- Email: [email protected]
Erosion and glow in the near-earth space environment
582
Nuclear Instruments and Methods in Physics Research B18 (1987) 582-586 North-Holland, Amsterdam
E R O S I O N A N D G L O W IN T H E N E A R - E A R T H S P A C E E N V I R O N M E N T R.G. A L B R I D G E , R.K. COLE, A.F. D A E C H 1), R.F. H A G L U N D Jr, C.L. J O H N S O N , H. POIS, P.M. S A V U N D A R A R A J , N.H, T O L K a n d J. Y E Center for Atomic and Molecular Physics at Surfaces, Department of Physics and A stronon~r, Vanderbilt Unwersltv, ' ". Nashville. T N 37235, USA t~ Martin Marietta Michoud Aerospace, New Orleans, LA 70189, USA
The phenomena of spaceglow and material erosion, which are observed on spacecraft that orbit the earth at low altitudes, are described. Also described are the physical environment of the low earth orbit and the methods by which laboratory research on these phenomena is conducted.
1. Introduction
When new frontiers of our physical environment are opened for exploration, scientists often discover new phenomena to be studied and new technological difficulties to be overcome. Such has been the case with the opening of the frontier of outer space. For example, spacecraft that orbit the earth at relatively low altitudes * (in the range of about 200 to about 600 km) experience two phenomena, glow and erosion, which present on the one hand intriguing scientific puzzles and on the other hand perplexing technological difficulties. Spaceglow is a luminous sheath extending some centimeters outward from surfaces that face the direction of motion of the spacecraft. The mechanisms of production of the glow phenomenon are of fundamental scientific interest and the means of eliminating it are of great technological interest, since spaceglow contributes to an unwanted background to optical measurements made by instruments in low earth orbit. Similarly, there is a twofold interest in the phenomenon of surface erosion which degrades solar panels, optical surfaces, and structural members. Such degradation can compromise the missions of short term flights and could render space platforms useless after exposure periods of months or years. By means of both laboratory and in-flight studies, investigators hope to learn the causes of these phenomena and to devise means of controlling their intensity or of circumventing their undesirable effects. We discuss here the physical environment of the low earth orbit, the nature of the glow and the erosion * The choice of low-altitude orbits is often based upon economic considerations, since large amounts of fuel would be required to repeatedly raise large payloads to higher orbits. For this reason, complex, long-term space platforms will operate in low earth orbits. 0168-583X/87/$03.50 9 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
phenomena, and the methods by which laboratory research is being carried out.
2. The environment of the low earth orbit
At a typical low-earth-orbit altitude of 300 km the atmospheric pressure is - 1 0 s Torr and the atomic and molecular components of the atmosphere have a combined total number density of about 109 cm- 3. The major constituent, comprising about 90% of the total particle density, is atomic oxygen which is created at lower altitudes from the photodissociation of molecular oxygen by ultraviolet radiation. Also present are molecular nitrogen, molecular oxygen, helium, argon, and hydrogen. The number densities of the atmospheric components as a function of altitude are shown in fig. 1, which is reproduced from a report of Leger et al. [1]. These densities vary with time of day; and the values for minimum activity can differ from the values for maximum solar activity by as much as one or two orders of magnitude. The densities plotted in fig. 1 fie between the minimum and the maximum values. In addition to the natural components of the atmosphere, the region surrounding a spacecraft can contain materials carried aloft by the craft itself. For example, materials can desorb from surfaces or can be emitted as exhaust gases from thruster engines. These processes contribute water, carbon dioxide, freons, cleaning agents, and other substances to the surroundings [2]. Also, the motion of the craft contributes to the environmental conditions in that atmospheric densities are increased on the side of the craft facing the direction of motion and decreased on the opposite side. Furthermore, although the thermal kinetic energies of the atmospheric components are quite small ( < 0.1 eV), the motion of the spacecraft results in collisions between
583
R.G. A/bridge et a L / Erosion and glow 700
600
m
500
q
400
300 km
200
100 S
10
7
6
10
9
8
10
10
10
I0
10
10
-3 NUMBER
DENSITY
cm
Fig. 1. The number densities of atmospheric components as a function of altitude. the spacecraft surfaces and the atmospheric molecules with sufficient relative speed that the molecules have an effective kinetic energy of a few electron volts. At a typical orbital speed of 8 kin/s, the effective kinetic energy of oxygen atoms is 5 eV and of nitrogen molecules is 9.5 eV. It is believed that neutral atomic oxygen, striking the craft with an energy of - 5 eV, plays an important role in spaceglow and erosion. The estimated atomic-oxygen density of - 10 9 atoms cm-3 and the spacecraft velocity of - 8 km s - t give an estimated oxygen-atom flux of -1015 oxygen atoms cm -2 s - t at space-shuttle altitudes. Since studies of material degradation carried out on shuttle missions are accomplished over a period of days, the total estimated atomic oxygen fluences for these experiments are of the order of 102~ oxygen atoms cm-
2.
