US2932754A - Electron tubes - Google Patents

Description

April 12, 1960 J. H. o. HARRIES ETAL 2,932,754

ELECTRON TUBES Filed July 30, 1957 2 Sheets-Sheet 1 FIG. 2

INVENTORS JOHN HENRY OWEN HARRIES BARBARA K. HARRIES AGENT April 12, 1960 HARR'ES 2,932,754

ELECTRON TUBES Filed July 30, 1957 2 Sheets-Sheet 2 FIG. 6

INVENTORS. |a JOHN HENRY OWEN HARRIES FIG 5 BARBARA K. HARRIES AGENT ELECTRON TUBES i Application July 30, 1957, Serial No. 675,080

' 5 Claims. Cl. 313-69) This invention relates to improvements in beam-forming electron tubes and has particular reference to pulsed beam tubes having novel electrode structures. 7

In conventional four-electrode tubes it it well known that the wires of a control grid intercept a sufficient amount of the electron discharge from the cathode as to cause undesirable grid current and resultant heating of the control grid and its supporting structure. Known previous attempts to overcome this problem have been based on knowledge that .the control grid wires are bombarded by electrons traveling directly from the cathode to the control grid. 7 Wehave discovered that grid current is caused not alone by such direct bombardment of the grid wires but also by the return of electronsafter they have passed thegrid wires. In fact we find that the primary cause of grid current in beamed triodes having screening grids at cathode potential, or screening grids whose potential with respect to the cathode is substantially less than the control grid-to-cathode potential during the conductive part of the cycle, is due to returning electrons rather than to direct bombardment. The present invention is directed to means for modifying the shape of the electric field in the cathode-anode space whereby the electron trajectories which were previously turned back toward the control grid are instead caused to be directed toward the anode. This is accomplished by providing the screen grid wires with radially extending fin-like cross-sectional shapes which not only direct the electrons from the cathode to the anode in the desired manner but also maintain a high value of mu and reduce the screen grid to anode capacity.

The present invention also relates to such a screen grid in combination with a cathode which is shaped to focus streams of electrons into beams and a control grid hava ing effective portions positioned relatively close to the cathode and as close to'the sides of the electron beams as is compatible with the beams not being directly intercepted by the control grid, with the radial fin-type screen grid being maintained at or near cathode potential and so spaced from the control grid and the anode that the screening factor or mu of the control grid is relatively high, and so that sufiicient positive field from the anode penetrates between the two grids so as to draw the electrons toward the anode when the anode potential is in the range of one to three times the maximum positive potential at which the control grid is driven during operation of the tube. p v Accordingly, a primary object of our invention is to provide an electron tube with a novel electrode structure which substantially reduces or eliminates undesirable grid current.

Another object is the provision in an electron tube of a screen grid having a predetermined shape which will form electric fields therearound which substantially reduce the turning back to the control grid of electrons "emitted by the cathode. I

Another object is to provide an electron tube with a novel screen grid which will not only increase the flow of electrons tom the cathode to the anode but will also maintain a high value of mu and reduce the screen gridto-anode capacity.

A further object is to provide an eificient beamed triode having an additional novel screen grid of the above character.

Other objects and advantages of the invention will become apparent from the following description taken" in connection with the accompanying drawings, wherein:

. Fig. l is an axial sectional view of the electrode structures of an electron tube embodying a preferred form of the invention;

Fig. 2 is a horizontal sectional view taken substantially on line 2--Z of Fig. 1;

Fig. 3 is an enlarged fragmentary horizontal sectional view of a portion of the electrode structure;

' Fig. 4 is a view similar to Fig. 3 illustrating a modified cathode structure;

Fig. 5 is a diagrammatic view illustrating the trapping of electrons by conventional grid structures and the prevention thereof by the present invention; and

Fig. 6 is a chart listingthe various spacings and sizes of electrode components in one successful tube embodying the invention. 7

Referring more particularly to the drawings, the electrode structures shownin Fig. l are adapted to be suitably mounted within an electron tube such as that shown and described in U.S. application Serial No. 690,356, filed October 15, 1957, and assigned to the common assignee, and for details of such tube structures as are not shown in the present drawings reference should be made to the copending application.

