2378 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 36, NO. 5, OCTOBER 2008 Modeling of FEEP Plume Effects on MICROSCOPE Spacecraft Jean-François Roussel, Thomas Tondu, Jean-Charles Matéo-Vélez, Enrico Chesta, Stéphane D’Escrivan, and Laurent Perraud Abstract—Several aspects of plume effects of field-effect electric propulsion (FEEP) were studied. We first estimated the contami- nation by cesium deposit due to charge exchange of fast ions and slow neutrals and to direct neutral impingement. Levels are rather low, with local maximums of a few tens or hundreds of angstroms per year, and not much more than 1–10 Å farther from thruster nozzles. Neutralization of the ions emitted by FEEPs was also addressed, both concerning FEEP ion space-charge compensation and spacecraft net current, i.e., the floating potential issue. With the presence of a cathode-grounded neutralizer, the spacecraft was shown to float somewhat negative with little dependence on the ambient environment. Index Terms—Cesium, field-effect electric propulsion (FEEP), floating potential, space vehicle propulsion, surface contamination. I. I NTRODUCTION F IELD-EFFECT electric propulsion (FEEP) [1] can be used for continuously adjustable micropropulsion. Contrary to the case of other types of electric propulsion (EP), such as plasma propulsion or ion propulsion, the propulsive fluid of FEEP thrusters is a reactive compound, which is potentially highly contaminant. For the FEEPs planned to be used on the MICROSCOPE mission, propulsive ions are cesium. Experi- mental investigations show that it could lead to very detrimental deposits, even inducing chemical reactions with polymers [2], [3]. Contamination is thus a primary concern when embarking such thrusters. Previous studies of cesium backflow from FEEP thrusters [4], [5] only dealt with the very near field of the plume and the return flux within few centimeters from its nozzle center. This region collects the vast majority of the return flux and is very important for the contamination and possible detrimental effects on the thruster itself. However, for the integration of the FEEPs on the spacecraft and possible damages to spacecraft coatings or arrays, the scale of concern is the system scale, with Manuscript received February 8, 2008; revised May 22, 2008. Current version published November 14, 2008. This work was supported by CNES R&T funding. J.-F. Roussel, T. Tondu, and J.-C. Matéo-Vélez are with the Space Environment Department (DESP), French National Aerospace Research Establishment (ONERA), 31055 Toulouse, France (e-mail: roussel@onera.fr; tondu@onera.fr; mateo@onera.fr). E. Chesta, S. D’Escrivan, and L. Perraud are with the Centre National d’Etudes Spatiales, 31401 Toulouse, France (e-mail: Enrico.Chesta@cnes.fr; Stephane.d-Escrivan@cnes.fr; Laurent.Perraud@cnes.fr). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPS.2008.2002541 much smaller return but possibly more detrimental effects to the platform. The scientific objective of the MICROSCOPE mission is to verify the equivalence principle to a relative accuracy of 10 −15 . The equivalence principle, the basis of Einstein’s general relativity, while it could be violated by more advanced gravity theories, states the equality of inertia and gravity masses, hence the universality of free fall. It will be verified by comparing the “free fall” of two different metal masses in the spacecraft over many orbits. This is to be achieved through a feedback loop en- suring quasi-perfect free fall of one of the masses through drag compensation. Any deviation from free fall of the other mass will be a relevant signal. This involves high-accuracy electrosta- tic accelerometers. The MICROSCOPE orbit shall be 700- km sun synchronous with ascending and descending nodes (orbit intersections with equator) at 6- and 18-h local time to avoid insulation changes and subsequent thermal constraints on the very sensitive payload (see [6] or http://microscope.onera.fr/). One of the constraints of the drag-free requirement of the MICROSCOPE mission is a control of the spacecraft floating potential or, at least, of its fast fluctuations. A nonzero potential modifies the thrust of an electric thruster, because the kinetic energy of the ions is modified before they leave the influence of the spacecraft (its sheath). Uncontrolled changes in potential, hence in thrust, would be detrimental to the drag compensation loop. Spacecraft floating potential was thus also to be studied carefully for this specific mission, which shall embark the FEEP ion guns and electron neutralizers. Although the question of the influence of FEEP on spacecraft floating potential was already raised in the literature (see, e.g., [7]), our specific ambient plasma and neutralizer conditions requested a specific assessment. These two topics are addressed in the next two sections: plume dynamics and its subsequent contamination first and then floating potential assessment next. II. LOCAL PLASMA ENVIRONMENT I NDUCED BY FEEP A. Plume Model The FEEP thrusters planned to be mounted on MICROSCOPE are manufactured by Alta S.p.A. Their general characteristics are supplied in Table I. The cesium plasma induced by MICROSCOPE FEEPs first necessitated building a model of these thrusters. Because the spacecraft mesh was planned to start at centimeter scale close to thruster exits, the approach consisted in getting plume data on that boundary around 1 cm from the FEEP slit and 0093-3813/$25.00 © 2008 IEEE