Experimentally Determined Plasma Perturbation by a Probe

The perturbation of a plasma by a probe with radius large in comparison to a mean free path is examined with a small movable probe. Electron density, electron temperature, and plasma potential are determined as a function of distance from the large probe. The experimental conditions corresponded well to those assumed by Waymouth; Waymouth's theory applied to the large‐probe electron current characteristic yields a value for electron density in the unperturbed plasma that is correct within a few percent. The electron temperature so determined is too low by 19%. A fundamental assumption of all theories, that electron temperature in the perturbed volume of plasma is independent of position, is found to be incorrect, at least in a plasma containing drift currents.