The Physics of High-Velocity Ions in the Hall Thruster Near-Field

Abstract

A study of the physics underlying ion trajectories within the near-field region of a Hall thruster plume is presented. In this context, high velocity ions are ions that have been accelerated through the full potential drop of the thruster (sometimes referred to as or beam ions). Results from an experimental survey of an SPT-70 thruster plume are shown, along with simulated data from a Hall thruster code and from a plasma sheath model. Two main features are examined: the central jet along the Hall thruster centerline, and the population of ions at angles. In the experimental portion of the investigation, three diagnostic instruments were employed: (1) a Faraday probe for measuring ion current density, (2) an ExB filter for mapping ions with the primary beam energy, and (3) a Retarding Potential Analyzer (RPA) for determining ion energy distributions. In the numerical portion, two codes were employed: (1) a hybrid-PIC Hall thruster code known as HPHall, and (2) a model of the plasma sheath near the exit plane of the thruster, which was developed by the author. A comparison between the measured and simulated data sets is made, to analyze the degree to which different mechanisms are responsible for the evolution of the thruster plume in the near-field region. This analysis shows that the central jet is both a function of symmetric expansion of the ion beam as well as asymmetry in the internal potential field of the thruster. Additionally, it is suggested that energy, angle ions could be generated given a specific internal electric field configuration, while oscillations are ruled out as the cause of these ions. The results from the sheath model show that while the sheath can change trajectory angles by 10 to 20 degrees, it can not fully explain the presence of high angle ions with energies.