Bruce A. Banks

Science Application International Corporation, Cleveland, Ohio

Christian A. Kneubel

BAB Technology, LLC, Olmsted Township, Ohio

Sharon K. Miller

Glenn Research Center, Cleveland, Ohio

Atomic Oxygen Energy in Low Frequency

Hyperthermal Plasma Ashers

NASA/TM—2014-218412

November 2014
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Bruce A. Banks

Science Application International Corporation, Cleveland, Ohio

Christian A. Kneubel

BAB Technology, LLC, Olmsted Township, Ohio

Sharon K. Miller

Glenn Research Center, Cleveland, Ohio

Atomic Oxygen Energy in Low Frequency

Hyperthermal Plasma Ashers

NASA/TM—2014-218412

November 2014

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
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NASA/TM—2014-218412 1 

Atomic Oxygen Energy in Low Frequency 

Hyperthermal Plasma Ashers 

Bruce A. Banks 

Science Application International Corporation 

Cleveland, Ohio 44135 

Christian A. Kneubel 

BAB Technology, LLC 

Olmsted Township, Ohio 44138 

Sharon K. Miller 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract 

Experimental and analytical analysis of the atomic oxygen erosion of pyrolytic graphite as well as 

Monte Carlo computational modeling of the erosion of Kapton H (DuPont, Wilmington, DE) polyimide 

was performed to determine the hyperthermal energy of low frequency (30 to 35 kHz) plasma ashers 

operating on air. It was concluded that hyperthermal energies in the range of 0.3 to 0.9 eV are produced 

in the low frequency air plasmas which results in texturing similar to that in low Earth orbit (LEO). 

Monte Carlo computational modeling also indicated that such low energy directed ions are fully capable 

of producing the experimentally observed textured surfaces in low frequency plasmas. 

Introduction 

Radio frequency (RF - 13.56 MHz) plasma ashers have been highly successful in identifying the 

suitability of atomic oxygen protective coatings for low Earth orbital (LEO) spacecraft materials such as 

the solar array blankets on the International Space Station (Refs. 1 to 3). However, recent atomic oxygen 

durability experiments performed in low frequency (LF - 30 to 35 kHz) plasma ashers have shown to be 

much more accurate predictors of the in-space atomic oxygen erosion yield (volume removed per incident 

atom) than thermal energy plasma ashers (Refs. 4 to 6). It is thought that the arrival flux is much more 

directed in LF ashers and at normal incidence as opposed to the RFs plasma ashers which produce a more 

isotropic arrival flux. Comparing the surface texture of polymers resulting from thermal energy exposure 

with hyperthermal energy, there is a visible increase in height-to-width aspect ratio of the surface 

features. This is also thought to give rise to large differences in atomic oxygen erosion yield for polymers 

with significant amounts of inorganic fill such as ash or pigment particles (Ref. 4). Figure 1 compares the 

surface texture of chlorotrifluoroethylene exposed to thermal energy atomic oxygen (~0.04 eV) in a RF 

air plasma with hyperthermal (~4.5 eV) atomic oxygen in LEO. 

The LF plasma ashers have consistently shown the development of higher aspect ratio textures on 

polymers than the RF plasma ashers indicating that the arriving atomic oxygen may be more directed and 

potentially at higher energies. This paper reports the results of tests conducted to confirm the 

directionality and measure the approximate energy of arriving atomic oxygen in LF plasmas. 
NASA/TM—2014-218412 2 

(a) (b) 

Figure 1.—Scanning electron microscope images of atomic oxygen textured 

chlorotrifluoroethylene exposed to a fluence of: (a) 1.25u1021atoms/cm2 in a 

thermal energy plasma and (b) 8.99u1021 atoms/cm2 at ~4.5 eV in LEO on 

the Long Duration Exposure Facility. 

(a) (b) 

Figure 2.—LF plasma ashers: (a) LF-6, 30 kHz and (b) LF-5, 35 kHz. 

Apparatus and Procedure 

Apparatus 

Experiments were conducted in two different LF plasma ashers operated on air at a pressure of 

~150 millitorr and a power of 35 W are shown in Figure 2. 

