Effects of electrode misalignment and flow rate changes on the thrust in an electron bombardment engine

The effects of electrode misalignment and ion current variations on the thrust from a single aperture of an electron bombardment ion engine have been investigated using a digital computer. Four types of perturbation have been considered: changes in ion emission rate, changes in axial spacing of the electrodes, transverse misalignment, and tilt between the electrodes. The positions of the plasma boundaries have been determined self consistently with the ion flow rate and electrode configuration for each case. A first-order analysis of the effects of perturbations that destroy the symmetry of the flow has been formed by a suitable combination of solutions obtained assuming planar and circular symmetry. It has been shown that the axial thrust varies linearly with moderate changes in the ion flow rate. Axial changes in electrode spacing produce thrust changes that depend on the operating level and are higher at the higher flow rates. For transverse displacements the changes in ion flow rate as a result of plasma sheath movements are negligible, and the change in thrust results primarily from the geometric changes of the trajectories. It has also been shown that for tilts of the magnitude likely to be encountered, the changes in the thrust are negligible.