A method is described for continuously measuring plasma potential over a 4-kV range using conventional components and equipment. The time resolution of the measurement is presently 1 msec, but could be made 0.1 msec without serious difficulty. The lithium-ion-beam transmission technique is used. The ion-beam energy is varied from 0 to 4 kV by a saw-tooth generator at a frequency of 1 kc/sec. A Faraday cup collects the ions which have acquired sufficient energy to overcome the plasma potential. When the current from the Faraday cup reaches a threshold level, the oscilloscope trace of the saw-tooth wave form is intensity-modulated, producing the desired plasma potential versus time relation.
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Waugh et al. (1964) studied this question.