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Laser particle characterization under interplanetary space conditions
J . Aerosol S e i . , Vol. 20, No. 8, pp. 1537-1539, 1989. Printed in Great Britain.
0021-8502/89 $3.00 + 0.00 Pergamon Press plc
Laser Particle Characterization under Interplanetary Space Conditions H a r a l d K o h l ], J o s e f G e b h a r t 2 a n d E b e r h a r d G r G n I
1Maz-Planck-lnJtitut f~r Kernphysik, P.O.Boz 103980, D-6900 Heidelberg, FRG 2GeseUschaft fg~r Strahlen- und Umtoelt/orJehung, Paul-Ehrlich-Str. ~0, D-6000 Frankfurt, FRG
INTRODUCTION Special experimental conditions for particle measurements with lasers exist in laboratory space experiments simulating space environment. Experiments with samples of sublimating ice-dust-mixtures are performed in the comet simulation (KOSI) project (Kochan et al., 1989), simulating physical processes at the surface of cometary nuclei. Dust and ice particles are ejected from the sample surface, when irradiated by artificial sunlight. They are propelled by the gas, which expands from the ice surface into the vacuum. Particles are within a wide size range from a few p m up to some mm, due to the production technique. Suspensions of mineral grains and water are sprayed with carbon dioxide gas into liquid nitrogen, building up small, almost spheric "dirty" ice particles. When this sample material is radiatevely heated within a cooled environment at about 10 -4 mbar pressure water and carbon dioxide ice sublimate. Accelerated surface particles, consisting both of icy particles and fluffy mineral aggregates of low densities ( ~ 0.4 gcm -3 ), which are accelerated, leave the sample with speeds up to a few m/sec. They are flying on parabolic trajectories in the vacuum chamber until they fall to the bottom in the end. Up to now for the KOSI experiments three diagnostic tools for particle characterization have been developped and applied: 1. Piezoelectric impact detectors (Kohl et al., 1989), similar to instruments used in space. They only record hard and compact particles with a still large ice content and sizes larger than about 150 pm. Impact signals give information on size and velocity of the particles. However, the large fraction of smaller particles are not detected. 2. Collector boxes at the bottom of the vacuum chamber (Thiel et al., 1988). They give information about spatial and angular distribution of the emitted particles, but do not have any time resolution, and therefore give no evidence for the particle flux. 3.
TV cameras (Kochan et al., 1988).
They do not reach satisfactory resolution for small particles.
Moreover it is rather complicated to evaluate the large number of particles on the screen wit~ respect to velocity and size of the grains. Thus for characterization of the particles in the KOSI experiment it is necessary to develop other detection methods, mainly in order to record particles in the size range below 100 pro. Physical parameters like size distribution, visual albedo of the particles, structure and surface properties are of interest as well as dynamical parameters like velocity, emission direction and possible acceleration within the space immediately in front of the sample. In order to avoid disturbance of the gas flow from the sample by the presence of mechanical instrumentation within the flow, and to increase sensibility of the measurement, a laser method based on particle light scattering has been chosen.
1537
1538
H. KOHL et al.
LASER MEASUREMENT
CONCEPTS
Concepts using laser light scattering measurements are common in particle characterization (Gebhart, 1989). In contrast to applications in terrestrial environments, however, a few fundamental physical differences have to be taken into account under simulated space conditions. Because of the low gas pressure aerodynamic sampling of the particles is not possible. Particle fluxes in the vacuum of i to 10 particles per cm 2 and second are expected, depending on the recorded size fraction. Particles are expected to consist of H20 or CO~ ice, of dusty ice conglomerates, of compact micron sized grains or of larger porous particle agglomerates. This agglomerates, or "filamentary sublimate residues" (Storrs et. al., 1988), are found by sublimation of ice-dust-mlxtures. As the sample in the KOSI experiments has a diameter of 30 cm, a relatively large detection area in front of the sample has to he installed. For this purpose a laser measuring system is set up and adjusted to the experimental conditiond of the KOSI project. The sensing area is installed about 20 cm above the sample surface (Fig. 1). Trajectories of emitted particles cross the illuminated volume with a few m/see at maximum. A large sensing area is realized by expanding the beam of a 7 mW He-Ne-laser. This can he achieved statically by cylindrical lenses and dynamically by a high frequency laser scanner. Within the vacuum chamber a light sheet of up to 10 cm width is formed. Laser light scattered on the particles passing through this sheet is collected by a wide angle lens and focussed onto a field stop (Fig. 2). By means of a lens system the scattered light is collimated and then passes an interference filter. After the optical filtering the light is directed onto the cathode of a photomultiplier. "Amplification of the signals takes place within a chamber. Thereafter electrical signals are transferred via electrical cables of up to 5 m length out of the vacuum chamber, where the signals can he counted and displayed on an oscilloscope. Data also are then processed and stored digitally by a PC system. i n t e r f e r e n c e filter pre-amplifier / photom~lier~
telescope optics
a c u s t o - o p t i c deflector
insolation - - ~
ol
_
oscilloscope F|g.
