A pulsed high flux source of nearly monoenergetic atomic oxygen was designed, built, and successfully demonstrated. Molecular oxygen at several atmospheres pressure is introduced into an evacuated supersonic expansion nozzle through a pulsed molecular beam valve. An 18 J pulsed CO2 TEA laser is focused to intensities greater than 10(9) W/sq cm in the nozzle throat to generate a laser-induced breakdown. The resulting plasma is heated in excess of 20,000 K by a laser supported detonation wave, and then rapidly expands and cools. Nozzle geometry confines the expansion to provide rapid electron-ion recombination into atomic oxygen. Average O atom beam velocities from 5 to 13 km/s were measured at estimated fluxes to 10(18) atoms per pulse. Preliminary materials testing has produced the same surface oxygen enrichment in polyethylene samples as obtained on the STS-8 mission. Scanning electron microscope examinations of irradiated polymer surfaces reveal an erosion morphology similar to that obtained in low Earth orbit, with an estimated mass removal rate of approx. 10(-24) cu cm/atom. The characteristics of the O atom source and the results of some preliminary materials testing studies are reviewed

Caledonia, George E.

Krech, Robert H.

English

NASA Technical Reports Server

N8 7- 26 189
PULSED SOURCE OF ENERGETIC ATOMIC OXYGEN
George E. Caledonia and Robert H. Krech
Physical Sciences Inc.
Research Park, P.O. Box 3100
Andover, MA 01810
ABSTRACT
A pulsed high flux source of nearly monoenergetic atomic oxygen has been
designed, built, and successfully demonstrated. Molecular oxygen at several
atmospheres pressure is introduced into an evacuated supersonic expansion
nozzle through a pulsed molecular beam valve. An 18J pulsed CO2 TEA laser is
focused to intensities > 10 9 W/cm2 in the nozzle throat to generate a laser-
induced breakdown. The resulting plasma is heated in excess of 20,000 K by a
laser-supported detonation wave, and then rapidly expands and cools. Nozzle
geometry confines the expansion to promote rapid electron-ion recombination
into atomic oxygen. We have measured average 0-atom beam velocities from 5 to
13 km/s at estimated fluxes to 10 18 atoms per pulse. Preliminary materials
testing has produced the same surface oxygen enrichment in polyethylene
samples as obtained on the STS-8 mission. Scanning Electron Microscope exam-
inations of irradiated polymer surfaces reveal an erosion morphology similar
to that obtained in low earth orbit, with an estimated mass removal rate of
-10-24
 cm3/a tom. The characteristics of the O-atom source and the results of
some preliminary materials testing studies will be reviewed.
INTRODUCTION
Satellites in low-earth orbit sweep at velocities of -8 km/s through a
rarefied atmosphere which consists primarily of atomic oxygen. Experimental
pallets flown on Shuttle missions STS-5 and STS-8 clearly demonstrated a
dependence of material degradation and mass loss on the ram direction atomic
oxygen exposure.(1-4) These experiments indicate that most hydrocarbons and
active metals are highly reactive, whereas material containing silicones,
fluorides, oxides and noble metals are moderately inert. For Kapton, ® an
important aerospace polymer, it was observed that about one in ten atomic
oxygen interactions lead to mass loss due to chemical reaction.(2,3)
The need for continued material degradation studies has been emphasized
in a study by Leger et a1.(5) where it was demonstrated that atomic oxygen
induced material degradation could have a severe impact on the performance of
Space Station. The EOIM-3 material pallet, with sophisticated instrumentation
to detect atomic oxygen reaction products and to study reaction mechanisms, is
scheduled to be deployed on a future Shuttle mission, and NASA is actively
pursuing the development of various hardening techniques to make materials
more impervious to the effects of energetic atomic oxygen interactons.
The recent setbacks in launch capability have reemphasized the need for a
high flux atomic oxygen source which can be used to study material degrada-
tion. Although in-flight experiments provide valuable test data, the large
135
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NOTE
VIEW OF RADIOMETER FROM ABOVE
VIEW OF DMA FROM SIDE
1O.1
Fig. 1. Schematic diagram of
0-a tom apparatus
matrix of materials and test parameters, and uncertain flight scheduling now
mandate that most of these studies be performed in a ground test facility.
A high flux source of atomic oxygen has been developed at Physical
Sciences Inc. (PSI) based on years of research in the area of pulsed laser
propulsion.(6-8) The basic concept is to rapidly introduce a burst of gas
into an evacuated nozzle and then to focus the output of a pulsed laser to
cause a breakdown at the nozzle throat. The subsequent laser-initiated
