The Thrust of a Collisional-Plasma Source

The thrust provided by a plasma source, open at one end, is calculated for an arbitrary ratio of collision to ionization rates. If the plasma is of a low collisionality, ion pumping is dominant, and the momentum of the jet exiting the source is carried mostly by the plasma. In the collisional regime, neutrals are accelerated through charge-exchange collisions with ions, leading to neutral pumping, so that most of the momentum of the jet is carried by the neutral gas. It is shown that these ion–neutral collisions increase the thrust for a given power. However, the conventionally defined efficiency for a thruster, reflecting also the propellant utilization, is lower in the collisional regime.