Special features of electric propulsion

In most electric thrusters, the propellant atoms (A) have to be converted into ions in a plasma discharge before being accelerated to high speed and then ejected. Ionization results from collisions between plasma atoms and electrons: A + e → A ⁺ + e + e. Nevertheless, only a fraction of the ergol injected into the system is transformed and therefore actually participates in thrust. The ionization efficiencyη _i of the ergol is therefore an important term in electric propulsion, as it largely conditions the thrust level. This term is written :