Field Emission Electric Propulsion Ion Optics of Slit Emitters and Beam Focussing

摘要

The trajectories of ions in a field emission thruster were simulated in order to investigate the effect of varying the electrode geometry, incorporating both a rear focusing electrode and a decelerating, neutralizing electrode. The rear electrode efficiently decreases the angular spread of the beam. A configuration including the neutralizing electrode was found, for which the beam divergence is 20 deg (half angle) while the average E-field in the gap is kept below 10kV/mm, minimizing the likelihood of breakdown between the thruster electrodes. A high current density does not increase the divergence but it may be impossible to achieve the necessary electrostatic fields without breakdown. If the requirement for a focussed beam is relaxed, electrode configurations and potential distributions which permit emission without violating the breakdown criteria can be realized. The resulting divergent beams carry up to 70% to 80% of the axial momentum carried by a beam parallel to the axis.