US3586901A - Electron gun for use in contaminated environment - Google Patents

Abstract

The cathode of an electron gun is protected from gaseous contaminants in its environment by employing a pair of apertured anodes in tandem. The anode nearer the cathode has a smaller aperture and is operated at a higher positive potential with respect to the cathode than the anode farther from the cathode. The number of positive ions attracted to the cathode is sharply decreased by the resulting electric field configuration.

Description

United States Patent 2,219,117 10/1940 Schade inventor Bennie A. Findeisen Baldwinsville, N.Y.

Appl. No. 830,310

Filed June 4, 1969 Patented June 22, 1971 Assignee General Electric Company ELECTRON GUN FOR USE IN CONTAMINATED ENVIRONMENT 9 Claims, 4 Drawing Figs. 1 U.S. C1 315/14, 313/82 R Int.Cl H01j 29/46 Field of Search... 315/14- References Cited UNITED STATES PATENTS 2,935,636 5/1960 Knechtli 313/82 2,971,108 2/1961 Dickinson et al. 313/82 2,983,842 5/1961 Hrbek 315/16 3,151,269 9/1964 Vestal 315/15 Primary Examiner-Rodney D. Bennett, Jr.

Assistant Examiner-Malcolm F. Hubler Attorneys-Marvin Snyder, W. .1. Shanley,.1r., Thomas A.

Briody, Frank L. Neuhauser, Oscar B. Waddell and Joseph B Forman ABSTRACT: The cathode of an electron gun is protected from gaseous contaminants in its environment by employing a pair of apertured anodes in tandem. The anode nearer the cathode has a smaller aperture and is operated at a higher positive potential with respect to the cathode than the anode farther from the cathode. The number of positive ions attracted to the cathode is sharply decreased by the resulting electric field configuration.

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0 f I CATHODE GRID FIRST SECOND TO ELECTRON BEAM TARGET-- ANODE ANODE g3 AXIAL DlSTANCE FROM CATHODE- 5E 38 (0...! LN 02 F|G.4 5% INVENTOR: 2% BENNIE A. FINDE'ISEN, 3m z I 1 I 0 2o 50 BY Mm ENERGY(ELECTRON VOLTS)- HIS ATTORN ELECTRON GUN FOR USE IN CONTAMINATED ENVIRONMENT INTRODUCTION This invention relates to electron discharge devices, I and more particularly to electron guns for use with light valve apparatus. 1

One form of light valve suitable for optical projection of electronically generated images onto a remote display surface comprises an evacuated enclosure containing an electron gun in predetermined alignment with a transparent disc within the enclosure. The disc is rotated through a reservoir of light modulating fluid to deposit a continuously replenished layer of fluid on the disc surface. An electron beam, generated by the electron gun, is directed through electrostatic beam deflecting and focusing means and is scanned across a portion of the light modulating fluid layer so as to selectively deform the layer. The fluid deformations thus formed constitute diffraction gratings which, in conjunction with a Schlieren optical system, selectively control passage of light from a light source through the disc and through an output window in the enclosure envelope in order to create visible images at a remote display surface on which the light impinges.

Electron guns operate most satisfactorily in good vacuum environment. If these environmental conditions are degraded,

operational lifetime of the electron guns is shortened. In a light valve of the type described, electron bombardment of the deformable fluid medium causes emission of vapor molecules and molecular dissociation of fluid molecules into constituent gas molecules. Because environmental conditions are compromised due to this buildup of contaminants, electron gun lifetimes are adversely affected as a result of two factors.

One factor tending to shorten electron gun lifetimes is the destructive action on the surface of the electron gun cathode which occurs when positive ions are attracted, with high energy, to the cathode. These positive ions are generated as a result of electrons from the electron beam striking gas molecules within the light valve enclosure. Bombardment of the cathode with the resulting high energy ions tends to shorten drastically the operational lifetime of the cathode and, consequently, the electron gun.