Preliminary tests of pyrolytic graphite placed in an aluminum box with a slit opening in the top 

produced a clear band of atomic oxygen textured graphite below the slit. This was clear indication that the 

preponderance of the atomic oxygen arrival was highly directed. In a thermal energy asher there would 

simply be a very distributed indication of arrival. Figure 3 shows the geometry and resulting textured 

pattern in the pyrolytic graphite target. 

As a result of the rather clear definition but narrow textured band, an experiment was constructed to 

produce a wider and more measurable width textured pyrolytic graphite band by using a greater 

separation between the slit opening and the pyrolytic graphite below. Figure 4 shows the geometry and 

resulting texture pattern. 

Although the texture is not as contrasting as with the close gap of Figure 3, its width is still 

reasonably measurable and was used to calculate an approximate range of hyperthermal energies based on 

various plasma temperature assumptions. 
NASA/TM—2014-218412 3 

(a) (b) 

 

(c) 

Figure 3.—Test of pyrolytic graphite in an aluminum box with a slit lid close to the graphite: (a) Exposure geometry, 

(b) Photograph of test, (c) Resulting texture pattern after exposure in a hyperthermal energy asher. 

(a) (b) 

(c) 

Figure 4.—Test of pyrolytic graphite in an aluminum box with a slit lid with a greater separation to the graphite: 

(a) Exposure geometry, (b) Photograph of test, (c) Resulting texture pattern after exposure in a hyperthermal 

energy asher. 
NASA/TM—2014-218412 4 

Procedure 

The measurement of the hyperthermal ion energy involves estimation of the angular distribution of 

ions arriving through the slit where the velocity and energy of ions are due to both thermal energy and 

hyperthermal contributions. As a result of the LF plasma dissociation of the diatomic oxygen molecules, 

either single oxygen atoms are produced or O2

+ diatomic oxygen molecules are produced. Measurements 

of the oxygen O+

 and O2

+

 populations from similar low pressure end Hall oxygen plasmas using E x B 

probes indicate a neglibile O+

 content and that almost all of the ions are O2

+

 (Ref. 7). If one assumes that 

the LF plasma creates an O2

+

 ion temperature of T qK then from the equipartition of energy theorem, there 

is 1⁄2݇ܶ of energy for each of three degrees of translational freedom, 1⁄2 ݇ܶ for each of two rotational axes 

of the diatomic molecule and two vibrational degrees of freedom of the O-O bond, totaling to ଻

ଶ

݇ܶ per 

O2

+

 ion. One can question as to whether the translational, rotational, and vibrational energy have the same 

consequences of contributing to oxidative erosion as would a monotonic O+

 ion of the same total energy, 

however it is probable that energy (whether kinetic, rotational, or vibrational) is all deposited upon impact 

and that all contribute similarly to the erosion process. This would be true whether or not there is impact 

dissociation of the O2

+

 ion. Thus the total thermal energy of the atomic oxygen ions (and neutrals) is 

given by ET where 

 ൌ ்ܧ

଻

ଶ

݇ܶ ൌ ଵ

ଶ

்ݒܯ

ଶ ൅ ʹ݇ܶ (1)

and 

ଵ

ଶ

்ݒܯ

ଶ ൌ 

ଷ

ଶ

݇ܶ (2)

Where ܯ = mass of the O2

+ ions = 5.32u10–26 kg 

ݒ் = Root-mean-squared (rms) thermal velocity of the O2

+

 ions 

k = Boltzmann’s constant = 1.38u10–23 joules/qK 

Thus 

்௞ටଷ = ்ݒ

ெ (3) 

The ions formed are accelerated by the low frequency electric field which is only in the up and down 

direction whereas the thermal velocity vector adds to the hyperthermal electric field in an isotropic 

manner. Figure 5 shows a schematic of the slit, graphite target, and velocity vector geometries. 

