An improved solid expellant plasma generator has been developed. The plasma generator includes a support housing, an electrode rod located in the central portion of the housing, and a mass of solid expellant material that surrounds the electrode rod within the support housing. The electrode rod and the solid expellant material are made of separate materials that are selected so that the electrode and the solid expellant material decompose at the same rate when the plasma generator is ignited. This maintains a point of discharge of the plasma at the interface between the electrode and the solid expellant material

Poe, Garrett D.

Rood, Robert

Stone, Nobie H.

NASA Technical Reports Server

35
(12) United States Patent
Stone et al.
(54) SOLID EXPELLANT PLASMA GENERATOR
(75) Inventors: Nobie H. Stone, Huntsville, AL (US);
Garrett D. Poe, Madison, AL (US);
Robert Rood, Union Grove, AL (US)
(73) Assignee: NeXolve Corporation, Huntsville, AL
(US)
(*) Notice: Subject to any disclaimer, the term of this
patent is extended or adjusted under 35
U.S.C. 154(b) by 432 days.
(21) Appl. No.: 11/852,007
(22) Filed:	 Sep. 7, 2007
(65)	 Prior Publication Data
US 2009/0066256 Al	 Mar. 12, 2009
(51) Int. Cl.
HOIJ 7124	 (2006.01)
(52) U.S. Cl . ............................. 315/111.11; 315/111.21
(58) Field of Classification Search ............ 315/111.21,
315/111.31, 111.71, 111.81, 111.9 1, 111.11;
118/722, 723 VE
See application file for complete search history.
(56)	 References Cited
U.S. PATENT DOCUMENTS
3,279,176 A	 10/1966 Boden
(1o) Patent No.:	 US 7,701,145 B2
(45) Date of Patent:	 Apr. 20, 2010
3,321,919 A 5/1967 Marolda
4,825,646 A 5/1989 Challoner et al.
5,942,854 A * 8/1999 Ryoji et al .	 ............	 315/111.21
6,424,091 131 * 7/2002 Sawada et al.	 .........	 315/111.81
6,640,535 132 11/2003 Gallimore et al.
2003/0152184 Al 8/2003 Shehane et al.
FOREIGN PATENT DOCUMENTS
Wo	 W09222189	 12/1992
* cited by examiner
Primary Examiner David Hung Vu
(74) Attorney, Agent, or Firm David E. Mixon; Bradley
Aram Boult Cummings LLP
(57)	 ABSTRACT
An improved solid expellant plasma generator has been
developed. The plasma generator includes a support housing,
an electrode rod located in the central portion of the housing,
and a mass of solid expellant material that surrounds the
electrode rod within the support housing. The electrode rod
and the solid expellant material are made of separate materi-
als that are selected so that the electrode and the solid expel-
lant material decompose at the same rate when the plasma
generator is ignited. This maintains a point of discharge of the
plasma at the interface between the electrode and the solid
expellant material.
25 Claims, 3 Drawing Sheets
34
https://ntrs.nasa.gov/search.jsp?R=20100028311 2019-08-30T10:40:11+00:00Z
U.S. Patent	 Apr. 20, 2010	 Sheet 1 of 3	 US 7,701,145 B2
10
22
Expellant, Neutrals, Ions, and Electrons
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O +
	
O O	 O
	
± O +.	 +
REA'.
O
O
Tether
FIG. 1
MM
N
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U.S. Patent	 Apr. 20, 2010	 Sheet 2 of 3	 US 7,701,145 B2
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U.S. Patent	 Apr. 20, 2010	 Sheet 3 of 3	 US 7,701,145 B2
2.0
1.8
1.6
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o X i,
X
50 100 150 200 250 300 350 400 450 500 600
Voltage Across Discharge (volts)
-	 -	 Igniter and Primary Plasma Generator Units o
-------------- Primary Plasma Generator Unit 	 x
Earlier TSS-1 R Satellite Thruster Unit	 n
FIG. 3
US 7,701,145 B2
1
	
2
SOLID EXPELLANT PLASMA GENERATOR
	
	
housing; and where the electrode rod and the solid expellant
material comprises separate materials that are selected so that
STATEMENT REGARDING FEDERALLY 	 the electrode and the solid expellant material decompose at
SPONSORED RESEARCH OR DEVELOPMENT
	
	
the same rate in order to maintain a point of discharge of the
5 plasma at the interface between the electrode and the solid
The U.S. Government has a paid-up license in this inven- 	 expellant material.
tion and the right in limited circumstances to require the	 Other aspects and advantages of the invention will be
patent owner to license others on reasonable terms as pro- 	 apparent from the following description and the appended
vided for by the terms of Contract Nos. NASS-02105 and 	 claims.
NASS-01153 awardedby the National Aeronautics and Space 10
Administration (NASA). 	 BRIEF DESCRIPTION OF DRAWINGS
FIELD OF THE INVENTION
	
