The plasma properties of the very-near-field (10-50 mm) plume of the D55 anode layer thruster (TAL) were measured, The D55 is the 1.35-kW TAL counterpart to the SPT-100 and was made by the Central Scientific Research Institute of Machine Building of Kaliningrad, Russia. The thruster was tested in the 6 m diameter x 9 m long vacuum chamber at the University of Michigan's Plasmadynamics and Electric Propulsion Laboratory, and the diagnostic probes were positioned using a three-axis translation table system. Water-cooled Hail probes, a Faraday probe, emissive probes, and langmuir probes were used to examine the near-field plasma properties. Water-cooled Hall probes were employed to explore the effect of the closed-drift current on the radial magnetic field. The change in the magnetic field during thruster operation was found to be less than 5% over the region examined, which indicated that the Hall current was limited to several tens of amperes. Evidence also indicated that the closed-drift current extended between 5 and 10 mm downstream of the anode. Ion current density profiles showed that the annular beam focuses within 40 mm of the thruster exit plane. Plasma potential measurements indicated that ion acceleration occurred primarily within 10 mm of the anode. The highest electron temperature measured in this investigation occurred immediately downstream of the anode, and the temperature decreased with axial distance from the thruster. The low-energy electrons were confined to the high-density core of the plasma beam.
