Ion Beam Modeling in FEEP Thrusters

A numerical model of the Ion beam associated with the operation of Field Emission Electric Propulsion thrusters Is presented. The objective oft be study Is to develop a computational tool capable of predicting the effects or thruster configuration parameters (geometry, voltage distribution between the electrodes) on thruster performance (beam divergence, ion exhaust velocity). A two-dimensional, steady model bas been adopted. The electric potential, electric field and the trajectories of the emitted particles are evaluated In a domain which extends from the emitter tip to a point where either beam neutralization takes place or the electric field becomes sufficiently low to assume that the trajectories can be reasonably approximated as straight lines. A simple emission model has been assumed, neglecting the detailed, microscopic description of the physical emission mechanism. The electric field Is evaluated by a finite element scheme; boundary conditions are provided by an auxUiary technique based on the placement of fictitious charges In proximity to the electrode surfaces. The beam Is simulated through a discrete number of charged particles; the effects or the associated space charge density distribution on the electric field Is also considered. The results of the simulations or typical thruster configurations are presented and discussed. The need to take the space charge effects Into account to attain a sufficiently accurate solution Is Indicated. Neutralization at a finite distance must be taken Into account to fully appreciate the effects or different voltage distributions between the electrodes, and It Is shown to Increase beam divergence. The capabUity or a thruster configuration with an additional, decelerating neutral electrode to focus the ion beam Is also Investigated.