US3287735A - Radiant energy apparatus - Google Patents

Description

SR Ems EEEERENEE sEARcH ROOM 392879735 BIG-121- MTRIM Nov. 22, 1966 H. R. DAY, JR

RADIANT ENERGY APPARATUS 5 Sheets-Sheet 1 Filed Aug. 28, 1962 1 7'? v e n t2: 2-":- /-/& ro/d 7?; Day a; 00 2 W #:t'arrvey.

Nov. 22, 1966 H. R. DAY, JR

RADIANT ENERGY APPARATUS 5 Sheets-Sheet 2 Filed Aug. 28, 1962 [r7 verv tor: Harold Rud u Ator'ney.

Nov. 22, 1966 H. R. DAY, JR 3,237,735

RADIANT ENERGY APPARATUS Filed Aug. 28' 1962 5 Sheets-Sheet 5 2 inventor: 25 J Hare/d IQDdy c/ifj Attorney.

United States Patent 3,287,735 RADIANT ENERGY APPARATUS Harold R. Day, Jan, Burnt Hills, N.Y., assignor to General Electric Company, a corporation of New York Filed Aug. 28, 1962, Ser. No. 220,011 16 Claims. (Cl. S M-74) This invention relates to the production of radiation within a closed chamber for effective utilization outside such chamber, and more particularly to the effective transmission of focused electron beams through an electron permeable closure onto a recording medium.

The apparatus according to the present invention finds particular utility in the production of diffraction phase gratings on a deformable medium such as a thermoplastic material. The general method, apparatus and medium are described and claimed in the copending application of William E. Glenn, In, Serial No. 8,842, filed February 15, 1960, now Patent No. 3,113,179, a continuation-inpart of application Serial No. 698,167, filed November 27, 1957 (now abandoned), and of application Serial No. 783,584, filed December 29, 1958 (now abandoned), all assigned to the assignee of the present invention. According to this system of recording, diffraction gratings are inscribed on the deformable thermoplastic medium with a controlled electron beam a few microns in diameter. Areas where electron charge is deposited by the beam are compressed due to electrostatic attraction between the surfaces of the recording medium, causing the formation of diffraction phase gratings. After the deformable medium is thus impressed with the desired intelligence, the deformable medium may be placed in a projection apparatus which blocks non-diffracted light while projecting an image of the intelligence recorded in the gratings. The recorded gratings may contain picture information such as, for example, a television image.

During diffraction grating recording, both the deformable medium and the electron gun forming the electron writing beam are conventionally housed in the same evacuated chamber for two reasons: (1) to maintain a long, mean-free path for the electrons so they dont suffer dispersion by too many collisions with gas molecules, and (2) to protect the cathode or filament of the electron gun from erosion by, and reaction with, gas molecules. The second reason is more pressing. However, it is difficult to maintain a vacuum under recording conditions because the recording medium produces gaseous products when bombarded with electrons. It therefore appears the electron gun and recording medium are to a large degree incompatible. The result is short cathode life and low cathode efiiciency.

Previous attempts to solve this problem by separating the electron gun and the recording medium as with an intervening electron permeable window have not been successful; therefore, diffraction recording in this manner has been considered impractical. Metal and metal oxide windows of the type employed, for example, in electron irradiating and sterilizing apparatus cause the electron beam to be highly scattered and absorbed, presumably due to collision of electrons with atoms of the window. The beam is then much too large to be used to form diffraction gratings. For example, a beryllium metal window having a thickness on the order of 0.001 inch, or somewhat less, will scatter an electron beam to a diameter of several mils, and the resulting beam will have an indistinct edge. This window thickness, required to prevent pinholes, also necessitates use of a hundred kilovolt magnitude electron beam for penetration.

The scattering caused by a window has been determined heretofore on a statistical basis relative to the number of atoms in the window, and scattering is predicted for any type or thickness of window. In passing through the conice ventional window, the electrons are found not only to scatter in direction but to change in velocity so that refocusing of the beam is impossible. Furthermore a photographic plate, even though placed immediately adjacent a conventional window, reveals impracticable scatte-ring which is not noticeably improved by focusing the beam at the plane of the window.

I have discovered that certain extremely thin windows, particularly when formed of atomically highly oriented material, produce little or no scattering as statistically predicted, but rather pass an electron beam or other radiant energy beam with surprisingly high definition. When the wavelength of the radiant energy beam is small compared to the inter-atomic spacing in thin windows, and particularly when a windows atomic construction is comparatively regular throughout its thickness, an increasing number of beam particles must be considered as passing through the window on a quantum mechanical basis without effectively suffering collision or scattering. These unscattered particles are coherent, that is unchanged in direction and velocity, making it possible to maintain focus or produce focusing after the beam has penetrated the window.

