Theoretical work on dusty plasmas was conducted in three areas: collective effects in a dusty plasma, the role of dusty plasmas in cometary atmospheres, and the role of dusty plasmas in planetary atmospheres (particularly in the ring systems of the giant planets). Laboratory investigations consisted of studies of dust/plasma interactions and stimulated molecular excitation and infrared emission by charged dust grains. Also included is a list of current publications

Whipple, E. C.

English

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_ . _  , 
FINAL REPORT 
"THE PHYSICS AND CHEMISTRY OF DUSTY PLASMAS: 
A LABORATORY AND THEORETICAL INVESTIGATION" 
Elden C. Whipple 
Principal Investigator 
From: January 1, 1983 - June 30, 1986 
8 
Center for Astrophysics and Space Sciences 
University of California, San Diego 
La J o l l a ,  California 92093 
Grant No.: NASA NAGW-399 
(NASA-CR-173963) THE PHYSICS A H L  CHL'MISTRY NE7- 14 1 4 7  C E  D U S l Y  PLASHAS: A L A R O R A ' I R P  A l i D  
l BEOBETICAL I NV  ES'I I G A ' I Z C I  F i n a l  Beport 
( C a l i f o r n i a  Uciv, ~ San Dieqc, La Jo l la , )  
6 F  U n c l a s  CSCL 201 63/75 43641 0 
https://ntrs.nasa.gov/search.jsp?R=19870004714 2020-03-20T12:22:38+00:00Z
FINAL REPORT ON NASA GRANT NAGW-399 
This final report summarizes briefly the research that has been 
carried out on NASA Grant NAGW-399, entitled "The Physics and Chemistry 
of Dusty Plasmas: A Laboratory and Theoretical Investigation". The 
vork was performed under the auspices of the University of California 
at San Diego. The principal investigator was Dr. Elden C. Whipple. 
The work is summarized under the two headings of: 
1. Theoretical Investigations, and 2. Laboratory Investigations. 
Copies of abstracts of all publications resulting from this work are 
enclosed as weil as copies of two UCSD reports: 
Reports on Laboratory Investigations of Dusty Plasmas", and the other 
entitled, "A Vhite Paper on Dusty Plasmas". 
one entitled, "Progress 
The latter report, "A White Paper on Dusty Plasmas" was prepared 
as a summary of a workshop held at the end of the grant period, on 
February 10-11, 1986, at the University of California at San Diego. The 
purpose of the workshop was to review the work on dusty plasmas that has 
been accomplished, and to make recommendations for further research. The 
availability of the White Paper was made known to the scientific 
community%y means of a "Meeting Report" published in Eos (Vol. 67, pg. 
658, Sept. 1, 1986). A number of copies of the White Paper have been 
distributed in response to requests from interested scientists. 
1. Theoretical Investigations 
Theoretical work on dusty plasmas has been in three areas: 
collective effects in a dusty plasma, the role of dusty plasmas in 
cometary atmospheres, and the role of dusty plasmas in planetary 
magnetospheres-- particularly in the ring systems of the giant planets. 
(1) Collective effects 
In a plasma/dust mixture, the average charge on a grain can be 
quite different that the charge on an isolated grain in a plasma when 
the intergrain distance is on the order of or smaller than the plasma 
Debye length. In the latter case the dust particles electrostatically 
screen each other and the average charge is sigificantly less than for 
an isolated grain. 
there is now a significant space charge in the plasma that balances the 
charge on the grains. In effect, the whole intergrain plasma becomes a 
plasma sheath. 
There can still be overall charge neutrality but 
The electrostatics of a dust/plasma system was analyzed using 
three different models: a permeable grain model (where it is assumed 
that the plasma can penetrate the grains), an impermeable grain model, 
and a capacitor model. The results of the three models are similar but 
1 
not identical. 
are determined primarily by a parameter 2 = 4 nX C, where 
X is the Debye length, N is the dust concentration, and C is the 
capacitance of the grains (where all the grains are the same size). 
This parameter is the approximate ratio of the charge per unit volume 
expected on isolated grains to the available charge per unit volume in 
the plasma. Vhen 2 is larger than unity, then depletion of the plasma 
occurs and the charge on the grains is significantly reduced. This 
work was published in the Journal of Geophysical Research (Vhipple et 
al, 1985). 
The charge on a grain and the grZin to plasma potential 
This work has been extended to include the effects of a dust 
size distribution. 
similar to what was obtained for grains of a single size in a plasma. 
