US 3697793 A
A single electrode has an insulated face upstream in the direction of the ion beam so that a charge builds up thereon to serve as a screen electrode. The electrode defines openings, and these openings have electrode metal on the sides thereof. The electrode metal serves as an accelerator electrode, when an accelerator voltage is applied thereto. When electrodes on different sides of the openings have different voltages applied thereto, they serve to deflect the ions so that they no longer form a beam symmetric about the arcs. The electrode is formed by producing bars which consist of two electrodes separated by electrically-insulative, glass-filled refractory, and with one edge of the bars optionally carrying an insulative nose thereon. These bars are assembled in egg crate configuration to define a plurality of electrode openings.
Description (OCR text may contain errors)
United States Patent 9 Kin 1 Oct. 10 1972  ION BEAM DEFLECTION SYSTEM Primary Examiner-Raymond F. Hossfeld  Inventor: Harry J. King Woodland Hills Attorney-James K. Haskell and Allen A. Dicke, Jr. 57 ABSTRACT  Asslgnee: git in-craft Company Culver A single electrode has an insulated face upstream in 1 a] the direction of the ion beam so that a charge builds  Filed: Feb. 9, 1970 up thereon to serve as a screen electrode. The electrode defines openings, and these openings have elec- [211 Appl' 9765 trode metal on the sides thereof. The electrode metal serves as an accelerator electrode, when an accelera- 52 us. Cl. ..3l3/63, 29/2517, 60/202, tor voltage is applied theretowhen electrodes on 313 217 313 21 ferent sides of the openings have different voltages ap- 511 Int. Cl ..F03h 5/00 plied thereto, they Serve to deflect the ions 50 that 58] Field of Search 60/202; 313/63, 217, they no longer form a beam symmetric about the arcs.
313/218 The electrode 18 formed by producing bars which consist of two electrodes separated by electrically-insula-  References Cited tive, glass-filled refractory, and with one edge of the bars optionally carrying an insulative nose thereon. UNITED STATES PATENTS These bars are assembled in egg crate configuration to 3 279 76 0/1966 B d 0/ 0 define a plurality of electrode openings.
9 Claims, 5 Drawing Figures PATENTEDIIBT 10 m2 3.697.793
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PATENTEDnm 10 I972 SHEET 2 [IF 3 ION BEAM DEFLECTION SYSTEM The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 U.S.C. 2457).
BACKGROUND This invention is directed to an improved ion beam deflection system, particularly arranged for the deflection of all of the beamlets issuing from a Kaufman-type electron bombardment ion thrustor, particularly when the accelerator electrode thereof is a perforated electrode.
The original electron bombardment ion thrustor, as disclosed in H. R. Kaufman U.S. Pat. No. 3,156,090, produces an ion beam which is of such nature that the thrustor can be employed to produce thrust. The thrust is produced on the ground, as well as in space applications, although the thrust produced finds its particularly important utility in space devices because of its high specific thrust. This same high specific thrust is obtainable on the ground and, thus, the thrustor is useful in vacuum chamber work where such thrusts are desired, although it is usually more economic to employ other types of thrust-producing devices in earthbound vacuum chambers. Another such thrustor is illustrated in Dryden U.S. Pat. No. 3,345,820. In this latter patent, the beamlets are accelerated through perforations in the accelerator electrode, so that the large plurality of small circular beamlets are produced. This ion optical structure is of greater efficiency than the plurality of parallel rod ion optics of the Kaufman patent.
Another patent showing means for ion beam deflection for the directing of thrust from such a thrustor is George R. Brewer and George A. Work U.S. Pat. No. 3,535,880. The Brewer and Work patent is limited to deflecting beam in a single plane, to thus provide mere steering in a single plane. Additionally, Harry J. King and James W. Ward U.S. Pat. No. 3,604,209, issued Sept. 14, 1971 shows the deflection of a small part of the total beam. It deflects only a row of beamlets as issuing from a perforated accelerator electrode. These patents show net electrostatic deflection of the ion beam issuing from the accelerator electrode, with the result of net thrust which is not axial of the engine. However, they are limited in direction and net amount of deflection, as discussed above.