The electrode structures embodying the present invention include a cathode structure 10, a control grid structure 1 1, a screen grid structure 12 and an anode structure 13, all of which are coaxial structures mounted one within another as shown and suitably supported sojas to maintain their respective predetermined space relations. The cathode structure embodies a hollow cylindrical cathode 14 within which is located a heating coil or filameat 15 (Fig. 2). The filament 15 is suitably connected to a source of electrical energy whereby it will radiantly heat the cathode 14 for liberation of electrons, as will be described. The outer surface of the cathode 14 is provided with a plurality of longitudinal grooves 16 which are spaced substantially equidistant apart throughout the circumference of the cathode. The side walls 17 of the grooves 16, in the preferred embodiment of this invention, extend substantially radially with respect to the longitudinal axis of the cathode, and the bottoms of the grooves 16 are preferably slightly concave, being curved in a transverse direction'as shown best in Fig. 2. Deposited upon the bottom of each groove 16 is a layer 18 of a selected electron emissive material such as one of the oxides commonly used for this purpose in the electron tube industry. When the cathode is heated and a suitable potential applied in the normal manner across the tube,

Opposite each land :19 between the respective grooves 16 is located a respective wire 20 of the control grid 11. The wires 20 extend parallel to the cathode and to each other and in predetermined spaced relation thereto, and are of circular cross-sectional shape of a diameter consistent with the widths of the lands 19 such that thewires 20 are as close to the sides of the electron beams emanating from the emissive material 18 in grooves 16 as is compatible with non-interceptionof the beams by the wires.

Radially spaced outwardly of each of the control grid wires 20 is a respective screen gn'd member or wire 21,

the wires 21 being located in predetermined spaced re latio'ns' to both the anode 13 and wires 20.

The anode 13 is a hollow cylindrical body of copper which enclosesthe grids and cathode. and forms in itself a large portionjof'tlie tube envelope. 7 1 The several electrodes are all supplied with various potentials by conductive means clearly shown and dc: scribed in the aforementioned copending application. The 'tube is adapted'to be operated byimpressingthe desired potential between thean'o'de and cathode, applying a relatively high positive potential to the control grid, and applying to the screen grid a zero potential with respect tothe cathode. 1 V 1 v V r Wehave found that'in an electron tube of thevcharacter described, thecross-sectional shape andfthe posinon of screen "grid 12, in relation to the other-electrodes and the anode, are very critical and assume particular importance in the consideration of the affect of the grids upon the trapping of electrons emitte'd'by the cathode.

Known prior attempts to reduce or eliminate grid current caused by interception of electrons by the control grid have generally consisted of various methods of pre venting direct bombardment of the control grid by electrons emitted by the cathode, such as by reducing the diameter of the control grid wires or removing the control grid wires far from the direct trajectories of the electrons. We have found that a very important cause of gridcurr'ent is mater trappedfelectrons, that is, electrons which have passed between -the Wires of the control grid and. areea searo be reversed back toward the control grid by the" affect of the screen grid upon the field between the grids. Fig. illustrates this undesirable :feature wherein a tube embodying conventional roundsectioned grid wires ,20 and 22 some of the electrons from material 18 will follow trajectories 23 and 23a until they approach a screen grid wire 22. Then the field around wire 22 will reverse the directions of the elec rrons, causing them to assume new trajectories 24 and 24k; whereupon they will collide with wire 20.

A tube of the presently described type is adapted. to be dperated by application to the control grid of a relatively. high positive potential, and to the screen grid of a zero potential with respect to the cathode. Therefore, electrons emitted by the material 18 in the cathode grooves 16 will be focused, under the influence of the side Walls 17 of the grooves, toward the anode 13. The control grid wires 20 are located just out of the direct trajectories ofthe electrons and, therefore, will not normally'intercept the electrons. The curvature of the emitting surface also aids in properly focusing the electron beam because of the fact that electrons leave an emitting surface substantially normal to the surface at the point of emission.

In this type of tube structure, the positive control grid normally will not intercept any primary electrons .until they have travelledpast this grid and have reversed their directions and been trapped. Therefore, the control grid Wires 20 may be conveniently of round crosssectional shape, either single wires or pairs of radially disposed and closely adjacent round-section wires.

We have found, however, that the screen grid wires 21- 'shouldbe provided with a particular predetermined cross sectional shape which will form a shaped electrical'field preventing a reversal of manyof the electron trajectories.

