US3917973A

US3917973A - Electron tube duplex grid structure

Info

Publication number: US3917973A
Application number: US487406A
Authority: US
Inventors: Jules S Needle; William H Sain
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1974-07-10
Filing date: 1974-07-10
Publication date: 1975-11-04
Anticipated expiration: 1992-11-04
Also published as: FR2278154A1; CH587564A5; JPS5132172A; GB1464201A; NL7508140A; DE2528562A1; JPS5812971B2

Abstract

A gridded electron tube suitable for generating high power at high frequency, has a cathode surface composed of a set of parallel emitting strips separated by non-emitting strips. Massive grid bars are aligned over the non-emitting strips, thereby reducing grid current interception and providing thermal dissipation. Attached to the side of the grid bars facing the cathode is an array of closely spaced fine wires which increase the electric field due to the grid at the emitting surface, and decrease the penetration of field due to the anode, thereby increasing the transconductance and amplification factor, decreasing the transit time of electrons, and improving the uniformity of emission current density.

Description

United States Patent [1 1 Needle et al.

[ Nov. 4, 1975 1 ELECTRON TUBE DUPLEX GRID STRUCTURE [75] Inventors: Jules S. Needle, Palo Alto; William H. Sain, Belmont, both of Calif,

[73] Assignee: Varian Associates, Palo Alto, Calif.

[22] Filed: July 10, 1974 [21] App]. No: 487,406

[52] US. Cl. 313/337; 313/348, 313/349; 313/350 [51] Int. Cl. HOlJ l/20;HO1.1 19/14 [58] Field of Search .1 313/337, 338, 348, 349, 313/350, 302

[56] References Cited UNITED STATES PATENTS 2,844,752 7/1958 Hoover 313/338 X 2,932,754 4/1960 Harries et a1. 313/302 X 3,562,576 2/1971 Rusterholz 313/302 3,573,535 4/1971 Hughes 313/350 3,725,717 4/1973 Leliovsky et a1 313/302 Lee et a1 313/348 Sain 313/338 ABSTRACT A gridded electron tube suitable for generating high power at high frequency, has a cathode surface composed of a set of parallel emitting strips separated by non-emitting strips, Massive grid bars are aligned over the non-emitting strips, thereby reducing grid current interception and providing thermal dissipation. Attached to the side of the grid bars facing the cathode is an array of closely spaced fine wires which increase the electric field due to the grid at the emitting surface, and decrease the penetration of field due to the anode, thereby increasing the transconductance and amplification factor, decreasing the transit time of electrons, and improving the uniformity of emission current density.

9 Claims, 12 Drawing Figures Patent Nov. 4, 1975 Sheet 1 of 3 FIG. I

U.S. Patent Nov. 4, 1975 Sheet 3 of 3 3,917,973

FIG. 9b

FIG. 9a

PRIOR ART POSITION 0N EMITTER STRIP Ewzma Z0625 Ewzma 2052 5 FIG. lOb

FIG. IOCI PRIOR ART ELECTRON TUBE DUPLEX GRID STRUCTURE FIELD OF THE INVENTION The invention pertains to gridded electron tubes such as widely used to generate radio frequency power. In high power, high frequency tubes the control grid next to the cathode is driven to a voltage positive with respect to the cathode for at least a portion of the radio frequency cycle. The grid can then collect electrons and become heated by their bombardment energy, causing impaired performance or possible failure by melting. Various tube characteristics are dependent on the grid structure. The amplification factor increases with the size of the grid wires and the ratio of their spacing from the cathode to their mutual spacing. Transconductance increases and electron transit time decreases with a decrease in cathodeto-grid spacing. In a tube design, these characteristics must be compromised with the mechanical and thermal limitations of the grid structure.

PRIOR ART A number of techniques have been used to reduce the harmful effects of grid current interception while maintaining other desirable characteristics of a gridded tube. A widely used technique is to inhibit the emission of the areas of the cathode directly under the grid wires, from which areas most of the electrons intercepted by the grid normally would arise. US. Pat. No. 3,814,972 issued June 4, I974 to william Sain describes a structure in which the non-emissive areas are simply not coated with the active material used for the emissive areas. US. Pat. No. 2,544,664 issued Mar. 13, l95l to LP. Garner et al. describes a structure in which the parts of the cathode electrode beneath the grid wires are cooled while the emissive parts are heated.