In accordance with various features of the embodiments of my invention herein illustrated, a thin electron permeable window is disposed, for example, between the cathode and the beam focusing and deflecting means located in a chamber'separate from the cathode. A particularly advantageous placement of the thin window is across a small aperture in an electron gun anode or other gun electrode. The recording medium, now located in the chamber remote from the cathode, will not contaminate the cathode. The chamber including the recording medium is desirably evacuated to provide a long meanfree path for electrons, but such evacuation need not be nearly so exacting as the vacuum in the chamber housing the electron emitting cathode. The gaseous products from the recording medium are easily tolerated in the additional chamber.

In accordance with another feature of the present invention, the thin electron permeable window is formed of highly oriented pyrolytic graphite or similar material having a regular or nearly monocrystalline structure. A Window of such material passes a high percentage of an electron beam or other radiant energy beam and does so without detectable scattering.

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. The invention, however, both as to organization and method of opera tion, together with further advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings wherein like reference characters refer to like elements and in which:

FIG. 1 is a side view partially broken away of an electron-beam, light-valve apparatus constructed in accordance with the present invention.

FIG. 2 is an enlarged broken away view, partially in cross-section, of the chamber including an electron gun and electron permeable exit window in accordance with the present invention,

FIG. 3 is an enlarged cross-sectional view of an electron gun anode and electron permeable window,

FIG. 4 is a plan View of an alternative construction according to the present invention wherein electrostatic focusing is employed,

FIG. 5 illustrates an electron permeable window mounted upon an insulated support ring, and

FIG. 6 illustrates an electron permeable window mounted on a thin metallic ring.

The electron beam apparatus as illustrated in FIGS. 1,

2 and 3 employs an electron permeable vacuum-tight window 1 of less than 500 Angstroms in thickness and preferably on the order of 50 to 200 Angstroms in thickness. This window separates first and second differentially-evacuated, glass-walled regions or chambers 2 and 3, the former including an electron gun at 4 for producing an electron beam 5 directed through the Window 1. The window is desirably formed of material Which is highly oriented atomically and has a regular or nearly monocrystalline structure. Pyrolytic graphite is preferred. This type of material, while providing an effective barrier to gas molecules, is believed to present a screen-like structure to an approaching electron beam whereby a well defined central electron beam readily passes through the materials atomic structure without the usual scattering.

The electron gun 4 of the present embodiment comprises a tungsten filamentary cathode 6, a grid electrode 7 and an accelerating anode 8. The cathode and grid are adjustably supported in chamber 2 and may be positioned by means of adjusting screws 9. Anode 3 is conductively supported on a centrally apertured anode plate 10. For purposes of the present invention, plate 10 is considered as permanently secured across the bottom of chamber 2 forming a gas-tight seal therewith. However, plate It is separable from chamber 3 as by loosening the bolt 11 which normally secures the chambers together.

The anode 8 supports the gas-tight but electron permeable window 1, the latter being disposed across a small central beam defining aperture 12 in the anode. The electron beam is narrowly focused and restricted at aperture 12, which is desirably on the order of 2 to 10 mils in diameter. Grid 7 is maintained negative with respect to the electron beam (by means not shown), and anode 8 is maintained quite positive with respect to the electron beam (by means not shown); the electric field produced by these two electrodes acts to focus the beam at aperture 12, provided the electrode voltages are properly adjusted in a manner well known by those skilled in the art. The aperture being small tends to restrict the electron beam, reducing aberration effects.

Several advantages accrue in locating the electron permeable window across this small aperture. The window, while retaining physical strength, may be substantially thinner when disposed over a very small aperture than would be the case if the window were located at some other point in the system across a larger aperture. The window then passes a large percentage of the electrons impinging thereon as an unscattered central electron beam, because of the thinness, and also because the beam velocity is at a peak at the positive anode. For example, an annealed pyrolytic graphite window, 200 Angstroms thick located at this point passes approximately 50% of a 15 kilovolt electron beam. The remaining 50% of the beam is not randomly scattered; separate and distinct diffraction images are produced which are radially separated from the central beam and account for the remaining 50%. These diffraction images are intercepted by a metal diaphragm 13 which is located downstream from the window 1, while the non-scattered central beam passes through an aperture 14, larger than aperture 12, located centrally of the diaphragm 13.

It is of course understood that other electron gun structures may be employed in place of the one herein illustrated. It is however generally desirable to place the electron permeable window across the smallest aperture through which the electron beam passes and where the beam has a high velocity as compared with elsewhere in the same apparatus. This will generally be at a crossover or focus point for the electron beam.