The rms velocity, vT, of the thermal velocity vector, VT, is given by ݒ் ൌ ȁ்ܸȁ where 

்ܸ = ݒ்ሾሺ ߠሻÁ ൅  ሺߠሻÉሿ (4) 

Where Á and É are unit vectors in the x and -y directions and ߠ is the angle of an O2

+ ion relative to straight 

downward. The maximum hyperthermal velocity, ݒு, is given by ݒு ൌ ȁܸுȁ where 

ܸு = ݒுÉ (5) 

and 

ଵ

ଶ

ுݒܯ

ଶ ൌ ݁ܧு (6) 
NASA/TM—2014-218412 5 

Figure 5.—Schematic of the geometries in the 

LF plasma asher, where VT = the thermal 

velocity vector, VH = the electric field vector, 

and VTot is the total velocity vector. 

Thus 

ݒு = ටଶ௘ாಹ

ெ (7) 

where 

e = electron charge = 1.6u10–19 coulombs 

EH = hyperthermal energy due to the acceleration of the O2

+

 ions into the slit opening. 

Thus the total velocity vector sum is 

்ܸ௢௧ = ்ܸ ൅ܸு (8) 

whose magnitude can be found from Equations (3) to (7). 

ݒ்௢௧= ቀ

ଷ௞்

ெ

൅  ଶ

ெ ඥ͸݇ܶ݁ܧுܿߠݏ݋ ൅ ଶ௘ாಹ

ெ ቁ

ଵ

ଶൗ

 (9) 

The total energy, ܧ்௢௧ǡ of the O2

+ ions, from Equations (1) and (9) is thus given by 

்௞଻ = ௧௢்ܧ

ଶ

(10) ுܧ݁ ൅ ߠݏ݋ܿுܧ݇ܶ݁͸ඥ൅ 
NASA/TM—2014-218412 6 

Based on the activation energy (Ref. 8) for an oxygen plasma erosion of carbon, the cosine 

dependence of atomic oxygen flux through the slit, the arrival of the atomic oxygen at an angle, )with 

respect to the surface normal, and the loss of flux due to the distance from the slit, the erosion depth, y, is 

given by 

 (11) ଶݔ൅ ଶ݄Ȁξ׎ଶݏ݋ܿ ೟೚೅ாబΤா݁ିן ݕ

where 

ܧ ଴ൌ activation energy 0.27 eV = 4.33u10–21 joules 

 ݄ ൌvertical distance from slit to the top surface of the pyrolytic graphite 

 (see Fig. 5) 

 ݔ ൌ horizontal distance from the edge of the slit (see Fig. 5) 

Based on the velocity vectors orientation and magnitude as well as the geometry shown in Figure 5 

௛  ൌ ׎ 

ξ௫మା௛మ (12)

௭

௛

ൌ  ௩೅ ୡ୭ୱ ఏ

௩೅ ୡ୭ୱ ఏା௩ಹ

 (13)

and 

 ߠ ൌ  ௫

௭ ൌ  ௫ሺ௩೅ ୡ୭ୱ ఏା௩ಹሻ

௛௩೅ ୡ୭ୱ ఏ (14) 

Solving for ߠ involves a quadratic equation which results in 

 ൌ ߠ 

௛௩ಹ௫േ௫ට௛మ௩ಹ

మ ିሺ௛మା௫మ ሻ൫௩ಹమ ି௩೅మ൯

௩೅ሺ௛మା௫మሻ (15)

By using Equation (11) and the related Equations (7), (8), (9), (10), (12), (14), and (15) to plot ݕ versus ݔ

one can find a reasonable match between the observed ݔெ௔௫ for the specific h shown in Figure 5. The 

choice of the r in the quadratic equation for sin ߠ turns out to be not important because it makes only a 

fraction of 1 percent difference in the resulting erosion. 

Results and Discussion 

Based on measurements shown in Figure 3, ݔெ௔௫ ൌ ͳǤͲ ܿ݉ͅand ݄ ൌ ʹǤͷ͹ܿ݉ , and using 

Equations (11) and its related equations, trials of hyperthermal energies and thermal energy temperatures 

were assumed which resulted in the plots of erosion depth shown in Figure 6. 