A second factor tending to shorten electron gun lifetimes is the clogging of apertures in plates within the electron guns due to accumulation of residues. Clogging of these apertures is believed to be caused by electron bombardment of the deformable fluid medium molecules which have condensed or otherwise collected on the apertured plates. This direct irradiation of the fluid molecules, together with the indirect effect of heat, causes molecular dissociation so as to evolve gas and leave a carbonaceous residue. Accumulation of this residue may eventually clog an aperture in an electron gun and distort or altogether block the electron beam.

The present invention is directed to means for sharply reducing the rate at which positive ions bombard electron gun cathodes and the rate at which carbonaceous residue builds up around apertures in the guns. These salutary effects are achieved by employing a pair of apertured anodes within the gun to establish an electric field that tends to minimize the number of positive ions bombarding the cathode. In addition, the second anode acts as a mechanical shield, reducing the arrival rate of residue-producing material at the aperture of the first anode.

Accordingly, one object of the invention is to extend the lifetime of an electron gun operating in a contaminated environment by protecting the electron gun cathode from exces sive bombardment by positive ions.

Another object is to extend the lifetimes of electron guns operating in contaminated environments by reducing aperture clogging in the electron gun anodes.

Another object is to provide an electron gun capable of operating satisfactorily in a contaminated environment as well as in a high vacuum environment.

Briefly, in accordance with a preferred embodiment of the invention, an electron gun for producing a beam of electrons at a substantially constant rate over an extended period of time is provided. The electron gun comprises a chamber in gaseous communication with an enclosure at a region of access thereto. Ideally, the enclosure is evacuated of gas; however, due to gaseous contaminants evolved within the enclosure, the enclosure is maintained at a finite, albeit subatmospheric, pressure. A cathode is situated within the chamber, along with a first regional isolation means interposed in the chamber between the cathode and the region of access to the enclosure and a second regional isolation means interposed in the chamber between the first isolation means and the region of access to the enclosure. Each of the first and second isolation means contains an aperture of predetermined size, the apertures being aligned along an axis common to the cathode and the region of access to the enclosure so as to permit the beam of electrons to pass from the cathode into the enclosure. Means are-provided for maintaining each of the first and second isolation means at predetermined positive potentials respectively with respect to the cathode, the positive potential of the second isolation means being lower in amplitude than the positive potential of the first isolation means so as to establish a potential hill between the 'second isolation means and the cathode.

BRIEF DESCRIPTION OF THE DRAWINGS The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself, however, both as to organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a sectional view of the electron gun of the instant invention;

FIG. 2 is a section of the electron gun viewed along line 2-2 as shown in FIG. 1;

FIG. 3 is a graphical illustration of the electric potential established along the longitudinal axis of the electron gun under normal operating conditions; and

FIG. 4 is a curve illustrating distribution of secondary emission electrons within the electron gun with respect to their energies.

DESCRIPTION OF TYPICAL EMBODIMENTS FIG. 1 illustrates an electron gun constructed in accordance with the teachings of the instant invention. The electron gun comprises a hollow chamber formed generally by a hollow annular body element 10 of ceramic material, an annularheader element 11 of ceramic material penetrated by a pair of electrically conductive pins 13, and a hollow annular spacer element 12 of ceramic material. The ceramic material of elements 10, 11 and 12 exhibits good electrical and thermal insulation characteristics. A typical ceramic material suitable for constructing elements 10, 11 and 12 comprises a dielectric such as Forsterite, a magnesium silicate of the composition 2M- gOSiO,.

The end of the hollow chamber opposite header II is defined by isolation means such as a snout member 14 which is joined to spacer element 12 as by brazing. Regional isolation means 14 is typically comprised of titanium, and carries a flange portion 15 to which an electrically conductive shroud 16 is attached. Shroud 16 may conveniently comprise stainless steel of about 10 mils thickness, welded to flange l5. Shroud 16 is designed to extend through an opening 17 in the envelope or outer glass wall 18 of a light valve. Opening 17 furnishes a region of access to the interior of the light valve enclosure, permitting gaseous communication between the interior of the light valve enclosure and the interior of the electron gun chamber. The electron gun is supported by a tubular member 20 which is welded to a flange portion 21 of partitioning means 14. Support member typically has a flared end portion 22 which is fritted to glass wall 18 of the light valve. Conveniently, support member 20 may comprise an alloy known as HC4, available from Allegheny Ludlum Corporation, Pittsburgh, Pennsylvania.

