US2913617A

US2913617A - Electron beam discharge device

Info

Publication number: US2913617A
Application number: US643622A
Authority: US
Inventors: Jewell G Tucker
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1957-03-04
Filing date: 1957-03-04
Publication date: 1959-11-17
Anticipated expiration: 1976-11-17

Description

Nov. 17, 41959` J. G. TUCKER ELECTRON BEAM DISCHARGE DEVICEv Filed March 4, 1957 FIG-.2.

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TARGET ELEOTRON SOURCE R M Rw Y N OT D m. o G WL T L WE B W H E J W m SOURCE ELECTRON l 2,913,617 Y ELEcrnoN BEAM DISCHARGE` DEvrcE Jewell G. Tucker, Owensboro, Ky., assignor `tofGfenel-al :Electric Company, a corporation of New Yorlr This invention relates to electron discharge devices of the electron beam type. c i

In many electron discharge devices the electron ow from the cathode` or electron source to the anode or target electrode is confined to a beam-likepath. Along this path are disposed an accelerating electrode, positively biased with resect to the cathode and situated between the-cathode and anode in a position to attract electrons from the cathode toward the anode, anda focus electrode, positioned between the accelerating electrode and cathode and biased at or near cathode potential for the purpose of converging electrons flowing from the cathode to the anode into a beam. Examples of such discharge devices are receiving tubesof the beam or sheet beam type and cathode-ray tubes. .Particularly at the higher frequencies, the quality of performance of such devices is determined to a large extent by the proportion of electrons leaving the cathode ,which arrive at the anode, and, where amplificationis desired and a control grid is employed, by the control Igrid-anode transconductance and the ratio of transconductanceto cathode current. Minimum electron flow from the cathode to electrodes'other than the anode is also important'where operation at a low noisefactor is desired.

Accordingly, a principal object of the present invention is to provide an electron discharge device of the electron beam type having improved performance in comparison with prior art devices, yet having a substantially simplified structural-form affording significant savings in manufacturing cost.` Y, i i

Another object is to provide an improved electron .discharge deviceof the beam 4type characterized by `low noise factor ope/ration', substantially increased` control grid-anode transconductance,` j and `a substantially increasedf'ratio of transconductance tocathodecurrent.i

.Another object is to provide, in an electron discharge device of the-electron beamftype having between the cathode and anode` a control grid, focusing electrode means, and laccelerating electrode means, an improved electrode structure enabling a substantial reduction in cathode-accelerator spacing without objectionable `electron flow to the accelerator means. i

These, and other objects of the present invention will be apparent from the following description taken in conjunction wit'h the accompanying drawing, and the scope of the invention will be defined in the appended claims.

l 2,913,617 Patented Nov. 17, 1959 voltage gradient, and focusing action suicient to give a well defined electron beam is maintained while the nonconducting covering on the accelerating electrode minimizes accelerator current for low noise operation. In another `form of the invention, the focus electrode and accelerating electrode are arranged to form the outer layers'of a sandwich of which the middle layer` is a sheet of electrically non-conductive material; thereby minimizing-accelerator-cathode spacing consistent with permissib le inter-electrode voltage gradient.

In the drawing: c

Fig. 1 is a partially broken away perspective View of an electron discharge device of the electron beam type constructed in accordance with one form of the present invention;

Fig. 2 is an enlarged fragmentary view of the device of Fig. 1, showing in transverse section the velectrode structure thereof, andpshowing schematically exemplary circuit connections thereof;-

. Fig. 3 is a view similar to Fig. 2, showing the electrode structure modified in accordance with another form of the invention; Fig. 4 is a diagrammatic view of another electrode arrangement to which the present, invention may be Fig. 5 is a `view similar'to Fig. 4 showing the applicaj tion of the present invention thereto.

Referring to Figs. 1 and 2, one embodiment of the invention is illustrated in connection with a discharge device of thefsheet beam typey wherein the electronilow from the cathode to the anode is confined to a beam having a relatively narrow horizontal dimension in comparison withV its vertical dimension. The discharge device of Fig. 1 includes anA enveloperZ enclosing a centrally located indirectly heated cathode-4. The cathode may have one ormore electron emissive surfaces, that shown having two in number, referenced as 6 and 8, and facing in opposite directions.` Surrounding the cathode in spaced relation -therewith is a control `grid 10 constituting of a somewhat flattened helix of spacedturns of newire wound on a pair of supporting rods 12, 14.A The remain- Briefly, according to one aspect of vthe invention, I provide an electron discharge device of the electron beam type in `which the separate focus electrode of the prior art is eliminated and its focusing action is suppliedby the accelerating electrode, the accelerating electrode being modified for this purpose by minimizing its spacing from ing electrode structure consistsof groups of electrodes, each disposed opposite one of the emissiveysurfaces` of the cathode. Each such group of electrodesincludes an anode 16, a planar focus electrode 18 having one or more electron windows orapertures 20 between the control grid 10 and anode 16 dimensioned according to the desired size and shape of the Velectron beam, anda planar acceleratorelectrode .22 situated between the focus electrode 18 andfanode 16 and having one or moreapertures ,24 drmensioned according to the desired sizeand rshape of the focused-electronv beam. :In thev discharge device shown, the anodes 16 arevfelectrically connected by a conductor shown 'schematicallyat 26 in Fig. 2, the acceleratorY electrodes 22 arelikewise connected by a conductor 28, and the focus electrodesflS are. likewise connected by:` conductor 30.' Theseveral electrodes are supported between insulating spacers` 32, 34 which maybe of mica, andwhich engage the ,wall ofenvelope 2. j,

