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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to rf plasma sources and the use of such sources for
cleaning surfaces in space, and more particularly to helicon wave plasma
sources suitable for cleaning spacecraft thermal radiators and telescopes.
2. Description of the Related Art
There is a need for a low power, self-contained cleaning system for
removing contaminants that build up on the exposed surfaces of a
spacecraft, without damaging the device being cleaned. For example,
thermal radiators are used to cool a spacecraft by radiating more energy
than they absorb. This results from their having a high emissivity in the
infrared (IR) wavelengths corresponding to blackbody radiation from the
warm spacecraft, and a low absorbtance over the wavelengths of the solar
spectrum.
Thermal radiators become contaminated in space due to the condensation of
hydrocarbon vapors outgassed from organic materials carried onboard the
spacecraft, such as adhesives, potting compounds, conformal coatings and
thermal blankets. Ultraviolet light from the sun causes
photopolymerization of the hydrocarbons, which would otherwise
re-evaporate to some degree; this generates high molecular weight films
that do not re-evaporate. These contaminants can greatly increase the
radiator's solar absorbtance, and thus reduce its cooling capacity. To
counter this, extra large radiator panels are typically used. The extra
radiators not only add weight and cost to the spacecraft, but also cool
the spacecraft excessively before they become contaminated. This requires
valuable onboard electrical power to be used to heat the radiators; the
greatest heating is needed during eclipse seasons, when power is least
available. Near the end of the spacecraft life, the radiators exhibit a
poor heat rejection performance that can cause the onboard electronics to
be subjected to large thermal extremes, thereby reducing their lifetime
and reliability.
Imaging optics such as telescopes that are used on spacecraft also become
contaminated in this way. The condensed hydrocarbon vapors form a scum
that causes absorption and scattering of the light being imaged by the
telescope, blurring the images. This contamination process is
significantly worsened if the telescope is designed to view the sun, which
result in the photopolymerization mentioned above.
Telescopes that are cooled to cryogenic temperatures, on the order of tens
of Kelvins, to permit observations in the infrared, are also subject to a
buildup of surface contaminants. Such cryotelescopes suffer from
condensation not only of hydrocarbon vapors, but also of water vapor,
carbon dioxide, ammonia and other cryocondensible gases. The frozen gases
absorb incident radiation, and in time become roughened by sublimation
roughening, increasing their optical scatter. Cryotelescopes have
previously been warmed to sublime the frozen gases. However, this renders
the instrument "blind" during the sublimation process, and consumes a
great deal of cryogen. The residual hydrocarbon contaminants have been
simply allowed to accumulate.
Previous attempts to develop a cleaner for space borne telescope optics
used high energy ion beams to remove the contaminants. However, this
resulted in damage to the delicate optical surfaces because of ion beam
sputtering. Radiators often use conductive coatings such as indium oxide,
and the potential for sputter damage to such coatings has also made ion
beam cleaning inapplicable to radiators.
Another approach involves the use of an ultraviolet lamp to create ozone
within the telescope tube. The ozone oxidizes hydrocarbon contaminants on
the telescope optics. Unfortunately, this approach requires the telescope
tube to be pressurized with oxygen gas, which imposes a substantial burden
upon the spacecraft in terms of telescope mass, large oxygen tankage and
cryogen loss due to convective warming during cleaning.
Another approach to cleaning spacecraft surfaces is described in U.S. Pat.
No. 4,846,425 to Champetier and assigned Hughes Aircraft Company, the
assignee of the present invention. This technique uses the negative charge
that is typically accumulated on a spacecraft, which collects more active
electrons than relatively inactive positive ions. Neutral oxygen is
released from the spacecraft, ionized by the background space plasma, and
drawn back to the spacecraft by its negative charging to react with the
surface contaminants. A very large oxygen supply is required, however,
because most of the oxygen escapes and is not drawn back to the
spacecraft. This is because the oxygen must be ionized within a few debye
lengths (about 10-100 m) from the spacecraft to be drawn back, and the
majority of the oxygen is not ionized within this zone.
The use of plasmas is well known in ground applications for removing
hydrocarbons. Nascent oxygen atoms and ions in the plasma oxidize the
hydrogen and carbon atoms that make up the contaminants, and the reaction
energy propels the volatile oxide from the contaminated surface. One type
of plasma source that has been used for this purpose is based upon a
Penning electron discharge; such a plasma source is described in U.S. Pat.