A flat or fin-like-shape has proved to be very successful for this purpose. With a fin-shaped wire 21', many of the electron trajectories'will not be reversed as in the case of a roundsection wire, but instead will be per- -.mitted to continue on toward the anode, as indicated by the portions of the trajectories designated 25 and 25a I in Fig. 5.

-It is, of course, necessary that the fin-shaped, grid wires 21 be strategically spaced from the control grid and from the anode, depending upon the potentials applied to these electrodes. ,The fin-shaped wires 21 should ex- .'4'- tend radially with respect to the center of the electrode structure, with one thintedge of each wire being directed toward the axis of the coaxial electrode structure. With such a structure, slight geometrical deviations as may occur in the manufacture of the tube will not cause significant ill effects such as are created upon even slight adjustments of round-sectioned wires. These considerations are based upon a static mu in the order of 100. A large increase of static mu will reduce anode to oath: ode potential gradient in areas between pairs of grid WiresZl and increase defocusing in these areas, thus tending to increase the possibility of trapping of electrons. Lower values of mu will, however, render trapping a less difiicult problem;

Since the use of fin-shaped' wires in the screen allows modifications in the sizes of other electrodes without resulting in changing the trajectories of the electrons emitted by thecathode, we have found that the diam- 16.1 of the control grid Wires may be increased "and the inside radius of the anode 13 may be decreasedgpro: vided that the potentials of these electrodes are reduced 'to a predetermined figureJ This results in a reduction 'walls 17 of grooves 16, in order to properly direct electrons so that they may be influenced by screen ,grid 12 in the proper manner. However, thejactual focusing of the electrons may be accomplished by other cathode structures or shapes such as shown,'for example, in Fig. '4

wherein the longitudinal grooves 26 in cathode 27are transversely curved, with the curvature extending tosubs'tantially the outermost surface 28. The emitting material 29 is restricted to only the deeper portions of each. groove 26 so that the uncovered or non-emitting portions 30 of the grooves will function to focus the 'elec" trons in a manner similar to the function of side Wall 17 in Fig. 3. The lands31 between grooves 26'may be provided. with shallow transversely curved parallel longitudinal grooves which allow control grid wires 20 to'be advantageously positioned closer to the cathode.

While the use of radial fin-type grid wires in various kinds of multi-grid tube structures is well known per se,

we have found that tin-shaped wires may be successfully used in a screen grid for reducing trapping of electrons and reversal of the trajectories thereof toward a control grid in a tube having a high mu, when combined with a properly'focusing cathode as taught herein. It is to be understood, however, that various modifications may bezmade within the scope of the'invention.

Therefore, the details shown and described are to'be interpreted. as "illustrative. v 7

Referring particularly to Figs. 4 and 6, tubes have been satisfactorily made and operated when using an anode having an inside radius (a) of 1.440, a fin-shaped screen grid the wires of which are .020" thick and (0)1060" wide, and a control grid whose wires are (e)..07() in diameter. The cathode lands are grooved to a depth of (g.).-.0l5" and the, control 7 grid wires are located (f) [Ol l v from the radius of the high points of the lands. The screen grid wires are (d) .025" from the control grid wires and are (b) .200" from the anode. The emitting groove of the cathode is (k) .020 deep and (.12) .673

wide, with'emitting material therein being (j) .QSSTwide:

These tubes have a mu of between and 100. It is pointed out here that the above dimensions are to he considered as approximate.

.We claim: V 1 1. In an-electron tube, electrode structures comprising a hollow'anode, a cylindrical cathode coaxial within the anode and having on its outer surface a plurality'of alternately located electron emittingian'd men-emitting areas and means for focusing electrons from the emitting areas into beams directed toward the anode, a control grid comprised of wires disposed adjacent to the nonemitting areas of the cathode, said cathode and anode being operable at different known potentials and the control grid being operable at a relatively high positive potential, and screen grid means located in predetermined spaced relation between the anode and control grid and comprised of members radially aligned with the wires of the control grid, said members being maintained during operation of the tube at substantially cathode potential and having cross-sectional shapes forming electric fields therearound which aid electron flow to the anode and prevent substantial trapping of electrons between grids.

