Three-dimensional measurement of a stationary plasma plume with a Faraday probe array

Recently, an increasing number of small satellites have chosen electric propulsion (EP) as the propulsion system for motion control in space flights. Research on the ejected high energy plasma plumes from EP thrusters has attracted significant interest due to their interactions with the components of satellites. This work demonstrates a probe array diagnostic platform (with 67 Faraday probes on the array) that enables the acquisition of the full three-dimensional morphology of the plume current density from the near-field to the far-field of EP stationary thrusters. The plume parameters of an ion thruster including the 3-D plume morphology, divergence angle, and thrust vector angle are measured using this diagnostic platform. From 250 to 700 mm from the thruster exit, the plume field exhibits a stable divergence angle of 43.6°. By fitting circles to the contours of each cut plane, the average thrust vector angle is calculated to be 1.3°, and the repeatability error for the circle center fitting is approximately 2.2%. The current density profiles on exit cut planes demonstrate an “elliptical distortion” phenomenon in the plume shape. Using the ellipse fitting method, it is found that an elliptical shape was formed in the near-field of the plume. As the plume propagates to the far-field, the distortion of the ellipse becomes more apparent (the aspect ratio is 1:1.14). Some possible causes may be the geo-magnetic field, background vacuum environment distribution, or thruster manufacturing error. The thrust vector data can provide a basis for the control strategy of the satellites. In addition, the 3-D plume current density data can give us a comprehensive understanding of the plume plasma physics. These results may also provide an accurate and rich database for parameters for future plume simulations.