Exemplary circuit connections for. the electrodes of the discharge device of Figl are best'shown in Fig. 2. The anodes 16 are connected to a source of suitable positive.D.C. potential 36. The accelerator electrodes 22K are biased at'or near anode supply potential byfa suit? able dropping resistor 38, wconnected betweenelectrodes 22 and source 36, and a bypass capacitor 40 connected between resistor 38 and ground. 'Ihe focus electrodes` 18 are biased at the potential of cathode 4, which is connected to ground through resistorc42. ControlV grid :10 receives signals through a coupling capacitor 44,V and a A 5 resister., 46i$ccnncted between .grid ,lofaml ground.. ance Without creating an objectionable inter-electrode In normal operation of the discharge device of Figs. 1

scribed, it is desirabler to have a high control grid-anode transconductance in order to providel adequate gain, and the anode, accelerating electrode and focus electrode currentsl should be as small as possible in order to achieve a low noise factor.Y Small focus and accelerating electrode currents have been difcult to obtain in discharge devices ofthe prior art, however, because the requirement for formation of an electron beam of the necessary definition, electron density, and homogeneity necessitates the placement of the electron-opaque portions ofk the focus electrode in the path of some of the electrons emanating from the cathode, and requires the edges of the accelerating electrode aperture 24 to closely surround the path of the electron beam, so that some electrons inevitably land on the focus and accelerating electrodes. Also, if the ,focus and

accelerator electrode apertures

20 and 24 are not in exact alignment, the accelerator will attract an abnormal number of electrons, or if the acceleration aperture 24 has a small burr, it will attract an abnormal number of electrons. This increase in accelerator current in turn increases the partition noise and lowers the grid-anode transconductance. Moreover, in prior art tubes the accelerating electrode and focus electrode are independently supported in spaced relation, and the transconductance is limited by the low vintensity of the accelerator electrode field at the cathode, which is in turn caused by the necessity of spacing the accelerating electrode a suflicient distance from the cathode to avoid excessive voltage gradients between control grid, focus electrode and accelerating electrode.

These limitations on improved performance are `avoided, according to one aspect of the present invention, by having each focus electrode- 18 and its associated accelerator electrode 22 of Figs. 1 and 2 constitute the outer layers of an integrated structure of sandwich construction, of which the center layer consists of a sheet or plate of electrically non-conductive material 50. Nonconductive sheet 50 is suitably apertured as at 48 to coincide with

electrodes

18 and 22, and preferably should have a high dielectric constant so that it may be quite thin. The sheet 50 may provide the principal mechanical support for the integrated sandwich structure, and may be, for example, a thin sheet of mica, while

electrodes

18 and 22 need not be self-supporting, and may be of metal foil or conductive paint clad or coated on layer 50. An important advantage of this construction is that the spacing of Vthe accelerating electrode 22 and cathode 4 is minimized because the interposition of the high dielectric layer 50 Vpermits accelerating electrode 22 and focus electrode 18 to be spaced much closer together, in comparison with prior'art devices, without an objectionably largeinter-electrode voltage gradient; The intensity of the accelerating electrode field at the cathode is thereby substantially increased, with Va corresponding increase in control grid-anode transconductance.

Further, in accordance with the invention, it has been discovered that an additional substantial increase in transconductance can be. obtained while maintaining good beam focusing action by eliminating altogether the conductive layer 18 which serves as the focus electrode in Fig. 2 and moving the resulting two-layer electrode structure 5.0, 22 as close to the cathode 4 as the limit of inter-electrode voltage gradient permits. The resulting structure is shown in Figure 3, which illustrates another form of the invention. In the discharge device of Fig. 3 the electrode structure disposed between the control grid and each anode 16 consists solely of a sheet or plate of electrically non-conductive material 64, provided with one or more suitable beam-forming apertures 62, and

faced on the surface thereof facing the anode with a layer of conductive material 60 forming the accelerating electrode. Sheets 60 and 64 may be mutually mechanically supporting, or either may have the necessary stiffness to provide suitable mechanical support for the other.