	
It should be noted that identical features in different draw-
ings are shown with the same reference numeral.
The invention relates generally to spacecraft propulsion. 15	 FIG.1 is a conceptual schematic diagram a solid expellant
More specifically, the present invention relates to a solid	 plasma generator in accordance with one embodiment of the
expellant plasma generator for spacecraft. 	 present invention.
FIG. 2 shows a cross-sectional diagram of an ionization
BACKGROUND ART 	 chamber located above the expellant block in accordance
20 with one embodiment of the present invention.
The application of electrodynamic tethers in space appli-	 FIG. 3 is a graph that shows the discharge current (amps) as
cations is presently limited by the lack of a suitable plasma 	 a function of the voltage across the generator discharge (volts)
contactor device. Previously, the NASA Tethered Satellite 	 in accordance with one embodiment of the present invention.
System used an electron gun to emit electrons back into the
conductive ionospheric plasma at the negative pole of the 25	 DETAILED DESCRIPTION
electrodynamic tether. Hollow cathode plasma generators
were used on the Plasma Motor-Generator (PMG) tether mis- 	 The solid expellant plasma generator of the present inven-
sion. However, both, the electron gun and the hollow cathode 	 tion uses a solid expellant to generate a conducting gas (i.e.,
are sensitive to contamination, depend heavily on spacecraft 	 plasma). The basic principle is that the expellant vapor cloud
conditioned electrical power, and are complex. In particular, 30 enables an electrical discharge that ionizes vapor particles.
the Hollow Cathode requires a high pressure vessel and the 	 The heat generated by the discharge, in turn, vaporizes addi-
associated pluming to regulate and handle the gaseous expel- 	 tional expellant, which replenishes the expanding cloud and
lant.	 maintains the conditions required for the electrical discharge
Consequently, aplasma device that does not rely heavily on	 to function. This device is robust and eliminates the need for
spacecraft resources (mass and conditioned electrical power) 35 highpressure vessels, heaters, pressure regulators, valves and
is desired. Also, the plasma device should be insensitive to 	 other pluming components. The generator is also insensitive
contamination and, therefore, not require special on-orbit	 to contamination, and does not require on-orbit conditioning
pre-operation conditioning or stand-by power and is capable 	 or regulated power.
of emitting large, multi-amp currents. In order to create such
	 The purpose of the solid expellant plasma generator is to
a plasma device, it is a requirement to emit large currents 40 provide a suitable plasma contactor device for the operation
while minimizing the use of electrical power and expellant 	 of electrodynamic tethers in space. The generator requires
mass. Additionally, the device should use the emission of 	 only a center electrode surrounded by a solid expellant block.
large currents at low power without the use of contamination- 	 The device is powered by unregulated electrical power devel-
sensitive, low work-function materials.	 oped by the electrodynamic tether in the generator mode, or
45 by unregulated solar-electric power in the motor mode.
SUMMARY OF THE INVENTION	 FIG.1 is a conceptual schematic diagram a solid expellant
plasma generator in accordance with one example of the
In some aspects, the invention relates to a solid expellant 	 present invention. As shown, the solid expellant plasma gen-
plasma generator, comprising; a stainless steel support hous-	 erator 10 includes a cylindrical shaped housing 12 that con-
ing; a 0.9 mm diameter copper-clad carbon electrode rod 50 tains an electrode rod 14 located in the center section of the
located in the central portion of the support housing; a block
	