The source of electrons within the electron gun is a cathode 23, preferably a dispenser type cathode. The cathode is supported by a pair of leads 24 which may be welded to pins 13. The cathode is raised to its operating temperature by passage of current through leads 24 which produces resistive heating of the cathode.

The electron gun is provided with a grid electrode 25 containing an aperture 26 to permit passage of the electron beam therethrough. The amplitude of electron beam current is controlled by the amplitude of voltage on the grid. Grid 25 is preferably comprised of titanium.

Regional isolation means 27, typically comprised of titanium, is situated between grid 25 and isolation means 14. A small aperture 28, typically in the order of4 mils in diameter, is situated in a button member 30 having a flanged portion 31 which is supported within a tubular heat choke 32. Button 30 may conveniently be comprised of vanadium, with an outside diameter of approximately 65 mils and flange 31 diameter of approximately 60 mils. Heat choke 32 is typically comprised of a refractory metal such as tantalum and may conveniently be welded to flanged portion 31 of button 30. The opposite end of heat choke 32 is attached to partitioning means 27 as by welding.

Electron beam shaping is accomplished by application of a pair of conductive straps 33, which are comprised of a refractory metal such as tantalum, to the face of button 30 opposite grid 25 as by welding. Straps 33 are typically in the order of 2 mils thickness, and are situated side-by-side with adjacent straight edges separated from each other by about 04 mils. this can be seen in FIG. 2, which illustrates a section of the electron gun shown in FIG. 1 as viewed along a line designated 2-2. Opening 28 is seen to be constricted by straps 33, so as to create a narrow slot through which the electron beam may pass. It can also be seen in FIG. 1 that cathode 23, aperture 26 in grid electrode 25, slot 28 in button 30, and the opening in isolation means 14 are all aligned along a common axis 37. This axis constitutes the path followed by electrons emitted by cathode 23 of the electron gun.

Cathode 23 of the electron gun shown in HO. 1 is maintained at an average potential of zero amplitude. This may be accomplished by coupling pins 13 to the secondary winding output leads of a transformer 35 driven from an AC power source 36. The centertap of the secondary winding and one side of the primary winding of transformer 35 are grounded.

Grid 25 may be maintained at a slightly negative potential with respect to ground. By controlling the amplitude of this negative potential, the amplitude of electron beam output current, which is the electron beam current in shroud 16, may be maintained constant. Thus, if the electron emission capability of cathode 23 should become degraded, the amplitude of negative potential on grid 25 of the electron gun may be decreased so as to permit enough additional ones of the electrons emitted by cathode 23 to pass through aperture 26 in the grid and thereby maintain the electron beam current in shroud 16 constant.

Isolation means 27 is maintained at a positive potential with respect to ground, so that button 30 and straps 33 are maintained at this positive potential due to electrical conduction through heat choke 32 This positive potential serves to attract electrons from cathode 23. Isolation means 14 is also maintained at a positive potential with respect to ground. However, the positive potential of isolation means 14 is slightly less than the positive potential of isolation means 27. Accordingly, a slight potential difference exists between isolation means 27 and isolation means 14.

In operation, dispenser cathode 23 is heated by current supplied from transformer 35 through pins 13 and leads 24. This causes emission of electrons toward aperture 26 in grid 25.

