Development and characteristics of ion thruster accelerator with single glass coated grid to provide increased ion extraction capability and larger diameter accelerator syste

Banks, B. A.

Nakanishi, S.

English

NASA Technical Reports Server

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Attent ion#  His ib  W i n n i m  H. Morgan 
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Patent Matters 
SUBJECT8 ' Announcement of NASA-Owned U. 8. Patents i n  STAR 
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t 
U. S. Patent No. 
Gavermnt or 
Corporate Employee 
Supplementary Corporate 
8oYtce (if applicable) a /r/R I 
W18A Patent Caae No. 
NOTE - If th i s  patent covers an invention made by a corporate 
employee of a NASA Contractor, the following is applicable: 
yes (J NO @ 
Pursuant t o  Section 305(a) of the National Aeronautics and 
Space Act, the name of the Administrator of NASA appears on 
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inventor (author) appears a t  the heading of Column No. 1 of 
the Specification, following the word8 a. .: . w i t h  respect to 
https://ntrs.nasa.gov/search.jsp?R=19710017166 2020-03-17T03:07:24+00:00Z
Jan. 5, 1971 B. A. BANKS ETA1 
ION THRUSTOR ACCELERATOR SYSTEM 
'1 4 
FIG. 2 
INVENTORS 
BRlJCE A. BANKS 
S H l GEO NAKANlSHl  
1 
ATTORNEYS 
3,552,124 United States Patent Ofice Patented 1971 
larger diameter accelerator systems to be used for low 
3,552,124 voltage operation. 
ION THRUSTOR ACCELERATOR SYSTEM These and other objects of the invention will be apparent 
B ~ ~ e $ ~ ~ ~ ; s ~ ~ ~ ~ ~ s ~ f ~ $ f " ; $ ~ ~  ~ $ ~ ~ ~ i f i " , i ~  from the specification which follows and from the draw- 
as represented by the *dministratcPr of the ~~~~~~l ing wherein like numerals are used throughout to identify 
Aeronautics and Space Administration iike parts. 
Filed Sept. 9,1968, Ser. No. 758,390 
h t .  Cl. F03h 5/00 DESCRIPTION O F  THE DRAWING 
U.S. CI. 60-202 1 Claim In the drawing: 
lo FIG. 1 is an-isometric view with parts bloken away 
of an ion thrustor having an accelerator qystcrn construcl- 
ABSTRACT O F  THE DISCLOSUPIE ed in accordance with the present invention, and 
A single coated grid accelerator system for an ion FIG. 2 is an enlarged sectional view taken along the 
thrustor. The grid is coated with an insulating material line 2-2 in 
that is bonded to its surface. 15 DESCRIPTION O F  THE PREFERRED 
EMBODIMENT 
In a typical electron bombardment ion thrustor of the STATEMENT OF OWNERSHIP type described in U.S. Pat. No. 3,154,090, mercury vapor 
The invention described herein was made by employees on is fed into an ionization chamber throuoh a distributor 
of the United States Government and may be manu- lv plate. Atoms of the vaporized propellant are then bom- 
factured and used by or for the Government for govern- barded by electrons emitted from a cathode. The path 
mental purposes without the payment of any royalties length traveled by electrons going from this cathode to 
thereon or therefor. an anode is zreatlv increased bv an axial ~~iaonetic field 
- .  - 
BACKGROUND O F  THE INVENTION 2j thus causing more ionization by electron bombardment. Such a conventional ion thrustor utilizes a double 
This invention is concerned with a single grid accelera- grid accelerator system having lboth a screen grid and 
tor system for an ion thrustor. The invention is particu- a n  accelerator grid. The screen grid serves to contain the 
lcrly directed to  a glass coated grid for use in-such a discharge plasma while forminithe necessary ion optics 
system. 30 to prevent direct impingement of accelerated ions onto 
Double grid accelerator systems have been used on the accelerator grid. Ions in the near vicinity of the screen 
ion thrustors. Such systems combine a screen grid with grid have a high probability of being accelerated 
an accelerator grid. A screen grid increases the thrustor through openings in the screen and the accelerator grids 
weight and complexity. Also problems of thermal buck- because of the high electric fields present at the grids. 
ling and warping have been encountered, and prevention 35 Thrust is produced as these ions accelerate through the 
of electrical shorting of the grids presents an insulating grid system. 
