US3843902A

US3843902A - Gridded convergent flow electron gun

Info

Publication number: US3843902A
Application number: US00439674A
Authority: US
Inventors: G Miram; G Kuehne
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1972-08-24
Filing date: 1974-02-04
Publication date: 1974-10-22
Anticipated expiration: 1991-10-22

Abstract

A gridded convergent flow electron gun employs a dimpled oxide coated thermionic cathode emitter facing a centrally apertured accelerating anode. A multiapertured control grid is interposed in the space between the dimpled oxide coated cathode and the anode for pulsing the electron beam. A multiapertured shadow grid is disposed overlaying the emitting surface of the cathode emitter with the apertures of the shadow grid being in alignment and in registration with the respective dimpled areas of the emitter and the corresponding apertures in the control grid for projecting a multiplicity of non-intercepting convergent flow beamlets through the individual holes in the control grid. The shadow grid is placed in nominal contact with the cathode emitter and is made of a material having essentially the same coefficient of thermal expansion as the cathode. In a preferred embodiment, the cathode includes a nickel base member and the shadow grid is made of nickel.

Description

United States Patent Miram et al.

[451 Oct. 22, 1974 1 GRIDDED CONVERGENT FLOW ELECTRON GUN [75] lnventors: George V. Miram, Daly City;

Gerhard B. Kuehne, Santa Clara, both of Calif.

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

[22] Filed: Feb. 4, 1974 [21] Appl. No.: 439,674

Related [1.8. Application Data [63] Continuation of Ser. No. 283,432, Aug. 24, 1972,

abandoned.

[52] US. Cl 313/454, 313/299, 313/338, 313/346 R, 313/348, 313/D1G. 1 [51] Int. Cl. ..H01j 1/48, 1101] l/94,l-10lj 19/48 [58] Field of Search 313/69 R, 70 R, 82 R, 337, 313/338, 346 R, 346 DC, 348, DIG. 1, 299

[56] References Cited UNITED STATES PATENTS 3,389,290 6/1968 Yoshida et a1 313/337 3,484,645 12/1969 Drees 1 313/348 3,558,967 l/l97l Miram 315/35 3,567,989 3/1971 Koshizuka 313/346 X 3,594,885 7/1971 Miram et al. 313/337 X Primary Examiner-John Kominski Assistant ExaminerSiegfried H. Grimm Attorney, Agent, or Firm-Stanley Z. Cole; Robert K. Stoddard; Harry E. Aine [57] ABSTRACT A gridded convergent flow electron gun employs a dimpled oxide coated thermionic cathode emitter facing a centrally apertured accelerating anode. A multiapertured control grid is interposed in the space between the dimpled oxide coated cathode and the anode for pulsing the electron beam. A multiapertured shadow grid is disposed overlaying the emitting surface of the cathode emitter with the apertures of the shadow grid being in alignment and in registration with the respective dimpled areas of the emitter and the corresponding apertures in the control grid for projecting a multiplicity of non-intercepting convergent flow beamlets through the individual holes in the control grid. The shadow grid is placed in nominal contact with the cathode emitter and is made of a material having essentially the same coefficient of thermal expansion as the cathode. In a preferred embodiment, the cathode includes a nickel base member and the shadow grid is made of nickel.

3 Claims, 3 Drawing Figures PAYENTEBM 22 m4 .FIG.|'

BEAM TRAJECTORIES;

' LINES EOUIPOTENTIAL FIG.3

GRIDDED CONVERGENT FLOW ELECTRON GUN This is a continuation of application Ser. No. 283,432 filed Aug. 24, 1972, now abandoned.

DESCRIPTION OF THE PRIOR ART Heretofore, gridded convergent flow electron guns of the type employing a dimpled oxide coated thermionic cathode emitter have been proposed for use in switch tubes, RF tetrodes, pentodes, and for use in linear beam microwave tubes. It has been proposed in such guns to employ a multiapertured control grid with the centers of the apertures in the control grid aligned along the beam path with the centers of the individual dimpled portions of the cathode emitter to form a multiplicity of Pierce type guns for projecting a multiplicity of convergent flow beamlets through the control grid in a non-intercepting manner.

It has also been proposed to provide a shadow grid disposed on or just overlaying the emitting surface of the cathode emitter for suppression of emission from the web-like portion of the thermionic cathode emitting surface which is in registration with the similar web-like portion of the control grid to prevent unwanted interception of emission on the control grid. Heretofore these shadow grids have been made of tungsten or molybdenum.

It has been found that when such a shadow grid is disposed above the surface of the cathode emitter that it produces a deleterious effect on the electrostatic focusing of the individual beamlets such that unwanted crossover of the electron trajectories is obtained in focusing the individual beamlets through the corresponding apertures in the-control grid. This was overcome in the prior art by placing the shadow grid directly on the surface of the cathode emitter and brazing the grid in place. However, the oxide coating could not be applied to the oxide coated nickel substrate member until the brazing operation had been completed. As a result, oxide coating applied to the dimpled regions of the cathode spilled onto the grid thereby destroying the focusing effect of the shadow grid. Another problem with the use of molybdenum and tungsten shadow grids for a nickel oxide coated cathode was that the difference in thermal expansion between the shadow grid and the nickel cathode resulted in undesired warping of the grid structure and consequent defocusing of the individual beamlets.