With respect to the electrode structure of Fig. 3, it is an important advantage that the spacing between the accelerating electrode 60 and cathode 4 is further reduced in comparison with Fig. 2 without creation of an excessive voltage gradient between electrodes, because the surface of the composite structurel 60, 64 which faces the cathode consists substantially exclusively of electrically non-conducting material 64. In one discharge device constructed according to the invention, for example, it was found that accelerating grid-cathode spacing could be reduced by as much as fifty percent, in comparison with corresponding prior art types. Accordingly, with such a construction, the control grid-anode transconductance is maximized. Surprisingly, the resulting significant increase in gain is achieved at substantially no cost in increased accelerating electrode current and noise factor, as measured by static performance figures. In one exemplary embodiment constructed according to the form of the invention shown in Fig. 3, for example,l the ratio of transconductance to cathode current as measured by static performance figures was of the order of 2000 micromhos per milliampere, Whereas in prior art beam-type discharge devices this ratio is normally less than 1000 micromhos per milliampere, The reason for this net gain in performance is not yet fully understood, but it is believed that the accelerating electrode 60, when positioned so close to the cathode, is able to provide some focusing effect as well as the desired accelerating effect, and it is also believed that under electron bombardment the non-conducting layer 64 charges to a negative potential, relative to the cathode, and thus exerts an effective focusing action on the electron cam. Accelerating electrode current is not objectionably increased apparently because the non-conducting layer 64 effectively shields the accelerating electrode 60 from electrons traveling from the cathode toward the anode, and once such electrons pass through the opening 62 in the accelerating electrode, they are traveling at suicient velocity and are formed into such a well-defined beam that very few, if any, deviate from the beam path enough to land on the accelerating electrode 60.

While the invention has been illustrated and explained in connection with an electron discharge device of the sheet beam type, it will be readily understood by those skilled in the art that the invention is equally applicable to other types of discharge devices, or in fact to any electrode arrangement wherein electrons are drawn from a source by an accelerating electrode past an intermediate focus electrode whose function it is to converge the electrons into a beam. Fig. 4 shows such a generalized environment, including an electron source 70, target 72, accelerating electrode 74 having one or more apertures 75, and spaced focus electrode 76 having one or more beam forming apertures 80, while the application of the presentl invention to such an environment is shown in Fig. 5. Inl the structure of Fig. 4 the focus electrode 76 and accelerating electrode 74 are separate and independent, the accelerating electrode is maintained by a bias supply, illustrated at potentiometer 78, at a substantially higher potential than the focus electrode 76 so as'to create the necessary electrostatic field therebetween for good focusing of electrons passing through the beam forming aperture 80, and the two

electrodes

74, 76 are spaced apart a suliicient distance to avoid the establishment of an excessive voltage gradient therebetween. In the structure of Fig. 5, however, in accordance with the present invention, the separate focus electrode 76 is completely eliminated and the accelerating electrode is faced on its surface facing the electron source 70 with a layer of electrically non-conductive material 82. The nonconductive surface 82 enables the accelerating electrode 74 and electron source 70 to be spaced much closer without creating an excessive voltage gradient between them, thereby maximizing the intensity of the accelerating field at the electron source 70. Also, like llayer 64 in Fig. 3, non-conducting layer 82 effectively focuses the electrons passing through aperture 75 into a well-defined beam of desired homogeneity and density, yet the shielding action of non-conducting layer 82 minimizes current to the accelerating electrode 74.

It will be appreciated by .those skilled in the art that the invention may be carried out in various ways and may take various forms and embodiments other than those illustrative embodiments heretofore described. It is to be understood, therefore, that the scope of the invention is not limited by the details of the foregoing description, but will be defined in the following claims.

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

l. In an electron discharge device of the electron beam type, an electron source, an electron target, and a composite focusing and -accelerating electrode disposed between the source and target, said composite electrode comprising a sheet of electrically conducting material situated in and transversely disposed with respect to the path of electron fiow from the source to the target, said sheet of conducting material having a single aperture therein through which electrons are adapted to flow and adapted to be maintained at an electron accelerating potential with respect to the source, and a covering of non-conducting material on the side of said conducting sheet facing said source, said covering extending over the entire surface of said sheet of conducting material facing said cathode and having a single aperture the periphery of which is coincident with the periphery of the aperture in the sheet of conducting material.