1
Experimental
setup of the space simulator including the laser mensurement unit.
-la
er
e l e c t r o n i c filter
~ ] -T-----T" L ' 1 tdata acquisition and storage
laser c o n t r o l
!
Laser p a r t i c l e characterization
objective
1539
Fig. 2 Proto-type of rec
e.
REDUCTION OF SUNLIGHT BACKGROUND Main experimental problems arise from the sunlight background in the chamber. As the artificial sun focusses intensive light with a power of about 1 solar constant (1.36 kW m -2 s - I ) onto the sample surface, scattered and reflected light brightens the whole interior of the vacuum chamber. Even the cold shrouds, which are painted black, reflect significant amount of sunlight. Furthermore most diagnostic instrumentation within the chamber is skidded by superisolation foils, especially designed for high reflectivity to avoid heating. Artificial sunlight is produced by Xenon high pressure lamps and has about the same spectral distribution as the sun (Kochan et al., 1988). The white spectrum is superposed by a few features of the noble gas' atomic transitions. Within the area of He-Ne- wavelength at 632 nm no peak occurs. To eliminate the background light, narrow-band optical filtering at laser wavelength seems to be most promising. In preliminary experiments the beam of a 7 mW He-Ne-laser was expanded to a sheet of light of about 6 cm width and dust particles of various sizes passed through the light beam. Light scattered by the particles was collected by an optical system installed at a distance of about 24 cm from the laser beam. For optical filtering an interference filter (A,,~® = 633.8 nm, ~
= 11.5 nm, T=,= --- 0.557) was put in
front of the multiplier tube. With this arrangement in a room with reduced daylight signal-to-nolse-ratios better than 10 could be achieved for dust particles of a few tens of microns. I]]uminatin 8 the surrounding of the optical system with full daylight raised the output voltage of the multiplier in such a way that a constant background signal was added which exceeded the particle signals by about a factor of 10. Further experiments with a combined system of optical filters, consisting of interference fiIters of different width and cut-off glass filters are in preparation. In addition it will be investigated how far the constant background level can be eliminated by a capacitive coupling between photomultiplier and electronics. REFERENCES
Gebhart, J. (1989). Technisches Messen 56, 192-203. Kochan, H., Benkhoff, J., Bischoff, A., Fechtig, H., Feuerbscher, B., Grin, E., Joo, F., Klinger, J., Koh], H., Krankowsky, D., Roessler, K., Seboldt, W., Thiel, K., Schwehm, G., Welshaupt, U. (1988). Proc. 19th Lunar Planet. Sci. ConL Houston. 487-492. Kochan, H., Feuerbacher, B., Joo, F., Klinger, J., Seboldt, W., Bischoff, A., DSren, H., St<~mer, D., Spohn, T., Fechtig, H., Grin, E., Kohl, H., Krankowsky, D., 1%oessler, K., Thiel, K., Schwehm, G., Weishaupt, U. (1989). Adv. Space P,es. 9, (3)313-(3)122. Kohl, H., Grin, E., Weishaupt (1989). submitted to Planet. Space Sci. Storrs, A.D., Fanale, F.P., Saunders, R.S., Stephens, J.B. (1988). Icarus 76, 493-512. Thiel, K., Kochan, H., 1%oessler, K., Grin, E., Schwehm, G., Hel]mann, H., Hsiung, P., KSlzer, G. (1989). Proc. Workshop on Analysis of returned comet nucleus samples, Milpltas.
