A single grid accelerator system for an ion thrustor is discussed. A layer of dielectric material is interposed between this metal grid and the chamber containing an ionized propellant for protecting the grid against sputtering erosion

Margosian, P. M.

Nakanishi, S.

English

NASA Technical Reports Server

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flUG 7 1973
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https://ntrs.nasa.gov/search.jsp?R=19730018972 2020-03-17T08:53:45+00:00Z
United States Patent
Margosian et al.
mi 3,744,247
[45] July 10, 1973
[54]
[75]
[73]
[22]
[21]
[63]
SINGLE GRID ACCELERATOR FOR AN ION
THRUSTOR
Inventors: Paul M. Margosian, Roslindale,
Mass.; Shigeo Nakanishi, Berea,
Ohio
Assignee: The United States of America as
represented by the Administrator of
the National Aeronautics and Space
Administration, Washington, D.C.
Filed: Sept. 14, 1971
Appl. No.: 180,473
Related U.S. Application Data
Continuation of Ser. No. 758,540, Sept. 9, 1968,
abandoned.
[52] U.S. Cl 60/202, 313/63, 313/217,
313/218,313/230,313/355
[51] Int. Cl .: F03h 5/00
[58] Field of Search 313/63, 207, 107,
313/230,355,218,217;60/202
[56] References Cited
UNITED STATES PATENTS
3,371,489 3/1968 Eckhardt 313/63 X
2,913,617 11/1959 Tucker 313/355 X
3,021,446 2/1962 Ekkers et al 313/218 X
3,345,820 10/1967 Dryden 313/63 X
3,500,122 3/1970 Sohl 313/63 X
2,068,287 I/I937 Gabor 313/207 X
2,094,450 9/1937 Geffcken et al 313/207 X
3,250,942 5/1966 Yoshida et al 313/107 X
3,297,902 1/1967 Beggs 313/355 X
Primary Examiner—Palmer C. Demeo
Attorney — N. T. Musial, Gene E. Shook et al.
[57] ABSTRACT
A single grid accelerator system for an ion thrustor. A
layer of dielectric material is interposed between this
metal grid and the chamber containing an ionized pro-
pellant for protecting the grid against sputtering ero-
sion.
3 Claims, 3 Drawing Figures
10
/
- i.
PATENTED JULIO 1973 3,744,247
10
20
FIG. I
XXVjy_jjfs'y sy
14
FIG. 2
\
14
FIG. 3 \
16
BY
«_-A/
INVENTORS
PAUL M. MARGOSIAN
SHIGEO NAKANISHI
ATTORNEYS
3,744,247
1 2
SINGLE GRID ACCELERATOR FOR AN ION merit ion thrustor showing an accelerator system con-
THRUSTOR structed in accordance with the present invention,
This is a continuation of application Ser. No. 758,540 FIG. 2 is an enlarged sectional view taken along the
filed Sept. 9, 1968, and now abandoned. line 2—2 in FIG. 1, and
STATEMENT OF GOVERNMENT OWNERSHIP '
 2 ™
The invention described herein was made by employ- tion.
ees of the United States Government and may be man- DFSrRIPTION OF THF PRFFFRRFD
ufactured and used by or for the Government for gov- F^BODIMF^NT
ernmental purposes without the payment of any royal- 10 MENT
ties thereon or therefor. An electron bombardment ion thrustor of the type
RACKr ROUND OF THF INVFNTION described in U.S. Pat. No. 3,156,090 utilizes mercuryBACKGROUND OF THE INVENTION
 as g propellant. Mercury vapor is supplied to an ioniza-
This invention is concerned with an improved ion ex- tion chamber through a distributor plate, and atoms of
tractor component for an ion thrustor. The invention is 15 mercury are bombarded by electrons emitted from a
particularly directed to a single grid accelerator system cathode. An axial magnetic field is supplied to increase
for an electron bombardment type ion thrustor. the path length traveled by electrons going from the
Accelerator grid systems used in electron- cathode toward an anode thereby increasing the ioniza-
bombardment thrustors have depended on a screen tion by electron bombardment.
grid and an accelerator grid as two main components. 20 Such an electron bombardment ion thrustor utilizes
Both are made of stainless steel or molybdenum and both a screen grid and an accelerator grid in its ion ex-
are mounted at close spacing relative to each other. traction portion. The screen grid serves to contain the
The grids are maintained at a potential difference of discharge plasma forming the necessary ion optics to
several thousand volts with respect to each other. This prevent direct impingement of accelerated ions onto
voltage applied over the close spacing provides an elec- 25 the accelerator grid. Ions in the near vicinity of the
trie Field required for the extraction of ions from the screen grid have a high probability of being accelerated
ionization chamber thereby providing thrust. The to openings in the grids because of the high electric
screen grid is operated at a positive potential and the fields present. Thrust is produced as these ions acceler-
accelerator at a negative potential. This "accel-decel" ate through the grid system.
mode of operation insures maximum ion extraction 30 Referring now to FIG. 1 there is shown an electron
while preventing back streaming of neutralizing elec- bombardment ion thrustor 10 of the aforementioned
irons into the thrustor. type. The ion thrustor 10 has an ionization chamber 12
One type of grid system employs a parallel wire ar- for containing mercury propellant that has been ion-
rangement. Such a system is shown in U.S. Pat. No. ized by electron bombardment in the manner previ-
3,156,090. A pair of perforated plates having aligned 35 ously described.
holes has also been utilized. The overall weight of these The ion thrustor further includes an accelerator sys-
double grid systems as well as the hardware required tem for accelerating propellant ions in the direction of
for proper mounting is objectionable. The required uni- the arrows in FIGS. 1 and 2. This accelerator system
form gap of close spacing between the grids further ere- utilizes a single grid 14 in the form of a perforated plate
ates fabrication problems. 40 of an electrically conducting material. The accelerated
. . ~.., ^^ ^..r. ,v,,,^x,-r..^^, propellant ions pass through holes 16 in this plate.SUMMARY OF THE INVENTION ^
 grjd 14 ? connect|d to a source of eP,ectrica,
These problems have been solved by the single grid power, such as a battery 18, as shown in FIG. 1. The
accelerator system constructed in accordance with the electrical power source 18 impresses a potential on the