Most of the oxygen atoms in space are in the atomic 3p ground state; however Ton- [3] reports that approximately one part per million ( - 103 excited atoms cm-3) are present in an excited ID metastable state (t = 147 s). Oxygen atoms in the 1D state are produced by photodissociation of 02, and by the interaction of electrons with O 2 or O; and the deexcitation of the metastable to the 3p ground state occurs by photon emission or by means of collisions with other particles, primarily N2. An oxygen atom in the metastable state is believed to be particularly reactive because of its ability to store and deliver a significant quantity (1.96 eV) of localized energy in addition to the 5 eV of translational kinetic energy.
The atomic oxygen may not act alone but synergistically with fluxes of electrons, ultraviolet photons, 02 , N~, N, NO, He, Ar, H20, and CO 2, as well as with surface adsorbates. The suprathermal energies associated with these fluxes are such that desorption can occur and endothermic chemical processes not common at laboratory temperatures can be initiated. Adsorbed gases such as H20 and CO 2 which emit in the infrared are a source of unwanted background radiation that hinders electromagnetic measurements made by onboard instruments.
3. Spaceglow and erosion
In-space studies of erosion and glow phenomena have been carried out by means of the space shuttles (primarily STS-5 and STS-8), the Solar Max Satellite and the Atmospheric Explorer Satellites. In general such experiments have measured gross feature~ of the phenomena such as total mass loss due to erosion and total glow emittance in a given wavelength range; however specific information related to the details of the processes responsible for the phenomena are difficult to obtain in space because the recording of such data is labor intensive. The spaceglow extends a few centimeters from the surfaces outward into the direction of the spacecraft's velocity vector. It displays a broad continuous spectrum which in the visible portion of the spectrum has a maximum intensity at about 7000/k. The glow has not VI. ELECTRON/ION/ATOM EMISSION
584
R.G, Alhrtdge et aL / Erosio,t and glow
yet been observed in ground-based laboratory_ experiments and the mechanisms that create it are not known. Furthermore, the relation between glow and material degradation is not understood; and existing data from space-based experiments show that there is not a clear relation between the intensity of the glow and the mass of the substrate ablated. Thus, some materials (e.g. MgF2 and black chemglaze paint) that exhibit intense glow do not evidence mass loss; and some materials (e.g. polyethylene) that erode significantly do not glow brightly [2]. There is, however, a correlation between emission intensity and atomic-oxygen density at altitudes above 180 km [2]. Also, at altitudes between 140 and 180 km the emission scales as the square of the neutral molecular density of oxygen and nitrogen. Green [2] reports that these data seem to indicate that the role of the surface in spaceglow is to act as a catalyst for reactions leading to electronically excited species or to act as an agent for the transfer of energy from external to internal modes of excitation. Green [2] divides explanations of the spaceglow phenomenon into three groups: (1) gas-phase collisions that give rise to chemical and plasma excitation processes, (2) surface-aided chemiluminescence reactions with ad-
sorbates, and (3) surface bulk reactions leading to material losses or composition changes. Kofsky and Barrett [4] have recently reviewed existing data and hypotheses relative to the spaceglow phenomena and have concluded that nitrogen dioxide is a likely candidate for the source of the glow. Data on erosion show that prefluorinated carbonbased polymers and silicon polymers have lower erosion rates than do organic materials, that filled materials have erosion rates that depend on the nature of the fillers, that organic polymers containing only carbon, hydrogen, oxygen, nitrogen, and sulfur react readily, that magnesium fluoride and magnesium oxides exhibit good stability, that metal oxides are generally resistant to erosion, that silver and osmium react rapidly, that metals other than silver and osmium show microscopic but not macroscopic changes, and that reactivities are relatively insensitive to surface temperature in the range 25-125~ [1]. Fig. 2, reproduced from data supplied by Dr. Ann Whitaker of the Marshall Space Flight Center, is an electron micrograph of a sample of FEP Teflon that was coated with 1000 ~, of inconel and silver and exposed on the Solar Max Satellite [5]. Note the pitting of the
Fig. 2. Electron micrograph of a sample of FEP Teflon exposed to the environment of the low earth orbit. (Courtesy of Dr. Ann Whitaker, Marshall Space Flight Center.)
