Ion Thruster Optical Performance Enhancement via Ion-emissive-surface Shaping

Numerical results that demonstrate how ion emissive surfaces should be shaped to induce the formation of well-focused, high-current beamlets are presented. Results obtained when the sheath shape and position are established and then held constant as operating conditions are changed are compared to corresponding results obtained with a conventional plasma sheath that moves. These comparisons show the shaped surface: 1) yields beamlets that remain focused over wider beamlet current ranges, 2) enables operation without a crossover limit, and 3) enables operation at lower net-to-total accelerating voltage ratios and, therefore, at lower specific impulses. It is also shown that even greater beamlet currents can be extracted before the perveance limit is encountered if the propellant utilization efficiency approaches 100% and it is no longer necessary to have a small accel hole diameter to limit neutral propellant loss. Such a propellant utilization efficiency is expected for the Emissive Membrane Ion Thruster because its membrane is able to pass only ionized propellant. d = diameter (ds – screen, da – accel, dd decel) E = electric field strength J = current (Jb – beamlet, Ji – impingement) r = radius (rs – screen, ra – accel) lg = grid spacing u = Bezier function independent variable V = voltage (Va – accel, Vd – decel, VN – net, VT – total) z = axial position