The second region or chamber, generally designated at 3, which now receives the electron beam, includes a transparent charge deformable medium 15. This medium may comprise a deformable oil of the type set forth in Patent 2,943,147, to William E. Glenn, Jr., issued June 28, 1960, and assigned to the assignee of the present invention, or a thermoplastic material as hereinafter described. The deformable medium 15, conveniently supported in a suitable light transmitting holder 16, is positioned in the path of electron beam 5. The electron beam causes electrostatic deformation in the medium for forming light-diffracting phase gratings therein.

Magnetic focusing coil 17 and deflection coils 17a act upon the electron beam 5 to produce a finely focused electron beam at the surface of the deformable medium 15, while the electron beam is deflected in a pattern such as that of a television raster. As the electron beam is deflected along a raster line, it is also influenced by deflection coils 17a in a manner to momentarily slow, speed up, and even halt the deflection of the electron beam along a raster line to cause varying charge density along the line where the beam impinges upon the medium 15. The varying charges produce deformations in the medium in the form of diffraction phase gratings capable of dilfracting light. This manner of deflection for establishing phase gratings containing three-color television information, for example, is set forth in Patent Re. 25,169, to William E. Glenn, Jr., which is assigned to the assignee of the present invention.

To provide read-out of the foregoing information, light from a light source 26 is concentrated for illumination of medium 15 by means of a condensing lens 18. Another lens 19 normally images a plurality of optical slits 2b, which are illuminated by source 26, upon a plurality of opaque bars 21. However, when diffraction intelligence is impressed on the medium 15, the phase gratings thus formed will ditfract the light from light source 26 so that diffracted light passes between the bars 21 through slits 22. A lens system 23 then acts to image the intelligence contained in the diffraction gratings upon a screen 24. Mirror 25 is included in the light path between lens system 23 and the screen, for conveniently providing a vertical image of the diffraction intelligence thereon.

The electron beam 5 is thus focused and deflected, in accordance with one feature of the invention, after having passed through electron permeable window 1. This is made possible because in passing through the extremely thin electron permeable window 1, the beam is substantially unaltered in direction and velocity, in contra-distinction to prior electron permeable windows. Thus a microscope inserted in the FIG. 1 apparatus at a glass window 2'7 reveals no detectable change in the cross-section of the electron beam. When a photographic plate is substituted for medium 15, and the electron permeable window 1 is alternately removed and reinserted, no change in the beam image is detected, except in its intensity.

Focusing coil 17 produces concentration of the electron beam into a smaller spot size at the deformable medium 15. For example, a beam one mil in diameter at aperture 12 can be concentrated by focusing coil 17 to a less than M; mil diameter at deformable medium 15, a size quite appropriate for establishing diffraction gratings. The focusing action produced may in the alternative be considered one of refocusing the electron beam or imaging aperture 12 upon medium 15. Without such focusing the beam would have be come much larger downstream, with or without a window.

It is to be chiefly noted in this connection that an electron beam passing through electron permeable windows, as heretofore known, could not be further focused or concentrated mainly because the beam electrons were randomly altered in velocity in passing through the window. Thus a focusing field appropriate to the velocity of some particular electron would not be appropriate for other electrons of the beam, thus causing a beam which could not be refocused. In the present invention, however, further focusing is as easily accomplished in the same manner as if the electron permeable window were absent.

The same phenomena are noted with regard to deflection. That is, electrons having varying velocities would be deflected to different degrees by a given deflection field. However, in accordance with the present invention, deflection in region 3 is readily and accurately accomplished because the beam electrons have not been differentially altered in velocity.

During operation of the apparatus, electron gun 4, isolated in chamber 2, remains unaffected by gaseous products produced outside this chamber when, for example, the electron beam strikes medium 15. The medium 15 comprising recording oils or thermoplastic material is found to have the property of evolving a carbonaceous oily vapor and gas ions when heated and subjected to electron bombardment. In accordance with the present invention, these products cannot reach the cathode or the gun electrodes and therefore form no objectionable coatings thereon. These gaseous products cannot then poison a cathode by chemical combination therewith, nor are deleterious dielectric layers deposited on the other electrodes which would alter their electron-optical characteristics. It is easily possible to maintain a good vacuum on the order of mm. Hg or less in the region 2 which includes cathode, while at the same time permitting a lesser vacuum of only 10- mm. Hg to lO mm. Hg, for example, in chamber 3.