housing 12. A block of solid expellant material 16 surrounds
of solid expellant material made of poly(butyl methacrylate) 	 the electrode rod 14. The solid expellant material 16 is cast in
with poly(tetraethylene glycol diacrylate) crosslinks that sur- 	 a shape to fill the interior of the housing 12. The electrode rod
rounds the electrode rod within the support housing; where	 14 is connected 18 to a high voltage tether (not shown) in
the electrode rod and the solid expellant material decompose 55 order to provide a source of electrical power to the electrode
at the same rate in order to maintain a point of discharge of the 	 rod 14. As the electrode rod 14 discharges 20, neutral par-
plasma at the interface between the electrode and the solid	 ticles, electrons, and ions are expelled in the form of a vapor
expellant material upon ignition of the generator; where the 	 cloud from the generator 22.
generator maintains an expellant mass utilization of 0.00325 	 The solid expellant plasma generator is capable of operat-
kg/amp-hr or higher; and where the operating voltage of the 60 ing in the configuration shown in FIG. 1. However, efficiency
generator is inversely proportional to the emission current 	 is improved by the addition of an "Ionization Chamber"
once the generator reaches a critical emmission current of 	 above the Expellant Block, as shown in FIG. 2. The ionization
approximately 0.4 amps. 	 chamber 30 consists of: an extension of the metallic support
In other aspects, the invention relates to a solid expellant 	 canister 32 beyond the top surface of the expellant block 34;
plasma generator, comprising: a housing; an electrode rod 65 an aperture 35 with a smaller diameter orifice (which has the
located in the central portion of the housing; a block of solid 	 effect of limiting the escape of the neutral vapor cloud); and a
expellant material that surrounds the electrode rod within the 	 pertinent magnet 36, which is in the shape of a hollow cylin-
US 7,701,145 B2
3
	