2. In an electron discharge device of the electron beam type, a cathode, an anode, and a composite focus and accelerating electrode disposed between the cathode and anode, said composite electrode comprising a sandwich consisting of a sheet of electrically non-conducting material situated in and` transversely disposed with respect to the path of electron fiow from the cathode to the anode, said sheet of non-conducting material having an aperture therein through 'which electrons are adapted to iiow, and a layer of electrically conducting material on the surface of said non-conducting sheet facing said anode said non-conducting material covering all portions of the surface of said conductive layer facing said cathode, said layer of `conducting material being adapted to be maintained at an electron accelerating potential with re* spect to the cathode. y

3. In an electron discharge device of the electron beam type, a cathode, an anode, an accelerating electrode comprising a sheet of electrically conductive material transversely disposed between the cathode and anode and adapted to be maintained at a positive potential with respect to the cathode for promoting electron flow from the cathode to the anode, said accelerating electrode having an aperture through which electrons flowing from the cathode to the anode are adapted to pass, and a layer of electrically non-conducting material covering only the entire surface of said sheet of conductive material facing said cathode, said non-conducting material extending to coincidence with the periphery of said aperture being adapted to build up an electron charge responsive to bornbardment by electrons emitted from said cathode, whereby the electrostatic field associated with said charge in the vicinity of said aperture in said accelerating electrode exerts a converging and focusing influence-forming electron flowing through said aperture into a beam.

4. In an electron discharge device of the electron beam type, a cathode, an anode, and a composite electrode for accelerating and focusing electron flow from said cathode of said anode, said composite electrode comprising a sheet of electrically non-conducting material situated in and transversely disposed with respect to the path of electron flow between the cathode and anode and having a single aperture through which electrons may pass, and a layer of conductive material on each face of said sheet of non-conducting material, each layer of conductive material having a single aperture coincident with the single aperture in said non-conducting sheet and coincidently covering the entire surface of said non-conducting sheet about the aperture therein.

5. In an electron discharge device of the electron beam type, a cathode, an anode, a control grid between the cathode and anode, and a composite focus and accelerating electrode disposed between the control grid and anode, said composite electrode comprising a sheet of electrically non-conducting material situated in and transversely disposed with respect to the path of electron fow from the cathode to the anode and having an aperture therein through which electrons are adapted to flow, a

Y first layer of conducting material on the side of said nonconducting sheet facing said anode adapted to be maintained at an electron accelerating potential with respect to the cathode, said non-conducting material masking the entire surface of said first layer facing said cathode and said non-conducting material and first layer having coincident apertures, and a second layer of conducting material on the side of said non-conducting sheet facing said cathode adapted to be maintained at a potential less than said first layer.

6. An electron discharge device comprising an envelope, a pair of electrically non-conducting discs supported within the envelope in spaced generally parallel relation, a cathode supported between the discs having an electron emitting surface facing in a direction generally parallel to the plane of the discs, an anode supported between the discs and spaced opposite the cathode, a con- Y trol grid supported between the discs situated between the cathode and anode, a plate of electrically non-conducting material supported between the control grid and anode, said plate having an aperture through which electrons are adapted to ow from the cathode to the anode, and a layer of electrically conductive material extending outwardly from the periphery of said aperture on the surface of said plate facing said anode and adapted to be maintained at an electron accelerating potential relative to said cathode, said layer of conductive material being disposed in coincident relation on the entire surface of said plate of non-conducting material and all portions of the surface of said conductive layer facing said cathode being covered by said plate of non-conducting material.

7. An electron discharge device comprising an envelope, a pair of electrically non-conducting discs supported within the envelope in spaced generally parallel relation, a cathode supported between the discs having an electron emitting surface facing in a direction generally parallel to the plane of the discs, an anode supported between the discs and spaced opposite the cathode, a control grid supported between'the discs situated between the cathode and anode, a plate of electrically non-conducting material supported between the discs and situated between the control grid and anode, said plate having an aperture through which electrons are adapted to flow from the cathode to the anode, a first layer of electrically conductive material on the surface of said plate facing said anode and adapted to be maintained at an electron accelerating potential relative to said cathode, and a second layer of electrically conductive material on the surface of said plate facing said cathode, said non-conducting material covering all ,portions of the surface of said first v conductive layer facing the cathode, said rst and second layers of conductive material each having an aperture 7 8 the periphery of which is coincident with the periphery of 2,686,885 Bailin Aug. 17, 1954 the aperture in the plate of non-conducting material. 2,712,087 'Fite et al. June 28, 1955 '2,735,032 Bradley Feb. 14, 1956 References Cited in the le of this patent 2,777,084 Laerty I an. 8, 1957 UNITED STATES PATENTS 5 FOREIGN PATENTS 2,455,851 Beggs Dec. 7, 1948 707,064 Great Britain Apr. 14, 1954