US2939028A - Electron gun for a cylindrical capacitor - Google Patents

Description

May 31, 1960 EJJ. COOK 2,939,028

ELECTRON GUN FOR A CYLINDRICAL CAPACITOR Filed Nov. 13, 1957 2 Sheets-Sheet 1 fr? ve n tor-.- 50! we rd J Coo/r,

by 7%.! M

His Attorngy.

May 31, 1960 E. J. COOK 2,939,028

ELECTRON GUN FOR A CYLINDRICAL CAPACITOR Filed Nov. 13, 1957 2 Sheets-Sheet 2 Zn ve 771.907: Edward J Cook,

Hi5 Attorney ELECTRON GUN FOR A CYLINDRICAL CAPACITOR Edward J. Cook, Scotia, N.Y., assignor to General Electric Company, a corporation of New York Filed Nov. 13, 1957, Ser. No. 696,125

6 Claims. (Cl. 313-84) drical capacitor devices such as traveling wave tubes.

A hollow beam should have constant interior and exterior radii so that it can be directed very close to the helical coil in a traveling wave tube where the electric field is strongest. But such a beam is difiicult to produce due to the mutual repulsion of the like charges on the electrons which tends to cause the beam to spread.

In an article entitled Axially Symmetric Electron Beam and Magnetic Field System, by L. A. Harris in the Proceedings of the Institute of Radio Engineers, p. 700, June 1952, a system was disclosed for eliminating the beam spread and thus for producing a stable flow by applying a strong electric field between the outer and inner cylinders of the cylindrical capacitor, which field overcomes any tendency for the beam to spread outward. The inner spreading due to the electric field and space charge forces was eliminated by rotating the electrons in cylindrical shells about the helix to produce centrifugal beam is passed through a magnetic field directed radially within the cylindrical capacitor at right angles to the beam. Preferably, this field imparts angular momentums to the electrons in a very short time so there is insufficient time for them to spread due to their mutual repulsion. Also, the field should be completely radial because axial components defocus the beam and thereby cause spreading. Further, it would be advantageous if the structure producing the radial field were completely external to the evacuated envelope for then it could be readily repaired and replaced if necessary.

Accordingly, an object of the present invention is to provide a cylindrical capacitor device with an improved electron gun.

Another object is to provide an improved electron gun for producing a stable flow of the hollow beam in a cylindrical capacitor device.

A further object of the present invention is the provi' sion of an electron gun for producing substantially planar equal angular momentums for all electrons of a hollow beam in a cylindrical capacitor device.

Still another object is to provide a cylindrical capacitor device in which the electrons obtain angular momentums in a very short time.

A still further object of the present invention is to provide a magnetic field structure for a cylindrical capacitor that produces a magnetic field having practically no axial components.

Another object is to provide a magnetic field structure that may be positioned on the exterior of the exacuated envelope of a cylindrical capacitor device.

I achieve these and other objects in one form of my invention by providing two magnetically-opposed, coaxial, cylindrical electromagnets on the exterior of an evacuated envelope ofa cylindrical capacitor containing an annular shaped cathode. The currents through the magnet windings are adjusted to immerse the cathode in a fringe field tangent thereto andorthogonal to the beam flow, which field produces the desired forces on the electrons.

The novel features believed characteristic of my invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawings in which:

Fig. 1 is a side view, partly in cross-section, of a traveling wave tube type cylindrical capacitor device embodying one form of my invention, and

Fig. 2 is an enlarged view of the electron gunCof Fig. 1.

In the several figures of the drawing, I have indicated corresponding elements by corresponding reference numerals to facilitate comparison. Now referring specifically to the embodiment illustrated in Fig. l I have illustrated a cylindrical capacitor of the traveling wave tube type comprising an electron gun section 7 in which a hollow electron beam is generated, a drift section 9 in which the electron beam interacts with the field of a slow wave structure, and an output section 11 in which the electrons from the beam are collected and the output signal derived.

.electrodes '16, and accelerated by an anode 18. If re-- quired, additional annular electrodes 20 can be placed outside the gun structure to deflect the beam to the appropirate entrance radius for drift section 9. Suitable operating potentials for electrode 16 and anode 18 are provided by circuits (not shown) connectedby leads 1 22 and 24, respectively, extending through insulators 26 and 28, respectively, in a conducting ring 30 that acts as an electrostatic shield for the section. -A non-magnetic end piece 32 mounts within an end portion of ring 30 to provide a seat for the magnetic structure.

The magnetic structure for the electron gun section 7 I comprises an inner cylindrical core 34 of magnetic material coaxial with an outer cylindrical core 36 of similar 7 material. A first winding 38 is wound between these two cores and a second winding 40 is wound around core 36 The energization of these windings and the resultant flux are controlled by means illustrated as variable resistors 42 and 44 connected to provide individual control of the current in the windings flowing from a battery 46 so that tion. The magnetic structure which is completed by end plate 48 mounts within end piece 32 and is held therein 1 by a set screw 50.

The drift section 9 of the cylindrical capacitor comprises a vacuum envelope 52 constructed from a conductive non-magnetic material that electrostatically shields the capacitor interior. An outer drift tube 54 of conductive material, which is coaxially mounted within envelope 52, is connected by a conductor 56 extending through an insulator 58 in. envelope 52 to an external circuit (not shown) that provides a. suitable operating 1 potential. A helical coil 66, mounted coaxially with and longitudinally adjacent to tube 54 to provide an input to the capacitor, is energized through a conductor 62 I extending through an insulator 64 in the envelope 52. A slow wave structure 66 which slows-the: axial velocity of an electromagnetic wave and which may be a helicalshaped conductor as is conventional with traveling wave tubes, is mounted along the axis of section 9. r I At the left end of drift section 9 an annular insulator 68 insulates andphysically'separates envelope 52 from Patented May 31, 1960- drift tube 54. Envelope '52 is secured by a mounting ring 70 bolted by bolts 72 to a plate 74 that also secures ring 30 of electron gun section 7.