The abrupt theoretical discontinuity of the erosion depth near ݔெ௔௫ ൌ 1.08 cm would not be expected to 

occur experimentally because we have assumed a single value (the root mean squared value) for the thermal 

velocity of atoms and ions where in reality there is a Maxwell Boltzmann distribution which would tend to 

make a smoother transition. The hyperthermal energy shown in Figure 6, of 0.26 eV, is consistent with the 

observed results shown in Figure 4(b) provided the assumed thermal plasma temperature is 300 qK. If one 

assumes a different plasma temperature then somewhat different hyperthermal energies result. Because there 

are two unknowns: hyperthermal energy and temperature of the thermal plasma a plot of hyperthermal 

energy versus plasma temperature can be made where the erosion stops at ݔெ௔௫ ൌ 1.08 cm which matches 

the experimental data. The result is shown in Figure 7. Thus assuming a thermal plasma temperature 

between 300 and 1000 qK, the hyperthermal energy is between 0.3 and 0.9 eV. This is a factor of ~6.5 

higher than for a thermal energy plasma of any temperature but the arrival is from a uniaxial direction which 

results in a textured surface morphology similar to LEO. 
NASA/TM—2014-218412 7 

Figure 6.—Predicted erosion depth, y, versus horizontal distance, x for a 300 qK plasma 

and a hyperthermal energy of 0.26 eV. 

Figure 7.—Predicted hyperthermal energy versus thermal energy plasma temperature. 

 

Ͳ0.40

Ͳ0.35

Ͳ0.30

Ͳ0.25

Ͳ0.20

Ͳ0.15

Ͳ0.10

Ͳ0.05

0.00

0.0 0.5 1.0 1.5

Relative erosion depth

Horizontal distance to the right of the edge of the slit, cm

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 200 400 600 800 1000 1200

Hyperthermal energy, eV

Plasma thermal temperature, oK
NASA/TM—2014-218412 8 

Figure 8.—Scanning electron microscope 

image of the surface of pyrolytic 

graphite exposed to atomic oxygen an 

LF hyperthermal plasma. 

Figure 8 contains a scanning electron microscope image of the surface of pyrolytic graphite exposed 

to the LF hyperthermal plasma. The texture is indicative of the very directed atomic oxygen flux and is 

much closer to the hyperthermal texture developed in LEO as shown in Figure 1(b) than the cratered 

surface from thermal energy asher exposure of CTFE in Figure 1(a). 

It has been debated whether directed sub-1 eV ions are capable of producing the texture shown in 

Figure 8 when the thermal energy (0.04 to 0.13 eV) is so close to the hyperthermal energy. Monte Carlo 

computational model predictions of the eroded surface profile were made using a model that has been 

developed to mathematically simulate the erosion of polymers by atomic oxygen on Kapton H polyimide 

(Refs. 9 to 11). The model is a two dimensional array of simulated polymer square cells (1000u1000 

cells) that are removed when atomic oxygen oxidizes them. The model allows great flexibility in 

assumptions so that the interaction parameters can be tuned to replicate observed erosion in a variety of 

atomic oxygen environments including ram or sweeping hyperthermal low Earth orbital atomic oxygen, 

or isotropic thermal energy oxygen representative of radio frequency plasma ashers. 

In the model, atoms are mathematically impinged upon a test surface in random locations upon a 

polymer surface or a surface that adjoins a polymer. Upon impact with a non-reactive surface such as 

aluminum or silicon dioxide, the atomic oxygen can recombine to become chemically inactive or remain 

atomic and scatter with reduced energy in either a specular, random cosine (Lambertian) direction, or a 

prescribed direction relative to the local surface normal. If the atoms are scattered, then they have another 

chance of reacting, recombining, or scattering. 

If the atomic oxygen impacts a polymer it can react producing volatile oxides thus causing the 

removal of a computational cell or it can be ejected without reaction in a similar manner as atoms 

scattering off of nonreactive materials. An energy dependent reaction probability, PE, for surfaces with 

volatile oxides is given by 

ܲா ൌ ܿ݁

ିாಲ ாൗ (16) 

Where c = a proportionality constant depending upon the polymer 

EA = activation energy, eV 

E = atomic oxygen impact energy, eV 

In addition to energy dependence of reaction there appears to be a reaction probability dependence on 

the arriving direction relative to the local surface normal direction. This reaction probability dependence, 

