Measurements of the space potential of electron cyclotron heated plasmas in the Advanced Toroidal Facility

A heavy ion beam probe has been used to measure the space potential of electron cyclotron heated plasmas in the Advanced Toroidal Facility. The results of the measured potential profiles are presented along with the radial electric field strength profiles derived from these measurements. The measured potential profiles have been compared to those predicted by a simple model of the plasma based upon an ambipolarity constraint on the ion and electron particle fluxes in steady state. The trends of the measured and modeled profiles are in agreement, although the strength of the electric field predicted by the model is much greater than that inferred from the potential profile measurement. When an oxygen-like impurity ion species is added to the model, the predicted electric field strengths are altered significantly and become much closer to the measured values. >