problem. Referring now to the drawing an ion thrustor shown 
A single grid accelerator system was suggested in an in FIG. 1 has an ionization chamber 10 for containing a 
attempt to solve these problems. Such a system may have p~opellant that has been ionized in any conventions? 
boron nitride clamped to the accelerator grid. It  was also 40 manner. By way of example, this propellant may be 
proposed to spray aluminum oxide onto the accelerator mercury that is ionized by electron bombardment as dc- 
grid. It was further suggested that a ceramic material be scribed in the aforementioned U.S. Pat. No. 3,156,090. 
cemented on the accelerator grid. The ion thrustor further includes an accelerator sys- 
Various difficulties were encountered in using these tem 12 constructed in accordance with the present invcn- 
grids. Because the boron nitride had to be clamped onto 45 tion for accelerating propellant ions in tbc direction of 
the grid it was in the form of thick sheet to prevent crack- the arrows in FIGS. 1 and 2. The accelcralar system 12 
ing. The use of a thick insulating coating reduces thrust utilizes a single grid 14 of an electrically conducting ma- 
because of the increased acceleration distance of the ions. terial that is connected to a source of electric power, suc l~  
The use of thick coatings also limited the size of the ion as a ,battery, as shown in FIG. 1. This electrical power 
thrustor because as the diameter of the grid increased 50 source impresses a potential on the grid 14 that is highly 
the insulator thickness had to be increased to prevent negative relative to the ionization chamber 10 
cracking. The flame sprayed aluminum oxide coatings An insulator 16 having an annular configuiation ex- 
degraded because it is extremely porous to mercury. The tends about the periphery of the ionization chamber 10 
ceramic cements are also porous and failed in a manner The insulator 16 serves to electrically isolate the single 
similar to that of the aluminum oxide coatings. 55 grid 14 from the metal housing forming the ~on~zat ion 
SUMMARY OF THE INVENTION chamber 10. An important feature of the invention IS that the elec- 
These problems have been solved in the accelerator trically conducting grid 14 is coated with an insulating 
system constructed in accordance with the present in- material 18. This insulating coating 18 1s bonded to the 
vention which utilizes a single glass coated grid. The glass 60 grid 14 as best shown in FIG. 2. 
forms a strong mechnical bond to a metal grid. When In a conventional double grid system of the type pre- 
the coating is of the required thickness it is electrically viously described, a plasma sheath is forrncd near the up- 
reliable. stream surface of the screen grid. In the single grid sys- 
It is, therefore, an object of the present invention to pro- tem of the present invention, the sheath is believed formed 
vide an ion thrustor having an accelerator system which 65 near the upstream surface of the insulating coating 18. 
utilizes a single grid thereby eliminating the problems en- Because of the high electrical resistance of the EnsuEating 
countered in a double grid system. coating 18, charges can build up on its surface to form 
Another object of the invention is to provide an im- a virtual screen grid. 
proved accelerator grid which enhances the ion extrac- In  this manner the plasma sheath is moved closer to the 
tion capability of an ion thrustor. 70 negative accelerator 14. This increases the field strength 
A still further object of the invention is to provide an for a given voltage which, in turn, increases the ion beam 
improved grid structure for an ion thrustor which enables current density. 
3,552,124 
3 4 
The grid 14 is electrically conductive to establish the fications may be made to the disclosed structure and ma- 
proper electrostatic field for accelerating the ions in the terials without departing i r o n  the spirlt of the ~i?veiitroa 
chamber 10. Because a portion of the electrically con- or the scope of the subjoined claims. For example, the 
ducting grid 14 is not covered by the insulating material size of the coated grid as well as the geometry of the per- 
