NUMERICAL-MODEL OF CESIUM ION-BEAMS IN-FIELD EMISSION ELECTRIC PROPULSION THRUSTERS

A numerical model of the ion beam in field emission electric propulsio n thrusters is presented. The objective of the study is to develop a c omputational tool capable of predicting the effects of thruster config uration parameters (geometry, voltage distribution between the electro des) on thruster performance (beam divergence, ion exhaust velocity). A two-dimensional, steady model has been adopted, A simple emission mo del has been assumed, neglecting the detailed, microscopic description of the physical emission mechanism. The electric field is evaluated b y a finite element scheme; boundary conditions are provided by an auxi liary technique based on the placement of fictitious charges in proxim ity to the electrode surfaces, The beam is simulated through a discret e number of charged particles; the effects of the associated space cha rge density distribution on the electric field is considered. The resu lts of the simulations of typical thruster configurations are presente d and discussed. The need to take space charge effects into account to attain a sufficiently accurate solution is indicated: the beam charge density modifies the electric potential distribution generated by the electrodes voltage difference, causing a decrease in the local electr ic field at emitter tip and an increase in beam divergence. Neutraliza tion at a finite distance must be taken into account to fully apprecia te the effects of different voltage distributions between the electrod es: a higher accelerator electrode voltage determines a lower ion exha ust velocity and a larger beam divergence, The capability of an additi onal, decelerating neutral electrode to focus the ion beam is also inv estigated, showing that only little improvement in beam containment is attained.