Emission characteristics and stability of a helium field ion source

Abstract

The emission characteristics and stability of a helium field ion source were studied. Detailed measurements of current emitted from the (111) plane of a tungsten emitter revealed a sharp peak in I–V characteristics that shifted toward a higher voltage when the helium pressure increased or the emitter temperature rose. Measurements of the pressure dependence of the ion current showed a decrease in current above the critical pressure, ∼1.3 Pa. To determine why the current decreased, we varied the helium flow rate and the distance between the emitter tip and aperture. The measured characteristics indicated that the decrease in current was not caused by a large flow of warm helium or by a charge exchange of emitted He+ ions with neutral He atoms inside the ion source, so we had to assume the cause lies elsewhere. We recorded ion current versus time characteristics to determine emission stability, which could be improved to ±10% within 60 min by reducing impurities in the helium gas.