SUMMARY In order to aid in the understanding of this invention, it can be stated in essentially summary form that it is directed to an electrode structure which includes unitary bars which are comprised of two electricallyseparated metal plates secured to and formed with a glass-filled, porous ceramic therebetween. For singleplane electrostatic deflection of an ion beam passing between the bars, parallel bars are sufficient. However, for ion beam deflection within conical limits, the bars are interlocking and interengaged at an angle to define electrode openings. The plates are positioned around the openings so as to permit the application of acceleration and deflection voltages.
The process by which the bars are formed includes the application and firing of a porous ceramic onto metal electrode plates, followed by assembly of a plurality of plates carrying the ceramic into a fixture. The porous ceramic is fired and glass-filled to provide a non-porous, insulative structure between a plurality of metal electrode plates.
Accordingly, it is an object of this invention to provide an ion beam deflection system, particularly for electron bombardment ion thrustors, where each of the beamlets issuing through the openings in a perforated accelerator electrode is electrostatically deflectable in more than one plane through the axis to direct the thrust of the thrustor. It is another object to provide an integrated structure comprised of an accelerator electrode and electrostatic deflection plates whereby the potential difference between individual conductive blades in the accelerator electrode causes deflection in selected conically-defined direction, and the overall voltage of the blades in the accelerator electrode with respect to the screen electrode of the thrustor causes acceleration of the ion beam.
It is still another object to provide effective ion beam deflection by providing electrostatic deflection in selected conically-defined direction at all of the beamlet openings in an electron bombardment ion thrustor having a perforated accelerator electrode.
It is another object of this invention to provide a single electrode structure which acts as the screen electrode in an ion beam device by buildup of charge on an upstream insulative space, and acts as an accelerator electrode by virtue of downstream metal plates which can be held at accelerating voltages. It is another object to provide a unitary structure comprised of a plurality of interlocking bars, each of the bars comprising a unitary structure of metal electrode plates and a ceramic insulative structure which has its pores filled with glass. It is a further object to provide a process by which a pair of metal electrode plates are secured to an insulative structure by firing a refractory coating on the plates, assembling them into the desired relationship and filling the porous-fired ceramic with glass.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an axial section through the fairly schematic electron bombardment ion thrustor, schematically showing the associated power supply and connections, which ion beam device incorporates the electrode of this invention.
FIG. 2 is a front elevational view of the electrode of this invention.
FIG. 3 is a section taken generally along the lines 3 3 of FIG. 2.
FIG. 4 is a perspective view of several of the bars of the electrode showing them as assembled in a fixture preparatory to the final assembly step.
FIG. 5 is illustrative of the process steps of assembly of the finished electrode.
DESCRIPTION FIG. 1 fairly schematically illustrates a Kaufmantype electron bombardment ion thrustor at 10. Thrustor 10 comprises a body 12 in which is positioned a cathode 14. The cathode 14 produces electrons which move toward cylindrical tubular anode 16. The electron path is lengthened by provision of a suitable magnetic field so that the electrons move in a spirallylengthened path toward anode 16. A material tobe ionized, such as a gas, is introduced into the interior space of the anode so that, by electron bombardment, the ionizable material is ionized. For example, pipe 18 is connected to supply ionizable vapor or gas through the cathode into the interior space.
The outlet of the chamber defined by body 12 is covered by screen electrode 20, which is at the potential of body 12. Screen electrode 20 preferably has a plurality of perforations therethrough to permit the ions to drift out of the interior body of the thrustor. Accelerator electrode 22 is positioned downstream of screen electrode 20. Accelerator electrode 22 has a plurality of openings therethrough, and the accelerator electrode is maintained at a suitable potential to accelerate the ions passing therethrough, to produce thrust. In order to prevent the buildup of a space charge, neutralizer 24 is positioned downstream of the accelerator electrode 22 to inject electrons into the ion beam to provide a zero electrical charge. This prevents buildup of a charge on the body 12 of the thrustor or the structure upon which it is mounted. Neutralizer 24 also prevents a buildup of ionized material around the thrustor.
Power supply 26 performs by discharge supply between anode l6 and cathode l4; beam supply between cathode l4 and ground, and accelerator supply between ground and accelerator 22. Power supply 26 is connected to pipe 18, which is electrically connected to cathode 14, in order to provide the cathode power. Anode 16 is also connected to the beam power supply. Furthermore, body 12 is connected to the power supply to provide the ground reference. Neutralizer 24 is supplied with power from the power supply in order to provide its electron beam. A cathode heater may also be required, and this would also be connected to power supply 26.