The carbonaceous deposits which would otherwise form upon the electrodes of the electron gun will now have a tendency to deposit upon the electron permeable window 1. While such deposits do not have the same debilitating effect upon the window, it has been found advantageous to keep the window free of such substances. An additional reason, then, for placing the electron permeable window at the beam defining aperture 12, where the electron beam is most restricted, is the heating produced in the window at this point. It is found this heating desirably produces evaporation of carbonaceous oil deposits from the window surface. The gas ions produced in region 3 which would have a tendency to bombard the cathode in the ordinary apparatus, also cannot reach the cathode because of the interposition of window 1.

The cathode 6 may be run at a lower temperature than heretofore found necessary while still producing an acceptable electron emission, and therefore its lifetime is multiplied several times over. The tungsten filamentary cathode in the drawing is illustrative only and is replaceable with higher emission cathodes, for example, of the alkaline earth metal oxide type. Such cathodes produce copious electron emission, but have heretofore been completely incompatible with the carbonaceous deformable medium located in the same apparatus.

An alternative embodiment of the present invention employing electrostatic focusing, and wherein the electron permeable window is supported, for example, on an insulating ring, is illustrated in FIG. 4. In the embodiment of FIG. 4, an electron gun comprises a cathode 6 having an electron emissive coating at 7 which may comprise a conventional thermionically emitting oxide. The cathode is internally heated by means of a separate filament 28. A grid 29, negative with respect to the cathode, includes an aperture 30 through which electron beam 5 passes. The electron beam is directed through an aperture 31 in a cylindrical anode electrode 32, anode 32 forming together with annular metal spacer 33 the end of the region or enclosure 2.

The aperture 31 is covered with a vacuum tight but electron permeable window 1 supported upon the anode with a refractory ceramic insulating spacer 34. The optics of the electron gun system are arranged such that an electron beam crossover or focus occurs at the window 1. The heating produced by the electron beam at the electron permeable window 1, preferably formed of pyrolytic graphite, is conserved to the Window through the use of an annular ceramic spacer 34 shown in detail in FIG. 5. Thus the electron permeable window tends to operate at a hotter temperature and more readily evaporates any gaseous products which may become deposited thereon. An alternative window supporting construction is illustrated in FIG. 6 wherein electron permeable window 1 is sup- 6 ported on a thin metallic annular member 35 which may be substituted for the ceramic member 34 in the FIG. 4 apparatus. The metallic annular member 35 is conductive but is preferably formed of a material thin in comparison to anode 32 so that it tends to conserve heat to the window 1.

Returning to FIG. 4, the electron beam 5 passing through window 1 is further focused and concentrated by means of an electrostatic lens including electrodes 36, 37 and 38. Again, it should be noted the electron beam in passing through the window has received essentially no scattering and therefore may receive further focusing and deflection. The electrostatic focusing system comprising electrodes 36, 37 and 38 acts to focus and concentrate the electron beam 5 to a spot size on the order of 5 microns upon a thermoplastic tape 40 where the tape passes over capstan 41.

A pair of electrostatic deflection plates 39 deflect the beam in a direction transverse to the movement of tape 40 between tape reels 42 and 43. A television type raster may then be formed by moving the tape from reel 42 onto reel 43 as the electron beam is deflected transversely of the tape. This transverse deflection may thus be velocity modulated in order to cause the formation of a charge pattern on the tape 40. The thermoplastic tape is heated by a heating means 44 before reaching capstan 41 so that in its softened condition it is capable of deformation in response to the charge pattern to develop diffraction phase gratings thereon.

The thermoplastic tape 40 is of the type set forth and claimed in the aforementioned application of William E. Glenn, ]r., Serial No. 8,842. Briefly this tape comprises a base material carrying a thermoplastic coating which coating is oriented towards the impinging electron beam. The base material is optically clear and smooth and may suitably comprise 4 mil thick optical grade polyethylene terephthalate sold under the name Oronar. The thermoplastic layer on the film is also optically clear, having a substantially infinite room temperature viscosity and a relatively fluid viscosity at a temperature of 100-150 C., to which temperature it is heated by means 44. One satisfactory thermoplastic material is a blend of polystyrene, m-terephenyl and a copolymer of weight percent of butadiene and 5 weight percent styrene. Specifically, the composition may be 70 percent polystyrene, 28 percent m-terephenyl and 2 percent of the copo-lymer. The film thickness can vary from about 0.01 mil to several mils, with the preferred thickness being about equal to the distance between desired depressions in the film for forming the phase gratings, e.g. approximately 5 microns or approximately 0.2 mil in the present instance.

