Impact of extractor aperture on the onset voltage of externally wetted electrospray sources

Abstract

Accurate prediction of onset voltage is essential for optimizing the emission characteristics of electrospray sources (e.g., by setting the range of emitted beam energies), yet existing analytical models often overlook the influence of practical geometric details such as an extractor aperture. This study establishes a direct relationship between extractor aperture radius and the onset voltage for externally wetted geometries. Simulation results provide an empirical model that can be represented in a simple analytical form and demonstrate that increasing the extractor aperture radius from 0 to 0.5 mm can increase the predicted onset voltage by 18%–63%, depending on the emitter apex radius of curvature. These predicted trends were experimentally confirmed through laboratory testing with electrochemically etched tungsten needle emitters; increasing the aperture radius from 0.2 to 0.75 mm for an emitter apex radius of curvature of 5.18 ± 0.25 μm raised the measured onset voltage by 40 ± 2%, consistent with the proposed scaling law prediction of 32 ± 11%. The results demonstrate the significant, and predictable, effect of aperture radius on source operation, emphasizing the necessity of accounting for this feature during the design of electrospray systems.