Accelerator electrode 22 conventionally has an overall average negative voltage of about 1.0 KV with respect to ground, and this voltage is supplied by line 28. The differential voltage power supply 32 has output lines 34, 36 and 38, 40. The differential voltage power supply is able to provide a potential difference between these pairs up to 500 volts, above and below the potential in line 28, for total electrostatic differentialdeflection voltage therebetween of 1,000 volts. This voltage differential can either be continuously variable from any value from zero up to 1,000 volts, or the differential voltage power supply can be the on-off type to provide either zero differential voltage or the illustrative 500 volts differential voltage. The difference is only in the complexity of the differential power supply.
The accelerator electrode 22 is shown in plan and section in FIGS. 2 and 3. It comprises ring 42 with which circular boss 44 is integrally formed. Ring 42 and boss 44 have a central hole 46 therethrough with the ring and boss comprising the supporting structure for the balance of the accelerator electrode. It is made of electrically-insulative material, having high temperature strength, such as ceramic.
Boss 44 has a plurality of slots 48, 50, 52 and 54 therein extending down to the top surface of ring 42. These slots are in the upright direction across the circular boss, both above and below hole 46. Similarly, it has slots 56, 58, 60 and 62 through the circular boss positioned substantially at right angles with respect to slots 48 through 54. The slots define the number of openings in the accelerator electrode, and define the number of the openings. In the present instance, nine openings are defined through which beamlets pass, and in which electrostatic deflection takes place. The use of nine openings in which electrostatic beamlet deflection control is accomplished is merely an exemplary number and any convenient number of openings can be defined by the appropriate number of slots and associated equipment.
Bars 64, 66, 68 and 70 are respectively positioned within slots 48, 50, 52 and 54. Similarly, bars 72, 74, 76 and 78 are respectively positioned in slots 56, 58, 60 and 62.
The preferred manner in which the bars 64 through 78 are produced is schematically illustrated in FIG. 5, where the making of bar 66 is illustrated. Bar 66 is illustrative of all the other bars and is comprised of metal plates 80 and 82. These metal plates act as electrostatic deflection electrodes in adjacent openings. The openings are defined by the intersection of the several bars, with the metal plates of the several bars being notched so that they can be assembled into an egg crate structure defining the accelerator electrode openings. The notching of the metal plates is arranged so that metal plates extending in alternate directions do not touch each other, but are spaced from each other, as is also seen in FIG. 3. A structure with 120 symmetry is also possible in which each aperture would be triangular and bounded by three electrodes.
The first step of producing the bars and the electrode structure is the coating of refractory upon the inner side of each of the bars 80 and 82, as is seen in the uppermost part of FIG. 5. This refractory is generally indicated at 84. The generalized composition of this refractory dielectric subcoat is shown in Table I.
TABLE I Parts by Weight -200 Mesh pre-fired refractory dielectric powder Thermal expansion coefficient,
2-4.5 X 10"/C 70-95 I-llgh resistivity glass (fusion temperature 850l050the substrate metal 30-5 A more specific formula for the refractory dielectric subcoat 84, generally described in Table l and particularly adapted for the coating of molybdenum, is given below. First, a basic high resistivity glass frit is mixed together in the proportions given in Table II below.
TABLE II Parts by Weight Magnesia (MgO) 10-14 Calcite (CACO 2-8 Alumina A1 0 10-15 Boric Acid (H 80 45-55 Silica (SiO 30-45 The refractory dielectric sub-coat 84 is then formed of the materials given in Table III.
TABLE II] Parts by weight 200 Mesh mullite powder 80 The mixture of materials of Table III is placed in a porcelain ball mill with alundum stones. The mixture is ball-milled for 4 hours. Following milling, the milled slip is passed through a 100 mesh screen. The metal plates 80 and 82 have their surfaces prepared to receive the refractory dielectric sub-coat by degreasing, sandblasting, liquid honing, electro-polishing or oxidation. Any convenient surface cleaning means can be employed, so long as the refractory dielectric subcoat properly adheres to the metal. The refractory dielectric subcoat slip is applied as a wet coating with a spray gun. From 0.006 to 0.010 inch can be applied each coating. The coating is dried between coats to build up thicknesses to as great as 0.015 inch. If thicker subcoats are required, each application of 0.015 inch should be thoroughly fused. For building the present accelerator electrode, about 0.010 inch of refractory dielectric subcoat 84 on each of the metal plates 80 and 82 is satisfactory. Other means, such as flame or plasma spraying may be used to coat the electrode with sub coat. The choice depends on type of metal for the electrode and the operating temperature. Basically, the thermal coefficients of expansion of metal and subcoat must match.