The negative grid potential is adjusted so as to produce an electron beam having the desired beam current amplitude through shroud 16. The electrons from cathode 23 impinge upon straps 33 and the region between the straps, thereby raising the temperature of button 30. The beam of electrons is constricted by the separation between straps 33 and the size of opening 28, so that the beam emerging from flanged portion 31 of button 30 is of small, shaped cross-sectional configuration. Although button 30 is heated to an elevated temperature by electron bombardment, the thermal conductivity of heat choke 32 is low enough to prevent an excessive rise in temperature of isolation means 27. However, the electrical conductivity of heat choke 32 is sufficiently high so that button 30 and straps 33 function as a first anode for the electron gun.

lsolation means 14 is also maintained at a positive potential with respect to the cathode, and hence functions as a second anode of the electron gun. Typically, the second anode may be operated at approximately 8,000 volts, while the first anode may be operated at a voltage which is from 50 to 150 volts higher than the amplitude of the second anode voltage. With a grid control voltage of about 120 volts, an electron beam output current of approximately 4.5 microamps may be obtained.

The portion of regional isolation means 27 surrounding button 30 acts, not only to assist in formation of the electric field of the first anode, but also to minimize thermal radiation from button 30 to ceramic material 10 and the remainder of the electron gun structure. This is desirable in that button 30 is maintained at a high temperature, discouraging condensation of vapors thereon. Moreover, isolation means 14 is at a somewhat lower temperature than isolation means 27, or, in other words, the second anode is maintained at a lower temperature than button 30. The advantage gained by operating the electron gun in this manner is that oil vapor and molecules from the deformable medium employed in the light valve tend to collect on the relatively cooler surfaces of the second anode and not on button 30. Since the diameter of the opening in isolation means 14 is in the order of mils, condensation of oil vapor and accumulation of oil molecules around this opening does not build up to a quantity sufficiently large to block the opening and thereby prevent the electron beam from passing into the interior of the light valve. On the other hand, ifthe oil vapor or oil molecules were to accumulate on the surface of button 30, opening 28 might become clogged in a relatively short time, due to the small size thereof.

A portion of snout member 14 protrudes somewhat into the beam exit opening of isolation means 27. This avoids perturbations in the path of the electron beam by eliminating any direct path which electrons and ions might follow to reach spacer member 12 and charge its surface nonuniformly. ln addition, isolation means 14 acts somewhat as a mechanical shield by reducing the arrival rate of deposit-producing material on aperture 28 from the interior of the light valve. Shroud l6 protrudes through opening 17 in envelope 18 of the light valve in order to provide a substantially constant electric field for electrons to pass from isolation means 14 into the interior of the light valve; that is, any stray electric fields built up on the surface of glass envelope 18 cannot cause perturbations in the path of the electron beam.

Due to impingement of the electron beam on the deformable fluid medium in the light valve, molecules of vapor are evolved by the deformable fluid medium, tending to contaminate the evacuated interior of the electron gun. Those molecules of the fluid which drift into the path of the electron beam are bombarded by electrons, both high energy primary electrons and low energy secondary emission electrons emanating from straps 33 through aperture 28, resulting in ionization of the molecules. In conventional electron guns, some of the positive ions thus created are attracted to the cathode and land thereon with high energy, causing erosion of the cathode surface. in the electron gun of the present invention, the positive ions which are formed and can return through aperture 28 to be accelerated into the cathode must have precisely the correct initial velocity imparted to them electron gun, there existsa finite probability that ions with such correct initial velocities will be created by both the primary electron beam and lower energy secondary emission electrons. However, in the electron gun of the present invention, the first anode, represented by button 30 of FIG. I, is at a higher potential than the second anode. represented by isolation means 14 of H6. 1, as illustrated graphically in FIG. 3. This creates a potential hill" 40 which each positive ion formed outside the region between cathode 23 and anode 14 must overcome in order to be attracted to the cathode. The height of this potential hill may conveniently be from 50 to 150 volts. Accordingly, most of the positive ions having the requisite kinetic energy requirements at the instant of formation to reach the cathode of a conventional electron gun are actually prevented from reaching the electron gun cathode because their kinetic energies are insufficient to overcome the potential hill existing between the first and second anodes of the electron gun; only the relatively few ions with kinetic energy above 50 to 150 electron volts along path 37 at the instant of emergence from the electric field of anode 14 can enter the electron gun chamber and strike the cathode thereof. In addition, the low energy secondary emission electrons cannot continue along path 37, but are electrostatically forced to return to member 27. Accordingly, the ionizing path length of these secondary emission electrons is greatly reduced, resulting in generation of a further reduced number of ions. The energy distribution of secondary emission electrons in the electron gun of FIG. 1 is illustrated in FIG. 4.