detonation wave will heat the major portion of the gas during the laser pulse
creating a high temperature plasma. This plasma will then expand through a
nozzle tailored to allow electron-ion recombination but not atomic recombin-
ation. As the gas expands its temperature and density will drop, however, its
directed velocity increases correspondingly, producing a thermally "cold,"
high energy beam of oxygen atoms at the nozzle exit. This technique has been
utilized to produce a high velocity 0-atom source for material degradation
studies. For example with 10-4g of gas and a 5J laser pulse we predict
formation of >10 18 oxygen atoms with a characteristic energy of 5 eV. For
comparison with other sources, if the source is pulsed at 10 Hz, an average
flux of >10 17
 0-a toms/cm2-s can be maintained on a 100 cm 2 target.
In our research effort at PSI we have constructed a small test facility
to demonstrate that high velocity oxygen atoms can be produced. A series of
measurements have been performed to demonstrate the presence of atomic oxygen
and the measured velocities agreed with theoretical predictions. Measurements
of the material degradation of 0-a tom irradiated targets are being performed,
and these experimental observations are described. The design of a larger
0-atom test facility presently under construction is also presented.
FACILITY DESCRIPTION
The present test facility consists of a stainless steel high vacuum
chamber in which oxygen gas is rapidly pulsed through a conical expansion
nozzle and laser heated by a pulsed CO2 laser to temperatures above 20,000 K.
QUARTZ WINDOWS
A schematic diagram of the appara-
tus is shown in Fig. 1. The vacuum
chamber is a standard 20 cm diameter
five-way stainless steel high vacuum
cross with Con-Flat flanges. Each
sidearm extends 20 cm from the center
of the chamber. A sixth 10.0 cm
diameter port was welded to the bottom
of the chamber for evacuation. The top
flange of the vacuum chamber has two
radiometers for time-of-flight velocity
measurements into the chamber. The
pulsed valve/nozzle assembly is mounted
on the end flange which also contains
vacuum feed-throughs for electrical
connections. The opposing flange holds
the laser focusing lens. Each side
136
LASER APPLICATIONS, LTD
10120 Hz. 18/14J
GOLD BEAM	
802 LENS/WINDOW
STEERING
MIRRORS	 \J	 NOZZLE
VALVE
Fig. 3® Laser/optics assembly
flange has two 5 cm diameter quartz view-ports for visual and spectroscopic
observations.
The bottom flange is connected to a 10 cm diffusion pump stack equipped
with an ionization gauge readout. The ultimate pressure in the chamber is
3 x 10-5 torr. In operation the chamber pressure is kept below 1 x 10 -4 Corr,
to prevent beam interaction with the background gas. (Mean free path at
10- torr is 50 cm which is greater than the chamber length and is sufficient
to provide a "collision free" environment®) The pumping speed of the vacuum
chamber is sufficient to allow introduction of a pulse of 10-4g of oxygen at
1 Hz into a background pressure below 1 x 10 -4 torr.
The pulsed valve/nozzle
assembly is shown schematically in
Fig® 2. The valve is a modified	 1/8" 0-RING TIP
Model BV-100V pulsed molecular
beam valve from Newport Research,	 VALVE
Inc. This valve allows the gener- 	 MECHANISM
a tion of short duration pulses of	 1 mm ORIFICE PLATE
gas at high f lowra tes which cannot 	
PULs
be continuously maintained under
high vacuum conditions due to
pumping speed limitations. The	 Fig. 2,
valve is operated with a 1 mm i.d.
orifice plate, and is bolted
COOLING
JACKET	
PRESSURE TRANSDUCERS
_ 20"_ _ _7 _ COZ LASER BEAM
VALVE	 r}m	 NOZZLE -P
0-atom source pulsed valve/
nozzle assembly
directly to a 100 mm long, 20 deg full angle aluminum expansion nozzle with a
1 mm i " d " throat. The choked flowrate of the valve/nozzle assembly is 0,19g
of oxygen per second per atmosphere stagnation pressure®
The nozzle has two flush mounted pressure transducers located 43 and
93 mm from the throat with 1 ps response time and 20 mV/psia sensitivity.
A Laser Applications Limited TEA CO2 laser is used to generate 18J pulses
of 10.6p radiation® The energy is delivered in a 2.5 ps pulse, with approx-
imately one-third of the energy delivered in the first 200 ns® The radiation
in the gain switched spike generates a laser induced breakdown in the high
pressure oxygen at the nozzle throat forming a plasma which continues to
absorb the radiation as long as the laser is on®
The laser beam is directed to
the test chamber by three gold
turning mirrors (Fig. 3) " A
barium fluoride flat between
the second and third mirror
reflects eight percent of the
beam to a calorimeter to moni-
tor the laser energy® A
300 mm focal length barium
fluoride lens is used to focus
the laser beam to -1 mm
diameter spot size at the
nozzle throat.
LUMONICS 50-0
ENERGY METER n
	
