Positive Column Formation in Ion Sources Employing Surface Ionization

In order to gain some insight in the problem of space charge neutralization of ion beams we make use of a previous calculation to discuss the unidimensional flow of positive ions in a tube where both ions and electrons issue from an emitter surface but only ion current is collected by means of an accelerating electrode. The resulting ion flow can be neutralized to varying degrees by the electrons present. We find that even in the absence of collisional interactions the potential distribution in the tube may closely resemble the electrode sheath and positive column structure familiar to glow discharges. This is the situation to be expected whenever the ion current has values far in excess of that predicted from a zero degree of neutralization space charge limited theory.