The ion energy distribution of each ion species in the SPT-100 plume was obtained at various angles off thruster axis at 1 m and 50 cm from the thruster exit using an ExB probe. Each peak of the measured ExB probe traces was fitted using a model based on the kinetic theory of gases, and the energy distribution functions f(EJ were obtained. From the fitting parameters, E, (the beam energy) and II (the exponential factor) of the distribution functions were found for each ion species at various

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... ations in the SPT-100 plume. The distributions of Xe" ions were close to Maxwellian. For each ion species, the angular profiles of & at 1 m and 50 cm from the thruster exit were similar both in shape and magnitude. The spread of the ion energy was calculated from the width of the distribution functions, and was approximately 38 eV near the thruster axis. Ion species fractions were calculated at each measurement. point by forming the first moments of the distribution functions. There was a region near the thruster axis, C 20 degrees off thruster axis, where the majority of plume ions were Xe" ions. Outside of this region, the fractions of Xe" ions increased significantly. It was found that an ExB probe was an effective technique to determine macroscopic parameters of each ion species in the plasma.