present invention. A dielectric material is interposed 45 grid 14 and is highly negative relative to the ionization
between the grid and the thrustor ionization chamber. chamber 12.
A metal grid has its upstream surface covered with di- An insulator 20 having an annular configuration ex-
electric material, or a dielectric grid may have its tends about the periphery of the ionization chamber
downstream surface covered with a metal. 12. The insulator 20 serves to electrically isolate the
It is, therefore, an object of the present invention to single grid from the metal housing forming the ioniza-
provide a single grid accelerator system for an electron tion chamber 12.
bombardment ion thrustor. An important feature of the invention is that the elec-
Another object of the invention is to provide an im- trically conducting grid 14 has the surface that faces
proved accelerator grid for an ion thrustor which is the ionization chamber 12 protected from sputtering
readily fabricated. erosion. This is accomplished by positioning a layer of
A still further object of the invention is to provide an insulating material 22 between the chamber 12 and the
electron bombardment ion thrustor which accom- grid 14 as shown in FIGS. 1 and 2.
plishes the ion extraction function in an improved man- The grid 14 is electrically conductive to establish the
ner. ' ,„ proper electrostatic field for accelerating the ions in
These and other objects of the invention will be ap- the chamber 12 in the direction of the arrow. The grid
parent from the specification which follows and from 14 may be of a perforated metal plate having the up-
the drawing wherein like numerals are used throughout stream surface covered with dielectric material 22 as
to identify like parts. shown in FIG. 2. A molybdenum grid 14 having a coat-
DESCRIPTION OF THE DRAWING 65 '^ *
FIG. 1 is an isometric view having parts broken away In general, deposited dielectric materials exhibit
along an axial quarter section of an electron bombard- some degree of porosity which could be significant in
3,744,247
long duration applications. The alternate embodiment
shown in FIG. 3 is preferred where long operating life
is of primary importance.
This embodiment utilizes a grid 14 in the form of a
perforated metal plate which is identical with the grid
shown in FIGS. 1 and 2. Instead of utilizing a single
covering of dielectric material as shown in FIG. 2, the
alternate embodiment utilizes layers of dielectric mate-
rial 24 that are separated by layers of metal 26. The
metal layers 26 are nonporous.
Vapor deposition of the metal 26 is preferred be-
cause the thickness of this layer can be controlled.
Such a deposition process enables larger overall poten-
tial differences between the positive plasma and the
negative base metal grid to be maintained.
It is further contemplated that a perforated plate of
dielectric material 22 could be used at the start of fabri-
cation. The downstream surface of this dielectric plate
would be covered with a metal to form the grid 14.
The single grid 14 mounts in the same location as the
previous grid systems and serves the same purpose.
However, this accelerator system accomplishes the ion
extraction in an improved manner. The single grid 14
is maintained at a negative potential by the electrical
power source 18. The dielectric layer 22 which is ex-
posed to the plasma in the ion chamber 12 assumes a
positive potential nearly equal in value to that of the
plasma potential. The electric field necessary for ion
extraction is established with a single component accel-
decel grid system rather than the conventional two
component system. Potential differences up to the
break down limits of the dielectric material in the layer
22 are possible.
While several preferred embodiments of the inven-
tion have been described, it will be appreciated that
various modifications may be made to the disclosed
structure without departing from the spirit of the inven-
tion or the scope of the subjoined claims. For example,
the size of the grid as well as the geometry of the perfo-
rations may be varied. It is further contemplated that
the geometry of the dielectric layer may be changed to
meet the special requirements.
We claim:
1. An electron bombardment ion thruster comprising
a source of ions comprising
a cylindrical housing forming a chamber,
S ionized mercury forming a plasma having a positive
potential in said chamber,
an aluminum oxide plate having a plurality of aper-
tures therein mounted on said housing thereby
covering one end of said chamber, said alumi-
10 num oxide plate having a first surface facing to-
wards said chamber and a second surface facing
away from said chamber, said apertures extend-
ing between said first and second surfaces, said
aluminum oxide plate having a positive potential
IS substantially equal to that of said plasma
whereby said plasma covers said first surface and
said apertures,
an annular insulator extending about the periphery
of said chamber in contact with said housing and
20 said first surface of said plate, and
an accelerator system comprising
a layer of molybdenum covering said second sur-
face of said aluminum oxide plate, and
means for applying a potential to said molybdenum
25 that is highly negative to said plasma whereby
ions from said plasma covering said apertures in
said aluminum oxide plate are accelerated
through said apertures to produce thrust.
2. An electron bombardment ion thruster having an
30 accelerator system as claimed in claim 1 including a
second aluminum oxide plate in contact with a surface
of said molybdenum layer opposite said first named
aluminum oxide plate, and
a second molybdenum layer covering said second
35 aluminum oxide plate.
3. An electron bombardment ion thruster as claimed
in claim 2 having a third aluminum oxide plate in con-
tact with the surface of said second molybdenum layer
opposite said second aluminum oxide plate, and
40 a third molybdenum layer in contact with said third
aluminum oxide plate.
45
50
55
60
65


Single grid accelerator for an ion thrustor

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