US2992347A - Oscillation suppressor for electron guns - Google Patents

Description

July 11, 1961 R. A. DEHN OSCILLATION SUPPRESSOR FOR ELECTRON GUNS Filed Nov. 5, 1959 0 6 llll /m enfor: RUdO/ph A. Dehn, by /w d &M

United States Patent i ce Patented July 11, 1961 2,992,3`47 OSCILLATION SUPPRESSOR FOR EIJECTRON GUNS Rudolph A. Dehn, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed Nov. '3, 1959, Ser. No. %50,701 8 Claims. (Cl. 313-82) This invention relates to an electron gun of the type useful for producing an electron beam in an electron tube apparatus and more particularly to the focusing devic'e of such a gun.

In certain electron tubes of more recent development, high energy electron beams are required for proper tube operation and to achieve such high energy, the electrons in the beam are accelerated from the cathode, from which they briginate by emission, by high potentials applied to electrodes spaced from the cathode. In controlling the beam, it is frequently advantageous to provide a focusin'g cup or electrode about the cathode and having an open side in the direction of the accelerating electrodes. The efiect of such electrodes is to confine ofocus the electrons into a beam of relatively small radial dimenson.

In the -apparatus of the type described, the cup is contained in a sealed conductive housing which is spaced from the cup. It frequently occurs that the transit time of the high energy electrons in travelling from the cathode across a portion of the region within the housing is so interrelated with the size and Volume within the housing, that it acts as a resonator so as to excite and establish modes of high frequency, spurious oscillations within the housing and including the region of the cathode and focusing cup. Such spurious oscillations are undesirable in that they interfere with the direct electric field in'the space between the cathode and accelerating electrode and thus, interfere with formation of the proper electron beam. In the absence of such spurious oscilla t-ons, proper beam focusing may be readily achieved. Focusing cups made entirely of metal have no efiect in suppressing or dissipating the oscillations of the generated field in that such cups present a too low resistauce to fields of the spurious oscillations. I

It is accordingly a principal object of my invention to facilitate high energy electron beam focusing without production of any spurious oscillations in the region of the beam focusing electrode.

It is another object of my invention to prevent the initiation or production of spurious high frequency electromagnetic energy waves in the region about an electron gun. i

While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which I regard as my invention, the invention may be better understood from the following description taken in connection with the accompanying drawings in which:

FIG. 1 is a cross sectional elevation of an electron gun according to my invention, and

FIG. 2 is an enlarged, partial View showingrin detail the film applied to portions of the focusing cup. In accordance with my invention, the production and efiect of spurious oscillations in the region of an electron gun are prevented or avoided by the provsion of a thin coating of conductive material of predetermined resistivity over the surface portions of an insulating, focusing cup which are in regions tending to produce a relatively high intensity, high frequency, electromagnetic field. Such a film coating is preferably of a refractory metal and is of a resistauce such as to generally match the wave impedance of the oscillations tending to be produced whereby currents tending to be established in the film due to the efiect of high frequency oscillations are most eiiectively loaded in the production of heat in accordance with the product of the resistauce of the path of the cunrent and the square of the value of the current. Accordingly, the oscillations themselves are prevented from occurring. i

Referring now more particularly to the drawings for a more detailed description of my invention, 10 represents generally the entire electron gun structure which embodies my invention which includes a housing comprising -a hollow, cylindrical, ceramc member 12 sealingly closed at one end by a wall 14, preferably of a suitable metal, and being in abutment with an end of a conductive cylindrical housing portion 16 at the other end.

To complete the housing, an apertured anode 18 is in abutment with the other end of metallic housing 16. The anode section 18 is provided with an aperture 20 for accommodating the flow of electrons in the beam produced by the electron gun. Other apparatus of the electron tube beyond the anode 18, for which the gun 10 provides the electron beam, is not shown, iasmuch as the same forms no essential part of the present invention.