BaFZ
BEAM SPLITTER
137
BEAM DIAGNOSTICS
Optical measurements have been conducted to characterize the laser
initiated 0-atom beam. Beam velocities from 5 to 13 km/s were obtained by
varying both the stagnation pressure and the laser energy. The velocities
were deduced by monitoring the time history of the 777.3 nm atomic oxygen line
emission with two filtered radiometers mounted on the top flange of the vacuum
chamber.
The spectral measurements of the O-a tom beam, and target interactions
have been obtained using a Princeton Instruments Optical Multichannel Analyzer
(OMA). The OMA head consists of a 1024 photodiode array with a gated S-20R
intensifier. The spectrograph is a Jarrel Ash 0.275m MARK X with interchange-
able grating, and a maximum resolution of 3A. The optical field of view in
the chamber was restricted to a 0.090 x 0.90 cm rectangle by a collecting
telescope matched to the f number of the spectrograph. Glass cut-on filters
were used to prevent second and higher order spectra from being recorded.
Spectra are recorded using the OMA in the gated mode. The diode array
was gated on 5 us after the laser trigger to prevent detection of scattered
breakdown radiation. A number of spectral scans of the radiation from the
expanded oxygen plasma were performed over the wavelength range of 400 to
800 nm. Results were highly reproducible. The strongest signals arose from
atomic oxygen emission at 777 nm. No evidence of 0 2
 emission was identified,
however, some atomic hydrogen emission was observed due to laser ablation of
the valve tip elastomers, and H2O impurities in the oxygen feed gas.
Characterization of the beam has been also performed by ballistic pendu-
lum measurements of the cold and hot flows. This has allowed quantization of
the atomic flow rate to within a factor of two. Refinements in the larger
test facility, primarily a time-of-flight mass spectrometer probe, will
further reduce this uncertainty.
PRELIMINARY MATERIAL DEGRADATION STUDIES
Preliminary material degradation studies have been conducted with the
small test facility. Materials irradiated have included polyethylene,
Teflon®, Kapton6
 (untreated and oxidation resistant treated samples), Mylare,
PEEK, PBT and Carbon Epoxy composites.
Ground-based testing facilities must produce a surface morphology and
mass removal rate similar to that produced on orbit to provide meaningful
erosion data. Typical erosion morphology of a polymeric sample is shown in
Fig. 4 (graciously provided by J.T. Visentine), for exposure levels of 5 to
9 x 1020
 0-a tom/cm2 , corresponding to -1 week on orbit. Kapton samples
irradiated in our facility to an exposure level of -3.3 x 10 20
 0-atom/cm2
 show
a remarkably similar surface morphology Fig. 5, and measurable mass loss
(-1.3 mg). (Fig. 6 shows a sample of Kapton 500H prior to exposure for com-
parison.) This morphology is not due to kinetic energy alone since another
sample of Kapton was bombarded with -5 eV Ar (Fig. 7) and showed no mass loss.
Similar results were obtained with low and high density polyethylene samples.
138
ORIGINALPAGE
F POOR QUALITY
PRIMP TO EXPOSURE
	