No. 4,800,281, also assigned to Hughes Aircraft Company. Penning-type
sources generally include either a filamentary cathode or a hollow cathode
for achieving thermionic electron emission. However, filamentary cathodes
predictably burn out over time, and are therefore unacceptable for use in
spacecraft applications in which replacement of the cathode is not
possible. For hollow cathodes, the electronic emissive material that is
used to coat the hollow cathode is often incompatible with reactive gases
such as oxygen that are desirable plasma fuels for cleaning optical
surfaces.
Another plasma source that has been used for ground-based cleaning
applications is the helicon wave source. This type of device operates by
coupling externally generated electric and magnetic fields into a plasma
that is confined by an axial magnetic field. An antenna consisting of two
loops diametrically positioned on the outside of the source tube produces
a transverse rf magnetic field, perpendicular to both the tube axis and a
constant axial magnetic field. The rf field excites a helicon wave in the
source tube, and energy is transferred from this wave to the plasma
electrons. The helicon wave theory is discussed in Chen, "Plasma
Ionization By Helicon Waves", Plasma Physics and Controlled Fusion, Vol.
33, No. 4, 1991, pages 339-364. The use of helicon wave plasma sources for
semiconductor cleaning is described in Singer, "Trends in Plasma Sources:
The Search Continues", Semiconductor International, July 1992, pages
52-57.
Helicon plasma sources use versions of an rf antenna that have complicated
wiring schemes to avoid establishing an rf magnetic field parallel to the
source tube axis. This type of antenna, commonly referred to as a Nagoya
Type III antenna, is described in Watari et al., "Radio Frequency Plugging
of a High Density Plasma", Physics of Fluids, Vol. 21, No. 11, November
1978, pages 2076-2081. The overall plasma sources are quite large and
massive, and consume too much power and gas to be considered for
spacecraft applications.
Other reactive plasma sources such as parallel-plate reactors are also used
to clean hydrocarbons in ground applications. In addition to excessive
weight and power consumption, those sources produce ion energies high
enough to risk damaging optical surfaces.
SUMMARY OF THE INVENTION
The present invention seeks to provide an improved method for removing
contaminants from the surface of a body in space with low power and gas
consumption requirements, and without damaging the surface being cleaned.
In particular, it seeks to provide a reactive rf plasma source that has
these characteristics and is small enough in size to be useful for
practical spacecraft applications. A neutralization of charge buildup on
the spacecraft surface is also desired.
These goals are achieved by generating a substantially spaced-charge
neutral plasma of a type that reacts with the contaminant to be removed
from the spacecraft surface, directing the plasma onto the contaminated
surface at an energy below the surface sputtering energy, and reacting the
plasma with the contaminant to remove it from the surface. A new type of
helicon wave source is used to provide the plasma at a low energy level,
less than 20 eV, at which sputter damage to delicate optical surfaces is
avoided. The new plasma source can also be made sufficiently compact and
light weight to be useful for spacecraft applications. This reduction in
scale is made possible by the use of permanent magnets to establish the
uniform axial magnetic field, as opposed to the prior use of
electromagnets with their attendant high power consumption, coupled with a
unique rf antenna design that is greatly simplified compared to the prior
Nagoya-type antennas.
The new antenna includes a pair of conductive rings that extend around the
tube and are axially spaced from each other. The rings are preferably
formed in a unitary construction with a conductive base bar that extends
generally parallel to the tube axis, with the rings rigidly supported by
and integral with the opposite ends of the base bar. A conductive rf feed
bar extends generally parallel to the tube axis between the two rings on
the opposite side of the tube from the base bar, with an input rf signal
delivered to opposite sides of an interruption in the feed bar. The two
rings are designed to divide an rf current from the feed bar symmetrically
and then recombine the current in the base bar, thereby avoiding the
generation of any substantial net axial rf magnetic field through the
tube.
In a preferred embodiment, the feed bar interruption is provided between
one of the rings and the adjacent end of the feed bar. Localized
enlargements of the ring and feed bar include openings to receive the
sheath and inner conductor of a coaxial rf feed cable, respectively. The
antenna is durable, easy to assemble to the tube, and contributes to the
reduction in overall size.
These and other features and advantages of the invention will be apparent
to those skilled in the art from the following detailed description, taken
together with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional view of a compact rf plasma source in accordance with
the invention;
FIG. 2 is a perspective view of an rf antenna used in the plasma source of
FIG. 1;
FIG. 3 is a sectional view of a spacecraft telescope with a mirror that is
cleaned with the compact rf plasma source of the invention; and
FIG. 4 is a simplified perspective view illustrating the exterior surface
of a spacecraft being cleaned with the new plasma source.
