Propagation of charge-exchange plasma produced by an ion thruster

A charge-exchange plasma is produced downstream of ion thrusters by collisions between energetic ions and neutrals escaping through the ion optics. The charge-exchange ions flow radially from the thruster beam due to electric fields produced by its density gradient. The propagation of the charge-exchange plasma after it leaves the thruster beam is the subject of this paper. Under the proper conditions there is an 'end-effect' of a long, cylindrical Langmuir probe which allows a significant increase in collected ion current when the probe is aligned with a flowing plasma. This effect is used to determine the charge-exchange plasma flow direction at various locations relative to the ion thruster. A portion of the charge-exchange plasma flows upstream of the ion thruster and can represent a contamination source to electrically propelled spacecraft. The ion current collected by the probe as a function of its angle with respect to the plasma flow allows determination of the plasma density and plasma flow velocity at the probe's location upstream of the ion thruster optics. The density value obtained from the ion current agree to within a factor of two of density values obtained by typical voltage-current Langmuir probe characteristics.