R.G. Albridge et al. / Erosion and glow
surface. In many instances the loss of material from the exposed samples is quite significant; some organic materials have lost as much as 10 gm of surface thickness over a period of a few days (fluences of - 102o atoms cm-2) [6]. It is significant that in some instances the amount of erosion has been correlated with total oxygen fluence [1].
4. Laboratory sources of atomic oxygen
Although in-flight experiments have the advantage of being directly related to the existing environmental conditions, they do not provide opportunities for rephcation and control, and they will be difficult to schedule in hght of the present quiescence of the space program. Thus, the development of a laboratory beam of 5-eV neutral oxygen atoms with fluxes corresponding to those encountered in low earth orbits has become of critical interest to space researchers. But the task of developing such beams is not an easy one for a variety of reasons: (1) recombination of atomic oxygen can occur by means of three-body colhsions on the inner walls of the containing vessel, (2) atomic oxygen fluxes of 10 t5 cm -2 s- l correspond to relatively high beam currents of - 5 g A on a circular target of 1 mm radius, (3) ion beams are difficult to neutralize such that the focus and the energy of the beam is maintained, and (4) 5-eV ions are too low in energy to easily control, but too high in energy to generate thermally. Two means of producing atomic oxygen from gaseous, oxygen-containing molecules are photodissociation and electron-impact dissociation. The photodissociation of 0 2 has a small cross section and the process produces neutral atoms with thermal velocities; thus this method is not advantageous for the production of 5-eV neutrals. Utilizing a cold-cathode electron source, Singh and coworkers [7] have generated oxygen beams by the electron-impact ionization of gaseous molecular oxygen. The beam is a mixed-ion beam (O § and O f ) rather than a neutral atomic beam, and there is a large flux of thermal neutrals (most likely 02) that is one or two orders of magnitude larger than the ion flux. In addition, the energy spread of the beam is large. Arnold and Pephnski [8] have obtained 1-eV atomic-oxygen beams with fluxes of 1015-1016 atoms cm -2 s - l by thermally dissociating molecular oxygen be means of a modified, commercially-available plasma torch. Less than 50% of the beam is dissociated. Such sources have the disadvantages of containing impurities from the nozzle, of creating a background of hght that makes optical spectroscopy impossible, and of being too low in energy. Chutjian and Orient [9] have published a schematic diagram of a device that is designed to produce a beam of neutral oxygen atoms by the photodetachment of electrons from O - ions. Cross and Cremers [10] have
585
described a nozzle source that is driven by a CO2-1asersustained continuous optical discharge. This source has attained plasma temperatures as high as 20000 K in rare gases and is predicted to provide 8.3 k m / s oxygen atoms from a hehum-oxygen mixture. At Vanderbilt we have developed a new method of producing low-energy neutral atomic oxygen beams through the neutralization of 02+ or O + ions. This method produces, pure beams of variable energy with very little energy spread and permits studies of surface interactions by means of optical spectroscopy. The acceleration, deceleration, focusing and velocity selection of beams of atomic particles are best accomphshed by means of electric and magnetic fields acting on charged ions; however, since in the present instance the desired end product is a beam of neutral atoms, the ions must be neutralized after the final stages of energy selection and focusing. Such neutralization is not easy to accomplish in such a way that the energy and the focus of the beam is maintained. We have accomplished such neutralization of oxygen ions with minimum disturbance of the beam. Following the neutralization, an electric field maintained between a pair of parallel plates sweeps aside any ions that have not been neutralized; or the charged components of