Following the spray coating and drying, the refractory subcoat is fused at 1,100 C. for 30 minutes in an argon atmosphere at a pressure of 1 inch of water above atmospheric pressure. Argon flow should be sufficient to carry away volatile products during heating. Following fusion, the refractory coating is ground flat, to the second step indicated in FIG. 5, Preliminarily to the coating, the egg crate notches were cut into the metal deflection plates. Thus, after grinding, the two plates 80 and 82 can be assembled together, as is seen shown in the third step of FIG. 5. The assembly together can be accompanied by, and preferably is accompanied by, the assembly of all the metal plates with their coatings together in a fixture. A suitable fixture is shown in FIGS. 2 and 3, where the ring structure of the finished electrode 22 serves as a fixture. Each bar is, thus, formed of a pair of metal plates carrying its refractory coating and, through notched inter-engagement, the several bars are assembled into the fixture, as shown in FIGS. 2, 3 and 4.
Optionally, insulating nose 86 is next sprayed onto the upstream face of the entire network of bars in the electrode. It is sprayed on of the same material given in Table III and under the same conditions. After a suitable insulating nose 86 is built up, the entire structure is fired, as discussed above for the fusion of the refractory subcoat.
The refractory dielectric subcoat is a porous structure and would absorb materials which would interfere with maintenance of high resistance between the electrostatic deflection plates. Thus, it is desirable that the dielectric refractory subcoat be filled with a suitable material to render it non-porous. A suitable material is a glass which has low viscosity at the melt temperature, thermal expansion close to mullite and high electrical resistivity. Two specific formulas are given in Table IV below.
TABLE IV Glass A Glass B Parts Parts by Weight by Weight Silica (SiO,) 25 20 Magnesia (MgO) 15.8 13 Alumina (A1 0,) 17.3 10 Boric Acid (H 67.3 80 Calcite (CACO 8.3 15
The low viscosity glass filler of either Formula A or Formula B is produced by dry blending of the ingredients of the selected formula, melting them at 1,350 C., followed by quenching in water and drying. The resultant shattered glass is ground with 3 to 6 percent enamel clay in sufficient water to produce a sprayable slip. Grinding is done in a porcelain ball mil with alundum stones. A 0.002 to 0.006 inch thick coating of this slip is applied over the previously-fired refractory dielectric subcoat. The refractory dielectric subcoat is then saturated with the selected low viscosity glass filler by heating the structure to 9001000 C. in an argon atmosphere.
ln some cases, a glass sealer coat on the surface is helpful, especially in those cases where the low viscosity glass filler coat has soaked into the refractory subcoats to the extent that the surface is left rough. In such a case, the low viscosity glass filler of Table IV can be applied to the external surface, in the same manner as when employed for filler, followed by firing at least C. lower than when it was originally used as a filler. Alternatively, and to produce a higher resistivity surface sealer glass coat, a high resistivity glass sealer coat is produced by mixing together the materials described in Table V TABLE V Parts by Weight Basic High Resistivity Glass Frit 80-95 The material in Table V is ball-milled together, in the same type of mill as previously described, for a period of 4 hours. The sprayable slip is sprayed on the refractory subcoat and filler and dried. Drying is followed by firing this coating to 875950 C. for 30 minutes in an argon atmosphere. After such treatment, a cross section of one of the bars resembles that shown in the bottom-most step of FIG. 5. Furthermore, the assemblage of all the coated plates into the fixture and into the grid work illustrated in FIGS. 2 and 4 has preferably been accomplished midway through the process so that the finished result in a unitary structure with a continuous fused refractory subcoat between the various metal plates, all made unitary by the low viscosity glass filler. Accordingly, the grid work becomes unitary, with openings therethrough defined by electrically-separate metal plates. The openings through the accelerator electrode are thus defined by the metal plates on the cross grinding. These metal plates are electrically connected so that all of the plates which define the upper side of the holes are connected together and to line 40, and all of the plates which define the lower sides of the holes are connected together and to line 38. Similarly, the plates which define the right sides of the holes (for example, plate 80) are connected together and to line 36 while the plates which define the left sides of the holes through the accelerator electrode (for example, plate 82) are connected together and to line 34. Thus, by application of