AFTER ATOMIC OXYGEN EXPOSURE
(10,OOOx)	 (10®000x)
Fig® 4. SEM photograms of STS-8 Kapton specimens
AFTER 12000 PULSES ( 3x1020/cm2 )
	
BEFORE EXPOSURE
Fig. 5® Kapton 500H after 12 000	 Fig® 6® Kapton 500H before
pulses (-3 x 10 20/cm^)	 exposure
Figa 7s Kapton 500H irradiated by 5 eV Ar for 12,000 pulses
139
Current capabilities provide for exposure rates of -1.5 x 10 20 atom/
cm2-hr at -5 eV (-1.6 Hz); and provide an exposure acceleration factor of 50
compared to LEO and 5000 for space station altitudes for -6 cm 2 samples. A
larger facility described below will allow for further accelerated testing.
DESIGN OF LARGE-SCALE TEST FACILITY
The small test facility produces high density O-atom beams, but cannot
irradiate large sample areas. Using the same techniques, we are constructing
a large chamber to allow the testing of larger samples (-15 cm diameter) at a
10 to 20 Hz pulse rate.
In order to accommodate
larger samples, or many smaller
ones, the new chamber will be
40 cm o.d. six-way cross
assembly. A chamber this size
allows for convenient sample
access, mass spectrometer probes
for beam and target product
analysis, in situ mass loss
measurement, velocity dependent
measurements and target irradi-
ation by a UV solar simulator.
A schematic of the chamber is
shown in Fig. 8.
PASSIVE EMISSION/OMA SYSTEM
i -
TARGET
/	 MASS
SPECTROMETER
/	 1	 CHOPPER WHEEL DRIVE (OCCASIONAL)
1
OCO2 LASERr^
LASER INDUCED
CRYOPUMP	 FLUORESCENCE DIAGNOSTIC
PIMP
Fig. 8. Schematic of new 0-a tom facility
A Varian cryopump with a speed of 3000 21s will maintain 10 -4 torr in
the chamber with a gas load of 10 -3 g/s. A cryopump was chosen to eliminate
any possible contamination of test samples by back diffusion of pump oil.
Two quadrupole mass spectrometer systems will be coupled to the main
chamber. A Balzers QMS 311 will be used to monitor the atomic beam. It will
sit in a differentially pumped chamber, separated from the main chamber by a
sampling orifice. This QMS will monitor the pulse to pulse reproducibility of
the 0-atom beam, and when cross calibrated with the laser induced fluorescence
diagnostic, will provide a real time quantitative 0-a tom detector. A second
QMS will be suspended from the top flange of the chamber to monitor the reac-
tion products of the atomic oxygen/interaction and will be mounted to allow
for rotation with respect to the target for angular product dependence
studies.
The pulsed CO2 laser system was manufactured by Laser Applications
Limited and provides 18J per pulse at 10 Hz. This allows a sufficient margin
for transmission and reflection losses to process -10 -4g bursts of gas into
5 eV 0-atoms. Furthermore an option for 20 Hz operation is available to
provide a higher throughput.
A Quantel 10 Hz YAG pump dye laser system will be used for multiphoton
laser induced fluorescence detection of atomic oxygen. Approximately 1 mJ at
226 mm can be obtained for the excitation of ground state atomic oxygen
140
(2p43p + 3p33p3p) with subsequent fluorescence around 845 nm from the
.3p3P $ 3 s
 S transitions in atomic oxygen, and the same technique can be used
to detect the 1D and 1S metastables.
SUMMARY
Our goal is to build a reliable ground based energetic 0-atom test
facility which will be able to meet the NASA material testing requirements.
The concepts of the design have been demonstrated both experimentally and
theoretically, and the technique exploits the characteristics of existing
commercially available high power laser technology. At present repetitively
pulsed CO2 lasers provide the most convenient source of laser radiation in
terms of reliability, cost and delivered energy.
With a 10 Hz pulse rate, the device will conservatively irradiate a
100 cm2
