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The performance of a feedback controlled electrostatic energy analyzer has been investigated for use as the basic detection mechanism in an ion beam probe plasma diagnostic system. This system provides the only direct, spatially resolved measurement of plasma space potential. The principles involved in analyzer operation are presented, leading to an analyzer system with a design entrance angle of 33Â°. The voltage correction function which results when the probing beam is swept across the plasma has been measured experimentally and is in excellent agreement with theoretical predictions. The measured amplitude and phase response of the system show that the entire analyzer/detector system may be modeled as a linear feedback system. Finally, a dual beam technique is described for making absolute space potential measuremermts and is substantiated with experimental data from an arc plasma using a Rb-K combination.