deflection voltages as described above, the exiting ion beam is deflected. By the employment of four plates at each opening, orthogonal deflection can be accomplished. While it is mechanically more difficult, other numbers than four plates can define each opening. For example, the openings can be triangular. With suitable deflection potentials applied to the three plates, orthogonal deflection can be accomplished. However, four plates for each opening are preferred because of simplicity. When employed with the screen'electrodes 20, the nose 86 is unnecessary. Under these circumstances, the finished bars can be substantially rectangular in form, as is illustrated in FIGS. 2 and 3. However, it may be desirable to eliminate the screen electrode 20. In those circumstances, the nose 86 is applied. Despite the negative accelerating potential on the plates of accelerator electrode 22, the insulated nose builds up its surface charge such that this upstream end of the electrode acts as a screen electrode. Thus, the upstream end acts as a screen electrode while the metal plates downstream thereof act as accelerator and deflection electrodes.
The specific details of the ceramic glass materials given in the tables above are particularly designed for employment with molybdenum as the metal. These glass and ceramic materials closely fit the molybdenum thermal expansion curve from to 1,000 C. However, certain stainless steels, tantalum and titanium are also useful as electrode metals in such devices. In such cases, the formulations of ceramic and glass are adjusted to correspond to the appropriate thermal expansion curve.
This invention having been described in its preferred embodiment, it is clear that it is susceptible to numerous modifications and embodiments within the ability of those skilled in the art and without the exer cise of the inventive faculty.
What is claimed is:
1. An electrode structure for an ion beam deflection system, said electrode structure comprising:
a plurality of first spaced bars extending in a first direction at generally right angles to an ion beam path;
each of said bars comprising first and second metal plates having glass-filled ceramic therebetween, said metal plates being adapted to have an acceleration voltage applied thereto and have a differential ion beam deflection voltage applied thereto so that the ion beam can be deflected as it passes through the openings defined by said bars in said electrode.
2. The electrode of claim 1 wherein said ceramic comprises a porous refractory subcoat having a coefficient of thermal expansion substantially identical to said plates, and a low viscosity glass filler substantially fill'n said orous refractor sub oat.
3. The e ectrode of clainl l w erein there are a plurality of second spaced bars extending in a second direction at an angle to said first spaced bars and generally at right angles to an ion beam path so that said bars define ion beam openings therebetween.
4. The electrode of claim 3 wherein said metal plates have notches therein, with the metal plates extending in the first direction having their notches interengaging with the notches and the metal plates extending in the second direction to maintain electrical isolation between the metal plates extending in the different directions.
5. The electrode of claim 4 wherein said bars extending in the first direction and said bars extending in the second direction are substantially in a common plane.
6. The electrode of claim 5 wherein said ceramic comprises a porous refractory subcoat having a coefficient of thermal expansion substantially identical to said plates, and a low viscosity glass filler substantially filling said porous refractory subcoat.
7. The electrode of claim 6 wherein said glass filler in said refractory subcoat is continuous between bars to form a unitary structure.
8. The method of producing an electrode for an ion beam deflection system comprising the steps of:
forming a plurality of notches in each of a plurality of metal electrostatic deflection plates; applying a porous refractory dielectric subcoat to one side of at least some of said deflection plates;
assemblying said metal electrostatic deflection plates in pairs with porous refractory dielectric subcoat between the metal plates of each pair of metal plates and assemblying the pairs of metal plates with other pairs of metal plates extending in another direction with interengaged notches so that a gridwork of bars of metal plates is formed, each of the plates being electrically separate;
filling the porous refractory dielectric subcoat with a low viscosity glass filler so that a unitary electrode structure is formed.
9. The process of claim 8 wherein said filling step comprises heating said porous refractory dielectric subcoat with glass filler material in contact therewith to a temperature sufficient to bring the glass filler to low viscosity, but below the fusion temperature of said refractory dielectric subcoat.