Analysis of Alkali Surface Ionization on Porous Substrates

Abstract

The evaporation of alkali atoms and ions from porous substrates is analyzed, making the basic assumption that the transport of alkali material occurs only by surface flow of densely packed alkali atoms close to the emitter surface. The alkali efflux velocity to the emitter surface depends on the flow rate per pore and the minimum pore radius within the surface-flow range. A concentration gradient exists radial to the pore exit which causes the neutral efflux and threshold temperature for surface ionization to be larger than the values characteristic of ionizers coated with vapor-deposited alkali. Alkali emission originates from an area surrounding and including the pore exit. The diameter of the emitting region depends on the alkali flow velocity from the pore and the atom and ion desorption energies. The emission region is divided into an inner zone that emits atoms and an outer zone which predominantly emits ions. This analysis predicts that the neutral efflux from the inner zone is inversely proportional to the pore density N and gives numerical values in good agreement with measured data. The neutral efflux from the outer zone follows the Saha-Langmuir equation, the total neutral efflux being the sum of both. The analysis leads to the conclusion that the threshold temperature depends on the minimum pore radius within the surface-flow range. The threshold temperature is also related to the pore density. This analysis may not be applicable to short straight capillaries because it depends on surface flow in the vicinity of the emitter.