18, sputtering erosion of the exposed surface of the grid 
- forations may be varied. It is further contcrnpldted lhat 
14 may be encountered. Where long life is of importance " the geometry of the coating may be changed to meet 
the electrically conducting grid I4 is a metal which is special requirements. A single grid accelelalor syslen? 
resistant to sputtering erosion. Molybdenum is a grid constructed in acco~dance with the invention can be used 
metal that has been found to be resistant to such erosion. with various types of thrdstors using othei pro1xlEtilai:t ma- 
The grid 14 is prefeiably of a metal which has a ther- terials. By way of example, such an accelerator system can 
ma1 expansion coefficient that matches the thermal ex- l0 be used with a contact ionization thrustor ulilizing ceslrrill 
pansion coefficient of the insulating material IS, which as a propellant. 
is preferably glass. Molybdenuln has been satisfactory for What is claimed is: 
grids 14 that are coated with glass. It  is also contern- I .  In an accelerator system for an 1011 1hru5tar hiv- 
plated that stainless steel, tungsten, and tantalum can be ing a chamber for containing an ionized propellant, the 
used for the grid 14. improvement comprising 
A slurry is prepared by suspending finely milled par- a single electrically conducted metal plaie having a plu- 
ticles of the insulating material 18 in a solution of water rality of apertures therein mounted a t  one end of said 
or some other organic or inorganic solvent. This s lu~ry is chamber, said plate having a first soiface facing to- 
sprayed onto one face and walls of the holes in a per- gg ward said chamber and n second suiface facing away 
forated plate forming the grid 14. The grid surfaces from said chamber, each of said apellures having a 
to be coated are initially oxidized prior to being covered wall extending between said first 5vilace a d  said 
with the sprayed slurry. second surface, 
The insulating coaling IS is preferably a type of glass a fused glass coating covering said first surface and said 
that forms a good seal with the material of the grid 14. 25 wall of each of said apertures for proieciing the same 
Corning glass 7052 has been found to be satisfactory for ftom sputtering erosion, and 
the slurry used in coating a molybdenum grid. means for applying a potential to said metal plate that 
'CTIilen the sprayed on slurry dries, the grid 14 is placed is highly negative to said chamber wlie~eby Ihe ioo- 
in an oven which is at a temperature hot enough to fuse ized propellant is accelerated through each of s a d  
the fine glass particles together and to seal the glass to the 30 apertures without contacting said wall Illereof. 
metal. Grcat care must be taken to insure the proper 
firing temperature to prevent the glass from flowing over References Cited 
and coveling the accelerator grid holcs or openings. If the UNITED STATES PATENTS 
temperature is too low the glass will not fuse or form a 
good bond. The gIass preferably has a working temper- $3 3,117,416 1/1964 Harries ------I------ 60-202 
ature greater than about 1400" IF so that the glass re- 3,156,090 11/1964 Kaufman ------------ 60-202 
mains a good insulator a t  the ion thrustor operating tern- 3,177,654 4/1965 Gradecak ------------ 60-202 
perature. Corning glass 7052 has a working temperature of 3,354,644 11/1967 Moore--------------- 60-202 
of about 2000" F. 3,452,237 6/1969 Wingerson --------- 60-202X 
After removing the grid from the firing oven it is per- 40 2, 998,523 8/1961 Muench et al. ---. -- 250-84 5 
mitted to cool slowly. FIG. 2 shows the glass coating 1% 3,015,032 12/1961 Hoyer et al. -------- 313-6lX 
after firing. By using a fused glass type seal, optimum re- 3,359,733 12/1967 Forbes -------------- 60-252 
sults are obtained between the insulator and the grid. The DOUGLAS WART, Primary Examiner glass coating is permanently i:npervious to mercury dif- 
fusion which causes other types of oxides or ceramic coat- 43 U.S. CI. X.R. 
ings to fail. 313-63 
While a preferred embodiment of the invention has 
been described it will be appreciated that various modi- 


Ion thruster accelerator system Patent

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