The operation of the FIG. 4 apparatus is quite similar to that of the FIG. 1 apparatus and need not be recited in detail. The electron permeable window 1 passes a central beam 5, as illustrated, and also tends to produce a number of electron diffraction images which are conveniently intercepted by diaphragms 45 and 46 of lens electrodes 36 and 38. These diaphragms are provided with centrally located apertures 47 and 48 for passing the central electron beam 5. Again, no observable scattering of the central beam takes place. Assuming a 10 kilovolt electron beam, electrodes 36 and 38 may be conveniently provided with a voltage of 8 kilovolts while electrode 37 is provided with a voltage of approximately 3 kilovolts. It is understood these voltages are exemplary only.

The electron permeable window may be disposed across either aperture 47 or 48 as an alternative to placement thereof across aperture 31 in the anode electrode. If the electron permeable window is placed across aperture 47 then diaphragm 46 will prevent passage of the outlying diffracted emissions. If the window is placed at aperture 48 an additional aperture (not shown) may be provided for stopping the additional r-ays. It is quite desirable from a standpoint of passing as much of the central electron beam as possible, that the window be placed across a small aperture in a high potential electrode where the electron 'beam attains a high velocity relative to other portions of the tube. Such electrode will frequently be a high positive voltage electrode but its actual physical location may vary from one electron beam apparatus to another. Frequently the high voltage electrode will be the last or exit electrode in what may be considered the electron gun or electron optical system. The window may also be secured on a separately provided apertured support in either the FIG. 4 or FIG. 1 apparatus.

Although the thin electron permeable window in accordance with the present invention may be formed from various materials, pyrolytic graphite, as a highly oriented and semi-single crystalline material, possesses numerous advantages and is generally preferred. The pyrolytic graphite, in addition to being an atomically regular material, is resistant to high temperatures and to oxidation. It also is mechanically strong and essentially gas tight. The pyrolytic graphite as employed for the electron permeable Window according to the present invention may be defined as a material made from carbonaceous gases by thermal decomposition or formed of carbonaceous material by evaporation and deposition on a surface. Briefly, a hydrocarbon gas, such as methane, is deposited on a surface heated to the range of 1800 to 2500 C. in a chamber wherein the carbon gas pressure may vary between 0.5 mm. and 760 mm. of mercury. The heated surface upon which the pyrolytic graphite is deposited is also preferably formed of a graphite substance.

Prior to admitting the hydrocarbon gas, such chamber is evacuated of other gaseous materials. The carbonaceous gas is then decomposed to a carbon vapor which deposits upon the heated surface. The graphite body deposited at these temperatures is a fine grained pyrolytic graphite which is quite free of unusually large gasnucleated particles. After deposition of the pyrolytic graphite, the surface with the graphite deposed thereon is allowed to return to room temperaure.

The body of pyrolytic graphite thus formed is preferably annealed at a temperature of at least 3500 C. and preferably in the range of 3500 to 3800 C. The annealing is carried out in a suitable enclosure in the presence on an inert gas such as argon which is desirably circulated through the enclosure. This anneal results in pyrolytic graphite with superior crystalline perfection and preferred orientation and results in nearly complete straightening of the planes of graphite forming the material. The pyrolytic graphite as herein described is more fully described and claimed in the patent application of Russell J. Diefendorf, Serial Number 199,467, filed June 1, 1962, and assigned to the assignee of the present invention.

A thin layer of pyrolytic graphite, suitable for an electron permeable window, is then separated from the above formed graphite body. One manner of securing extremely thin flakes of pyrolytic graphite material is to immerse the material in benzene while peeling thin layers therefrom with a needle under a microscope. The graphite tends to separate into thin planes possibly formed of a number of flat crystallites and useable as Window material. A method found more effective is the separation of the graphite material using a pressure sensitive adhesive, for example, ordinary cellophane tape having a pressure sensitive adhesive layer. The cellophane tape is impressed upon a body of pyrolytic graphite material and then stripped therefrom. A thin layer of graphite will adhere to the adhesive layer. A second piece of tape is then impressed upon the graphite material carried by the first tape. The tapes are then stripped from one another. An even thinner layer of graphite now adheres to the pressure sensitive adhesive of each tape. A fresh tape is then applied to one of these layers and then stripped off again resulting in a yet thinner layer of pyrolytic graphite. This procedure is continued until a graphite layer is secured which is readily transparent. The thickness of the graphite layer can be gauged with an interferometer or with a light absorption metering device, but with practice the indication of light transparency to the eye is usually sufficient for determining the proper graphite layer thickness, under 500 Angstroms and preferably less.

The adhesive material is now dissolved by immersing the cellophane tape including the attached graphite layer into a solvent solution. A typically satisfactory solvent for the self-adhesive layer on conventional cellophane tape is an equal part mixture of toluene (C H CH pyridine (C H N) and chloroform (CHCl The pyrolytic graphite flakes are found to separate freely from the tape in this solution.