Disposed within the housing '16 is a focusing cup 22 made of a suitable refractory ceramic and supported by a tubular member 24, which in turn is rigidly Secured in an aperture of end wall '14 as by suitable soldering or weldng. The interier end of tubular member 24 is received in an eniarged portion 26 of an opening 28 in the base of the cup 22 and may be bonded thereto by any suitable means. The exterior end of tube 24 is sealed by a cap 30 which is flanged at 32 to abut a complementary fiange` 34 at the end of the tube 24 and is further provided With a supporting insulator 36 therewithin. -For conducting electrical energy within the gun, insulator 36 is apertured to accommodate a conductive rod 38 extending along the interior of the tube. For producing a beam of electrons, cup 22 is opened at 40 and is provided with a recess 42 for accommodating a cathode -44 having an outwardly concave surface. The cathode is coated with a suitable emission enhancing materal 46 and is supported in the bottom of the recess 42 by a cylindrical foil support 48 extending from the periphery of the cathode to the bottom of` the recess 42 and Secured at each end. For raising the temperature of cathode 44 for copious thermionic emission, a heater 50 is mounted in close proximity to the side thereof remote from the concavej, coated side and electrical power is supplied to the heater 50 through the rod 38 at the end of which is an electrical wire 52 extendng from the rod end, to one end of the heater. A suitable insulating grommet 54 disposed in -a conductive supporting member 56 spanning the aperture 28, accommodates the wire 52. The other end of heater 50 is connected to the support ing member 56 which in turn is in conductive contact with tube 24 through a wire 58 attached to the support ing member 56 and to the tube 24. j

In Operating the electron gun, a high potential diiierence is established between the housing 16 and the anode 18 on the one hand and the cup 22 and parts conductively connected therewith on the other hand. To minimize adverse eflects of such high potentials at certain locations, a corona shield is formed at the end of housing 16 as shown at 60 and a complementary corona shield is formed at 62 by a cylindrical member 64 in contact with the end 14 and extending therefrom toward the housing 16. These corona shields provide a smoothly curved surface exposed to the regions of high electric field, thus minimizing the concentration of such fields at sharply pointed conductive edges within the gun.

The region within the housing 16 is of a high electric field because housing 16 and the cup 22 are, respectively, at aceclerating anode and ground potentials. Electrons traveling between cathode 44 and anode 18 may be of such a velocity as to have a transit time interrelated with the dimensions of housing 16 so as to eXcite modes of high frequency oscillations as in a cavity resonator.

In accordance with a feature of my invention, to prevent or minimize such oscillations, the cup 22 is made of an insulating ceramic material such as alumina, beryllia, forsterite or steatite, for example, and is coated with a conductive material along portions of high electric field intensity to provide an eifective load to such high frequency oscillations to completely suppress the high frequency electromagnetic field tending to be generated in the region. To this end, as shown more clearly in FIG. 2 of the drawing, the cup may be coated by a conductive material along portions 64, 66, 68, and 70; The material so utilized may be 'any material which in a thin coating is conductive but presents a resistance which effectively matches the wave impedance of the oscillations tending to be created and to dissipate the energy of such fields. This material further possesses required properties of compatibilty with other tube components as to matters such as low vapor pressure and has adequate potential breakdown resistance. Materials producing particularly good results and considered preferable are molybdenum, tantalum and tungsten or any other suitable high refractory metal. The use of these metals facilitates operation of the gun at higher potentials because of higher breakdown potentials thereof and they are also possessed of qualities such as low vapor pressure at higher tube Operating temperatures.

In accordance with my invention, film thicknesses for optimum loading, may be varied over a rather wide range in accordance with the demands of the particular tube structure and the particular modes and frequencies of oscillations tending to be produced therein. Film resistivities from 100 ohms per square to 1,000 ohms per square are contemplated as being optimum values in different situations and under some circumstances resistvities from to 10,000 ohms per square may be utilized effectively.