DETAILED DESCRIPTION OF THE INVENTION
A preferred embodiment of a compact rf plasma source that can be used for
orbiting spacecraft is shown in FIG. 1. It includes a cylindrical plasma
tube 2, formed from a material such as alumina, ceramic or glass, within
which the plasma is generated. An rf antenna 4 is provided around the
exterior of the plasma tube and provides a flow path for rf current that
generates an oscillating magnetic field (a magnetic dipole) in a plane
perpendicular to the tube's axis. The antenna could at least theoretically
be located around the interior of the plasma tube, but that would add to
the surface area within the tube and thereby increase the rate of plasma
recombination loss. As described below, the antenna has a unique
configuration that avoids the generation of an oscillating field along the
tube axis, and is much simpler in design than the Nagoya-type antennas
previously used for this purpose.
The remainder of the assembly is held in place between upper and lower
stainless steel endcaps 6 and 8, which are notched to retain the opposite
ends of the tube 2. A series of tie bolts 10 are used to secure the
endcaps together. An outer wire mesh shield 12 provides additional
structural support and protection for the assembly. The antenna 4 is
secured against slippage on the plasma tube, such as by a ceramic-to-metal
braze.
A number of permanent bar magnets, of which two magnets 14 and 16 are
illustrated although typically six-ten magnets would be employed, are
provided at various azimuthal positions around a source and housed in a
magnet tube 18 that forms a protective housing. The magnets, which can be
formed from samarium cobalt or neodymium-iron-boron, establish a magnetic
field within the tube that is generally parallel to the tube axis. A
cylindrical shell magnet might also be used in place of the bar magnets.
The magnetic field is kept generally uniform within the tube through the
use of pole pieces 20 and 22 at the upper and lower ends of the assembly.
The pole pieces, which are preferably iron or other high permeability
magnetic material, are shaped to provide the desired field uniformity;
similarly shaped pole pieces are employed in known ion thrusters. The
magnet tube 18 is preferably formed from stainless steel to avoid shorting
the magnetic circuit.
A plasma source gas is supplied from a gas reservoir 24 through a valve 26
and nipple 28 into the back end of the plasma tube. Oxygen is commonly
used as a source gas because it reacts with hydrocarbon contaminants, but
other sources such as nitrogen, CF.sub.4, argon, air or water vapor could
also be used. In general, any gas or vapor that reacts with the
contaminant to be removed and does not condense at the plasma source's
operating temperature (estimated at about 100.degree. C.) could be
considered. Instead of using a valve 26, a heater might be employed to
control the gas or vapor flow rate into the plasma tube.
An rf source 30 is coupled to the antenna 4, most conveniently via a
coaxial cable 32. The cable includes an outer sheath 34 that is connected
to one of the antenna electrodes 36, and an inner conductor 38 that
extends through terminal 36 and is connected to a second antenna terminal
40.
If desired, the structure shown in FIG. 1 can be modified to add an
additional permanent magnet structure, downstream from the magnet
structure illustrated in the figure, to provide a more consistent field
gradient that encourages outward plasma drift through the tube's open
discharge end 42. This type of magnet configuration is used, for example,
in the plasma generator of U.S. Pat. No. 4,977,352 to Williamson, one of
the present inventors, assigned to Hughes Aircraft Company.
It has been found that the plasma source illustrated in FIG. 1 can be
successfully used to generate a highly reactive plasma that will clean
contaminated spacecraft surfaces with low rates of power and gas
consumption, and at an energy level below that at which sputter damage to
optical surfaces is produced. This is accomplished with a structure that
is considerably smaller and lighter than ground based helicon wave plasma
sources currently available.
In a typical application of the invention, a 22 mm diameter plasma source
is supplied with gas at a flow rate of 0.5-20 sccm (standard (temperature
and pressure)cm.sup.3 /minute). Higher or lower fluences can be obtained
by increasing or decreasing the gas flow rate or power. The input rf power
is in the 5-25 W range, while the magnets provide an axial field of about
150 milliTesla (the field is somewhat stronger at the pole faces). With an
rf frequency of 100 MHz, a maximum oxygen ion current of 50 mA is
generated. The energy of the plasma ions and neutrals ranges up to about
15 eV, which is sufficiently below the approximate threshold of 20 eV at
which sputter damage to optical surfaces can commence.
A novel rf antenna configuration that replaces the Nagoya-type antenna and
is instrumental in enabling a more compact plasma source is shown in FIG.