0021-8502/89 $3.00 + 0.00 Pergamon Press plc
Laser Particle Characterization under Interplanetary Space Conditions H a r a l d K o h l ], J o s e f G e b h a r t 2 a n d E b e r h a r d G r G n I
1Maz-Planck-lnJtitut f~r Kernphysik, P.O.Boz 103980, D-6900 Heidelberg, FRG 2GeseUschaft fg~r Strahlen- und Umtoelt/orJehung, Paul-Ehrlich-Str. ~0, D-6000 Frankfurt, FRG
INTRODUCTION Special experimental conditions for particle measurements with lasers exist in laboratory space experiments simulating space environment. Experiments with samples of sublimating ice-dust-mixtures are performed in the comet simulation (KOSI) project (Kochan et al., 1989), simulating physical processes at the surface of cometary nuclei. Dust and ice particles are ejected from the sample surface, when irradiated by artificial sunlight. They are propelled by the gas, which expands from the ice surface into the vacuum. Particles are within a wide size range from a few p m up to some mm, due to the production technique. Suspensions of mineral grains and water are sprayed with carbon dioxide gas into liquid nitrogen, building up small, almost spheric "dirty" ice particles. When this sample material is radiatevely heated within a cooled environment at about 10 -4 mbar pressure water and carbon dioxide ice sublimate. Accelerated surface particles, consisting both of icy particles and fluffy mineral aggregates of low densities ( ~ 0.4 gcm -3 ), which are accelerated, leave the sample with speeds up to a few m/sec. They are flying on parabolic trajectories in the vacuum chamber until they fall to the bottom in the end. Up to now for the KOSI experiments three diagnostic tools for particle characterization have been developped and applied: 1. Piezoelectric impact detectors (Kohl et al., 1989), similar to instruments used in space. They only record hard and compact particles with a still large ice content and sizes larger than about 150 pm. Impact signals give information on size and velocity of the particles. However, the large fraction of smaller particles are not detected. 2. Collector boxes at the bottom of the vacuum chamber (Thiel et al., 1988). They give information about spatial and angular distribution of the emitted particles, but do not have any time resolution, and therefore give no evidence for the particle flux. 3.
TV cameras (Kochan et al., 1988).
They do not reach satisfactory resolution for small particles.
Moreover it is rather complicated to evaluate the large number of particles on the screen wit~ respect to velocity and size of the grains. Thus for characterization of the particles in the KOSI experiment it is necessary to develop other detection methods, mainly in order to record particles in the size range below 100 pro. Physical parameters like size distribution, visual albedo of the particles, structure and surface properties are of interest as well as dynamical parameters like velocity, emission direction and possible acceleration within the space immediately in front of the sample. In order to avoid disturbance of the gas flow from the sample by the presence of mechanical instrumentation within the flow, and to increase sensibility of the measurement, a laser method based on particle light scattering has been chosen.
1537
1538
H. KOHL et al.
LASER MEASUREMENT
CONCEPTS
Concepts using laser light scattering measurements are common in particle characterization (Gebhart, 1989). In contrast to applications in terrestrial environments, however, a few fundamental physical differences have to be taken into account under simulated space conditions. Because of the low gas pressure aerodynamic sampling of the particles is not possible. Particle fluxes in the vacuum of i to 10 particles per cm 2 and second are expected, depending on the recorded size fraction. Particles are expected to consist of H20 or CO~ ice, of dusty ice conglomerates, of compact micron sized grains or of larger porous particle agglomerates. This agglomerates, or "filamentary sublimate residues" (Storrs et. al., 1988), are found by sublimation of ice-dust-mlxtures. As the sample in the KOSI experiments has a diameter of 30 cm, a relatively large detection area in front of the sample has to he installed. For this purpose a laser measuring system is set up and adjusted to the experimental conditiond of the KOSI project. The sensing area is installed about 20 cm above the sample surface (Fig. 1). Trajectories of emitted particles cross the illuminated volume with a few m/see at maximum. A large sensing area is realized by expanding the beam of a 7 mW He-Ne-laser. This can he achieved statically by cylindrical lenses and dynamically by a high frequency laser scanner. Within the vacuum chamber a light sheet of up to 10 cm width is formed. Laser light scattered on the particles passing through this sheet is collected by a wide angle lens and focussed onto a field stop (Fig. 2). By means of a lens system the scattered light is collimated and then passes an interference filter. After the optical filtering the light is directed onto the cathode of a photomultiplier. "Amplification of the signals takes place within a chamber. Thereafter electrical signals are transferred via electrical cables of up to 5 m length out of the vacuum chamber, where the signals can he counted and displayed on an oscilloscope. Data also are then processed and stored digitally by a PC system. i n t e r f e r e n c e filter pre-amplifier / photom~lier~
telescope optics
a c u s t o - o p t i c deflector
insolation - - ~
ol
_
oscilloscope F|g.