Cleaning and leaving bare nickel islands between the dimpled portions of the cathode emitter, after coating of the emitting surface, generally reduced the amount of emission from the uncoated portion of the cathode emitter. However this eliminated the focusing effect of the shadow grid and because of the poorer focusing effect of the islands, approximately 2 percent beam interception was obtained on the control grid. For highpower applications, 2 percent interception is unacceptable. The prior art grid structure is disclosed and claimed in US. Pat. No. 3,558,967 issued Jan. 26, 1971 and assigned to the same assignee as the present invention.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved gridded convergent flow electron gun of the type employing a dimpled oxide coated thermionic cathode emitter.

In one feature of the present invention, a shadow grid made of a material having substantially the same coefficient of thermal expansion as that of the base member of the oxide coated dimpled cathode is disposed in nominal contact on the surface of the oxide coated cathode for focusing the individual beamlets through the corresponding apertures in the control grid.

In another feature of the present invention, the shadow grid and the cathode base member are both made of nickel.

Other features and advantages of the present inven tion will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal sectional view of an electron gun incorporating features of the present invention,

FIG. 2 is an enlarged view of a portion of the structure of FIG. 1, and

FIG. 3 is an enlarged view of a portion of the structure of FIG. 2 delineated by line 3-3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown an electron gun I incorporating features of the present invention. The electron gun I is generally of the type disclosed and claimed in US. Pat. No. 3,558,967 issued Jan. 26, 1971. The disclosure of this patent is hereby expressly incorporated by reference into the present description for a detailed description of the electron gun.

Briefly, the electron gun 1 includes a spherically concave nickel cathode emitter base member 2 with its axis of revolution axially aligned with a central aperture 3 in an accelerating anode electrode 4 which is axially spaced from the cathode base 2. The electron gun I is of the Pierce type to provide a convergent flow electron beam 5 which is projected through the aperture 3 in the anode 4 in a substantially non-intercepting manner.

A multiapertured control grid 6 is interposed in the region between the anode 4 and the cathode base member 2 in relatively close spacing, as of 0.039 inches, from a concave emitting surface 7 of the cathode base member 2 for controlling the flow of electrons from the cathode emitting surface 7 through the anode 4. In a typical example the control grid 6 is made of molybdenum or tungsten and the web of the grid has an axial thickness, as of 0.020 inches.

A centrally apertured focusing electrode 9 is disposed surrounding the periphery of the beam path 5 and is disposed between the anode 4 and cathode 2 to facilitate focusing of the beam 5 through the anode aperture 3. The focus electrode 9 is operated at cathode potential. The control grid includes a solid web portion at its outer periphery which is fixed as by brazing to one end of a cylindrical insulator 11 as of beryllia ceramic which in-turn is joined to the cathode focus electrode 9, as by brazing, to provide physical support and electrical isolation for the control grid relative to the focus electrode 9.

A concentric heat shield and heat choke structure 12 serves to support the cathode 2 from the focus electrode 9. More particularly, the heat shield structure 12 is joined at one end to the focus electrode 9 and at the other end to a cathode support sleeve 13- which has the cathode base member 2 affixed at one end thereof for closing off that end of the sleeve 13. A cathode heating element 14 is potted in an electrically insulative thermally conductive structure 15, as of ceramic, and affixed to the backside of the cathode base member 2 for heating the cathode base member 2 to its operating temperature, as of 800 C. The potted heater element is retained within the cathode sleeve 14 via a retaining wall and heat shield structure 16. v

Referring now to FIG. 2, the cathode emitter and grid structures are shown in greater detail. The cathode emitting surface 7 of the'cathode base member 2 is constituted of a multiplicity of dimpled regions 17 each being generally spherically concave and each having a radius of curvature substantially less than that of the composite cathode emitting surface 7. The individual concave cathode emitting surfaces 17 are coated first with sintered nickel powder and then with a suitable oxide emissive material, such as powders of barium and strontium oxides, in the conventional manner of forming an oxide coated cathode;

v The center of each of the individual lesser concave emitters 17 is aligned with the center of the corresponding aperture in the control grid 6 taken in the direction of the beam path. After the composite cathode emitting surface 7 has been oxide coated, the web-like islands between the individual concave emitting surfaces 17 are cleaned, as by brushing, of any oxide material therefrom.

. A shadow grid structure 18 which is made of a mate rial having the same coefficient of thermal expansion as that of the base member 2 is placed in nominal contact with the island web-like portions of the composite cathode ernitting surface 7 and affixed to the cathode emitter base member 2, as by spot welding at a number of places around the periphery of the shadow grid 18. The grid 18 may also be tacked to the concave face 7 of the base member 2 at a number of points, such as 6 or 7 points. Spot welding or tacking may be achieved by spot welding in the conventional manner or by laser beam welding.