PC is given by 

ܲ஼ = ሺ ߚሻ௡ (17) 

where ߚ = the angle between the arriving atomic oxygen direction and the local surface normal 

݊ = cosine exponent = 0.5 based on optimization (Ref. 12) 
NASA/TM—2014-218412 9 

TABLE 1.—MONTE CARLO INTERACTION PARAMETERS (REF. 10) 

Interaction parameter Value 

Atomic oxygen initial impact reaction probability 0.09 

Activation energy, EA, in eV for energy dependent reaction probability 0.26 

Atomic oxygen reaction probability dependence exponent upon angle of impact, n, where 

the reaction probability = PE•(cos En where E is the angle between the arrival direction and 

the local surface normal and PE is the energy dependent reaction probability at normal 

incidence 

0.5 

Probability of atomic oxygen recombination upon impact with protective coating 0.25 

Probability of atomic oxygen recombination upon impact with polymer 0.35 

Fractional energy loss, f, upon impact with polymer 0.45 

Fractional energy loss upon impact with protective coating 0.28 

Degree of specularity as opposed to diffuse scattering of atomic oxygen upon non-reactive 

impact with protective coating where 1 = fully specular and 0 = fully diffuse scattering 

0.045 

Degree of specularity as opposed to diffuse scattering of atomic oxygen upon non-reactive 

impact with polymer where 1 = fully specular and 

0 = fully diffuse scattering 

0.5 

Temperature for thermally accommodated atomic oxygen atoms, qK 300 

Limit of how many bounces the atomic oxygen atoms are allowed to make before an 

estimate of the probability of reaction is assigned 

25 

Thermally accommodated energy/actual atom energy for atoms assumed to be thermally 

accommodated 

0.9 

Initial atomic oxygen energy, eV 4.5 

Thermospheric atomic oxygen temperature, qK 1000 

Atomic oxygen arrival plane relative to Earth for a Maxwell-Boltzmann atomic oxygen 

temperature distribution and an orbital inclination of 28.5q

Horizontal 

(a) (b) 

Figure 9.—Monte Carlo predicted Kapton H erosion profiles for a: (a) 300 qK thermal energy plasma 

and (b) 0.26 eV hyperthermal plasma. 

An analysis of the results of low Earth orbital (LEO) reaction of atomic oxygen interacting with 

protected Kapton H at coating defect sites, allowed the Monte Carlo interaction parameters to be optimized 

to produce erosion prediction results that replicated LEO results (Ref. 10). These interaction parameters 

were used for simulation of atomic oxygen erosion of Kapton H polyimide and are listed in Table 1. 

Figure 9 compares the Monte Carlo predicted Kapton H erosion profiles for a 300 qK thermal energy 

plasma and a 0.26 eV hyperthermal plasma for the same number of atoms submitted. 

This indicates that texturing can be achieved with oxygen atoms arriving at far less than the 

hyperthermal energy of LEO (~4.5 eV). The directed nature of the atomic oxygen arrival flux is probably a 

significant cause for the closer simulation of the erosion yield of pigmented polymers such as white Tedlar.

Summary 

The atomic oxygen texturing of graphite was experimentally and analytically examined in an effort 

to quantify the magnitude of hyperthermal energy in low frequency (30 to 35 kHz) plasma ashers. The 

results indicate hyperthermal energies in the range of 0.3 to 0.9 eV are produced in the low frequency air 

plasmas unlike high frequency (13.56 MHz) air plasmas in which the energy is more in the thermal range. 

The hyperthermal energy in the low frequency ashers is a factor of ~6.5 higher energy than for a thermal 

energy plasma of any temperature, and the arrival is from a uniaxial direction which results in a textured 

surface morphology similar to LEO. Monte Carlo modeling also indicates that such low energy directed 

atoms and ions are fully capable of producing the experimentally observed textured surfaces. 
NASA/TM—2014-218412 10 

References 

1. Banks, B.A., Mirtich, M.J., Rutledge, S.K., and Swec, D.M., “Sputtered Coatings for Protection of 

Spacecraft Polymers,” presented at the 11th ICMC Conference, San Diego, California, April 9-13, 

1984; published in Thin Solid Films, 127, 1985. 

