Beam focusing of low-energy ion source by aperture displacement

Abstract

For low-energy plasma ion source (<1 keV), the narrow acceleration gap and application of high accel–decel voltage ratio are useful for obtaining high brightness ion beam. This paper describes these effects in a triode multiaperture extraction system on the characteristics of geometrical focusing by aperture displacement experimentally. Beam parameters such as focal length and beam divergence were estimated from a two-dimensional (axial and radial) power density profile by a Gaussian beamlet model. It is found that this model agrees very well with the beam power density profile. The relation of focal length with aperture displacement roughly agrees with the analytical model derived by Conrad. However, the effective gap width for the model is not simply determined from the aperture geometry.