Development Status of High-Thrust Density Electrostatic Engines

Abstract

Ion thruster technology offers the highest performance and efficiency of any mature electric propulsion thruster. It has by far the highest demonstrated total impulse of any technology option, demonstrated at input power levels appropriate for primary propulsion. It has also been successfully implemented for primary propulsion in both geocentric and heliocentric environments, with excellent ground/in-space correlation of both its performance and life. Based on these attributes there is compelling reasoning to continue the development of this technology: it is a leading candidate for high power applications; and it provides risk reduction for as -yet unproven alternatives. As such it is important that the operational limitations of ion thruster technology be critically examined -and in particular for its application to primary propulsion -its capabilities relative to thrust density and thrust-to-power ratio be understood. This publication briefly addresses some of the considerations relative to achieving high thrust density and maximizing thrust-to-power ratio with ion thruster technology, and discusses the status of development work in this area being executed under a collaborative effort among NASA Glenn Research Center, The Aerospace Corporation, and the University of Michigan. NomenclatureA b = beam area, m 2 AGI-Engine = Annular-Geometry Ion-Engine DMH-Engine = Dual-Mode Hybrid-Engine d s = screen electrode aperture diameter, m EP = electric propulsion F = thrust, N F A = thrust density, N/m 2 g = acceleration due to gravity, m/s 2 GRC = Glenn Research Center HET = Hall-Effect Thruster I a = accelerator electrode impingement current, A I b = beam current, A I bps = beam power supply current, A I d = discharge current, A I nk = neutralizer keeper current, A 2 I sp = specific impulse, seconds J b = ion beam current density, A/m 2 l e = effective acceleration length, m l g = interelectrode gap, m m = ion mass, kg MAGI-Engine = Multi-ring Annular-Geometry Ion-Engine NEXT = NASA's Evolutionary Xenon Thruster NGEPT = Next-Generation Electric Propulsion Thruster NSTAR = NASA Solar electric propulsion Technology Application Readiness PG = pyrolytic graphite P in = thruster input power, kW (unless otherwise specified) q = ion charge, C SOA = state of the art TAC = The Aerospace Corporation t s = screen electrode thickness, m UM = University of Michigan V a = accelerator electrode voltage, V V b = beam voltage, V V bps = beam power supply voltage, V V d = discharge voltage, V V nk = neutralizer keeper voltage, V V t= total accelerating voltage, V α = thrust-loss correction factor due to doubly-charged ions β = thrust-loss correction factor due to beam divergence ε i = discharge losses, W/A ε o = permittivity of free space γ = total thrust-loss correction factor η u = total propellant utilization efficiency

High Thrust-to-Power Annular Engine Technology

“…A potential effective means of maximizing ion engine F/Pin is by development of a non-conventional design: the Annular Engine (AE). 1 This design approach has several potential advantages: it should enable higher thrust density operation since the annular discharge chamber increases the effective anode surface area for electron collection as compared to a conventional cylindrically-shaped engine of equivalent beam diameter. This means that the engine…”

“…7,8 With minor magnetic circuit modifications, higher throughput (current density) and lower American Institute of Aeronautics and Astronautics discharge losses should be readily obtainable; at least to 150 W/A, and likely lower, to a limit of about 80 W/A. 1 At 150 W/A, the cross-over point where the F/Pin parameter for ion engines exceeds that for HETs is reduced from 2,600 seconds (for SOA engines) to about 2,300 seconds Ispwith further improvements in εi lowering the crossover further. In Fig.…”

“…However, improvements in the ion engine F/Pin parameter would yield higher performance than any other EP technology in the 4-12+ kW power range, over the broadest obtainable range in specific impulse (Isp). 1 This, in combination with its excellent demonstrated ground/in-space correlation of performance in primary propulsion applications in both geocentric and heliocentric environments, make it an attractive technology option for continued investments.…”

“…HETs). [1][2][3][4][5] These demonstrations however did not involve purposeful modifications to the technology to optimize performance or the F/Pin parameter. This is because these engines were intentionally designed for operation at low thrust densities for the purpose of achieving extremely long life times, in support of space science missions.…”

Investigation of Ion Beam Extraction Mechanism for Higher Thrust Density of Ion Thrusters

“…NASA GRC already started working on the development of high-thrust density ion engines for high power applications in collaboration with the Aerospace Corporation, and the University of Michigan. 3,4) Although they focus on the application to high-power primary propulsion, the technology would also be applicable to micropropulsion. In our early studies, we developed a two-dimensional particle model for a micro RF ion thruster and investigated the plasma parameters and ion beam profiles.…”