In these prior art structures the grid wires are off to the sides of the emitting areas and the streams of electrons from them. A positive grid voltage thus tends to produce less electric field at the centers of the emitting areas than at the side, ofthese areas which are closer to the grid elements. Also, a positive grid voltage tends to divert electrons from the intended path between the wires and toward the wires themselves.

The structure described by Garner et al is enormously complex and expensive since each emitting element must be individually supported, heated and insulated. Also, in this type of structure, there is an inherent compromise between the grid interception and the amplification factor and the transconductance. To achieve high amplification factor, the grid wires should be close together compared to the grid-cathode spacing, implying that the wires must be thinner to maintain the ratio of open passages for the electron flow. Also, as the wires are made closer it becomes harder to focus the electrons through the openings, so increased energy is dissipated on the finer wires and consequently they overheat. On the other hand, high transconductance is achieved by having the grid wires close to the cathode. If this is done without decreasing their mutual spacing, the amplification factor decreases and the increase in transconductance is very limited. If one scales down in geometric ratio the wire size. mutual spacing and cathode-grid spacing, the amplification factor remains relatively constant and the transconductance increases. Also, the electron transit time through the grid struc ture is reduced and the high frequency performance of the tube is thereby improved. However, a point is soon reached where the mechanical tolerances of construction and the lack of rigidity and thermal dissipation ability of the fine wires make the structure impractical. The aforementioned Sain patent discloses a method of overcoming some of these problems. By making the grid elements as strips elongated perpendicular to the cathode instead of round wires, their heat dissipation and rigidity are increased, with no appreciable increase in intercepted current because the area exposed to the cathode is not increased. Furthermore. the amplification factor, determined by the percentage if electric field from the anode which penetrates through the grid to the cathode, increases. Left unsolved were the prob lems of transconductance vs. grid size. the divergence of the electron stream by the positive grid potential, and nonuniform emission.

SUMMARY OF THE INVENTION An objective of the present invention is to provide a means for achieving the electrical benefits of a close spaced grid while maintaining the mechanical and thermal advantages of a coarse grid. A further objective is to provide a higher amplification factor than is practical with prior-art grids. A further objective is to provide uniform emission from the emitting areas. A further objective is to provide improved focusing of electron streams between the primary grid elements. These objectives are achieved by aligning massive grid bars in front of non emitting areas on the cathode and joining the bars on the side facing the cathode by a secondary array of closely spaced, fine grid wires. These fine wires may be close to the cathode because they are rigidly supported at frequent intervals by the massive bars which also cool them by thermal conduction. The added interception by the secondary grid is countered by the ability to make the wires exceedingly fine and by the low positive grid voltage needed to draw current. Also, the improved electron optics reduces interception by the massive primary grid bars. The amplification factor increases approximately as the product of the amplification factors of the two grids so that great isolation of input and output voltages is achieved.

The principles and practical realization of the invention will be illustrated with reference to the embodiments in the figures.

BRIEF DESCRIPTION OF THE FIGURES FIG. I shows, partly in section through the tube axis, a cylindrical triode tube embodying the features of the invention.

FIG. 2 shows in perspective a portion of the grid structure of the tube in FIG. 1.

FIG. 3 shows in perspective an alternative grid structure according to the invention.

FIG. 4 is a partial section of FIG. I perpendicular to the tube axis.

FIG. 5 is an enlarged section of a portion of the cathode and grid region as indicated by line 55 in FIG. 4.

FIG. 6 is a view similar to FIG. 5 but showing an alternative structure of the cathode.

FIG. 7 shows a section of the cathode and grid region of a planar tube embodying the invention.

FIG. 8 shows a section of a tetrode tube embodying the invention.

FIG. 9a is an illustration of the electric field pattern in the grid region of a prior art tube.

3 FIG. 9b is the field pattern in a tube embodying the invention.

FIG. 10a is an illustration of the pattern of emission density from the cathode of a prior art tube.

FIG. 10b is the emission density pattern ofa tube embodying the invention.

