Electrode Refraction and Ion Beam Collimation

Abstract

Efficient electrode structures for acceleration of ions in electrical propulsion systems are commonly designed so as to establish a potential function at the beam edge identical with that characteristic of space charge limited flow. Since the accelerated beam is intended to generate thrust, it must emerge from an aperture in the accelerator with a high degree of collimation. Consideration must therefore be given also to refraction by the transverse fields unavoidably associated with electrode apertures. In general, an aperture separating a region in which ions undergo acceleration from a region of lower or negative acceleration acts as a divergent lens; if the final stage is decelerating, the exit aperture acts as a convergent lens of greater strength than the aperture preceding it. It can be shown that plane parallel flow in the acceleration region is incompatible with full collimation of the ejected ion beam. The radius of curvature of the beam as it enters the final electrode aperture, the degree of final beam collimation, and the current density that can flow at a given exit voltage are therefore interrelated. This forces a compromise between degree of collimation and perveance, and determines the maximum thrust attainable at a given specific impulsemore » from an emitter of given radius. These considerations apply not only to single stage ion guns but also to multistage (including accelerate-decelerate) systems. Application of these considerations to the design of a particular accelerate-decelerate electrode configuration is described. (auth)« less