The depletion of electrons and the limiting 05 the charge on the dust 
grains depends on the same parameter 2 = 4 nX N 
is the average capacitance of the dust grains in the plasma. This 
work has been reported on at the 1985 Fall AGU meeting (Eoupis, 1985), 
and a paper describing the results is in preparation. 
The results of the analysis are qualitatively 
where now e 
(2) Cometary Atmospheres 
Electromagnetic forces play an important role in the dynamics of 
smaller cometary dust grains because of the fact that the grains are 
immersed in a plasma and radiative environment and are therefore charged 
to some electrostatic potential. These charged grains experience forces 
from magnetic fields and convectional electric fields in both the 
undisturbed solar wind and in the region of the shocked solar wind 
outside a cometary ionopause. 
The charge on a dust grain is determined by a balance between 
various charging currents: the most important are photoemission, the 
current caused by impact of plasma electrons and ions, and secondary 
emission of electrons caused by electron impact. 
relatively long for small dust grains with the result that for dust 
grains in some environments a time-dependent calculation of the grain 
charge must be performed in order to follow its orbit accurately. 
The charging times are 
A simple model of the particles and fields around a comet was 
used to calculate the trajectories of the smallest (micron and submicron 
sized) dust grains that were expected to be released from a cometary 
nucleus (Horanyi and Hendis, 1985). It was shown that electromagnetic 
forces associated with the motion of the grains through the magnetized 
plasma played a crucial role in their dynamics. 
the existence of asymmetrical envelopes of the sunward dust clouds and 
also the possible existence of wavy dust features far down the comet 
tail, similar to the wavy dust feature observed in the dust tail of 
Comet Ikeya-Seki 1965f. 
The calculations showed 
This work was extended by taking into account the motion of the 
comet around the sun, since this effect is important for dust trajectory 
times of more than a few days, and the calculations vere applied to the 
2 
motion of dust grains in the tail of Comet Giacobini-Zinner (Horanyi and 
Hendis, 1986a). It was found that the distribution of the grains-- 
particularly at the lowest end of the mass spectrum--in a plane normal 
to the orbital plane is entirely different from what had been expected 
for uncharged grains. The recent in-situ observations from the ICE 
spacecraft of the distribution of fine dust in the environment of Comet 
Giacobini-Zinner were consistent vith the calculations. 
Similar calculations were performed for Comet Halley (Flammer, 
Jackson, and Hendis, 1986; Horanyi and Hendis, 1986b). In the first of 
these papers, the authors show that observed sporadic variations of 
the brightness of Comet Aalley at large heliocentric distances (8-11 AU) 
are closely correlated with the encounter of high-speed solar wind 
streams. They proposed that during such periods the night side of the 
comet is electrically charged to numerically large negative potentials, 
with consequent electrostatic levitation and blow-off of fine charged 
dust grains lying on the surface of the cometary nucleus. 
paper the authors simulated the fine dust distributions near Halley's 
comet during both its 1910 and its 1986 appearances. The dust cloud 
morphology as seen from earth in 1910 corresponded to larger grains, 
where the effect of the electrical forces was too small to be clearly 
observed. For the 1986 flybys of the comet by the various cometary 
spacecraft i t  was found that the distribution of the lower end of the 
dust mass spectrum was largely determined by the direction of the 
interplanetary magnetic field. 
spacecraft detected the smallest grains much farther away from the 
comet than expected, well outside their bounding paraboloids defined by 
classical theory based on radiation pressure deceleration. These 
observations are consistent with the numerical calculations. 
In the second 
The dust detectors on the Vega 
( 3 )  Planetary Magnetospheres 
The stimulus for a great deal of the research on the physics of 
dusty plasmas has come from the observations of the ring systems of the 
planets, and particularly the observations of the fine structure in 
Saturn's rings. One of the early activities under this grant was the 
participation in the writing of two reviews: one, "Dust-Magnetosphere 
Interactions" (Grun, Horfill and Hendis, 1984) appeared as a chapter 
in the Book "Planetary Rings" (eds. R. Greenberg and A .  Brahic), and 
the other on electrodynamic processes in the Saturnian ring system 
(Hendis et al, 1984) appeared as a chapter in the book "Saturn" (ed. 
T. Gehrels). The various interactions between magnetospheric particles, 
fields and dust grains, and pertinent observations were reviewed in 
these articles. 
information about the mass concentrations of particulates in Jupiter's 
and Saturn's magnetospheres. The drift of dust particles induced by 
systematic and stochastic charge variation and by the plasma drag was 
described. Sputtering and mutual collisions affect the sizes of grains. 