In preparation for the implacement of the pyrolytic graphite window, the aperture across which the window is to be disposed is now drilled with a very small diameter hole. Anode 8 illustrated in FIG. 3, and for example formed of stainless steel, is drilled with a perpendicular hole 12, 5l0 mils in diameter. The hole is cleaned and polished so as to be burrless especially in the direction which will subsequently adjoin chamber 3, i.e., the surface upon which the window 1 will rest. The anode is then secured to anode plate 10, and the entire chamber 2 including the electron gun is oriented with the anode end of the gun upright. The chamber 2 is attached to an evacuation means through a tip off valve (not shown) causing an inflow of air through the small aperture 12. Now a small flake of pyrolytic graphite is transported from the solvent on a fine mesh screen and, with the aid of microscope means, is disposed upon the aperture 12. The fine mesh screen is removed. The evacuation means will cause the pyrolytic graphite layer to bow downwards slightly thereby insuring its continued implacement. However, the small pyrolytic graphite window is found to adhere substantially permanently to the anode structure even if the vacuum in region 2 is discontinued. The evacuation of region 2 can now be completed.

It is postulated the comparative transparency of pyrolytic graphite material to a beam of electrons is due to its regular atomic structure. In pyrolytic graphite, planar graphite crystallites have a preferred orientation, and are arranged so their layers are generally parallel to the body suface or the surface upon which they were deposited. The annealing of a pyrolytic graphite body at a temperature above 3500 C. results in an article of superior crystalline perfection and superior orientation. The annealing appears to result in complete straightening of the grains of the material in planes substantially parallel to the surface, to the extent of a nearly perfectly oriented structure. The orientation of the resulting material is effectively quite similar to a single crystal material. As far as the electron optics of the material are concerned, longitudinal metal grain boundaries which appear to divide most metals into a mosaic of variously oriented crystal structure, are either not present in the pyrolytic graphite as annealed, or else such boundaries are so substantially infrequent as not to affect the electron optics deleteriously. Seen under a microscope, small crystalline units of the graphite at first appear to become larger and then the boundaries appear to nearly vanish in annealing. To the eye, the body of pyrolytic graphite appears as a mirror and behaves otherwise as a single crystal.

In placing a window of such material in an apparatus, such as the apparatus of FIG. 1, double images sometimes occur for some window positions and beam orientations, indicating the possible presence of a grain boundary. However a very slight reorientation of the window or of the electron beam easily eliminates one of these images, and it is thought that the electron beam then panatrates the window without encountering such a grain boundary.

The graphite window flake is considered as being composed of one or more planar crystallite layers the planes of which are substantially perpendicular to the direction of the electron beam. The electron beam impinging upon the thin layer of crystalline material appears to pass straight through as a central beam of somewhat reduced intensity. The remaining beam is not scattered randomly near the central beam, but is diffracted by the atomic planes of the window material. This electron diffraction produces electron beam images at points radially separated from the central beam for example as viewed upon medium 15. The electron diffraction images are found to be as distinct as the central beam but radially displaced sufficiently from the central image so they may be easily intercepted by diaphragm 13. The central beam, which itself exhibits no scattering effects, passes through aperture 14 and the diffraction images are intercepted by the diaphragm 13.

The electron beam preferred according to the present invention is a low velocity beam as compared with most prior apparatus wherein a very high velocity electron beam has been required in order to pass through a window. Thus the electron beam in the instant apparatus has an energy on the order of 10,000 electron volts, this energy being quite sufficient for producing central electron beam penetration of 50% through the extremely thin pyrolytic graphite. Very high energy beams (50 100 kv.) are not only unnecessary according to the pres ent invention but frequently tend to have undesirable effects upon deformable media.

At the electron beam energies herein indicated, the electrons making up the beam are considered to have a wavelength appropriate for passing through the graphite crystalline structure without scattering. The dual nature of an electron-having wave properties as well as corpuscle properties-is well known in quantum mechanics. The wavelength of an electron is related to the velocity of an electron by the formula:

