Ion Thruster - Electrostatic Ion Thrusters - Gridded Electrostatic Ion Thrusters

Gridded Electrostatic Ion Thrusters

Gridded electrostatic ion thrusters commonly utilize xenon gas. This gas has no charge and is ionized by bombarding it with energetic electrons. These electrons can be provided from a hot cathode filament and when accelerated in the electrical field of the cathode, fall to the anode (Kaufman type ion thruster). Alternatively, the electrons can be accelerated by the oscillating electric field induced by an alternating magnetic field of a coil, which results in a self-sustaining discharge and omits any cathode (radio frequency ion thruster).

The positively charged ions are extracted by an extraction system consisting of 2 or 3 multi-aperture grids. After entering the grid system via the plasma sheath the ions are accelerated due to the potential difference between the first and second grid (named screen and accelerator grid) to the final ion energy of typically 1-2 keV, thereby generating the thrust.

Ion thrusters emit a beam of positive charged xenon ions only. To avoid charging-up the spacecraft, another cathode is placed near the engine, which emits electrons (basically the electron current is the same as the ion current) into the ion beam. This also prevents the beam of ions from returning to the spacecraft and thereby cancelling the thrust.

Gridded electrostatic ion thruster research (past/present):

• NASA Solar electric propulsion 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)