Electromagnetic effects were discussed which lead to the halo of 
Jupiter's ring, the dust distribution in Saturn's E-ring, and to 
levitated dust in the B-ring spokes as well as on the moon. 
Energetic particle absorption signatures reveal 
3 
Plasma effects on the dust grains in Saturn's ring system were 
discussed by Aoupis and Hendis (1983) who pointed out that a dust disc 
in a planetary magnetosphere constitues a dust-ring current which is 
subject to the finite-resistivity tearing mode instability. They 
suggested that the observed fine ringlet structure of Saturn's rings 
might be a relic of this process operating in the past. Hendis, Hill 
and Boupis (1983) and also Hendis et a1 (1984) brought together several 
aspects of electrodynamic interactions in a discussion of the formation 
and evolution of the B-ring spokes, the formation of P-ring waves, 
eccentricities in certain isolated ringlets, and the origin and 
morphology of the broad diffuse E-ring. Ip and Hendis (1983) looked at 
the consequence of the ring current in Saturn's D-ring driving 
field-aligned currents near the dawn and dusk terminators and closing 
through the Saturnian ionosphere. They suggested that depletion of the 
content of Saturn's ionosphere might be caused by this effect. Finally, 
Northrop et a1 (1986) suggested that the newly discovered "gossamer" 
ring of Jupiter, composed largely of micron-sized grains and existing in 
a very narrow band near synchronous radius, is probably the result of a 
"gyrophase" drift tovards synchronous orbit of very fine, charged 
grains. 
2. Laboratory Investigations 
(1) Studies of DustIPlasma Interactions 
By-Tech Research Corporation, under sub-contract to UCSD, has 
developed an experimental system to study the interaction of small 
(0.01 to 1 micron diameter) particles with a background plasma. A 
complete description of the experimental system and of the results 
that have been obtained is given in the enclosed report entitled, 
"Progress Report on An Experimental Investigation of Dusty Plasmas". 
A paper describing this system and experimental results is being 
prepared for publication. 
In the experiment, a beam of dust particles is generated and 
injected into a plasma and the interactions of the dust particles with 
the plasma are measured. To produce the particles, a refractory metal is 
evaporated in a vacuum oven and nucleated in a cooled chamber. 
has been used in the experiment because of its low melting point. By 
differentially pumping the dust cloud through a series of orifices, a 
beam of dust particles (up to several thousand per is produced 
vhich can the be di ected th ou h a plasma where the electron densities 
range from 10 to 10 per fnd where the electron temperature 
is on the order of a few eV. 
Bismuth 
9 6 
The charge distribution on the grains in the plasma is 
determined by measuring the current carried by the grains as a function 
of retarding voltage when they impact the collecting electrode of a 
retarding potential analyzer. In the case when the grains have come to 
an equilibrium potential with the plasma before impacting the collector, 
the shape of this current/voltage curve gives directly (after 
differentiation) the particle size distribution. The size distribution 
4 
determined in this fashion agrees very well with size distributions 
derived from electron micrographs of collected dust particles on a 
glass slide. 
An important aspect in the evolution of dust grains in space is 
the collisional processes that occur, including elastic collisions, 
inelastic colllisions which change internal energies, sticking 
collisions, and disruptive collisions. Such processes have never been 
studied before for microscopic grains in the velocity range that is 
available here--on the order of tens of meterslsec. It is important to 
understand how these collision processes can be affected by the charge 
on the grains. 
After determining that the charging process of the grains in the 
plasma was well understood, the experimental system was modified so that 
two dust beams could be produced and simultaneously injected into the 
plasma in intersecting trajectories. The intent here was to obtain 
grain/grain collisions so that collision processes could be studied. 
A diagnostic scheme involving Rayleigh scatting of light from the grains 
using a 5 mV Belium/Neon laser was used to determine the dust beam 
density profile and the precise locations of the beams. It was 
estimated that about 1 collision per sec between grains could be 
expected in the intersecti n volume between the two beams (with grain 
concentrations of 5000 cm , a grain radius of .05 microns, and beam 
velocities of 50 m/sec with the two beams at right angles). 
-9 
In order to obtain information about the collision processes 
i t  was necessary to detect and measure the properties of the individual 
collision products. At the end of the grant period, two schemes were 
being developed: an improved laser scattering technique that could 
detect the individual collision products in flight; and an eiecrron 
multiplier detection system which would detect the collision product 
on impact through the secondary electrons i t  would produce. Initial 
measurements indicated that the collision products were being detected, 
but further work was required to optimize and characterize the system 
and to measure particle collision rates. 
