page_title: Ion thruster
section_title: Gridded electrostatic ion thrusters
hierarchical_section_title: Ion thruster / Electrostatic ion thrusters / Gridded electrostatic ion thrusters
context_page_description: An ion thruster or ion drive is a form of electric propulsion used for spacecraft propulsion. It creates thrust by accelerating ions using electricity.
  An ion thruster ionizes a neutral gas by extracting some electrons out of atoms, creating a cloud of positive ions. These ion thrusters rely mainly on electrostatics as ions are accelerated by the Coulomb force along an electric field. Temporarily stored electrons are finally reinjected by a neutralizer in the cloud of ions after it has passed through the electrostatic grid, so the gas becomes neutral again and can freely disperse in space without any further electrical interaction with the thruster. Electromagnetic thrusters on the contrary use the Lorentz force to accelerate all species in the same direction whatever their electric charge, and are specifically referred to as plasma propulsion engines, where the electric field is not in the direction of the acceleration.
  Ion thrusters in operational use have an input power need of 1–7 kW, exhaust velocity 20–50 km/s, thrust 25–250 millinewtons and efficiency 65–80% though experimental versions have achieved 100 kilowatts, 5 newtons.
context_section_description: Gridded electrostatic ion thrusters commonly utilize xenon gas. The gaseous propellant begins with no charge; it is ionized by bombarding it with energetic electrons, as the energy transferred ejects valence electrons from the propellant gas's atoms. These electrons can be provided by a hot cathode filament and accelerated through the potential difference towards an anode. Alternatively, the electrons can be accelerated by an oscillating induced electric field created by an alternating electromagnet, which results in a self-sustaining discharge without a cathode (radio frequency ion thruster).
  The positively charged ions are extracted by a system consisting of 2 or 3 multi-aperture grids. After entering the grid system near the plasma sheath, the ions are accelerated by the potential difference between the first grid and second grid (called the screen grid and the accelerator grid, respectively) to the final ion energy of (typically) 1–2 keV, which generates thrust.
  Ion thrusters emit a beam of positively charged xenon ions. To keep the spacecraft from accumulating a charge, another cathode is placed near the engine to emit electrons into the ion beam, leaving the propellant electrically neutral. This prevents the beam of ions from being attracted (and returning) to the spacecraft, which would cancel the thrust.
  Gridded electrostatic ion thruster research (past/present):
  NASA Solar Technology Application Readiness (NSTAR), 2.3 kW, used on two successful missions
  NASA's Evolutionary Xenon Thruster (NEXT), 6.9 kW, flight qualification hardware built
  Nuclear Electric Xenon Ion System (NEXIS)
  High Power Electric Propulsion (HiPEP), 25 kW, test example built and run briefly on the ground
  EADS Radio-Frequency Ion Thruster (RIT)
  Dual-Stage 4-Grid (DS4G)
caption: Figure 2: A diagram of how a gridded electrostatic ion engine (multipole magnetic cusp type) works