US3040200A - Electron discharge device - Google Patents

Description

R. A. SHAFFER ETAL 3,040,200

ELECTRON DISCHARGE DEVICE Filed Nov. 24, 1959 June 19, 1962 'Fi .l

9 Fig.3

INVENTORS Robert A. Shaffer a JoYmes L. Mclnt re WITNESSES /qx L ATTORNEY United States Patent 3,040,200 ELECTRON DISCHARGE DEVICE Robert A. Shatter, Elmira, and James L. McIntyre, Horseheads, N.Y., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Nov. 24, 1959, Ser. No. 855,107 6 Claims. (Cl. 31365) This invention relates to electron discharge devices in general and particularly to television camera tubes employing the return beam principle.

The present invention will be discussed and described in connection with an image or-thicon type television camera tube since this is the most widely used tube which operates upon the return beam principle. However, it will be obvious that the present invention may be applied to any tube of the return beam type.

In image orthicon type television camera tubes, a low velocity electron beam is scanned across a target electrode to discharge or neutralize a charge pattern thereon which comprises a distribution of positive electrostatic charges. The scanning electron beam approaches the target at nearly zero velocity. The negative beam will strike and discharge the positively charged areas of the target and charge the target areas negatively until the surplus portion of the beam is repelled back along the tube axis. Thus, an electron beam of uniform density scans the target while a nonuniform or modulated beam is returned to the cathode end of the tube where it is collected and amplified as the output signal.

Generally, the modulated or information bearing beam is attracted to the first dynode of an electron multiplier structure which is concentric with and encircles the electron gun of the tube. The first dynode of the electron multiplier structure generally comprises a secondary electron emissive surface having a minute aperture therein through which electrons from the electron gun are emitted. One reason for the requirement that the electron aperture be quite small is that light from the thermionic cathode should be prevented, as much as possible, from reaching the photocathode of the tube. The emissive surface is generally supported by a cup-shaped member which is mechanically secured to the electron gun structure or the supporting tube thereof. Subsequent dynodes of the electron multiplier structure generally encircle the supporting tube of the electron gun.

in the past it has been the case that the space between the electron gun cathode and the first dynode of the multiplier assembly was totally enclosed except for the electron beam aperture. This aperture is necessarily made quite small so as to limit the cross-sectional area of the electron beam. For example, it is common for the beam aperture to be on the order of about .0015 inch in diameter. In the fabrication of a structure of this type it is necessary that the gun structure be outgassed, to prevent harmful efiects due to gases which would otherwise be evolved at tube operating temperatures. Temperature restrictions imposed by the materials employed and the fact that the cathode-first dynode space is accessible by only one aperture prevent the adequate outgassing of the cathode-first dynode space without gases uniting with the secondary electron emitting material around the aperture of the dynode and appearing in the output signal as image defects. To avoid the contamination of the secondary electron emitting material we provide an electron gun structure for return beam devices wherein the side wall of the supporting cup of the first dynode is provided with apertures making the outgassing of the cathode to first dynode cavity easier while avoiding possible gas contamination of the secondary electron emitting surface.

it is therefore a principal object of our invention to provide a return beam imaging device having a cathode to first dynode cavity which may be readily outgassed.

Another object is to provide a return beam imaging device having a first dynode with a secondary electron emitting surface which is free of contamination.

Another object is to provide a return beam imaging device from which may be derived an output signal which is free from defects due to contamination of the secondary emissive surface of the first dynode.

Another object is to provide a television camera tube from which signals may be derived for the formation of an image of high quality.

These and other objects of this invention will be apparent from the following description taken in accordance with the accompanying drawing, throughout which like reference characters indicate like parts, which drawing forms a part of this application and in which:

FIG. 1 is a sectional view of a television camera tube embodying the present invention;

FIG. 2 is a sectional view of the electron gun and first dynode structure of the device shown in FIG. 1; and

FIG. 3 is an enlarged sectional View of a portion of the structure of FIG. 2.

