Source of Fast Oxygen Atoms for Testing Satellite Materials

Physical Sciences Inc., Andover, MA

We have developed an apparatus that generates a pulsed beam of fast oxygen atoms, under Small Business Innovation Research (SBIR) funding by NASA's Jet Propulsion Laboratory, to provide a ground test facility to qualify materials to be used in low Earth orbit (LEO). This development was in response to a number of early space-shuttle observations of material erosion and property change resulting from interaction with the LEO atmosphere, which is predominantly monatomic oxygen. These interactions occur at orbital speeds ≈8 km/s. To our knowledge, ours is the only system that provides a high-flux beam of neutral oxygen atoms with wide-area operating capability at the desired speed of 8 km/s. Sets of materials (2.5-by-2.5-cm samples) have been simultaneously bombarded by 8-km/s oxygen atoms at fluences of up to 6 ×10²¹ cm^-2 in our facility.

The hyperthermal-oxygen-atom source is housed in two stainless-steel six-way crosses, including an 8-in. (20-cm) cross source chamber housing a pulsed oxygen-valve/nozzle assembly, connected to a 16-in. (41-cm) cross expansion chamber. A cryopump attached to the large cross maintains a base pressure of 3 ×10^-7torr (4 × 10^-5 Pa).

A Summary highlights the properties of the O-Beam.

The oxygen atoms are generated in a pulsed laser discharge in pure O₂. A 12-J/pulse CO₂ laser is focused into a conical expansion nozzle, which has been partially filled with O₂ by a pulsed-beam valve. The resulting plasma, ignited at the throat, expands out the nozzle, dissociating the molecular oxygen in front of it. The nozzle was designed to allow recombination of ions and electrons while the slower kinetics of atom/atom recombination maintains a highly dissociated beam. The result, at 8 km/s, is an approximately 50-µs pulse of highly dissociated oxygen (>80 percent atoms) with less than 1 percent ion content (the beam is charge-neutral).

The speed (5 to 12 km/s) of atoms in the beam is selected by varying the time delay between the pulsing of the O₂ valve and the triggering of the CO₂ laser. The delay determines the mass of O₂ processed by the pulsed discharge; this mass is inversely related to the speed of the atoms in the beam.

The fidelity of the LEO simulation is critical for evaluation of materials, and so considerable effort has been expended in characterizing the beam. The measured beam properties are summarized in the accompanying table. A variety of diagnostics have been developed for use with the system.

Our O-beam facility has been employed on several occasions to pretest materials that were subsequently flown in space experiments. The ground-test data and flight-test data were found to be in substantial agreement. A most recent large ground test was for the SEE (Space Environments and Effects) program on materials for the EOIM-3 (Evaluation of Oxygen Interactions with Materials Experiment -3) flight. We irradiated 84 materials following a careful protocol of pre- and post-test weighing. The Jet Propulsion Laboratory performed subsequent surface analysis. With a few understood exceptions, as before, the ground-test and flight-test results for these materials were found to be in substantial agreement, demonstrating our ability to simulate LEO conditions.

Our facility is used to provide material-evaluation services. To date, we have tested over 2,000 samples for both government and industrial customers. The facility has also been utilized to investigate other phenomena of importance to LEO operations, including contamination cleanup, scattering distributions and accommodation coefficients, shuttle glow, and excitation of emissions from contaminant gasses.

The basic concept of the fast-atomic-oxygen source has been extended to form fast atomic beams of other species; for example, fluorine, chlorine, and nitrogen atoms, for potential applications in the semiconductor industry.

Physical Sciences Inc. designs, builds, and licenses customized versions of our fast-atom source. Such systems are presently operational at the Center for Studies in Research and Technology, in Toulouse, France, and the European Space Research and Technology Center in Noordwijk, the Netherlands. A system has also recently been delivered to NASDA Tsukuba Space Center in Japan.

This work was performed by George Caledonia and Robert Krech of Physical Sciences Inc. under an SBIR contract monitored by NASA's Jet Propulsion Laboratory. For more information, please call (978) 689-0003 or E-mail to caledonia@ psicorp.com. SBIR0003