Numerical simulation techniques were used to track the evolution of species-specific momentum throughout a 5-kW class Hall thruster plume, with inclusion of magnetic field effects. All significant particle interactions were included in the model, and specific emphasis was placed on the dependence of velocity distribution functions and total plume momentum on the density of background xenon gas. To obtain the results, the coordinate-dependent influence of plume parameters such as ion flux density, neutral density, electron temperature and plasma potential was included in the calculations. Plasma potential increased significantly when the magnetic field was present. Results show that significant plume interactions occur, with initial ion beam momentum becoming substantially distributed between ion and neutral populations as the ion beam is attenuated, and increasing total momentum flux across a thruster-centric hemispherical surface at increasing downstream distance. Velocity distribution functions continue to change with propagation distance beyond 1m and exhibit angular and pressure dependence. Collision-based reduction of average ion momentum occurs in the plume as average neutral momentum increases. Integrated momentum flux variation with background pressure is obtained, due mainly to slow ions produced by charge exchange and electron impact ionization. These ions gain much more velocity than beam ions for an equivalent potential drop, but their trajectories are generally more radial than axial. Axial momentum contributions are sensitive to details of the plume potential map, with individual ions contributing negatively or positively depending on birthplace and resulting trajectory. The predicted increase of plume momentum with rising background gas density has similarities to the result of an earlier 1-D analytical study, although of lower magnitude and no longer monotonic.
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