Errors in ion and electron temperature measurements due to grid plane potential nonuniformities in retarding potential analyzers

Abstract

The behavior of a retarding potential analyzer in a laboratory plasma has been studied to investigate the effects of potential nonuniformities in the grid planes. Significant effects are found for the kind of grids used in the Ogo 1, 3, and 4 traps. It is found that in order to explain the laboratory results the retarding grid potential in the usual planar approximation must be replaced by an effective potential, which may be obtained from the laboratory data. Since the potential distribution about a grid of parallel wires is available analytically, this model is used to calculate the ion current through such a retarding grid. When ion trajectories are taken into account, these calculations show that the effective grid retarding potential is very closely related to the saddle point of the potential near the retarding grid. Applying these corrections to the Ogo 4 O+ temperatures results in a downward revision of about 32%, bringing them into much better agreement with radar backscatter results.