the beam are repelled by a positive potential on the target. By this means we have produced neutral atomic-oxygen beams with energies below 1 keV and have measured optical spectra of beam-surface interactions for beam energies of 1.5 keV. We are presently stepping the beam energy down to the region of 5 eV. The method of neutralization is applicable to all ions; hence the technique discussed here can be used to produce neutral nitrogen atoms and other neutral species that might play a role in the chemistry and physics of space. Our method of production of neutral atomic oxygen beams permits the energy of the beam to be varied over a wide range (from a few eV to tens of keV), provides a monocnergetic beam that is both well-focused and clean (i.e. void of other particles), and permits the beam to be used coincident with other means of excitation (e.g. UV radiation and electron impact) so that synergistic effects can be studied. The ion beams used in our work are produced from a mixture of 10% oxygen and 90% helium by means of a commericially-available Colutron ion source. After the beam is neutralized, it impacts with the target in an ultrahigh vacuum chamber (10-9 Tort), and the results of the interaction are studied in one of two ways: (1) excited species created in the interaction emit radiation which is measured by means of a grating spectrometer, or (2) ground-state neutral species are resonantly excited by means of a tunable dye laser and the fluorescent radiation is measured. By these means it is possible to obtain information about the identities and the energy states of the species desorbed from the surface of the sample. Such specific information about the atomic VI. ELECTRON/ION/ATOM EMISSION
586
R.G. ,41bridge et aL / Erosion and glow
processes occurring at the surface permit deductions to be made about the nature of the 'macroscopic phenomena observed, such as erosion and glow. It is important to note that measurements of this type usually provide information not only about the substrate materials but also about the adsorbed overlayers on the substrates. Our experience shows that the chemical nature of the ovedayer is determined in part by the nature of the substrate, that the overlayer can in some instances retard erosion, and that under particle bombardment the overlayer is eroded away so that the spectra measured vary with time. Also, our method of studying surface dynamics provides important data about interactions involving overlayers. Our previous work has shown that an important aspect of surface dynamic s is that the surface mediates the efficient transfer of external kinetic energy to interhal electronic energy and that desorption will occur only if the energy is sufficiently localized in time and space. These conclusions are relevant to the interpretation of spaceglow as a phenomenon dependent upon the energy transfer to and the subsequent desorption of adsorbed species such as nitrogen compounds. The conclusions are similarly important to the interpretation of erosion as the desorption of the substrate material following the deposition and the localization of energy on the surface.
This work was funded by Martin Marietta Michoud Aerospace under Contract A71052-A2.
References [1] L.J. Leger, J.T. Visentine and J.A. Schliesing, Proc. AIAA 23rd Aerospace Sciences Meeting, AIAA-85-0476, Reno (1985). [2] B.D. Green, Proc. AIAA Shuttle Environment and Operations II Conf., AIAA-85-6095, Houston (1985). [3] M. Torr, private communication (1986). [4] I.L. Kofsky and J.L. Barrett, Nucl. Instr. and Meth. B14 (1986) 480. [5] A. Whitaker, private communication (1986). [6] W.S. Slemp. B. Santos-Mason, G.F. Sykes Jr and W.G. Witte Jr, Proc. AIAA 23rd Aerospace Sciences Meeting, AIAA-85-0421, Reno (1985), [7] B. Singh, L.J. Amore, W. Saylor and G. Racette, Proc. AIAA 23rd Aerospace Sciences Meeting, AIAA-85-0477, Reno (1985). [8] G.S. Arnold and D.R. Peplinski, AIAA Journal 24 (1986) 673. [9] A. Chutjian and O. Orient, NASA Tech. Briefs (March/ April, 1986). [10] J.B. Cross and D.A. Cremers, Proc. AIAA 23rd Aerospace Sciences Meeting, AIAA-85-0473, Reno (1985).