(2) 
Charged Dust Grains 
Charged dust grains can have a large electric field at their 
Stimulated Molecular Excitation and Infrared Emission by 
surface because of their small size. If a molecule passes in the near 
vicinity of such a grain this electric field can change the rotational 
energy of the molecule. Once the molecule is in an excited rotational 
state, it might be possible for rotation-vibration coupling to distribute 
the energy into vibrational motion. This energy when released vould be 
emitted in the infrared region of the spectrum, and it is possible that 
some of the unidentified infrared emissions observed in a wide range of 
astrophysical objects might be a result of this excitation mechanism. 
efficiency of this mechanism. In the experiment, the charged grains 
An experiment was designed and built at UCSD to test the 
5 
were simulated by thin wires in a metallic brush. 
larger in diameter than the interstellar grains, were charged to a 
correspondingly higher potential in order to keep the magnitude of the 
electric field at their tips comparable to the expected fields at the 
surfaces of the dust grains. The experiment was designed to be carried 
out at either cryogenic or room temperature in order to determine the 
effects of molecules frozen on the grain surface. 
proposed that the unidentified infrared emission lines might correspond 
to stretching and bending modes of the various bonds of carbon and 
hydrogen atoms, hydrocarbons with dipole moments were primarily used as 
test gases. 
microns vas used to look for infrared emissions. 
The wires, while 
Since it had been 
A spectrometer covering the spectral range from 1 to 14 
The experiment was not successful in detecting infrared 
emissions that could be unambiguously ascribed to the proposed 
excitation mechanism. 
suggestive features at some of the wavelengths appropriate to the 
astronomical unidentified infrared emissions. However, there were still 
difficulties with background subtraction and with instrument stability 
which prevented conclusive results from being obtained before the end of 
the grant period. 
Preliminary spectra were obtained which had some 
3.  Publications 
Flammar, K. R., B. Jackson and D. A .  Mendis, "On the Brightness 
Variations of Halley's Comet at Large Heliocentric Distances", 
Moon, and Planets, 35, 203, 1986. 
Earth, 
Grun, E., G. E. Horfill and D. A. Hendis, "Dust-Magnetosphere 
Interactions, in Planetary Rings (Eds. R. Greenberg and A. Brahic), 
University of Arizona Press, p.  275, 1984. 
Horanyi, M., and D. A. Mendis, "Trajectories of Charged Dust Grains 
in the Cometary Environment", The Astrophysical Journal, 294, 357, 1985. 
Horanyi, M., and D. A. Hendis, "The Dynamics of Charged Dust in the Tail 
of Comet Giacobini-Zinner", J. Geophys. Res., 91, 355, 1986a. 
Horanyi, H., and D. A .  Mendis, "The Effects of Electrostatic Charging on 
the Dust Distribution at Halley's Comet", The Astrophysical Journal, 
- 307, 800, 1986b. 
Houpis, H. L. F., and D. A. Hendis, "The Fine Structure of the Saturnian 
Ring System", The Moon and the Planets, 29, 39, 1983. 
Houpis, H. L. F., "Electrostatic Charge on a Dust Size Distribution in 
a Plasma", EOS, 66, 948, 1985, (Abstract). 
6 
. 
Ip, V.-H., and D. A. Mendis, "On the Equatorial Transport of Saturn's 
Ionosphere as Driven by a Dust-Ring Current System", Geophys. Res. 
Lett., 10, 207, 1983. 
Hendis, D. A., J. R. Hill and 8. L. F. Houpis, "Charged Dust in Saturn's 
Hagnetosphere", J. Geophys. Res., 88, 929, 1983. 
Hendis, D. A., J. R. Hill, U.-B. Ip, C. K. Geortz and E. Grun, 
"Electrodynamic Processes in the Ring System of Saturn", in Saturn 
(Eds. T. Gehrels and M. S. Hatthews), Univ. of Ariz. Press., p. 546, 
1984. 
Hendis, D.A., J. R. Hill, W. Fillius, and T. G. Northrop, "A Note on Jupiter's 
Gossamer Ring", submitted to Nature, 1986. 
Whipple, E. C., T. G. Northrop and D. A. Hendis, "The Electrostatics of 
a Dusty Plasma", J. Geophys. Res., 90, 7405, 1985. 
Whipple, E. C., "A White Paper on Dusty Plasmas", UCSD Report, April, 
1986. 
Yadlowsky, E. J., and R. C. Hazelton, "Progress Reports on an 
Experimental Investigation of Dusty Plasmas", UCSD and By-Tech Research 
Corporation Report, 1986. 
7 


The physics and chemistry of dusty plasmas: A laboratory and theoretical investigation

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