Low Energy Sputtering Experiments using RBS
EMSL Project ID
3510
Abstract
The recent successful demonstration of the NASA Solar electric propulsion Technology Application Readiness (NSTAR) ion thruster on the Deep Space 1 mission has paved the way for future electrostatic propulsion missions. Ion thrusters are low thrust, high specific impulse devices that require long operating times, on the order of ten thousand hours, to impart the necessary impulse to a spacecraft. Many modes of failure of an electrostatic ion thruster are due to long term damage accumulation from ion bombardment sputtering. The cathode, cathode keeper, screen grid, and accelerator grid are the engine parts that are susceptible to failure due to sputtering damage. Sputtering yields for xenon-molybdenum systems are not well known for low ion energies especially energies less than 100 eV. Therefore, predictions of damage that engine parts will sustain from low energy sputtering are highly uncertain. The discharge cathode in particular sustains most its damage from low energy sputtering so predicting its lifetime is difficult.Low energy sputtering results from two different processes. Doubly charged ions impinge on cathode potential surfaces after accelerating through the potential difference between the cathode and anode potential surfaces, approximately 25 volts, giving them an energy of about 50 eV. Some low energy ions are also the result of charge exchange collisions that take place near the ion grid optics. A high energy ion collides with a low energy neutral atom leading to a charge exchange between the two particles. The newly formed low energy ion may not be properly focused through the grid aperture or may back stream into the grid leading to impingement and grid erosion.
A need exists for more research on sputtering caused by heavy ions at low energies. Most past sputtering research has been done with light ions. In current literature for xenon-molybdenum systems, there are little data for sputtering taking place below energies of 100 eV. These data are crucial for the analysis of cathode and grid optics erosion necessary for predicting thruster lifetime. Even 100 eV ion energy tests exhibit a large spread in the published data.
The experiments that I propose are a continuation of the research that NASA Glenn researcher Maris Mantenieks conducted. Molybdenum samples will undergo sputtering by xenon ions produced with an ion gun in an ultra high vacuum facility. The sputtered Mo will be collected on strips of aluminum foil. At Pacific Northwest National Laboratory, with assistance from Dr. Shutthanandan, these strips of foil will be tested with Rutherford Backscattering Spectrometry (RBS) to determine the amount of Mo deposited leading to a calculation of the sputtering yield. Tests will be conducted using ion energies of 100 eV to compare the data to previous tests performed by Mantenieks and others. More tests will be performed at 70 eV and possibly lower energies. The aluminum foil samples will be brought to PNNL to be tested in June during the week of RBS time allocated to the experiment.
Project Details
Project type
Exploratory Research
Start Date
2003-06-15
End Date
2005-06-20
Status
Closed
Released Data Link
Team
Principal Investigator