Fast Oxygen Atom Testing Services

PSI operates several aerospace testing facilities at their Andover, Massachusetts headquarters to simulate the interactions of spacecraft with the low earth orbit (LEO) environment. At the core of these facilities is the PSI patented FAST™ atomic oxygen source that produces an intense beam of oxygen atoms to simulate the chemical environment found on low earth orbit. It is the only atomic oxygen source that provides a high-flux beam of neutral oxygen atoms with wide-area coverage at 8 km/s.

With its fast oxygen atom test facility, Physical Sciences Inc. has contributed to the development of a database to understand the low earth orbit (LEO) environment. Increased LEO activities - such as NASA shuttle flights - in the last decade has renewed interest in high-velocity, gas-gas, and gas-surface interactions under rarefied conditions. Early space shuttle flights provided intriguing observations of material erosion, significant near-field gaseous contamination, and localized radiative signatures. Many of these observations were believed due to, or affected by, the interaction of atmospheric oxygen atoms with shuttle surfaces or with the self-induced contaminant cloud surrounding the shuttle. These interactions occur at orbital velocity, ~8 km/s, thus providing the collisional energy to initiate chemical interactions not accessible at thermal energies.

A significant database on fast oxygen atom interactions is needed to properly understand and control interactions between spacecraft and the LEO environment. The required database will not only quantify spacecraft material erosion or modification due to impact from fast oxygen atoms, but it will also determine the fate of the erosion products, evaluate radiative signatures from the erosion or interaction with surface contaminants, and measure the concomitant heat and momentum transfer to the surface. Furthermore, the interactions of the ambient molecules will help define the local environment around the spacecraft; therefore, scattering distributions from surface interactions, as well as collision cross sections for gas-gas-elastic and inelastic processes, must also be determined.

PSI has developed a unique high-flux, wide-area source of 8 km/s oxygen atoms that is being used to develop the required database. (A schematic of the Fast-2 embodiment of our source is shown in Figure 1.)

Figure 1. Schematic of Fast-2 System

Our device uses a combination of fast-acting valves, laser-induced breakdown, and hypersonic expansion within a vacuum chamber to produce a pulsed, expanding beam of relatively mono-energetic oxygen atoms that can be directed toward material or gaseous targets.

The beam source is used to evaluate the interaction of fast oxygen atoms with materials for purposes of LEO qualification. A wide variety of materials have been tested in our facility up to oxygen atom fluences of 6 �- 10²¹ cm^-2. We can provide simultaneous exposure of numerous samples and employ kapton witness samples as a calibration standard.

Figure 2 is an example of material behavior we observed. The test fluence level is similar to that experienced in a one-week shuttle flight. Significant erosion was observed in this carbon-fiber-reinforced plastic, with noticeably different erosive micro-structures in the fiber and matrix components.

Figure 2. Scanning electron micrograph analysis of carbon-fiber-reinforced plastic irradiated in PSI Fast Oxygen Atom Source. Top left contrasts virgin and irradiated materials. Remaining views emphasize erosion patterns at increased magnification, as shown.

A brochure describing our material testing services, including information on test procedures, fluences, and rates is available on request. PSI also builds and delivers custom-designed fast atom sources for material testing applications.

Our source is also being used to investigate other phenomena important to LEO applications. These studies include:

• Evaluating fast oxygen atom/surface accommodation coefficients and scattering distributions
• Measuring visible and infrared surface glows arising from fast oxygen atom surface / adsorbate interactions
• Determining fast oxygen atom/gas visible and infrared excitation cross sections
• Specifying fast oxygen atom surface contaminant scouring efficiencies.

Figure 3 is an example of our ability to simulate LEO phenomena in our laboratory.

Figure 3. PSI Fast Atom Facility Simulation

Here, flight measurements of the spectral distribution of the shuttle glow, a visible radiation signature observed above ram surfaces, is contrasted with a laboratory measurement taken with the PSI fast oxygen source impacting an NO-dosed surface. The agreement is excellent between the two data sets, clearly validating a postulated mechanistic interpretation of the glow.

PSI is prepared to undertake specific R&D efforts in all of these areas. Moreover, our laser-induced breakdown technique has been expanded to form fast beams of other species, e.g., nitrogen atoms, for similar investigations.

Robert H. Krech Physical Sciences Inc. 20 New England Business Center Andover, MA 01810-1077 Telephone: (978) 689-0003 Fax: (978) 689-3232 krech@psicorp.com