Electron Emission for Electric Propulsion: Reducing Power by Mitigating Space Charge Limits

Abstract

Electron emission is a requirement for most forms of space electric propulsion, as well as many other space applications. For many such missions minimizing system power is critical to the viability of the mission. The generation of electrons requires an amount of power that varies depending on the source (cold cathode and thermionic sources are considered here, not hollow cathode or other sources requiring consumables) but regardless of the source, the electrons must leave as a beam that has sufficient power (velocity) to escape the spacecraft. The space charge limit refers to the maximum current that can cross a gap (the sheath between the spacecraft and the surrounding plasma for example) at a given velocity, and thus is a practical lower bound to the beam power required for a given current. For many systems, especially those using low-power electron sources, this means that power must be added to the electron beam, usually with biased grids. The minimization of this electron beam power requirement is the topic of this paper. Analytic solutions for the space charge limit in simple geometries have been developed up to three dimensions, and these demonstrate some basic performance tradeoffs. Less practical for the analytical approach are more complicated geometries and the addition of spacing and timing factors. These complications, however, can allow significant improvement in the space charge limit. Therefore, using particle-in-cell computer simulations (with the XOOPIC code developed at Berkley), a number of techniques for mitigating the space charge limit have been investigated, ranging from atypical geometries to spatial and chronological phasing of emission. Some of the advantages and disadvantages of some techniques, and the implications for electron emission system design, will be presented here.