A=wavelength h=Plancks constant m=electron mass, and v=velocity of an electron The electrons in an approximately kilovolt beam will be found to have a wavelength which is short as compared to the inter-atomic spacing of a metal such as pyrolytic graphite. The complete beam of electrons of course has a very large diameter compared to this interatomic spacing of the window material, but may be looked upon as passing through the essentially singular crystalline atomic structure as through an atomic mesh screen or a properly oriented honeycomb. If the electron beam has low energy, on the other hand, the electron wavelength is comparable to the inter-atomic spacing and appears not to produce ready penetration of the window material. However, at extremely low electron velocities, e.g., as obtained by thermionic emission with no substantially accelerating electrodes acting upon the beam, ready penetration of the window is again observable. Thus, a thermionic filament o-r cathode placed in the region 2 of the illustrated apparatus will produce electrons capable of easily passing through the thin electron permeable window. In such an instance, the window itself may be utilized as an essentially cold cathode emitting surface but one providing a copious supply of electrons. Such an emitting surface is useful in many different discharge type devices. The ready penetration of the low velocity electrons through the window takes place when the electron has a long wavelength, compared to the inter-atomic spacing in the window. First order diffraction, if it takes place, is so far displaced radially as to be unobservable. It thus appears that penetration can take place for electron wavelengths which substantially differ from the inter-atomic spacing of the crystalline window material.

Although pyrolytic graphite is preferred as a window material, it is understood the invention in all its aspects is not limited to pyrolytic graphite. Other substances are also suitable to varying degrees when reduced to extreme thinness. These materials, while frequently resulting in some scattering, .do produce surprisingly good results in thicknesses below 500 Angstroms and preferably below 200 Angstroms. In such thicknesses such windows produce much less scattering than would be predicted. Thus aluminum oxide (A1 0 produces surprisingly little scattering as employed in an extremely thin electron permeable window. One disadvantage, however, of the aluminum oxide or alumina is its susceptibility to the intense heat caused by the electron beam due to thermally insulating nature of the material. For this reason it is sometimes desirable when using an alumina window to mount such window beyond the beam focal point or crossover in order to reduce heating. Aluminum oxide material of which a thin window is formed may be procured by anodic oxidation and obtained in a self-supporting film as described in an article by Louis Harm's, Journal of the Optical Society of America, vol. 45, No. 1, page 27.

Other materials which are suitable to varying degrees include pyrolytic boron nitride and tantalum oxide. Pyrolytic boron nitride, for example, has a regular atomic structure as compared to many metals, metals in general providing relatively poor electron transmission because of their usual heterogeneous internal structure. The preferred materials are in general somewhat refractory and their elementary constituents have a low atomic number, i.e. carbon, nitrogen, boron and aluminum, electron absorption increasing with higher atomic numbers. Evaporated carbon in extremely thin layers is sometimes satisfactory.

According to the illustrated embodiments of the present invention, electron beams have been described as passing through a thin window arrangement. However, in its broader aspects the present invention comprehends passage of other forms of radiant energy through an extremely thin radiant energy permeable window. For example X- rays are found to pass through thin windows according to the present invention with little or no central beam scattering.

In accordance with the present invention as above stated, numerous advantages are secured. An electron gun or other radiant energy producing means can be accommodated in its own isolated and sealed off region whereby the electron beam passing through the permeable window is directable in a second chamber, and may be focused therein to a very high degree. The second region may, if desired, constitute the atmosphere, within the limits dictated by air scattering and ionization. The electron gun or other radiant energy producing means, being isolated, is not subjected to cathode poisoning by gaseous products found in the second region. The radiant energy generating means will, moreover, not be deteriorated by ion bombardment from the second region. Also, the various electrodes in the first region are not subjected to deposition of foreign dielectric material and the like and therefore their electrical properties are not impaired with age.

While I have shown and described several embodiments of my invention, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from my invention in its broader aspects; and I therefore intend the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A recording apparatus comprising a deformable recording medium, means for heating said medium, an electron gun producing a narrowly focused electron beam directed towards said medium for recording diffraction phase gratings on said medium, and an evacuated chamber including said electron gun and provided with a window of thin crystalline electron permeable material having a high- 1y oriented and regular atomic structure between said electron gun and said recording medium said window passing said electron beam with substantially no defocusing thereof.

2. The apparatus acoerding to claim 1 wherein said window is formed of pyrolytic graphite.

3. The apparatus according to claim 1 wherein said window is less than 500 Angstroms in thickness.

4. Apparatus for producing phase diffraction gratings comprising an image receiving surface capable of de formation when subjected to electrostatic charge, means for directing a stream of electrons at said image receiving surface for producing said electrostatic charge thereon, and a thin layer of pyrolytic graphite between said electron beam producing means and said image receiving surface through which said stream of electrons passes without appreciable loss of coherence.

5. Radiant energy apparatus comprising an enclosure, a source of directed radiant energy within said enclosure, and an exit window of thin pyrolytic graphite included as a part of said enclosure through which said radiant energy is transmitted with substantially no change in direction thereof.

6. Radiant energy apparatus comprising first and second regions closed to one another except for a small aperture, means for producing a beam of radiant energy in a first of said regions directed towards a second of said regions through said aperture, and a window of thin crystalline material having a highly oriented and regular atomic structure between said first and second regions disposed across said aperture and through which said beam passes with substantially no change in direction thereof.