Referring now to FIGURE 1, there is shown a television camera tube for producing a video signal for transmission. The camera tube comprises an envelope 10 of a suitable material such as glass having at one end thereof a light transmissive face plate 12. On the inner surface of the face plate 12 is disposed a layer of photoemissive material 14. The photocathode 14 may be of any suitable material well known in the art such as cesium antimom'de. Positioned adjacent the photocathode 14 is an accelerating electrode 16 and axailly spaced therefrom is an annular electrode 18 for supporting a target electrode 20. The target electrode comprises a suitable dielectric material such as glass. Between the target electrode 20 and the photocathode 14 is a very fine screen mesh 22 closely spaced from the surface of the target 29. The screen mesh 22 is conductively supported by the annular electrode 18.

An object 24 which is to be televised is focused by an appropriate optical system, represented at 26 in FIG. 1, as an optical image on the photocathode 14. An electron image is emitted from the photocathode 14 in accordance with the optical image imposed thereon. By reason of an accelerating field provided between the photocathode 14 and the target 20, and due to the effect of a focusing coil 28 surrounding the envelope 10 of the tube, a uniform focusing field focuses and accelerates electrons from the photocathode to the target 20. The electrons strike the target 20 at a high velocity causing a secondary emission greater than unity fromthe surface of the target 20. The fine mesh screen 22 acts as a collector electrode of secondaries emitted from the target surface.

The emission of secondary electrons from the surface of the target 2t: leaves areas of the target positively charged to an extent related to the density of electrons impingent thereon. In this matter an electrostatic charge image is formed on the target 20.

The opposite side of the target 2% is scanned during tube operation by low velocity electron beam 34 emitted from an electron gun structure 49 mounted at the opposite end of the tube envelope 10. The tube is also provided with an alignment coil 32, an inner surface conductive coating 34, a decelerating electrode 36 and defiection coil 38 which are well known in the art. The deflection coil 38 causes the electron beam 36 to scan the surface of the target 2t}. Electrons from the beam 30 land on the target 20 to maintain the surface thereof at cathode potential. The remainder of the beam approaching the target at close to zero velocity is repelled by the target surface to form a return beam 31 which passes back along substantially the same path as the incident beam 30 and strikes a large dynode surface forming the first stage 70 of multiplier unit which comprises one or more additional dynodes 45, 4-6, 47, 48, 49 at successively higher positive potentials from which is derived the tube output signal.

Referring now to FIG. 2 and FIG. 3 there is shown an electron gun 5t! comprising a supporting tube 51 which is substantially a hollow cylinder. Within the supporting tube 51 is disposed a cathode 52 upon a cathode support 54 which is spaced from and supported by insulating spacers 64 which are mechanically secured to the supporting tube 51. The cathode 52 comprises a layer of thermionically emissive material. Within the cathode support 54 is disposed a heater coil 69 for heating the thermionic cathode 52 in the well known manner. A cathode lead 56 is provided to extend through the lower end of the supporting tube 51 for the application of a suitable potential to the cathode. This potential is usually ground potential. Also within the supporting tube 51 and disposed above the cathode 52 is a first grid 61 having an aperture 65 therein aligned with the cathode 52. The first grid 61 is supported by an insulating spacer 64. Above the first grid 61 is disposed a second grid 52 having an aperture 6-6 also in alignment. The second grid is supported by and spaced from the first grid by an insulating spacer 63. The exterior of the supporting tube 51 is provided with a flange portion 67 extending outwardly therefrom at the cathode end of the supporting tube 51. At the upper end of the supporting tube is provided a first dynode member indicated generally at 70.

The first dynode is provided with a fiat surface 72 and a cup shaped support 74 afiixed thereto. The cup shaped member 74 is provided with a flange portion 75 which is secured to the flange portion 67 on the supporting tube 51, although they are shown separated in FIG. 2. The flat surface 72 of the first dynode comprises a suitable metallic cap having a fine grained matt surface which is over a supporting plate 73 and securely fastoned thereto. Upon this surface 72 of the metallic cap is provided a coating of secondary electron emitting material 71. The secondary electron emitting material coated on the dynode surface may be any suitable material such as chromium, beryllium oxide, aluminum oxide and others. Aperture 81 is provided in the surface of the metallic member 72 with its secondary emitting coating 71. Apertures 79 and 80 are provided in mignment with aperture 81 in the members 73 and 74. The cup support member 74 is provided with a side Wall 77 having aper tures 76 disposed therein. The side wall 77 fits closely to the supporting tube 51 when the flanges 67 and 75 are joined. However, the spacing is adequate for the full utilization of the apertures 76.