In a typical example, the shadow grid 18 is made of nickel and the surface of the shadow grid 18 which faces the base member 2 is preferably polished to facilitate reflection of thermal energy back to the cathode base member 2. Thus, in a preferred embodiment, the shadow grid 18 has the same thermal expansion as that of the cathode base member 2 and is placed only in nominal contact with the emitting surface 7 of the cathode base member 2, such that a relatively poor thermally conductive joint is provided between the shadow grid 18 and the cathode base member 2.

a In operation, the cathode base member 2 is heated by heater 14 to approximately 800 C and the shadow grid 18 preferably operates between 50 and 100 C cooler than the cathode base member 2.

The thickness of the web of the shadow grid 18 is chosen, as shown in FIG. 3, to provide electrostatic focusing of the individual beamlets through the corresponding aperture in the control grid 6 in a substantially non-intercepting manner. The shadow grid 18 is non-emissive. This serves to shadow the web of the control grid. In a typical example, use of the nickel shadow grid reduces the unwanted interception on the control grid 6 to approximately 0.08 percent beam interception.

What is claimed is:

1. In an electron gun:

thermionic cathode emitter means having, a base member with a concave emitting surface, said concave emitting surface being constituted of a plurality of lesser individual concave oxide coated cathode emitter surfaces, said lesser oxide coated emitter surfaces being concave in each of two orthogonal directions withradii of curvature substantially less than that of said composite cathode emitter surface;

an accelerating electrode means having a central aperture of substantially smaller cross-sectional area than the area of said emitting surface of said cathode emitter, said central aperture being disposed in axial alignment with the center of said concave emitting surface of said cathode emitter for accelerating and converging a stream of electrons through said central aperture of said accelerating electrode into a unitary beam of electrons;

control grid means comprising a multiapertured concave control grid structure disposed overlaying the concave emitting surface of said cathode emitter with the centers of individual apertures in said control grid structure being disposed in alignment along the convergent beam path with the centers of individual lesser concave cathode emitting surfaces to form a plurality of individual convergent beam electron guns projecting the beam through said control grid in a plurality of nonintercepting beamlets which converge into the unitary beam after passage through said control grid;

shadow grid means comprising a second multiapertured grid structure of substantially identical hole pattern to that of said control grid structure, said shadow grid structure being carried from said cathode emitter and operated at the same potential as said cathode emitter for inhibiting thermionic emission from the underlying regions of said cathode emitter shadowed by said shadow grid and for focusing the lesser beamlets through the respective aligned apertures in said control grid structure, and said shadow grid being made of a materail having essentially the same coefficient of thermal expansion as that of said cathode emitter, said shadow grid being joined to the adjacent surface of said cathode emitter at a plurality of discrete points spaced over the surface of the cathode emitter, the remaining portions of said shadow grid being adjacent, but free of attachment to, said cathode emitter.

2. The apparatus of claim 1 wherein said shadow grid jacent lesser concave emitting surfaces, whereby a relatively poor thermally conductive path is provided between said shadow grid and said cathode emitter to allow said shadow grid to operate at a lower temperature than said cathode emitter in use.

Claims

1. In an electron gun: thermionic cathode emitter means having, a base member with a concave emitting surface, said concave emitting surface being constituted of a plurality of lesser individual concave oxide coated cathode emitter surfaces, said lesser oxide coated emitter surfaces being concave in each of two orthogonal directions with radii of curvature substantially less than that of said composite cathode emitter surface; an accelerating electrode means having a central aperture of substantially smaller cross-sectional area than the area of said emitting surface of said cathode emitter, said central aperture being disposed in axial alignment with the center of said concave emitting surface of said cathode emitter for accelerating and converging a stream of electrons through said central aperture of said accelerating electrode into a unitary beam of electrons; control grid means comprising a multiapertured concave control grid structure disposed overlaying the concave emitting surface of said cathode emitter with the centers of individual apertures in said control grid structure being disposed in alignment along the convergent beam path with the centers of individual lesser concave cathode emitting surfaces to form a plurality of individual convergent beam electron guns projecting the beam through said control grid in a plurality of nonintercepting beamlets which converge into the unitary beam after passage through said control grid; shadow grid means comprising a second multiapertured grid structure of substantially identical hole pattern to that of said control grid structure, said shadow grid structure being carried from said cathode emitter and operated at the same potential as said cathode emitter for inhibiting thermionic emission from the underlying regions of said cathode emitter shadowed by said shadow grid and for focusing the lesser beamlets through the respective aligned apertures in said control grid structure, and said shadow grid being made of a materail having essentially the same coefficient of thermal expansion as that of said cathode emitter, said shadow grid being joined to the adjacent surface of said cathode emitter at a plurality of discrete points spaced over the surface of the cathode emitter, the remaining portions of said shadow grid being adjacent, but free of attachment to, said cathode emitter.

2. The apparatus of claim 1 wherein said shadow grid and said cathode base members are made of nickel.

3. The apparatus of claim 2 wherein said remaining portions of said shadow grid are disposed in nominal contact with the concave surface of said cathode emitter in the web-like regions thereof defined between adjacent lesser concave emitting surfaces, whereby a relatively poor thermally conductive path is provided between said shadow grid and said cathode emitter to allow said shadow grid to operate at a lower temperature than said cathode emitter in use.