C.7.2 Electric propulsion thrusters

In ion engines the propellant gas, is fed into a discharge chamber where it is ionized by means of electron bombardment interactions. A set of grids located at the end of the chamber then accelerates the positive ions by means of electric fields applied between them. Exhaust velocities of 30 to 50 km/s are achieved on these engines.

The typical Hall thruster (or SPT) has the shape of an axially symmetrical annular shell. The propellant gas is injected into the discharge chamber from the anode side. The electrons emitted by the neutralizer cathode are prevented from streaming directly to the anode by a radial magnetic field. Their resulting azimuthal motion within the discharge chamber provides both the collisions with the neutral gas which form the ionized plasma and the Hall effect which accounts for the ejection of the positive ions at typical speeds of 10 to 20 km/s.

FEEP systems have very high specific impulse (8 000 sec) and high throtability. Clusters of ions are emitted from a spike or an edge on which a strong electric field is set.

Most thrusters use Xenon as a propellant. Some use metallic propellants such as Lithium, Indium or Caesium.