2. Rutledge, S.K., Banks, B.A., DiFilippo, F., Brady, J.A., Dever, T.M., and Hotes, D., “An Evaluation 

of Candidate Oxidation Resistant Materials for Space Applications in LEO,” NASA-TM-100122, 

presented at the Workshop on Atomic Oxygen Effects sponsored by NASA JPL, Pasadena, 

California, November 10-11, 1986. 

3. Banks, B.A., Miller, S.K., and de Groh, K., “Low Earth Orbital Atomic Oxygen Interactions with 

Materials,” paper presented at the Second International Energy Conversion Engineering Conference 

sponsored by the American Institute of Aeronautics and Astronautics, AIAA-2004-5638, 

NASA TM -2004-213223, Providence, Rhode Island, August 16-19, 2004. 

4. Banks, B.A., Simmons, J.C., de Groh, K.K., Miller, S.K., “The Effect of Ash and Inorganic Pigment 

Fill on the Atomic Oxygen Erosion of Polymers and Paints,” Proceedings of the “12th International 

Symposium on Materials in the Space Environment (ISMSE 12),” Noordwijk, The 

Netherlands (ESA SP-705, February 2013). 

5. Stambler, A. Inoshita, K., Roberts, L., Barbagallo, C., de Groh, K., and Banks, B., “GroundLaboratory to In-Space Atomic Oxygen Correlation for the Polymer Erosion and Contamination 

Experiment (PEACE) Polymers,” NASA/TM—2011-216904, Jan. 2011. 

6. Banks, B.A., Dill, G.C., Loftus, R.J., deGroh, K.K., and Miller, S.K., “Comparison of Hyperthermal 

Ground Laboratory Atomic Oxygen Erosion Yields With Those in Low Earth Orbit,” NASA/TM—

2013-216613, December 2013. 

7. Rutledge, S.K., Dever, J.A., Banks, B.A., and Olle, R.M., “The Impact of Negative Grounding of 

Solar Arrays on the Sputtering of Array Surfaces in LEO,” presented at the ASME International 

Solar Energy Conference on Space Solar Power for Orbital and Lunar Applications, Lahaina, 

Maui, Hawaii, April 4-8, 1992. 

8. Banks, B., Miller, S., de Groh, K., and Demko, R., “Atomic Oxygen Effects on Spacecraft 

Materials,” NASA/TM—2003-212484, June 2003 and Proceedings of the 9th International 

Symposium on Materials in a Space Environment, Noordwijk, The Netherlands, June 16-20, 2003, 

ESA SP-540, September 2003. 

9. Banks, B.A., Auer, B.M., Rutledge, S.K., Gebauer, L., and Sechkar, E.A., “Monte Carlo Modeling 

of Atomic Oxygen Interaction with Protected Polymers for Projection of Materials Durability in 

Low Earth Orbit,” Proceedings of the MRS 1992 Spring Meeting, San Francisco, California, April 

27-May 1, 1992. 

10. Banks, B.A. and Stueber, T.J., “Monte Carlo Computational Techniques for Prediction of Atomic 

Oxygen Erosion of Materials,” presented at the NATO Advanced Research Workshop on Computer 

Modeling of Electronic and Atomic Processes in Solids, Wroclaw, Poland, May 20-23, 1996, 

published in Computer Modeling of Electronic and Atomic Processes in Solids, Proceedings of the 

NATO Workshop, edited by R.C. Tennyson & A.E. Kiv, Kluwer Academic Publishing, 1997. 

11. B. Banks, T. Stueber, and M. Norris, “Monte Carlo Computational Modeling of the Energy 

Dependence of Atomic Oxygen Undercutting of Protected Polymers,” NASA TM 1998-207423, 

Fourth International Space Conference, ICPMSE-4, Toronto, Canada, April 23-24, 1998. 

12. Banks, B., Miller, S., de Groh, K., and Demko, R., “Atomic Oxygen Effects on Spacecraft 

Materials,” NASA/TM—2003-212484, June 2003 and Proceedings of the 9th International 

Symposium on Materials in a Space Environment, Noordwijk, The Netherlands, June 16-20, 2003, 

ESA SP-540, September 2003.