ion thruster

Electrostatic thrusters (“ion engines”) are the best developed type of electric propulsion de­ vice, dating in conception to the 1950’s, and having been demonstrated in space in 1964 on a suborbital flight of the SERT I spacecraft. The early history and concepts are well doc­ umented, and evolved through progressive refinements of various types of ion beam sources used in Physics laboratories, the improvements being essentially dictated by the needs for high efficiency, low mass and long life for these sources to be used in space. Of the various configurations discussed in the literature, only the electron bombardment noble gas type, plus (in Europe) the radio-frequency ionized thruster and (in Japan) the Electron Cyclotron Resonance thruster, have survived. Other interesting concepts, such as Cesium Contact thrusters and duo-plasmatron sources have been largely abandoned, and two new special devices, the Field Emission Electrostatic Propulsion (FEEP) and Ionic Liquid Ion Source (ILIS) have been added to the roster. The electron bombardment thruster itself has evolved in the same time interval from rela­ tively deep cylindrical shapes with uniform magnetic fields produced by external coils and with simple thermionic cathodes, to shallow geometrics using sharply nonuniform mag­ netic field configurations, produced by permanent magnets, and with hollow cathode plasma bridges used as cathode and neutralizer. While typical ion production cost is 400-600 eV for Hg at 80% mass utilization fraction, additional work with ring-cusp thrusters has yielded for example a cost of 116 eV in Xenon at the same utilization. Such reductions make it now possible to design for efficient operation (above 80%) with environmentally acceptable noble gases at specific impulses below 3000 sec, a goal that seemed elusive a few years back. The major uncertain issues in this field seem now reduced to lifetime (measured in years of operation in orbit) and integration problems, rather than questions of cost and physical principle or major technological hurdles. Extensions to higher power (tens of kW) and higher specific impulse (to 7,000 8,000 s) are now being pursued by NASA for planetary missions requiring high Δv. Principles of Operation Electrostatic thrusters accelerate heavy charged atoms (ions) by means of a purely electro­ static field. Magnetic fields are used only for auxiliary purposes in the ionization chamber. It is well known that electrostatic forces per unit area (or energies per unit volume) are of the order of 1 2 ε 0 E 2 , where E is the strength of the field (V/m). Typical maximum fields, as limited by vacuum breakdown or shorting due to imperfections, are of the order of 10 6 V/m, yielding maximum force densities of roughly 5 N/m 2 . This low force density is one of the major drawbacks of electrostatic engines, and can be compared to force densities of the order of 10 4 N/m 2 in self-magnetic devices such as MPD thrusters, or to the typical gas pressures of 10 6 − 10 7 N/m 2 in chemical rockets. Simplicity and efficiency must therefore compensate for this disadvantage. The main elements of an electrostatic thruster are summarized in the figure below. Neu­ tral propellant is injected into an ionization chamber, which may operate on a variety of principles: electron bombardment (shown in the figure), contact ionization, radio-frequency ionization, etc. The gas contained in the chamber may only be weakly ionized in the steady state, but ions are extracted preferentially to neutrals, and so, to a first approximation, we may assume that only ions and electrons leave this chamber. 1 The ions are accelerated by a potential difference V T applied between perforated plates (grids) and this same potential keeps electrons from also leaving through these grids. The electrons from the ionization chamber are collected by an anode, and in order to prevent very rapid negative charging of the spacecraft (which has very limited electrical capacity), they