Magnetic Field Effects on the Plume of a Diverging Cusped-Field Thruster

Abstract

The Diverging Cusped-Field Thruster (DCFT) uses three permanent ring magnets of
\nalternating polarity to create a unique magnetic topology intended to reduce plasma losses
\nto the discharge chamber surfaces. The magnetic field strength within the DCFT discharge
\nchamber (up to 4 kG on axis) is much higher than in thrusters of similar geometry, which is
\nbelieved to be a driving factor in the high measured anode efficiencies. The field strength
\nin the near plume region is large as well, which may bear on the high beam divergences
\nmeasured, with peaks in ion current found at angles of around 30-35 from the thruster
\naxis. Characterization of the DCFT has heretofore involved only one magnetic topology.
\nIt is then the purpose of this study to investigate changes to the near-field plume caused by
\naltering the shape and strength of the magnetic field. A thick magnetic collar, encircling
\nthe thruster body, is used to lower the field strength outside of the discharge chamber
\nand thus lessen any effects caused by the external field. Changes in the thruster plume
\nwith field topology are monitored by the use of normal Langmuir and emissive probes
\ninterrogating the near-field plasma. Results are related to other observations that suggest
\na unified conceptual framework for the important near-exit region of the thruster.