The number of positive ions striking the cathode may be reduced even further by insuring that all positive potentials employed in the light valve are kept below the positive potential of the first anode 30. if this is done, then there is no location anywhere in the light valve where ions may be formed with potential energy sufficient to cause them to be accelerated by an applied electrostatic field to a level of kinetic energy sufficient to overcome the potential hill between anodes 30 and 14.

The electrostatic field between the first and second anodes also serves to trap neutral gas molecules which become polarized due to the influence of this field, facilitating additional removal of gaseous contaminants from the electron gun. in addition, a sieve material or filter to assist in trapping gas molecules may be placed, if desired, in the interspace between the first and second anodes.

The foregoing describes an electron gun having an extended lifetime when operated in a contaminated environment. Clogging of anode apertures in the electron gun due to accumulation of contaminants thereon is minimized, and the cathode of the electron gun is protected from excessive bombardment by positive ions. The electron gun is capable of operating satisfactorily not only in a contaminated environment, but also in a high vacuum environment.

While only certain preferred features of the invention have been shown by way of illustration, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.

lclaim:

1. In an electron discharge device for producing an electron beam in a contaminated environment, said device including a cathode, means for protecting the cathode from bombardment by ions of contaminants, said means comprising:

a first anode containing a first aperture of predetermined area therein, said first anode being disposed at a predetermined distance from said cathode;

a second anode containing a second aperture of area greater than said predetermined area, said second anode being spaced entirely at a greater distance from said cathode than said first anode;

said apertures in said first and second anodes being aligned along an axis in common with said cathode;

means for maintaining said first anode at a predetermined positive potential with respect to said cathode; and

means for maintaining said second anode at a predetermined positive potential with respect to said cathode; and the positive potential of said second anode being lower in amplitude than the positive potential of said first anode so as to maintain a potential in the region between said first and second anodes which varies between the amplitudes of positive potential on said first and second anodes in accordance with axial location between said first and second anodes.

2. The apparatus of claim 1 including grid means interposed between said cathode and said first anode, said grid means containing an aperture therein in alignment along said axis, and means for maintaining said grid at a negative potential with respect to said cathode.

3. An electron gun for producing a beam of electrons at a substantially constant rate over an extended period of time, said'electron gun comprising:

a chamber in gaseous communication with an enclosure at a region of access to the interior of said enclosure, the interior of said enclosure being maintained at subatmospheric pressure;

a cathode situated with said chamber;

first regional isolation means interposed in said chamber between said cathode and said region of access and making substantially continuous physical contact with said chamber about the entire periphery of said chamber, said first regional isolation means containing an aperture of predetermined size therein;

second regional isolation means interposed in said chamber between said first regional isolation means and said region of access and making substantially continuous physical contact with said chamber about the entire periphery of said chamber, said second regional isolation means containing an aperture of predetermined size therein;

said apertures of said first and second regional isolation means being aligned along an axis common to said cathode and said region of access so as to permit said beam of electrons to pass from said cathode into the interior of said enclosure;

means for maintaining said first regional isolation means at a predetermined positive potential with respect to said cathode; and a means for maintaining said second regional isolation means at a predetermined positive potential with respect to said cathode, the positive potential of said second regional isolation means being lower in amplitude than the positive potential of said first regional isolation means.

4. The electron gun of claim 3 wherein the size of the aperture in said second regional isolation means is greater than the size of the aperture in said first regional isolation means.

5. The electron gun of claim 3 including electron beam current control means interposed between said cathode and said first regional isolation means, said electron beam current control means containing an aperture in alignment along said axis, and means for maintaining said electron beam current control means at a negative potential with respect to said cathode.

6. The electron gun of claim 4 including electron beam current control means interposed between said cathode and said first regional isolation means said electron beam current control means containing an aperture in alignment along said axis, and means for maintaining said electron beam current control means at a negative potential with respect to said cathode.

