXT. Preliminary results reveal magnetic and density fluctuation signals with a strong dependence on minor radius position. Phase shifts near 180 degrees have been observed between neighboring peaks and are consistent with a model in which the signal changes sign across a rational surface. Density fluctuation signal peaks occur near magnetic fluctuation signal peaks when both are observed. Results from a different set of plasmas in which soft X-ray sawteeth are observed are also reported. Potential sawtooth changes as large as 70 V have been observed in these plasmas. An HIBP injects a beam of Tl/sup +/ particles into the plasmas. Tl/sup ++/ ions created in a 0.1 cm/sup 3/ sample volume in the plasma are detected after exiting the plasma. By measuring the energy of the Tl/sup ++/, the potential in the sample volume is determined. The number of Tl/sup ++/ ions at the detector is proportional to the density in the sample volume. It also depends on beam attenuation effects along the entire beam trajectory. Attenuation effects are more significant for fluctuations with long correlation lengths.<>