2. The antenna 4 is formed from a conductive material such as copper,
preferably in a unitary integral construction. The antenna includes a pair
of rings 44 and 46 at its opposite ends whose inside diameter is
approximately equal to the plasma tube's outer diameter. The two rings are
electrically and mechanically connected by a base bar 48, which extends
parallel to the common ring/plasma tube axis. The opposite ends of the
base bar 48 merge into the rings, and rigidly support them to keep them
mutually separated. An rf feeder bar 50 extends between the rings on the
diametric opposite side of the antenna from base bar 48. The feeder bar 50
has an interruption 52, allowing the rf signal to be connected across the
opposite sides of the interruption. This interruption is preferably
between one of the rings 46 and the remainder of the feed bar 50, which
extends integrally up from the other ring 44. However, if desired the feed
bar can extend in from both rings, with the interruption near its middle.
The terminals 36 and 40 consist of enlarged areas on the opposed ends of
the feed bar 50 and upper ring 46. A relatively large axial opening 54 is
formed through the feed bar terminal 36 to accommodate the outer sheath of
the coaxial cable 32, while a smaller axial opening 56 is formed through
the ring terminal 40 to accommodate the cable's inner conductor. Set screw
openings 58 and 60 are provided in the terminals perpendicular to the
cable openings 54 and 56 so that the cable sheath and inner conductor can
be secured in place with set screws. Although the upper ring 46 is
illustrated as progressively expanding in width from the base bar 48 to
the terminal 40, its width can be held equal to that of the base bar, with
a tab on the opposite side of the ring for terminal 40.
The illustrated antenna configuration results in a symmetrical flow of rf
current that produces substantially zero net oscillating magnetic field
parallel to the tube axis. This is because the antenna provides
symmetrical clockwise and counter-clockwise current flow paths around the
plasma tube. As illustrated by the arrow 61, the current during one-half
of each rf cycle flows from the terminal 36 through the feed bar 50, and
divides equally in opposite directions around the lower ring 44. The ring
currents recombine at the base bar (arrow 62), and again divide equally in
opposite directions around ring 46 at the upper end of the base bar (arrow
64). The upper ring 46 provides a return path to the rf source via
terminal 40. This current flow reverses during the other half of the rf
cycle, but it still divides symmetrically around the rings and thus avoids
the production of a net oscillating magnetic field in the axial direction.
Typical dimensions for the antenna are a 28 mm inside diameter, an overall
axial length of 57 mm, and a base bar/feed bar thickness of 1 mm.
The application of the invention in cleaning an optical surface of a
spacecraft telescope is illustrated in FIG. 3. The telescope housing 66 is
mounted to the spacecraft's outer skin 68, with light entering the
telescope through an opening 70 in the skin. The telescope is shown as
consisting of a primary mirror 72, a secondary mirror 74 and a focal plane
array 76. Light baffles 78 are positioned along the edges of the telescope
tube 80 to reject scattered light.
An rf plasma source 82 in accordance with the invention is supplied with
gas and/or vapor from a supply tank 84, and is energized by an rf power
source 86. The plasma source 82 is shown mounted on the tube 80 that
carries the baffles 78, and oriented to direct a cleaning plasma 90 onto
the mirror surface.
If the telescope's optical components are warm, such as from solar heating
and heat radiated from the spacecraft, an oxygen plasma can be used to
remove both condensed hydrocarbon vapors that are outgassed onto the
mirror surface from organic materials carried onboard the spacecraft, and
the photopolymerized hydrocarbons that result from exposure to the sun. In
a demonstration of the invention, a buildup of ultraviolet-absorbing scum
on an optical surface had caused a high-resolution telescope-spectrometer
(HRTS) to lose its ability to image the sun in its designed far-UV (122
nm) range after only a few orbital periods. An unsuccessful attempt was
first made to remove the scum by scrubbing with a solvent. However, an
exposure to an oxygen plasma produced with the invention visibly removed
the contaminant layer. In other cleaning experiments, the reflectance of a
mirror similar to that used on HRTS at 122 nm had degraded from a pristine
level of 0.68 to a contaminated level of 0.33, but was restored by oxygen
cleaning with the invention to a reflectance of about 0.64. A window had a
transmittance at 122 nm of 0.64 in its pristine state, which deteriorated
to 0.21 after contamination. After an oxygen plasma cleaning of one side
the transmittance increased to 0.39, and was restored to 0.64 when both
sides of the window were cleaned.
For hydrocarbon and silicone-containing contaminants that commonly occur
from spacecraft outgassing, a plasma formed from an oxygen and CF.sub.4
mixture can be used. This type of mixture has previously been used in
ground applications. It has also been found that liquid compounds
containing both oxygen and flourine, such as hexafluoro acetone,
hexafluoro acetone hydrate or trifluoro acetic acid, can be used as a
plasma source, thereby reducing the complexity of the equipment that would
otherwise be required to handle two separate plasma source components.
