Ion Beamlet Divergence Characteristics of Two-Grid Multiple-Hole Ion-Accelerator Systems

Abstract

An experimental investigation was performed to determine the divergence characteristics (distortion factor, divergence angle, and divergence factor ) of ion beamlets that were commonly formed using multiple-hole, ionaccelerator systems of ion thrusters. An isolated ion beamlet was observed three dimensionally using arrayed Faraday probes, and its divergence characteristics were determined as a function of geometrical grid parameters and grid voltages. Because xenon was used as a propellant and the grid voltages were chosen being conscious of a recent ion-thruster technology trend, the results can be used for designing ion thrusters by simply translating geometrical grid parameters. The results did not contradict those in previous studies that used argon and one- or two-dimensional beam diagnostics. Nomenclature da = accelerator-hole diameter, mm ds = screen-hole diameter, mm dsp = beamlet-separator-hole diameter, mm Fd = distortion factor, s/ S fD = divergence factor Jb = beamlet current, A Ji = ith probe current, A L = axial distance between accelerator grid and probes, cm lcc = hole center-to-center distance, mm le = effective acceleration length, mm lg = screen-to-accelerator-grid separation, mm lsp = accelerator-grid-to-separator separation, mm NP/H = normalized perveance per hole, A/V 3/2 R = net-to-total voltage ratio, Vn/ Vt Ri = ith probe radial location measured from beamlet