7. In an electron beam apparatus, an electron gun comprising a plurality of electrodes including a cathode and an anode electrode having a small aperture for passing said electron beam, and a pyrolytic graphite Window disposed across said aperture in said anode electrode, said window passing said electron beam with substantially no defocusing thereof.

8. In an electron beam apparatus, an electron gun comprising a plurality of electrodes including a cathode and an anode electrode having a small aperture for passing said electron beam, a pyrolytic graphite window disposed across said aperture in said anode electrode, and an insulating refractory mounting ring between said window and said anode electrode, said window passing said electron beam with substantially no defocusing thereof.

9. Radiant energy apparatus comprising first and second differentially evacuated regions, means for producing an electron beam in a first of said regions, a thin crystalline electron permeable material having a highly oriented and regular atomic structure situated between said regions, said material passing said beam with no appreciable loss of coherence thereof, means for focusing and deflecting said electron beam in the second of said regions, and an image receiving surface disposed within the second of said regions, said surface lying in the path of said electron beam to enable said focusing means to focus said electron beam upon said image receiving surface.

10. An electron permeable window for passing a stream of electrons, said window being formed of a material having a regular and highly oriented atomic structure whose inter-atomic spacing is substantially different from the wavelength of electrons passing therethrough, said stream of eleptrons having a crosssectional dimension 12 which is large compared to the atomic spacing of said atomic structure.

11. The electron permeable window according to claim 10 wherein the window is formed of material having an atomic structure whose inter-atomic spacing is large compared to the wavelength of electrons passing therethrough, and which has crystallite planes oriented substantially perpendicularly to said stream of electrons.

12. The electron permeable window of claim 10 formed of pyrolytic graphite.

13. A radiant-energey permeable window for use in radiant-energy projecting apparatus, said window having a regular and highly oriented atomic structure whose inter-atomic spacing is substantially different from the wavelength of radiant energy to be passed therethrough and being constituted of a pyrolytically deposited substance of the class consisting of graphite, boron nitride, and tantalum oxide.

14. Radiant energy apparatus comprising first and second differentially evacuated chambers, a radiant energy permeable window providing communication between said chambers, and means for producing a beam of energy in a first of said chambers directed towards a second of said chambers through said window, said window having a regular and highly oriented atomic structure whose interatomic spacing is substantially different from the wavelength of radiant energy to be passed therethrough and being constituted of a pyrolytically deposited substance of the class consisting of graphite, boron nitride and tantalum oxide, so as to pass said beam with substantially no change in direction thereof.

15. Radiant energy apparatus comprising means for producing a narrowly focused electron beam, means for deflecting said electron beam, and a thin pyrolytic graphite window between said means for producing said electron beam and said means for deflecting said electron beam, said window passing said electron beam, said widow passing said electron beam with substantially no defocusing thereof.

16. Radiant energy apparatus comprising an electron gun producing a narrowly focused electron beam for forming an image, an evacuated first chamber housing said electron gun, a second chamber adjacent said first chamber, said second chamber having higher internal pressure than said first chamber, a thin pyrolytic graphite window between said chambers through which said electron beam passes with substantially no defocusing thereof, and means for further focusing and deflecting said electron beam after passage thereof through said window.

References Cited by the Examiner UNITED STATES PATENTS 1,943,109 1/1934 Coolidge 313-74 2,698,928 1/1955 Pulvari 340173 2,820,168 1/1958 Stiff 313-74 2,927,959 3/1960 Mast u--- 178-75 2,950,388 8/1960 White 31374 2,985,866 5/1961 Norton 340-173 3,099,762 7/1963 Hertz 313--74 3,113,179 12/1963 Glenn 340-173 FOREIGN PATENTS 519,015 3/ 1940 Great Britain. 627,063 7/1949 Great Britain.

TERRELL W. FEARS, Acting Primary Examiner.

IRVING SRAGOW, BERNARD KONICK, Examiners,

J. F. BREIMAYER, M. K. KIRK, Assistant Examiners.

Claims (1)

1. 4. APPARATUS FOR PRODUCING PHASE DIFFRACTION GRATINGS COMPRISING AN IMAGE RECEIVING SURFACE CAPABLE OF DEFORMATION WHEN SUBJECTED TO ELECTROSTATIC CHARGE, MEANS FOR DIRECTING A STREAM OF ELECTRONS OF SAID IMAGE RECEIVING SURFACE FOR PRODUCING SAID ELECTROSTATIC CHARGE THEREON, AND A THIN LAYER OF PYROLYTIC GRAPHITE BETWEEN

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