Charge transport characterization of the alternative low power hybrid ion engine (alphie) with particle-in-cell simulations

Abstract

The Alternative Low Power Ion Engine (alphie) is a high specific impulse plasma thruster where both electrons and ions flow combined through the open spaces of its two-grids system. Ionizing electrons from its external cathode travel in toward the ionization chamber accelerated by the potential drop established between the grids, whereas ions exit toward the opposite direction and are later neutralized by electrons emitted from the cathode. In this configuration, the ion current is not space-charge limited, contrary to conventional gridded ion engines where only ions are transported through the grids. Since previous approaches that consider only one charged species are not valid for the alphie two-grids system, the characterization of geometrical parameters is studied using a fully particle-in-cell simulation. This numerical scheme calculates the velocities and positions of electrons and ions within a structured axial-symmetric mesh, which mimics one hole of the grids. The charged particle densities, currents, and their axial and radial mean velocities along the simulation domain are evaluated. Additionally, important reductions in the computational time have been obtained using different time scales for ions and electrons. Numerical results show that moderate changes in the thickness and separation of the grids with respect to the values employed in the prototype currently tested in the laboratory do not significantly affect the exhaust ion current or the ion beam collimation. Since the average velocity of the axial ions is essentially insensitive to small modifications of the geometric parameters, so will the specific impulse and thrust delivered.