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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.