Nuclear Instruments and Methods in Physics Research B18 (1987) 582-586 North-Holland, Amsterdam
E R O S I O N A N D G L O W IN T H E N E A R - E A R T H S P A C E E N V I R O N M E N T R.G. A L B R I D G E , R.K. COLE, A.F. D A E C H 1), R.F. H A G L U N D Jr, C.L. J O H N S O N , H. POIS, P.M. S A V U N D A R A R A J , N.H, T O L K a n d J. Y E Center for Atomic and Molecular Physics at Surfaces, Department of Physics and A stronon~r, Vanderbilt Unwersltv, ' ". Nashville. T N 37235, USA t~ Martin Marietta Michoud Aerospace, New Orleans, LA 70189, USA
The phenomena of spaceglow and material erosion, which are observed on spacecraft that orbit the earth at low altitudes, are described. Also described are the physical environment of the low earth orbit and the methods by which laboratory research on these phenomena is conducted.
1. Introduction
When new frontiers of our physical environment are opened for exploration, scientists often discover new phenomena to be studied and new technological difficulties to be overcome. Such has been the case with the opening of the frontier of outer space. For example, spacecraft that orbit the earth at relatively low altitudes * (in the range of about 200 to about 600 km) experience two phenomena, glow and erosion, which present on the one hand intriguing scientific puzzles and on the other hand perplexing technological difficulties. Spaceglow is a luminous sheath extending some centimeters outward from surfaces that face the direction of motion of the spacecraft. The mechanisms of production of the glow phenomenon are of fundamental scientific interest and the means of eliminating it are of great technological interest, since spaceglow contributes to an unwanted background to optical measurements made by instruments in low earth orbit. Similarly, there is a twofold interest in the phenomenon of surface erosion which degrades solar panels, optical surfaces, and structural members. Such degradation can compromise the missions of short term flights and could render space platforms useless after exposure periods of months or years. By means of both laboratory and in-flight studies, investigators hope to learn the causes of these phenomena and to devise means of controlling their intensity or of circumventing their undesirable effects. We discuss here the physical environment of the low earth orbit, the nature of the glow and the erosion * The choice of low-altitude orbits is often based upon economic considerations, since large amounts of fuel would be required to repeatedly raise large payloads to higher orbits. For this reason, complex, long-term space platforms will operate in low earth orbits. 0168-583X/87/$03.50 9 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
phenomena, and the methods by which laboratory research is being carried out.
2. The environment of the low earth orbit
At a typical low-earth-orbit altitude of 300 km the atmospheric pressure is - 1 0 s Torr and the atomic and molecular components of the atmosphere have a combined total number density of about 109 cm- 3. The major constituent, comprising about 90% of the total particle density, is atomic oxygen which is created at lower altitudes from the photodissociation of molecular oxygen by ultraviolet radiation. Also present are molecular nitrogen, molecular oxygen, helium, argon, and hydrogen. The number densities of the atmospheric components as a function of altitude are shown in fig. 1, which is reproduced from a report of Leger et al. [1]. These densities vary with time of day; and the values for minimum activity can differ from the values for maximum solar activity by as much as one or two orders of magnitude. The densities plotted in fig. 1 fie between the minimum and the maximum values. In addition to the natural components of the atmosphere, the region surrounding a spacecraft can contain materials carried aloft by the craft itself. For example, materials can desorb from surfaces or can be emitted as exhaust gases from thruster engines. These processes contribute water, carbon dioxide, freons, cleaning agents, and other substances to the surroundings [2]. Also, the motion of the craft contributes to the environmental conditions in that atmospheric densities are increased on the side of the craft facing the direction of motion and decreased on the opposite side. Furthermore, although the thermal kinetic energies of the atmospheric components are quite small ( < 0.1 eV), the motion of the spacecraft results in collisions between
583
R.G. A/bridge et a L / Erosion and glow 700
600
m
500
q
400
300 km
200
100 S
10
7
6
10
9
8
10
10
10
I0
10
10
-3 NUMBER
DENSITY
cm
Fig. 1. The number densities of atmospheric components as a function of altitude. the spacecraft surfaces and the atmospheric molecules with sufficient relative speed that the molecules have an effective kinetic energy of a few electron volts. At a typical orbital speed of 8 kin/s, the effective kinetic energy of oxygen atoms is 5 eV and of nitrogen molecules is 9.5 eV. It is believed that neutral atomic oxygen, striking the craft with an energy of - 5 eV, plays an important role in spaceglow and erosion. The estimated atomic-oxygen density of - 10 9 atoms cm-3 and the spacecraft velocity of - 8 km s - t give an estimated oxygen-atom flux of -1015 oxygen atoms cm -2 s - t at space-shuttle altitudes. Since studies of material degradation carried out on shuttle missions are accomplished over a period of days, the total estimated atomic oxygen fluences for these experiments are of the order of 102~ oxygen atoms cm-
2.