 area with >10 17 atoms/cm2-s. In less than 2 hr of operation under
these conditions, energetic oxygen atom fluences of 7 x 10 20 /cm2 will be
possible over the exposed area. For comparison this 0-atom fluence is equiv-
alent to that seen by a ram direction shuttle surface for an entire week-long
mission in low earth orbit (250 km) and is equivalent to the fluence seen by a
ram surface of the Space Station at 500 km altitude during the course of an
entire year (during a year of average solar activity). Although questions can
be raised by accelerated testing, this new facility will provide the ability
to identify materials most affected by 0-atoms (which can then be tested
further on-orbit), to systematically separate effects of UV irradiation,
temperature, and 0-atom energy, to study the rate of change of recession/
material loss as surfaces change character, and to identify failure points
(most susceptible to erosion) in assemblies such as solar cells. The
importance of such effects has been clearly demonstrated in recent Space
Shuttle flights.(5, 9-11).
With this new test facility we can perform quantitative erosion testing
of materials, components, and even small assemblies (such as a solar cell
array) in order to determine components or interfaces which are most vulner-
able to 0-atom erosion. This ground-based facility will allow accelerated
testing to identify structures or materials which are in most critical need of
protection so that remedial strategies or protective coatings can be developed
and even tested in this ground test facility at a fraction of the cost and in
a much more timely manner than on-orbit.
REFERENCES
1. Leger, L.J., Spiker, I.K., Kuminecz, J.F., and Visentine, J.T., "STS
Flight 5 LEO Effects Experiments--Background Description and Thin Film
Results," AIAA Paper 83-2531-CP, October 1983.
2. Leger, L.J., Visentine, J.F., and Kuminecz, J.F., "Low Earth Orbit Atomic
Oxygen Effects on Surfaces," AIAA Paper 84-0548, January 1984.
141
3. Visentine, J.T., Leger, L.J., Kuminecz, J.F., and Spiker, I.K., "STS-8
Atomic Oxygen Effects Experiments," Paper presented at AIAA 23rd
Aerospace Sciences Meeting, Reno, NV, January 1985.
4. Green, B.D., Caledonia, G.E., and Wilkerson, T.D., "The Shuttle
Environment: Gases, Particulates and Glow," J. Spacecraft and Rockets
22, 5 Sept-Oct 1985, 500-511.
5. Leger, L.J., Visentine, J.T., and Schliesing, J.A., "A Consideration of
the Atomic Oxygen Interactions with Space Stations," AIAA 85-0476,
January 1985.
6. Simons, G.A. and Pirri, A.N. "The Fluid Mechanics of Pulsed Laser
Propulsion," AIAA J. 15, 835 (1977).
7. Rosen, D.I., Pirri, A.N. Weiss, R.F., and Kemp, N.H., "Repetitively
Pulsed Laser Propulsion: Needed Research," In progress in Astronautics
and Aeronautics Vol. 89 (1985), ed. L.H. Caveny, pp. 95-108.
8. Rosen, D.I. Kemp, N.H., Weyl, G., Nebolsine, P.E., and Kothandaraman, G.,
"Pulsed Laser Propulsions Studies," Physical Sciences Inc. Technical
Report TR-184, 1981.
9. Hall D.F. and Stewart, T.B., "Photo-Enhanced Spacecraft Contamination
Deposition," AIAA 85-0953, June 1985.
10. Peterson, R.V. Hanna, W.D., and Mertz, L.O., "Results of Oxygen Atom
Interaction with Kevlar and Fiberglass Fabrics on STS-8," AIAA 85-0990,
June 1985.
11. Knopf, P.W. Martin, R.J., McCargo, M., and Dammann, R.E., "Correlation of
Laboratory and Flight Data for the Effects of the LEO Atomic Oxygen on
Polymeric Materials," AIAA 85-1066, June 1985.
ACKNOWLEDGMENT
This work is funded by NASA under Contract NAS7-936 and monitored by the
Jet Propulsion Laboratory. The authors appreciate the efforts of
Dr. David I. Rosen who acts as a technical reviewer for the program at PSI.
142


Pulsed source of energetic atomic oxygen

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