Ion thrusters used for electric propulsion in space need to be operated for long durations in vacuum chambers on Earth in order to verify and test their operation and durability. Thrust is produced by the ejection of high-energy ions. In ground-laboratory testing, the high-speed ions ultimately impact the end wall of the vacuum chamber and some of the chamber wall (typically a metal) is sputtered, causing the ejection of material that can travel back and deposit on the ion thruster. This is called backsputter. Backsputter rates for metals are high enough that even during a short test a coating can develop on the downstream surface of the ion thruster. This coating can short the ion-acceleration optics and insulators, leading to early atypical breakdown and failure of the ion thruster. To reduce the chance of this atypical type of breakdown, carbon targets are installed for the ions to impact. Carbon has one of the lowest sputter yields of any material and can greatly reduce the amount of backsputter on the ion thruster, but for life testing of thrusters, which involves months of operation, even this low rate of backsputter can cause problems. Several configurations of a honeycomb target were evaluated at the NASA Glenn Research Center to determine if changing the geometry of the target could reduce the backsputter rate by changing the line-of-sight arrival of the backsputtered material and trapping some of the sputter ejecta.
Configuration of honeycomb tests in relation to an ion source.
Long description of figure.
A test was conducted in a small vacuum chamber at Glenn to compare backsputter ejecta from a standard Grafoil target to that from a high-aspect-ratio aluminum honeycomb backed with Grafoil. Two configurations were tested. The first (top part of the figure) simulated magnetic bending of the beam into the honeycomb and measured the sputter ejecta deposited source location. The second (bottom part of the figure) involved placing the honeycomb at various angles with respect to the beam and measured the ejecta deposited near the source. Fused-silica substrates partially covered with Kapton tape were used to measure the thickness of sputter-deposited material arriving at that location.
The results of the inclined honeycomb testing indicated that angling the axis of the honeycomb cells at or less than 5° with respect to the ion source did not attenuate the sputter ejecta. In fact, for higher angles, there was actually greater backsputter than that observed for normal incidence on Grafoil at the same location.
However, simulating the arrival of a magnetically bent beam of ions provided substantial reductions in backsputtered ejecta in comparison to Grafoil. The backsputter rate with respect to Grafoil was reduced to 18 percent for a bend of approximately 15° and to 3 percent at a bend of 75°. Thus bending the beam into the honeycomb so that the thruster does not have a view of the surfaces where ions are impinging enables the honeycomb to trap most of the sputter ejecta and to reduce the backsputter ratein comparison to when the target is bombarded head on. The low-backsputter technique does rely upon the use of magnetic bending of the broad ion beam with sufficient angle to cause trapping. Although the magnetic bending requirement adds complexity to the vacuum facility in the region of the downstream ion beam, the technique does demonstrate that there are low-backsputter options to conventional planar targets. This encouraging result may enable greater accuracy in ion-thruster life prediction based on long-life ground testing.
Find out more about the research of Glennís Electro-Physics Branch: http://www.grc.nasa.gov/WWW/epbranch/ephome.htm
Sharon K. Miller, 216-433-2219, Sharon.K.Miller@nasa.gov; Donald A. Jaworske, 216-433-2312, Donald.A.Jaworske@nasa.gov; and Bruce A. Banks, 216-433-2308, Bruce.A.Banks@nasa.gov
Authors: Sharon K. Miller and Elisa M. Vogel
Headquarters program office: Exploration Systems
Programs/Projects: Project Prometheus, or other applications where electric propulsion is used
Last updated: October 16, 2006
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