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By the use of simultaneous fast toroidal and slow poloidal sweeps of a heavy ion beam, plasma potential profiles in JIPP T-IIU tokamak are measured at the rate of 120 spatial profiles per second. A new method to eliminate the error due to the change of out-of-plane entrance angle caused by large plasma current, is successfully applied. One of the key factors is the very homogeneous characteristics of the energy analyzer along the wide slit length. A shaped electrode system, instead of guard rings with a resistor chain, successfully increases the homogeneity of an analyzer. The error due to the significant change of an in-plane entrance angle during a poloidal sweep is carefully minimized and calibrated by a secondary beam ionized by neutral gas introduced in a vacuum vessel. The depth of the measured potential at the plasma center reaches more than 1.5 keV in ohmic plasmas with the ion temperature of 600 eV. In some cases, a rather wide region of positive potential and a sharp decrease of the potential in the center of the plasma are observed. (author). (ERA citation 19:034889)