Most of the oxygen atoms in space are in the atomic 3p ground state; however Ton- [3] reports that approximately one part per million ( - 103 excited atoms cm-3) are present in an excited ID metastable state (t = 147 s). Oxygen atoms in the 1D state are produced by photodissociation of 02, and by the interaction of electrons with O 2 or O; and the deexcitation of the metastable to the 3p ground state occurs by photon emission or by means of collisions with other particles, primarily N2. An oxygen atom in the metastable state is believed to be particularly reactive because of its ability to store and deliver a significant quantity (1.96 eV) of localized energy in addition to the 5 eV of translational kinetic energy.
The atomic oxygen may not act alone but synergistically with fluxes of electrons, ultraviolet photons, 02 , N~, N, NO, He, Ar, H20, and CO 2, as well as with surface adsorbates. The suprathermal energies associated with these fluxes are such that desorption can occur and endothermic chemical processes not common at laboratory temperatures can be initiated. Adsorbed gases such as H20 and CO 2 which emit in the infrared are a source of unwanted background radiation that hinders electromagnetic measurements made by onboard instruments.
3. Spaceglow and erosion
In-space studies of erosion and glow phenomena have been carried out by means of the space shuttles (primarily STS-5 and STS-8), the Solar Max Satellite and the Atmospheric Explorer Satellites. In general such experiments have measured gross feature~ of the phenomena such as total mass loss due to erosion and total glow emittance in a given wavelength range; however specific information related to the details of the processes responsible for the phenomena are difficult to obtain in space because the recording of such data is labor intensive. The spaceglow extends a few centimeters from the surfaces outward into the direction of the spacecraft's velocity vector. It displays a broad continuous spectrum which in the visible portion of the spectrum has a maximum intensity at about 7000/k. The glow has not VI. ELECTRON/ION/ATOM EMISSION
584
R.G, Alhrtdge et aL / Erosio,t and glow
yet been observed in ground-based laboratory_ experiments and the mechanisms that create it are not known. Furthermore, the relation between glow and material degradation is not understood; and existing data from space-based experiments show that there is not a clear relation between the intensity of the glow and the mass of the substrate ablated. Thus, some materials (e.g. MgF2 and black chemglaze paint) that exhibit intense glow do not evidence mass loss; and some materials (e.g. polyethylene) that erode significantly do not glow brightly [2]. There is, however, a correlation between emission intensity and atomic-oxygen density at altitudes above 180 km [2]. Also, at altitudes between 140 and 180 km the emission scales as the square of the neutral molecular density of oxygen and nitrogen. Green [2] reports that these data seem to indicate that the role of the surface in spaceglow is to act as a catalyst for reactions leading to electronically excited species or to act as an agent for the transfer of energy from external to internal modes of excitation. Green [2] divides explanations of the spaceglow phenomenon into three groups: (1) gas-phase collisions that give rise to chemical and plasma excitation processes, (2) surface-aided chemiluminescence reactions with ad-
sorbates, and (3) surface bulk reactions leading to material losses or composition changes. Kofsky and Barrett [4] have recently reviewed existing data and hypotheses relative to the spaceglow phenomena and have concluded that nitrogen dioxide is a likely candidate for the source of the glow. Data on erosion show that prefluorinated carbonbased polymers and silicon polymers have lower erosion rates than do organic materials, that filled materials have erosion rates that depend on the nature of the fillers, that organic polymers containing only carbon, hydrogen, oxygen, nitrogen, and sulfur react readily, that magnesium fluoride and magnesium oxides exhibit good stability, that metal oxides are generally resistant to erosion, that silver and osmium react rapidly, that metals other than silver and osmium show microscopic but not macroscopic changes, and that reactivities are relatively insensitive to surface temperature in the range 25-125~ [1]. Fig. 2, reproduced from data supplied by Dr. Ann Whitaker of the Marshall Space Flight Center, is an electron micrograph of a sample of FEP Teflon that was coated with 1000 ~, of inconel and silver and exposed on the Solar Max Satellite [5]. Note the pitting of the
Fig. 2. Electron micrograph of a sample of FEP Teflon exposed to the environment of the low earth orbit. (Courtesy of Dr. Ann Whitaker, Marshall Space Flight Center.)