As shown in FIG. 2, the cathode 52 is disposed in an almost entirely enclosed space bounded by the enclosure made up of the cathode 52, the cathode support 54, the insulating spacer 64, the first grid 61, the insulating spacer 63, the second grid 62, the supporting tube 51 and the first dynode structure 70. This space will be hereinafter referred to as the cathode to first dynode space. In prior art devices this space was entirely enclosed except for the aperture 81 provided for the emission of electrons from the cathode to the target.

In fabricating evacuated electron tubes it iswell known that metal parts must be heated to an elevated temperature during exhaust processing in order to remove adsorbed gas from the metallic surfaces within the tube. This is necessary because in operation temperatures may be reached which would cause such gas to evolve and disturb tube operation. This outgassing is necessary with the gun structures of return beam image devices such as that shown in the drawing and naturally such outgassing occurs when the gun structure is complete and in place. Therefore, in prior art devices outgassing of the cathode to first dynode space had to be accomplished 4 through aperture 81. Because it is the only aperture in the structure the outgassing of the cathode to first dynode space cannot be done without gases being adsorbed on the first dynode structure including the secondary electron emissive portion.

Either of two factors may contribute to the adsorp tion of gas on the secondary electron emitting surface when outgassing is carried out solely through the electron beam aperture 81. In order to form a cap 72, which provides the supporting surface for the secondary emitting material, the surface must be properly prepared by drilling the aperture, cleaning the surface, and sandblasting the surface. For this purpose it has long been the practice to use silver as the material for the cap 72 because of its ease of workability. However, because of the low softening point of silver, the cap 72 could not be raised to the temperature necessary for thorough outgassing. Other gun components were, therefore, at a more elevated temperature than the silver cap 72 with the result that gases evolved from the other surfaces and caused to pass through the aperture 81 would tend to be readsorbed on the cooler silver caps 72 on which the secondary emitting material 71 is disposed. To avoid this particular problem it has been suggested to use a high temperature material, such as nickel, for the supporting caps 72 so that the entire gun structure may be outgassed at elevated temperatures. Some sacrifice in the workability of the cap material is, of course, incurred by this modification. Moreover, it has been found that this procedure itself is inadequate in solving the problem of contamination of the emissive surface because at the elevated outgassing temperatures the secondary electron emitting material becomes so highly reactive that upon the passage of gas atoms through the aperture 81 and over the secondary electron emitting surface a high probability of chemical reaction prevails. Therefore, no adequate solution to the problem has been found which permits outgassing solely through the electron beam aperture.

In accordance with our invention, apertures are provided in the side wall 77 of the cup support 74 of the first dynode member 7 0. Gases may be exhausted through these apertures without the necessity of causing them to pass by the secondary electron emitting surface where the probability of adsorption or chemical reaction is high. In this manner, image defects due to contamination of the surface by the gases altering the secondary electron emissive properties of that portion of the surface are avoided.

To further explain our invention let us consider a typical gas atom which is evolved from the cathode support 54 both in prior art devices and in a device in accordance with our invention. In a device according to the prior art the gas atom from the cathode support 54 would be caused to evolve therefrom by the application of heat radiated inwardly from the supporting tube 51. Since at the. time heat is applied for outgassing, the envelope is exhausted, the gas atom would tend to seek an escape route from the cathode to first dynode space. Since there was only one means of exit the atom would necessarily be brought to the vicinity of the aperture 81 of the first dynode member. When a low melting point alloy is used, because of its ease in working, for the metallic support member 72 of the first dynode, a similarly high heat is not applied to this region. Therefore, there is a fairly high probability that the thermal energy of the gas atom would drop to such an extent that it would be readsorbed on one of the cooler surfaces in the region of the ape1' ture 81. This readsorption could take place either before or after the gas atom has passed through the aperture 81. If it happened before the atom had passed through the aperture no great deterioration in tube operation would result. However, if the gas atom were readsorbed after passing through the aperture 81 it would most likely be adsorbed on the secondary electron emitting surface 71 forming a region of non-uniformity in secondary electron aos aoo emitting characteristics. Therefore, when the dynode surface was struck by the return electron beam secondary electron emission would not take place in that area in the same ratio as was occuring over the remainder of the dynode surface. Therefore, the output signal would fail to truly represent the image on the target 18.