The output section 11 of the cylinder capacitor includes an annular-shaped collector 76, positioned toremove the electrons from the capacitor. Also, an end plate 78 is mounted to a mounting plate 80 by bolts 8-2. A conductor 84, extending through an insulator 86 in plate 80, provides an outlet for electrons collected by collector 76. A coaxial cable 88, the interior conductor of which is joined to slow wave structure 66, guides the output voltage to a utilization circuit (not shown).

In the enlargement of Fig. 2 the only additional struc tures not shown in Fig. l are sources of potential 90 that provide potentials for some of the electrodes and for the outer drift tube 54. Of course these sources of potential need not be separate, but instead could be a voltage dividing arrangement connected across a single source of potential.

One principal point of interest of Fig. 2 is the shape of the magnetic flux field which is tangent to the cathode and orthogonal to the beam, where the beam crosses it. It is'a fringe field produced by directing the currents through windings 3 8 and 40 such that the fields external to these windings are opposed. Because of the opposing fields, no fields exist at any appreciable distance from the windings and there is only a fringe field near the windings resulting from their physical separation. Because the flux is tangent to the cathode 15 none of it passes through the actual emitting surface and thus the same flux links all of the electrons in the beam even though the magnetic structure is placed outside the evacuated envelope 32. As can be shown by Buschs theorem, mentioned in the above-cited article, this is the environment required to provide the same angular momentum for all of the electrons in the beam. The flux, being orthogonal to the beam, provides no axial component and thus produces no radial magnetic deflecting forces.

Due to the opposing fields, the resultant fieldcan be adjusted to essentially zero beyond the anode. The electrons then obtain their full angular momentum in a distance so short that the electrons reach the equilibrium region between drift tube 54 and slow wave structure 66 before their mutual repulsion can spread the beam.

The use of a plurality of magnets instead of a single one as in some of the prior art structures is advantageous because these magnets not only provide a better field pattern but also one that may be adjusted in direction. This directional feature is important because it permits immersion of the electron gun in a field tangential to the cathode while permitting the field away from the cathode to be orthogonal to the electron trajectories regardless of whether 'or not the beam. is parallel to or at an angle with the axis of the capacitor such as in a convergent beam capacitor.

Other embodiments, of the present invention should be apparent in view of the above discussion. For example, permanent magnets, and more magnets than two, can be employed. Further, there are cylindrical capacitor devices other than traveling wave tubes in which the present invention can be employed. Actually, this invention is useful in almost any hollow beam application in which it is desired to maintain the beam within certain cylindrical limits.

Although the invention has been described with respect to certain specific embodiments, it will be appreciated that many other modifications and changes may be made by those skilled in the art without departing from the spirit of the invention. I intend, therefore, by the appended claims, to cover all such modifications and changes as fall within the true spirit and scope of the invention.

a hollow beam of electrons having predetermined trajectories, a magnetic structure comprising magnetic means for immersing said cathode in a magnetic field, and means for controlling the flux produced by said magnetic means to render the flux lines at said cathode tangent thereto and the flux lines away from said cathode orthogonal to the electron trajectories in the hollow beam where the hollow beam crosses the flux lines.

2. In an electron tube having a cathode for producing a hollow beam of electrons having predetermined trajectories, a magnetic structure comprising a plurality of magnets for immersing said cathode in a magnetic field, two of said magnets producing opposed magnetic fields, and means for controlling the flux produced by each magnet for rendering the resultant field tangent to said cathode and orthogonal to the electron trajectories in the hollow beam whereby the electrons obtain equal angular momentums.

3. In a cylindrical capacitor having a cathode for producing a .hollow beam of electrons having predetermined trajectories within an evacuated envelope, a magnetic structure comprising two electromagnets mounted external to said evacuated envelope for im- 'mersing said cathode in fringe flux lines, said two electro magnets producing opposed magnetic fields, and means for individually controlling the energization of said electromagnets for producing a resulting fringe field that is tangent to the cathode and orthogonal to the trajectories low beam, magnetic means for immersing said cathode in a magnetic field, and means for adjusting the energiia tion of said magnetic means to render its flux lines tangent to said cathode and orthogonal to the trajectories of the electrons in the hollow beam.

5. A cylindrical capacitordevice comprising a cylindrical evacuated envelope, a cylindrical outer drift tubecoaxially mounted within said evacuated envelope, a cylindrical inner drift tube coaxially mounted within said outer drift tube, an annular cathode mounted at one end of said cylindrical capacitor for producing a hollow electron beam, a collector mounted at the other end of What I claim as new and desire to secure by Letters said cylindrical capacitor for collecting the electrons produced by said cathode, two electromagnets mounted on the exterior of said evacuated envelope and adjacent said cathode for producing a fringe field immersing said cathode, said two electromagnets producing opposed magnetic fields, and means for controlling the energization of said electromagnets individually wherebythe re sultant field is tangent to said cathode and orthogonal to the electron trajectories in the hollow beam.

6. In an electron discharge device with an annular-' shaped cathode for producing a hollow beam, a magnetic structure comprising two cylindrical magnets concentric with said annular-shaped cathode, with one magnet conthereto.

References Cited in the file of this patent UNITED STATES PATENTS 2,306,875 Fremlin Dec. 29, 1942 2,407,905 Rose Sept. 17, 1946 2,602,148 Pierce July 1; 1952 2,694,159 Pierce Nov. 9,1954 5 2,791,711 May 7,1957 1 2,812,467 Kompfner Nov. 5,1957

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