DESCRIPTION OF THE INVENTION In FIG. 1 is shown a practical embodiment of the invention in a cylindrical triode tube. The tube has a vacuum tight envelope consisting of; a metallic cup 10, as of copper, a portion 11 of whose inner surface serves as the tube anode, a sealed-off exhaust tubulation 12,

concentric insulators

13, 14, 15, 16, as of ceramic, isolating the electrical connections to the electrodes, and a series of thin metallic flanges 17, l8, 19, 20, 21, 22, hermetically sealed to the insulators and to anode ll and heater lead-in 23.

A cooling jacket 24 surrounds anode cup 10 and is bonded to cup 10, as by soldering, to provide thermal conductivity. Final assembly of the tube is accomplished by sealing, as by welding, flange l9 bearing the anode subassembly to flange bearing the cathodegrid assembly. A spiral radiant heater 25 is connected at one end to heater lead-in 23 and at the other to flange 22 by a lead 26 supported by a ceramic insulator 27. A hollow cathode cylinder 28, as of nickel, is supported coaxially to heater 25 by a thin metallic heat dam 29, as of iron-nickel-cobalt alloy, mounted on flange 21. The upper end of cathode 28 is closed by two spaced metallic discs 30 having a reflecting heat shield 3l between them.

Coaxially spaced outside cathode 28 is a cylindrical grid structure 32 comprising massive axially directed metallic bars 33, preferably of a refractory metal such as molybdenum, uniformly spaced about the circumference. At their top ends the bars are joined, as by spot welding, to a flanged disc 34. A ceramic plug 35 passes through the center ofdisc 34 and cathode end discs 30 to maintain axial alignment. The bottom ends of bars 33 are joined to a flange 36 mounted on envelope flange 20.

On the inside of bars 33 facing cathode 28, a cylindrical woven mesh of fine metallic wires 37 as of tungsten is bonded to bars 33, as by diffusion brazing with a plated gold film. The mesh 37 is preferably arranged diagonally to bars 33 so that individual wires form ap proximate helices, allowing them to expand thermally without substantially deforming the structure.

FIG. 2 shows an inside view of a portion of the grid structure.

FIG. 3 is an alternative grid structure in which only one set of parallel helical wires 37' is used in place of the mesh 37. The multiple helix 37 presents a desirable smooth surface on a small scale but is harder to fabricate than the mesh structure 37.

FIG. 4 shows in detail the electrode structure of the tube of FIG. 1, and FIG. 5 shows an enlarged portion of the cathode and grid structures which are the essence of the present invention. The surface of cathode 28 facing grid structure 32 comprises axial strips 38 of emissive material, such as a mixed oxide of barium, strontium and calcium. Between emissive strips 38 and opposite grid bars 33 are strips 39 of non-emissive material, such as uncoated nickel. In the example shown. the emissive strips 38 are recessed below non-emissive strips 39 in a direction away from the grid 32. The re cessed structure provides improved focusing of the 4 electron beams 40 between bars 33 by the covergence of the electric field leaving the edges of emissive areas 38, corresponding to the converging electron trajecto ries 40.

Grid bars 33 are, in this example. elongated in cross section in the direction perpendicular to the cathode 28, whereby their rigidity and thermal capacity are greater than that of a round wire of the same width, but the intercepted current is not increased appreciably. Also, the elongated bars increase shielding of the cathode from the electric field of the anode l1 and thus increase the amplification factor of the tube.

FIG. 6 shows an alternative structure in which nonemissive strips 39' form a continuous smooth surface with emissive strips 38. This structure has more uniform emission density than that shown in FIG. 5 and may be easier to build.

P10. 7 shows an embodiment of the invention as a planar tube. It will be apparent to those skilled in the art that the invention concerns the detailed structure of the cathode-grid region and may be embodied in tubes having many different overall geometries, including planar and cylindrical.

FIG. 8 shows a tetrode incorporating the invention. Screen grid wires 41 are aligned with grid bars 33, between grid structure 32 and anode 11. The electron paths are focused between wires 41, so the screen grid collects very little current.

FIG. 9 shows the electric fleld lines in a small section of electrode structure. FIG. 9a corresponds to prior-art tubes, such as described in Sain in U.S. Pat. No. 3,814,972 referenced above. It is seen that, as electrons approach the grid from the cathode side, they experience a diverging transverse component of field 45 which will tend to defocus the electron beam. Leaving the grid on the anode side they experience a focusing transverse component 42, but by then some electrons may have been drawn to the grid bars 33. FIG. 9b illustrates the fields according to the present invention. Since the fine grid 37 forms a smooth, essentially equipotential surface, there is no divergent field and the electrons pass grid 37 flowing essentially perpendicular to it. Between grid bars 33 the field has only converging transverse field components 42'.