In instances where a high softening point material, such as nickel, is used for the support cap 72 of the first dynode, the gas atom from the cathode sleeve would not tend to be readsorbed due to decrease in thermal energy because the temperature of the whole structure may be maintained at a uniformly high value. However, because of the high temperature of the secondary electron emitting surface 7-1, the materials thereon would be highly reactive and upon the gas atoms passage through the aperture 81, bringing it into proximity with the highly reactive emitting materials, the probability of a reaction is high.

Now considering the probable course of a gas atom on the cathode support 54 in a device constructed in accordance with the present invention. As in the previous case heat would be applied and would be radiated inwardly from the supporting tube 51 striking the cathode support 54 to cause the gas atom to be evolved therefrom. Because of the simultaneous exhaustion of the tube the gas atom would tend to rise toward the region of less pressure necessarily bringing it up toward the first dynode assembly.

Because of the apertures 76 in the side wall 77 of the cup shaped member 74 of the first dynode, the gas atom would tend to pass through these apertures. Since the side Wall need not be maintained at a lower temperature than that ordinarily used for outgassing the probability is high that the gas atom would readily pass through the apertures and be exhausted from the tube. Otherwise the gas atom would merely be readsorbed on the side Wall and would not substantially alter tube operation. In the practice of our invention, outgassed atoms are not likely to come close to the secondary electron emitting surface. Therefore, it is not significant whether a high temperature material or a low temperature material is used for the support cap 72.

In accordance with the present invention, the positioning of the apertures 76, utilized to enable out assing of the gun structure Without contamination of the emissive surface 71, is not highly critical. However, certain considerations do to some extent, limit their location. The thermionic electron emissive material of the cathode 52 must be heated to a relatively high temperature for emission to occur. This heating is provided by the resistance heating wire 69. At the temperatures at which electron emission occurs from the material 52, it is also generally the case that the material is luminescent. The cathode to first dynode space is therefore lighted by visible or invisible radiation which is readily refiected by the relatively smooth surfaces bounding the cathode to first dynode space. Typically, the cathode material 52 is, for example, a mixture of barium, calcium and strontium orides. The operating temperatures of this cathode are about 800 to 900 C. At such temperatures the cathode material is above its red heat.

The return beam dynodes 45, 46, 47, 48 and 49 are generally of the well-known venetian blind type. The emissive material on these dynodes is, for example, a mixture of silver and magesium oxides. It has been found that such material is photosensitive. Therefore, any stray radiation striking the dynodes 45, 46, 47, 4S and 49 will cause secondary electron emission which will appear in the output signal as an image defect. For that reason, it is necessary that radiation from the cathode be prevented from reaching the secondary dynodes of the return beam multiplier. Location of the outgassing apertures 75 must therefore be such that a light path is not provided between the cathode 52 and the dynodes 45, 46, 47, 48 and 49. For that reason the apertures 76, in accordance with a preferred embodiment of the present invention, are found to be conveniently situated in the Wall of the first dynode support cup. In this position the apertures, and hence the dynodes, are substantially light shielded by the cylindrical extremity 53 of the supporting tube 51 which serves as a light shielding means. Alternate locations such as providing apertures in the first grid 61 and the Wall of the supporting tube 51 are not preferred because of the difii culty of providing adequate light shielding without interposing additional elements which would further complicate tube fabrication.

As can be seen from the foregoing the size and shape of the aperture 76 in the side wall 77 are not critical. It is the fact that the apertures are provided in the side wall for outgassing that is "significant rather than their particular geometry. Despite their simplicity, the presence of the apertures in the side wall 77 serves the very important purpose of preserving the secondary electron emitting characteristics of the dynode surface 71 so that an accurate output signal may be derived representative of the scene on which the camera tube is focused.