7. The electron gun of claim 3 wherein all positive electrical potentials employed within said enclosure are lower in amplitude than the positive potential of said first regional isolation means.

8. The electron gun of claim 4 wherein all positive electrical potentials employed within said enclosure are lower in amplitude than the positive potential of said first regional isolation means.

9. The electron gun of claim 6 wherein all positive electrical potentials employed within said enclosure are lower in amplitude than the positive potential of said first regional isolation means.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 586 a 901 Dated ne 22 1971 Bennie A. Findeisen Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, line 23, "with" should read within Signed and sealed this 6th day of March 1973.

(SEAL) Attest:

EDWARD M. FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents FORM POWSO (m'sg) USCOMM-DC wave-pee II US GOVERNMENT PRINTING OVHLE 1990-366 Lil

Claims (9)

1. In an electron discharge device for producing an electron beam in a contaminated environment, said device including a cathode, means for protecting the cathode from bombardment by ions of contaminants, said means comprising: a first anode containing a first aperture of predetermined area therein, said first anode being disposed at a predetermined distance from said cathode; a second anode containing a second aperture of area greater than said predetermined area, said second anode being spaced entirely at a greater distance from said cathode than said first anode; said apertures in said first and second anodes being aligned along an axis in common with said cathode; means for maintaining said first anode at a predetermined positive potential with respect to said cathode; and means for maintaining said second anode at a predetermined positive potential with respect to said cathode; and the positive potential of said second anode being lower in amplitude than the positive potential of said first anode so as to maintain a potential in the region between said first and second anodes which varies between the amplitudes of positive potential on said first and second anodes in accordance with axial location between said first and second anodes.

2. The apparatus of claim 1 including grid means interposed between said cathode and said first anode, said grid means containing an aperture therein in alignment along said axis, and means for maintaining said grid at a negative potential with respect to said cathode.

3. An electron gun for producing a beam of electrons at a substantially constant rate over an extended period of time, said electron gun comprising: a chamber in gaseous communication with an enclosure at a region of access to the interior of said enclosure, the interior of said enclosure being maintained at subatmospheric pressure; a cathode situated with said chamber; first regional isolation means interposed in said chamber between said cathode and said region of access and making substantially continuous physical contact with said chamber about the entire periphery of said chamber, said first regional isolation means containing an aperture of predetermined size therein; second regional isolation means interposed in said chamber between said first regional isolation means and said region of access and making substantially continuous physical contact with said chamber about the entire periphery of said chamber, said second regional isolation means containing an aperture of predetermined size therein; said apertures of said first and second regional isolation means being aligned along an axis common to said cathode and said region of access so as to permit said beam of electrons to pass from said cathode into the interior of said enclosure; means for maintaining said first regional isolation means at a predetermined positive potential with respect to said cathode; and means for maintaining said second regional isolation means at a predetermined positive potential with respect to said cathode, the positive potential of said second regional isolation means being lower in amplitude than the positive potential of said first regional isolation means.

4. The electron gun of claim 3 wherein the size of the aperture in said second regional isolation means is greater than the size of the aperture in said first regional isolation means.

5. The electron gun of claim 3 including electron beam current control means interposed between said cathode and said first regional isolation means, said electron beam current control means containing an aperture in alignment along said axis, and means for maintaining said electron beam current control means at a negative potential with respect to said cathode.

6. The electron gun of claim 4 including electron beam current control means interposed between said cathode and said first regional isolation means said electron beam current control means containing an aperture in alignment along said axis, and means for maintaining said electron beam current control means at a negative potential with respect to said cathode.

7. The electron gun of claim 3 wherein all positive electrical potentials employed within said enclosure are lower in amplitude than the positive potential of said first regional isolation means.

8. The electron gun of claim 4 wherein all positive electrical potentials employed within said enclosure are lower in amplitude than the positive potential of said first regional isolation means.

9. The electron gun of claim 6 wherein all positive electrical potentials employed within said enclosure are lower in amplitude than the positive potential of said first regional isolation means.

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