FIG. 10 illustrates the variation of emission density across the width of the emissive strips for a constant positive grid voltage. The emission current density is determined by the electric field. FIG. 10a shows the density variation in the prior art structure of FIG. 9a. The field is highest near the edges 43 of the emissive strip 38, because the edges are closer to the grid bars 33. At the center 44, where the emissive surface is far ther from the bars 33, the field and hence current density are lower. The non-uniform emission density is undesirable because the cathode must be run hot enough to supply the highest density, but the low density regions are not contributing fully to the useful current. With the structure according to the present invention as shown in FIG. 9b, the emission as illustrated in FIG. 10b is practically uniform across the strip because the grid is effectively a smooth surface. Also, the field strength is higher because the secondary grid 37 effectively covers the space between bars 33. Thus. the transconductance of the tube is increased and the electron transit time between cathode and grid is reduced.

As an example of the benefits of the present invention, a certain triode tube constructed according to its teachings has a cut-off amplification factor of 2000.

This factor is the ratio of the minimum negative grid voltage to the positive anode voltage which just stops emission current from the cathode. in a prior art triode similar in all respects except not having the fine mesh secondary grid array, the amplification factor was only 200. The improvement provides a tube of increased utility as an on-off"switch which can be controlled by a low grid voltage. In another use as a high frequency grounded-grid amplifier, the increased shielding of the cathodegrid space from the anode voltage eliminated harmful regenerative feedback which had produced distortion of the amplified signalv While the invention has been described as embodied in certain exemplary tubes, it is intended not to be limited to them but to cover any apparatus according to the following claims What is claimed is:

1. An electron tube comprising: a cathode. an anode, and at least one grid electrode therebetween; the sur face of said cathode facing said grid composed of areas of electron-emissive material alternating with intervening areas of relatively non-emissive material; said grid electrode comprising, a set of conducting bars spaced from said cathode surface and aligned adjacent said non-emissive areas, an array of conducting wires of size and spacing less than said bars. said wires crossing the 6 spacing between adjacent bars and joined electrically and mechanically to the area of said bars facing said cathode surface.

2. The apparatus of claim 1 wherein said emissive areas and said non-emissive areas are parallel strips.

3. The apparatus of claim I wherein corresponding points of said cathode areas lie in a plane.

4. The apparatus of claim I wherein corresponding points of said cathode areas lie on a right circular cylinder and said conducting bars extend parallel to the axis of said cylinder.

5. The apparatus of claim I wherein said emissive areas are recessed in said cathode surface. with respect to said non-emissive areas, in the direction opposite said grid.

6. The apparatus of claim I wherein the cross-section dimension of said bars is elongated in the direction perpendicular to said cathode surface.

7. The apparatus of claim I wherein said array of wires comprises a set of equally spaced wires.

8. The apparatus of claim 1 wherein said array of wires is woven mesh.

9. The apparatus of claim 1 wherein said wires in said array pass diagonally between said bars.

Claims

1. AN electron tube comprising: a cathode, an anode, and at least one grid electrode therebetween; the surface of said cathode facing said grid composed of areas of electron-emissive material alternating with intervening areas of relatively nonemissive material; said grid electrode comprising, a set of conducting bars spaced from said cathode surface and aligned adjacent said non-emissive areas, an array of conducting wires of size and spacing less than said bars, said wires crossing the spacing between adjacent bars and joined electrically and mechanically to the area of said bars facing said cathode surface.

3. The apparatus of claim 1 wherein corresponding points of said cathode areas lie in a plane.

4. The apparatus of claim 1 wherein corresponding points of said cathode areas lie on a right circular cylinder and said conducting bars extend parallel to the axis of said cylinder.

5. The apparatus of claim 1 wherein said emissive areas are recessed in said cathode surface, with respect to said non-emissive areas, in the direction opposite said grid.

6. The apparatus of claim 1 wherein the cross-section dimension of said bars is elongated in the direction perpendicular to said cathode surface.

7. The apparatus of claim 1 wherein said array of wires comprises a set of equally spaced wires.

8. The apparatus of claim 1 wherein said array of wires is woven mesh.