2. In an electron tube, electrode structures comprising a hollow anode, a cylindrical cathode coaxial within the anode and having on its outer surface a plurality of alternately located electron emitting and non-emitting areas and means for focusing electrons from the emitting areas into beams directed toward the anode, a control grid comprised of wires disposed adjacent to the non-emitting areas of the cathode, said cathode and anode being operable at different known potentials and the control grid being operable at a relatively high positive potential, and a screen grid located in predetermined spaced relation between the anode and control grid and comprised of members radially aligned with the wires of the control grid, said members being adapted to be maintained during operation of the tube at substantially cathode potential and being ribbonlike in shape and extending parallel with the wires of the control grid and adapted at said potential to form electric fields therearound which aid electron flow to the anode and prevent substantial trapping of electrons between grids.

3. In an electron tube, electrode structures comprising a hollow anode, a cylindrical cathode coaxial within the anode and having on its outer surface a plurality of longitudinally extending parallel grooves separated by lands, the grooves containing supplies of heat effective electron emissive material, the lands being non-emissive, the sides of the grooves being substantially free of emissive material so as to aid in focusing electrons from the emissive material into beams directed toward the anode, a control grid comprised of wires disposed adjacent and parallel to the lands of the cathode, the wires being round-see tioned and of a diameter consistent with the widths of the lands such that the wires are as close to the sides of the electron beams as is compatible with non-interception of the beams by the wires, said cathode and anode being operable at different known potentials and the control grid being operable at a relatively high positive potential, and a screen grid comprised of members radially aligned with the wires of the control grid and adapted to o'perate at substantially the same potential as the cathode and having thin fin-like cross-sectional shapes forming at said potential electric fields therearound which aid electron flow to the anode and prevent substantial trapping of electrons between grids.

4. In an electron tube, electrode structures comprising a hollow anode having an inner surface of predetermined radius, a cylindrical cathode coaxial within the anode and having on its outer surface a plurality of longitudinally extending parallel grooves separated by lands, the bottoms of the grooves containing electron-emissive material and the lands being non-emissive, the sides of the grooves being substantially free of emissive material so as to aid in focusing electrons from the emissive material into beams directed towardthe anode, a control grid co'rnprised of wires disposed adjacent and parallel to the lands 1 the cathode, the wires being round-sectioned and of a diameter consistent with the sizes of the non-emitting areas of the cathode such that the wires are as close to the sides of the electron beams as is compatible with non-interception of the beams by the wires, said cathode and anode being operable at different known potentials and the control grid being operable at a relatively high positive potential, and a screen grid adapted to be maintained during operation of the tube at substantially the same potcntial as the cathode and comprised of ribbonlike members located radially parallel with the wires of the control grid and having one of their thin edges directed toward the axis of the structure and thereby having their sides extending radially of the axis so as to form at said potential electric fields therearound which aid electron flow to the anode and prevent substantial interception and trapping of electrons between grids.

5. In an electron tube, electrode structures comprising a hollow anode having an inner surface of predetermined radius, a cylindrical cathode coaxial within the anode and having on its outer surface a plurality of axially extending parallel grooves separated by lands, the bottoms of the grooves containing electron emissive material and the lands being non-emissive, the sides of the grooves being free of emissive material and having their surfaces extending radially with respect to the axis of the structure to aid in the proper focusing of electrons from the emissive material into beams directed toward the anode. a control grid comprised of wires disposed adjacent and parallel to the lands, the wires being round-sectioned and of a diameter consistent with the sizes of the non-emitting areas of the cathode such that the wires are as close to the sides of the electron beams as is compatible with noninterception of the beams by the wires, said cathode and anode being operable at different known potentials and the control grid being operable at a relatively high po'sitive potential, and a screen grid adapted to operate at substantially the same potential as the cathode comprised of ribbonlike members located radially parallel with the wires of the control grid and having their flat side surfaces extending radially with respect to the axis of the structure and parallel to the surfaces of the sides of the grooves and forming at said potential electric fields therearound which aid electron flow to the anode and prevent substantial interception and trapping of electrons between grids.

References Cited in the file of this patent UNITED STATES PATENTS 2,058,878 Holst et al. Oct. 27, 1936 2,070,816 Von Wedel Feb. 16, 1937 2,130,280 Knoll Sept. 13, 1938 2,494,670 Rajchman Ian. 17, 1950 2,512,858 Hegbar June 27, 1950

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