CALCULATED IV CHARACTERISTICS OF A GOLD LIQUID-METAL ION-SOURCE FOR APROTOTYPE EMITTER MODELED AS A CONE SPHERE

The operational properties of liquid metal ion sources (LMIS) have bee n studied extensively the last three decades. Nevertheless, debate sti ll exists on the emitter shape and the mechanism(s) responsible for io n emission. It is generally believed that prior to onset, the applied electric field causes a (cuspidal) deformation of the liquid emitter s urface. This study will use the cone-sphere as a prototype for the emi tter surface. The advantage of the cone-sphere model is its ability to replicate analytically the emitter shapes seen experimentally, since one can independently choose the radius of curvature, R, the ''form fa ctor'' k (which determines the amount of ''necking'') and the asymptot ic cone angle. The I-V characteristics of a cone-sphere model of a Au LMIS were calculated using the image hump (IH) and charge exchange (CE ) models for the field evaporation process. Field evaporation is assum ed to be thermally activated and follow an Arrenhius expression of the form exp(-Q(n)/kT), where Q(n) is the activation energy. The multidim ensionality of the source is accounted for in the calculation of the e xact field variation on the three-dimensional surface of the liquid em itter. The calculation of Q(n) in the image hump model follows a stand ard procedure. However, the evaluation of Q(n) for the charge exchange model differs from that done by others. First, the atomic energy curv e for Au is calculated using the universal binding energy curve of Smi th et al. [J. R. Smith, J. Ferrante, and J. H. Rose, Phys. Rev. B 25, 1149 (1982)]. The intersection of the atomic and ionic potential energ y curves is determined as a function of field. The quantity Q(n) is th en the energy difference between the minimum of the atomic energy curv e and its value at the intersection. The I-V characteristics for Au ob tained using the CE model are in better qualitative agreement with exp erimental results for Au and Cs than the IH model in the voltage range from 1 to 10 kV. In paricular, the calculated curves exhibit a defini tive onset voltage, a sharp rapid rise, and saturation behavior.