1
Experimental
setup of the space simulator including the laser mensurement unit.
-la
er
e l e c t r o n i c filter
~ ] -T-----T" L ' 1 tdata acquisition and storage
laser c o n t r o l
!
Laser p a r t i c l e characterization
objective
1539
Fig. 2 Proto-type of rec
e.
REDUCTION OF SUNLIGHT BACKGROUND Main experimental problems arise from the sunlight background in the chamber. As the artificial sun focusses intensive light with a power of about 1 solar constant (1.36 kW m -2 s - I ) onto the sample surface, scattered and reflected light brightens the whole interior of the vacuum chamber. Even the cold shrouds, which are painted black, reflect significant amount of sunlight. Furthermore most diagnostic instrumentation within the chamber is skidded by superisolation foils, especially designed for high reflectivity to avoid heating. Artificial sunlight is produced by Xenon high pressure lamps and has about the same spectral distribution as the sun (Kochan et al., 1988). The white spectrum is superposed by a few features of the noble gas' atomic transitions. Within the area of He-Ne- wavelength at 632 nm no peak occurs. To eliminate the background light, narrow-band optical filtering at laser wavelength seems to be most promising. In preliminary experiments the beam of a 7 mW He-Ne-laser was expanded to a sheet of light of about 6 cm width and dust particles of various sizes passed through the light beam. Light scattered by the particles was collected by an optical system installed at a distance of about 24 cm from the laser beam. For optical filtering an interference filter (A,,~® = 633.8 nm, ~
= 11.5 nm, T=,= --- 0.557) was put in
front of the multiplier tube. With this arrangement in a room with reduced daylight signal-to-nolse-ratios better than 10 could be achieved for dust particles of a few tens of microns. I]]uminatin 8 the surrounding of the optical system with full daylight raised the output voltage of the multiplier in such a way that a constant background signal was added which exceeded the particle signals by about a factor of 10. Further experiments with a combined system of optical filters, consisting of interference fiIters of different width and cut-off glass filters are in preparation. In addition it will be investigated how far the constant background level can be eliminated by a capacitive coupling between photomultiplier and electronics. REFERENCES
Gebhart, J. (1989). Technisches Messen 56, 192-203. Kochan, H., Benkhoff, J., Bischoff, A., Fechtig, H., Feuerbscher, B., Grin, E., Joo, F., Klinger, J., Koh], H., Krankowsky, D., Roessler, K., Seboldt, W., Thiel, K., Schwehm, G., Welshaupt, U. (1988). Proc. 19th Lunar Planet. Sci. ConL Houston. 487-492. Kochan, H., Feuerbacher, B., Joo, F., Klinger, J., Seboldt, W., Bischoff, A., DSren, H., St<~mer, D., Spohn, T., Fechtig, H., Grin, E., Kohl, H., Krankowsky, D., 1%oessler, K., Thiel, K., Schwehm, G., Weishaupt, U. (1989). Adv. Space P,es. 9, (3)313-(3)122. Kohl, H., Grin, E., Weishaupt (1989). submitted to Planet. Space Sci. Storrs, A.D., Fanale, F.P., Saunders, R.S., Stephens, J.B. (1988). Icarus 76, 493-512. Thiel, K., Kochan, H., 1%oessler, K., Grin, E., Schwehm, G., Hel]mann, H., Hsiung, P., KSlzer, G. (1989). Proc. Workshop on Analysis of returned comet nucleus samples, Milpltas.