Measurement of the radial electric field in the Advanced Toroidal Facility using a heavy ion beam probe

A heavy ion beam probe has been used to measure the radial electric field in electron cyclotron heated (ECH) plasma in the Advanced Toroidal Facility (ATF). These beam probe measurements are the first direct measurements of the plasma potential in the core region of a large stellarator. Measurements have been made of the potential profile in ECH plasmas under a variety of conditions. These have included density scans in which the line averaged density was varied from 5.0 x 1012 cm -3 to 1.0 x 1013cm-3. The majority of profile measurements have been made in the standard magnetic configuration. This is the configuration in which ATF is generally operated. It corresponds to the approximate center of the magnetic configuration operating space of ATF. A small number of measurements have also been made in other magnetic configurations. The measured potential profiles have been compared to those predicted with a simple model of the plasma in which the electric field is determined by requiring that the electrons and hydrogen ions, whose transport across the magnetic field lines is automatically identical in AFT, be so in steady state. This is the ambipolarity constraint which allows for a determination of the electric field. The trends of the measured profiles are consistent with those of the modeled profiles, but there exists large discrepancies between the predicted electric field strengths and those derived from the beam probe measurements. Several possible differences between the assumptions of the original simple model and the experimental conditions have been investigated. Only one, that of including an oxygen-like impurity charge species, significantly alters the predictions and brings them closer to the measured values