R.G. Albridge et al. / Erosion and glow
surface. In many instances the loss of material from the exposed samples is quite significant; some organic materials have lost as much as 10 gm of surface thickness over a period of a few days (fluences of - 102o atoms cm-2) [6]. It is significant that in some instances the amount of erosion has been correlated with total oxygen fluence [1].
4. Laboratory sources of atomic oxygen
Although in-flight experiments have the advantage of being directly related to the existing environmental conditions, they do not provide opportunities for rephcation and control, and they will be difficult to schedule in hght of the present quiescence of the space program. Thus, the development of a laboratory beam of 5-eV neutral oxygen atoms with fluxes corresponding to those encountered in low earth orbits has become of critical interest to space researchers. But the task of developing such beams is not an easy one for a variety of reasons: (1) recombination of atomic oxygen can occur by means of three-body colhsions on the inner walls of the containing vessel, (2) atomic oxygen fluxes of 10 t5 cm -2 s- l correspond to relatively high beam currents of - 5 g A on a circular target of 1 mm radius, (3) ion beams are difficult to neutralize such that the focus and the energy of the beam is maintained, and (4) 5-eV ions are too low in energy to easily control, but too high in energy to generate thermally. Two means of producing atomic oxygen from gaseous, oxygen-containing molecules are photodissociation and electron-impact dissociation. The photodissociation of 0 2 has a small cross section and the process produces neutral atoms with thermal velocities; thus this method is not advantageous for the production of 5-eV neutrals. Utilizing a cold-cathode electron source, Singh and coworkers [7] have generated oxygen beams by the electron-impact ionization of gaseous molecular oxygen. The beam is a mixed-ion beam (O § and O f ) rather than a neutral atomic beam, and there is a large flux of thermal neutrals (most likely 02) that is one or two orders of magnitude larger than the ion flux. In addition, the energy spread of the beam is large. Arnold and Pephnski [8] have obtained 1-eV atomic-oxygen beams with fluxes of 1015-1016 atoms cm -2 s - l by thermally dissociating molecular oxygen be means of a modified, commercially-available plasma torch. Less than 50% of the beam is dissociated. Such sources have the disadvantages of containing impurities from the nozzle, of creating a background of hght that makes optical spectroscopy impossible, and of being too low in energy. Chutjian and Orient [9] have published a schematic diagram of a device that is designed to produce a beam of neutral oxygen atoms by the photodetachment of electrons from O - ions. Cross and Cremers [10] have
585
described a nozzle source that is driven by a CO2-1asersustained continuous optical discharge. This source has attained plasma temperatures as high as 20000 K in rare gases and is predicted to provide 8.3 k m / s oxygen atoms from a hehum-oxygen mixture. At Vanderbilt we have developed a new method of producing low-energy neutral atomic oxygen beams through the neutralization of 02+ or O + ions. This method produces, pure beams of variable energy with very little energy spread and permits studies of surface interactions by means of optical spectroscopy. The acceleration, deceleration, focusing and velocity selection of beams of atomic particles are best accomphshed by means of electric and magnetic fields acting on charged ions; however, since in the present instance the desired end product is a beam of neutral atoms, the ions must be neutralized after the final stages of energy selection and focusing. Such neutralization is not easy to accomplish in such a way that the energy and the focus of the beam is maintained. We have accomplished such neutralization of oxygen ions with minimum disturbance of the beam. Following the neutralization, an electric field maintained between a pair of parallel plates sweeps aside any ions that have not been neutralized; or the charged components of