Very-near-field plume study of a 1.35 kW SPT-100

Ion current density and electron number density profiles were measured in the very-nearfield plume of a 1.35 kW stationary plasma thruster (SPT-100) loaned from Space Systems/Loraland Fakel Enterprises. In order to take these measurements without significantly disturbing plasma flow while data were collected, a small Faraday probe and a single Langmuir probe were constructed and used with high-speed electronics for fast spatial sweeps. The data showed that there were distinct peaks in ion current density and electron number density related to the position of the thruster discharge chamber exit in the very-nearfield plume. These peaks decreased and broadened as the axial position from the thruster exit increased. The Faraday probe data showed that a diverging annular ion beam came out of the discharge chamber. The inner boundary converged at or near 100 mm downstream of the exit plane. The Langmuir probe data gave insight to the competing effects between magnetic field confinement and diffusive processes.