ELECTROHYDRODYNAMIC INSTABILITIES AND THE ENERGY SPREAD OF IONS DRAWNFROM LIQUID-METALS

This paper aims at shedding some light on the beam energy spread versu s current curves of liquid metal ion sources (LMISs). Although Knauer' s collisionless space-charge broadening model originally gave almost p erfect agreement with experiment, deviations were discovered later at low and high currents. To make matters worse, as more metals were inve stigated, complete disagreement was found in certain cases. Although t he low-current deviation from Knauer's model is not well understood an d only tentative explanations can be given, the high-current deviation has been explained in terms of a self-sustaining instability that set s in at a critical current, the value of which varies greatly from met al to metal. This instability violates a basic premise of Knauer's mod el, namely that the ions' paths do not cross after their emission. Cal culations show that, for metals for which there is total disagreement with Knauer's model, the instability sets in almost from the onset. An important result of this work is the development of a stability crite rion for LMISs. It is shown that a low evaporation field and a high su rface tension are two principal factors that improve stability. The st ability is also affected, to a lesser extent, by the liquid density an d charge-to-mass ratio of the ions.