the beam are repelled by a positive potential on the target. By this means we have produced neutral atomic-oxygen beams with energies below 1 keV and have measured optical spectra of beam-surface interactions for beam energies of 1.5 keV. We are presently stepping the beam energy down to the region of 5 eV. The method of neutralization is applicable to all ions; hence the technique discussed here can be used to produce neutral nitrogen atoms and other neutral species that might play a role in the chemistry and physics of space. Our method of production of neutral atomic oxygen beams permits the energy of the beam to be varied over a wide range (from a few eV to tens of keV), provides a monocnergetic beam that is both well-focused and clean (i.e. void of other particles), and permits the beam to be used coincident with other means of excitation (e.g. UV radiation and electron impact) so that synergistic effects can be studied. The ion beams used in our work are produced from a mixture of 10% oxygen and 90% helium by means of a commericially-available Colutron ion source. After the beam is neutralized, it impacts with the target in an ultrahigh vacuum chamber (10-9 Tort), and the results of the interaction are studied in one of two ways: (1) excited species created in the interaction emit radiation which is measured by means of a grating spectrometer, or (2) ground-state neutral species are resonantly excited by means of a tunable dye laser and the fluorescent radiation is measured. By these means it is possible to obtain information about the identities and the energy states of the species desorbed from the surface of the sample. Such specific information about the atomic VI. ELECTRON/ION/ATOM EMISSION
586
R.G. ,41bridge et aL / Erosion and glow
processes occurring at the surface permit deductions to be made about the nature of the 'macroscopic phenomena observed, such as erosion and glow. It is important to note that measurements of this type usually provide information not only about the substrate materials but also about the adsorbed overlayers on the substrates. Our experience shows that the chemical nature of the ovedayer is determined in part by the nature of the substrate, that the overlayer can in some instances retard erosion, and that under particle bombardment the overlayer is eroded away so that the spectra measured vary with time. Also, our method of studying surface dynamics provides important data about interactions involving overlayers. Our previous work has shown that an important aspect of surface dynamic s is that the surface mediates the efficient transfer of external kinetic energy to interhal electronic energy and that desorption will occur only if the energy is sufficiently localized in time and space. These conclusions are relevant to the interpretation of spaceglow as a phenomenon dependent upon the energy transfer to and the subsequent desorption of adsorbed species such as nitrogen compounds. The conclusions are similarly important to the interpretation of erosion as the desorption of the substrate material following the deposition and the localization of energy on the surface.
This work was funded by Martin Marietta Michoud Aerospace under Contract A71052-A2.
References [1] L.J. Leger, J.T. Visentine and J.A. Schliesing, Proc. AIAA 23rd Aerospace Sciences Meeting, AIAA-85-0476, Reno (1985). [2] B.D. Green, Proc. AIAA Shuttle Environment and Operations II Conf., AIAA-85-6095, Houston (1985). [3] M. Torr, private communication (1986). [4] I.L. Kofsky and J.L. Barrett, Nucl. Instr. and Meth. B14 (1986) 480. [5] A. Whitaker, private communication (1986). [6] W.S. Slemp. B. Santos-Mason, G.F. Sykes Jr and W.G. Witte Jr, Proc. AIAA 23rd Aerospace Sciences Meeting, AIAA-85-0421, Reno (1985), [7] B. Singh, L.J. Amore, W. Saylor and G. Racette, Proc. AIAA 23rd Aerospace Sciences Meeting, AIAA-85-0477, Reno (1985). [8] G.S. Arnold and D.R. Peplinski, AIAA Journal 24 (1986) 673. [9] A. Chutjian and O. Orient, NASA Tech. Briefs (March/ April, 1986). [10] J.B. Cross and D.A. Cremers, Proc. AIAA 23rd Aerospace Sciences Meeting, AIAA-85-0473, Reno (1985).