A 2.45 GHz electron cyclotron resonance (ECR) plasma source with a magnetron magnetic field configuration has been developed for the generation of high flux of hyperthermal neutral beams (HNB). The plasma potential in ECR plasma is required to obtain the HNB energy. In this study, the axial profile of the plasma potential was deduced from the ion kinetic energy distribution (IED) which was measured using an ion energy analyzer. It was found that the IED was broadened with a full width half maximum (FWHM), representing the difference between the maximum plasma potential and the minimum plasma potential. (C) 2010 Elsevier B.V. All rights reserved.