possible, i.e. with a narrow air gap, to achieve a saturated corona current condition that produces maximum thrust. However, if the emitter is too close to the collector it tends to arc across the gap. Ion propulsion systems require many safety precautions due to the required high voltage. Emitter The emitter wire is typically connected to the positive terminal of the high voltage power supply. In general, it is made from a small gauge bare conductive wire. While copper wire can be used, it does not work as well as stainless steel. Similarly, thinner wire such as 44 or 50 gauge tends to outperform more common, larger sizes such as 30 gauge, as the stronger electric field around the smaller diameter wire results in lower ionisation onset voltage and a larger corona current as described by Peek's law. The emitter is sometimes referred to as the "corona wire" because of its tendency to emit a purple corona discharge glow while in use. This is simply a side effect of ionization. Air gap The air gap insulates the two electrodes and allows the ions generated at the emitter to accelerate and transfer momentum to neutral air molecules, before losing their charge at the collector. The width of the air gap is typically 1 mm / kV. Collector The collector is shaped to provide a smooth equipotential surface underneath the corona wire. Variations of this include a wire mesh, parallel conductive tubes, or a foil skirt with a smooth, round edge. Sharp edges on the skirt degrade performance, as it generates ions of opposite polarity to those within the thrust mechanism. See also Hall-effect thruster Ion thruster Magnetoplasmadynamic thruster Plasma actuator References Sources DR Buehler, Exploratory Research on the Phenomenon of the Movement of High Voltage Capacitors. Journal of Space Mixing, 2004 FX Canning, C Melcher, E Winet, Asymmetrical Capacitors for Propulsion. 2004. GVi Stephenson The Biefeld Brown Effect and the Global Electric Circuit. AIP Conference Proceedings, 2005. External links Electrostatic Antigravity on NASA's "Common Errors in propulsion" page NASA: Asymmetrical Capacitors for Propulsion Electric aircraft Electrostatic motors Propulsion American inventions fr:Propulsion électrocinétique