Probe theory - the orbital motion approach

A review is given of the orbital motion limited (O.M.L.) theory of cylindrical and spherical Langmuir probes. In many cases the O.M.L. theory is invalid and the second orbital motion theory presented here, that of Bohm, Burhop and Massey, shows that the concept of an absorption radius must be introduced. The extensive numerical extensions to this theory are briefly discussed. Recent experimental work is referred to in which measurements of ion currents to cylindrical probes showed better agreement with the ABR (radial-motion) theory than with the calculations of Laframboise. It is suggested that the mean free path must be greater than the probe radius by a large factor if the calculations of the Laframboise are to apply; the numerical value depends on the ratio of probe potential to ion temperature.