The use of the invention to clean an exterior spacecraft surface, such as a
thermal radiator, is illustrated in FIG. 4. The body to be cleaned is
illustrated generically by a cube 92 having a contaminated upper surface
94; the lower surface would also typically be contaminated. In actual
practice, the typical construction for a thermal radiator is thin
transparent silica with silver coating on the spacecraft side. Solar
panels 96 and 98 are supported respectively by yokes 100 and 102 on
opposite sides of the spacecraft. The solar panels rotate once each day to
track the sun, while the spacecraft antennas track the earth. A plasma
source 104 is shown mounted on one of the solar panels 96 at an angle to
the yoke 100, so that an oval shaped spot 106 is exposed to the plasma and
cleaned. A similar plasma source could be provided on the other solar
panel 98. The diurnal rotation of the solar wing allows the entire
radiator panel to be cleaned in a single day; calculations indicate that
one such cleaning per month would keep the radiators clean even in
worst-case contamination situations. The yokes provide a transmission path
for unregulated DC voltage generated by the panels into the spacecraft,
and for the return of a regulated voltage to the rf source 107 used for
the plasma source.
It has been discovered that special advantages are possible with this type
of cleaning by using water vapor plasma instead of oxygen. When ionized in
the rf discharge, water vapor decomposes into a number of species that
include the highly reactive radicals and radical ions H+, H, OH-, OH, O
and O+. Water vapor plasmas have been found to clean hydrocarbons with an
effectiveness equal to that of oxygen plasmas.
The easy storage of water offers a major advantage for its use as a
cleaning agent in applications such as spacecraft radiator cleaning.
Instead of the relatively costly and heavy high pressure cylinder required
for oxygen storage, water can be contained in a very compact, light weight
vessel at low pressure. The vessel can be kept warm by thermal-blanket
design so that the vapor pressure of the water is adequate to supply the
plasma generator. An additional advantage of water tankage is that
micrometeorite hits on the tank, while ultimately fatal to the cleaning
system, pose no hazard to the spacecraft. The water would slowly be lost
into space, as opposed to a punctured high pressure oxygen tank that could
produce a gas jet with sufficient force to generate unacceptable torques
on the spacecraft. The plasma generator 104 is illustrated as including a
plasma generation section 104a, to the rear of which a water storage tank
104b is directly coupled.
Water plasma cleaning is not suitable for a cryotelescope application,
since any water molecules that fail to become ionized in the rf discharge
would freeze onto the optical surfaces and could contaminate these
surfaces faster than they could be cleaned. Cryotelescope cleaning,
however, requires only small oxygen tanks, since the surface area to be
cleaned is very small in comparison to radiator panels (typically not more
than about 0.03 m.sup.2 vs. 5 m.sup.2 or greater). In addition, the oxygen
tank associated with a cryotelescope cleaner would be protected from
micrometeorites by the spacecraft structure.
The invention is believed to operate on the helicon wave principal, with
electromagnetic helicon waves launched on the plasma and propagating down
the magnetic field lines to be absorbed through an electron damping
process. This exchange of energy between the wave and electrons leads to
energy transfer to the electrons, increasing their temperature and thereby
sustaining the plasma.
Numerous conventional mechanisms can be employed to initiate the plasma.
These include briefly turning up the power to generate an electric field
high enough to break down the gas, waiting momentarily for a cosmic ray to
excite electrons sufficiently, briefly increasing the gas pressure so that
it breaks down more easily (with or without an increase in power),
providing a sharp electrode source to initiate the plasma, and adding a
radioactive component to the gas to produce a plasma-initiating
radioactive decay.
The invention can also provide a spacecraft charging protection function. A
spacecraft surface that is not exposed to the sun can acquire a high
negative charge, on the order of -20 kV. This is illustrated by the
negative charge symbol 108 on the spacecraft surface 110 in FIG. 4. The
charge differential between the plasma and the negative surface charge
produces an electric field 112, diverting positive ions from the plasma to
neutralize the localized charge 108. This is accomplished with only a
relatively small amount of charge diversion, so that the plasma which
cleans surface 94 remains essentially charge neutral. A similar charge
protection function is disclosed in U.S. Pat. No. 4,800,281 for a
Penning-discharge plasma source.
While several illustrative embodiments of the invention have been shown and
described, numerous variations and alternate embodiments will occur to
those skilled in the art. Such variations and alternate embodiments are
contemplated, and can be made without departing from the spirit and scope
of the invention as defined in the appended claims.
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