An ion thruster is a form of electric propulsion used for
spacecraft propulsion that creates thrust by accelerating ions. Ion thrusters
are categorized by how they accelerate the ions, using either electrostatic or
electromagnetic force. Electrostatic ion thrusters use the Coulomb force and
accelerate the ions in the direction of the electric field. Electromagnetic ion
thrusters use the Lorentz force to accelerate the ions. Note that the term
"ion thruster" frequently denotes the electrostatic or gridded ion
thrusters only.
The thrust created in ion thrusters is very small compared
to conventional chemical rockets, but a very high specific impulse, or
propellant efficiency, is obtained. This high propellant efficiency is achieved
through the very frugal propellant consumption of the ion thruster propulsion
system. They do, however, use a large amount of power. Given the practical
weight of suitable power sources, the accelerations given by these types of
thrusters is of the order of one thousandth of standard gravity.
Due to their relatively high power needs, given the specific
power of power supplies, and the requirement of an environment void of other
ionized particles, ion thrust propulsion is currently only practical beyond
planetary atmosphere (in space).
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.