4
der, and fits over the exterior of the support canister 32. This 	 than 1.8 amps. Above a critical emmission current, the oper-
has the effect of increasing the path length of ionizing elec- 	 ating voltage of the generator is inversely proportional to
trons before they escape the discharge region. The increased 	 emission current and decreases to less than 100 volts at 1.8
vaporpressure, as aresult ofthe support canister extension 32 	 amps. In testing, the critical emission current was approxi-
and aperture 35, and the increased path length of the elec- 5 mately 0.4 amps. Also, the expellant mass utilization of a test
trons, result in improved ionization efficiency of the vapor-	 model was 0.00325 kg/amp-hr.
ized expellant and therefore improved expellant mass utiliza- 	 The solid expellant plasma generator can be used in any
tion. Also shown in FIG. 2 is the electrode 38 and power	 application where a plasma generator is required. Examples
supply wire 40 that are attached by an electrical connector 42. 	 of potential applications include: the control of spacecraft
They are cover by high voltage insulation 46 and high tem- io charging on commercial satellites placed in synchronous
perature insulation 44. The power supply wire 40 is held in	 orbit where differential charging is a known problem; and the
place by a capture nut 48.	 vapor deposition process used, for example, in the semicon-
It should be noted that alternative embodiments of the 	 ductor industry. In should be clear that the present invention
present invention may utilize different configurations of fea-	 could also be used in any other applications know in the art
tures and different types of materials. For example, the elec- 15 that require a plasma generator.
trode rod may be located in alternative locations other than	 Key advantages of the present invention include the gen-
the center of the housing. In one such embodiment multiple	 eration of a self-sustained, self-fuelled electrical discharge.
rods may be used in the housing to provide enhanced current	 Additionally, the expellant material produces multiple vapor
capacity. Further, the solid expellant may be a variety of 	 particles per solid particle which are chemically benign. Con-
materials such as a wax (e.g., paraffin wax) or a polymeric go sequently, the vapor particles do not react chemically with
material such as: polyethylene; nylon; poly(methyl meth- 	 satellite surfaces or structures. Finally, the unique design of
acrylate); poly(butyl methacrylate); crosslinked polyethyl- 	 the center electrode, effectively "burns" at the same rate as the
ene; or teflon. One particular material that may be used is poly 	 expellant block is depleted in order to maintain a stable dis-
(butyl methacrylate) with poly (tetraethylene glycol diacry- 	 charge.
late) crosslinks. It is generally preferred to use polymers that 25	 While the invention has been described with respect to a
do not melt and that depolymerize without ashing. The elec- 	 limited number of embodiments, those skilled in the art,
trode may be made of such materials as: copper-clad carbon; 	 having benefit of this disclosure, will appreciate that other
uncoated carbon; tungsten; copper; or stainless steel. The key 	 embodiments can be devised which do not depart from the
characteristic of the materials of the electrode and the solid 	 scope of the invention as disclosed here. Accordingly, the
expellant is that they must be matched so that each material 30 scope of the invention should be limited only by the attached
decomposes at the same rate. The materials used for the 	 claims.
housing may be either conductive or non-conductive. Mate- 	 What is claimed is:
rials such as aluminum or stainless steel may be used. The key 	 1. A solid expellant plasma generator, comprising:
characteristic of the housing is that it provides structural 	 a stainless steel support housing;
support for the solid expellant material, the discharge cham- 35	 a 0.9 mm diameter copper-clad carbon electrode rod
ber, etc.	 located in the central portion of the support housing;
The function of the solid expellant plasma generator is 	 a block of solid expellant material made of poly(butyl
analogous to that of a candle in which the heat of the flame 	 methacrylate) with poly(tetraethylene glycol diacrylate)
melts the tallow which, in turn, fuels the flame. The center 	 crosslinks that surrounds the electrode rod within the
electrode is biased sufficiently that an electrical discharge is 40	 support housing;
initiated between it and the vapor cloud. The heat of the 	 wherein the electrode rod and the solid expellant material
electrical discharge decomposes additional material from the 	 decompose at the same rate in order to maintain a point
surface of the solid expellant block in the immediate vicinity 	 of discharge of the plasma at the interface between the
of the center electrode. The expellant maintains the vapor 	 electrode and the solid expellant material upon ignition
cloud at a sufficient column density to ensure a high collision 45	 of the generator;
rate between the energetic charged particles, accelerated 	 wherein the generator maintains an expellant mass utiliza-
away from the center electrode, and the neutral vapor par-	 tion of 0.00325 kg/amp-hr or greater; and
ticles in the cloud. Each ionizing collision creates a charged 	 wherein the operating voltage of the generator is inversely
pair ... a positive and a negative particle. Some of the	 proportional to the emission current once the generator
particles that are of opposite polarity to the center electrode 50	 reaches a critical emmission current of approximately
will be accelerated back into the electrode, which is main-	 0.4 amps.
tained at an emissive temperature by the resulting collisions. 	 2. A solid expellant plasma generator, comprising:
As previously mentioned, the center electrode is designed 	 a housing;
so that it decomposes at the same rate as the block of solid 	 an electrode rod located in the central portion of the hous-
expellant is depleted. This maintains the point of discharge at 55	 ing;
the interface between the electrode and solid expellant mate- 	 a block of solid expellant material that surrounds the elec-
rial. This is key to achieving a reliable ignition of the genera- 	 trode rod within the housing; and
tor. Also, it is critical for maintaining stable and efficient 	 wherein the electrode rod and the solid expellant material
evaporation/sublimation of the electrode/expellant of the 	 comprises separate materials that are selected so that the
generator.	 60	 electrode and the solid expellant material decompose at
In some embodiments, the center electrode is a 0.9 mm	 the same rate in order to maintain a point of discharge of
copper-clad carbon rod. In alternative embodiments, the elec- 	 the plasma at the interface between the electrode and the
trode is a 0.9 mm solid tungsten rod. The performance char- 	 solid expellant material.
acteristics of the solid expellant plasma generator as shown in 	 3. The generator of claim 2, wherein the housing is made of
FIG. 3 which shows the discharge current (amps) v. the volt-  65 a non-conductive material.
age across the generator discharge (volts). The performance 	 4. The generator of claim 2, wherein the housing is made of
of the generator shows an emission current capacity of more 	 a conductive material.
US 7,701,145 B2
5
5. The generator of claim 2, wherein the housing is made of
aluminum.
6. The generator of claim 2, further comprising a plurality
of electrodes located in the central housing.
7. The generator of claim 2, wherein the electrode is made
of copper clad carbon.
8. The generator of claim 2, wherein the electrode is made
of uncoated carbon.
9. The generator of claim 2, wherein the electrode is made
of tungsten.
10. The generator of claim 2, wherein the electrode is made
of copper.
11. The generator of claim 2, wherein the electrode is made
of stainless steel.
12. The generator of claim 2, wherein the solid expellant
material is Teflon.
13. The generator of claim 2, wherein the solid expellant
material is wax.
14. The generator of claim 2, wherein the solid expellant
material is paraffin wax.
15. The generator of claim 2, wherein the solid expellant
material is polymeric material.
6
16. The generator of claim 2, wherein the solid expellant
material is polyethylene.
17. The generator of claim 2, wherein the solid expellant
material is nylon.
5	 18. The generator of claim 2, wherein the solid expellant
material is poly (methyl methacrylate).
19. The generator of claim 2, wherein the solid expellant
material is poly(butyl methacrylate).
20. The generator of claim 2, wherein the solid expellant
io material is crosslinked polyethylene.
21. The generator of claim 2, wherein the solid expellant
material is a non-melting polymeric material.
22. The generator of claim 2, wherein the solid expellant
material is a non-ashing polymeric material.
15	 23. The generator of claim 2, wherein the generator has a
discharge rate of 0.00325 kg/amp-hr or higher.
24. The generator of claim 2, wherein the generator has an
emission current capacity of greater than 1.8 amps.
25. The generator of claim 24, wherein the generator has an
20 operating voltage of less than 100 volts when the emission
current capacity is greater than 1.8 amps.


Solid expellant plasma generator

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