Sheath Around a Spherical Probe in a Photoelectron Environment

A model and an experiment are applied to finding the current-voltage characteristics of a spherical probe in vacuum when the sphere emits photoelectrons and there are ambient photoelectrons. An orbit-motion-limited kinetic model is used for each of the two electron populations. Poisson's equation is solved to find the potential profile with and without a potential barrier (virtual cathode) from the electron space charge. The model current-voltage characteristics are compared with data from an experiment. The measured derivative of the probe characteristic does not show the expected effect of the virtual cathode suggesting that the potential minimum is absent in the experiment, perhaps as a consequence of the presence of ions.