While the present invention has been shown in one embodiment only, it will be obvious to those skilled in the art that it is not so limited but is susceptible of various changes and modifications without departing from the spirit and scope thereof.

We claim as our invention:

1. A return beam imaging device comprising an electron gun having a cathode, a return beam electron multiplier first dynode member, said dynode member comprising a supporting cup and a secondary electron emissive surface, said secondary electron emissive surface having an aperture for the passage of electrons from said cathode, and said supporting cup having a side wall having one or more apertures therein, said electron gun comprising one or more other members, said one or more other members, said cathode and said dynode member substantially enclosing a space except for said apertures in said side wall and said aperture in said secondary electron emissive surface, said supporting cup of said dynode member being disposed so that the apertures therein are unobstructed to permit outgassing said electron gun without utilizing the aperture through said secondary electron emissive surface.

2. A return beam imaging device comprising a cathode for the generation of an electron beam, a first dynode having a receptive surface for the reception of a return electron beam, said electron beam from said cathode having a path through said receptive surface of said dynode and apertured means provided between said cathode and said receptive surface apart from said electron path, said apertured means being disposed so that the apertures therein are unobstructed to permit outgassing the space between said cathode and said receptive surface without gas contamination of said receptive surface.

3. An electron discharge device comprising an electron P source in an enclosure formed by members including a bers of said enclosure to permit gas flow therethrough dur ing outgassing. V

4. An electron discharge device comprising an electron gun having a thermionic electron source for formation of an electron beam, a first member having an aperture in the path of said electron beam for the emission of said beam from said electron gun, a second member having one or'more outgassing apertures disposed apart from the path of said electron beam for the outgassing of said electron gun without utilizing said aperture in said first mem- 75 her, one or more light sensitive electron emissive electrodes disposed external to said electron gun and light shielding means disposed to substantially prevent radiation emitted by said thermionic electron source from passing through said outgassing apertures and striking said light sensitive electron emissive electrodes, said second member being spaced from said light shielding means to permit gas flow therebetween during outgassing.

5. A return beam imaging device comprising an electron gun having a thermionic cathode, a return beam electron multiplier disposed around said cathode including a first dynode comprising a cup-shaped member and a secondary electron emissive portion supported by said cup-shaped member, said secondary electron emissive portion having a beam aperture through which the beam from said electron gun emerge said cup-shaped member secured to a support structure and having one or more outgassing apertures therein to permit outgassing of said electron gun without contamination of said secondary electron emissive portion, said return beam multiplier also including a plurality of additional dynodes having thereon a photosensitive electron emissive material, said support structure spacedly disposed between said cathode and said additional dynodes and between said cathode and said out gassing apertures in said cup-shaped member to permit gas flow through said outgasing apertures.

6. A return beam imaging device comprising an electron gun having a thermionic cathode, a return beam electron multiplier disposed around said cathode including a Cal first dynode comprising a cup-shaped member and a secondary electron emissive portion supported by said cup-shaped member, said secondary electron emissive portion having a beam aperture through which the beam from said electron gun emerges, said cup-shaped member having a wall and a first flange portion, a supporting tube substantially supporting the components of said electron gun and having a second flange portion extending therefrom secured to said first flange portion, said cup-shaped member having a plurality of apertures in said wall to permit outgassing of said electron gun Without utilizing said beam aperture, said supporting tube extending substantially parallel and spaced from said wall having said apertures therein to permit gas flow therebetween, said return beam multiplier also including a plurality of additional dynodes disposed around said supporting tube and having thereon a photosensitive electron emissive material shielded from radiation from said cathode by said supporting tube.

References Cited in the file of this patent UNITED STATES PATENTS 2,146,366 Batchelor Feb. 7, 1939 2,540,621 Johnson Feb. 6, 1951 2,802,133 Haas Aug. 6, 1957 FOREIGN PATENTS 725,693 Great Britain Mar. 9, 1955

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