As pointed out hereinabove, for most efiective prevention of spurious oscillations the film would be of a thickness to most effectively match the wave impedance of the spurious oscillations tending to be generated. However, in a practical embodment of the invention, certain portions of the film which are in portions of higher applied direct electric field, are preferably made somewhat thicker than is optimum for spurious oscillation revention. Such thicker film portions are better able to withstand the destructive effects of occasional arcing that occurs at such regions by reason of the higher applied fields without being sufliciently mismatched as to materially lose their effectiveness in preventing spurious oscillations.

In the embodment of my invention shown in the drawings, along the film portion 64, extending from the bottom of the cup interier to the curved portion 66 and the film along portion 68, extendng along the exterior of the cup from portion 66 to curved portion 70, the applied direct electric field intensity is not as great as that along portions 66 and 70. Thus, the coatings of film along the portions 66 and 70 are preferably of lower resistivity than the coatings along portions 64 and 68, so as to reduce high voltage `arcing possibilities. The relatively higher resistivity of the film along portions 64 and 68 achieves damping of the high frequency oscillations.

While the present invention has been described by reference to particular embodiments thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the in vention. I, therefore, aim in the appended claims to cover `all such equivalent variations as come within the true spirit and scope of the foregoing disclosure.

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

l. An electron gun for producing a focused beam of electrons comprising a cathode, `a ceramic focusing cup enclosing the cathode and an apertured anode, and means for preventing spurious oscillations in said gun structure when said anode is operated at a high direct potential with respect to the remaining electrodes comprsng a coating of high refractory metal along the portions of sad focusing electrode in regions of high field intensity.

2. An electron gun for producing a focused beam of electrons comprising an apertured anode, a focusing electrode and a cathode disposed within said foousing electrode, said focusing electrode being a ceramic insulating member having a coating of refractory metal along portions thereof in regions of high field intensity.

3. A focusing cup for an electron gun comprising a hollow ceramic member open at one end for receiving a thermionically emissive cathode and portions of said member being coated with a high refractory conductive material.

4. An electron gun comprsing a hollow, conductive member and `an insulative focusing cup within said member and spaced therefrom and portions of said cup being coated with a thin film of conductive material.

5. An electron gun comprising a hollow, conductive member having an anode at one end and an insulative focusng cup within said member spaced therefrom, an electron emissive cathode within said cup for producing a beam of electrons in response to a potential applied to said anode positive with respect to said cathode, means for preventing the production of spurious oscillations within said member by electrons traversing the space between said cathode and anode and including a thin coating of refractory, conductive material along portions of said cup tending to be in electromagnetic fields of high intensity.

6. An electron gun comprising a hollow, conductive member having an anode at one end and an insulative focusing cup within said member spaced therefrom, an electron emissive cathode within said cup for producing a beam of electrons in response to a potential applied to said anode positive with respect to said cathode, means for preventing the production of spurious oscillations within said member by electrons traversing the space between said cathode and anode and including a thin coating of refractory, conductive material along portions of said cup tending to be in electromagnetc fields of high intensity, the resistance of said film being matched to the wave impedance of spurious oscillations tending to be produced in said member.

7. An electron gun for producing a focused beam of electrons comprising a ceramc focusing cup enclosing a thermionically emissive cathode, an anode spaced from said cup, said cup being contained in a conductive enclosure and means for suppressing spurious oscillations in said enclosure when said anode and enclosure are at a high direct potential with respect to said cup comprising a coating of high refractory conductive material along the portions of said focusing cup of high field intensity, the portions of said cup in fields of greater intensity having coatings of lower resistivity than portions thereof in the lo p te ti l fields.

5 6 8. An elect'on gun comprising a hollow, conductve intensity, the resistance of said film being between 10 and member having an anode at one end and an insulatve 10,000 ohms per square to prevent the generation of focusing cup within said member spaced therefrom, an spun'ous osc'uations in said member electron emissive cathode within said cup for producing a beam of electrons in response to a potential applied 5 References Cited in the file of this patent to said anode positive with respect to said cathode, means for preventing the production of spurious oscillations UNITED STATES PATENTS within said member by electrons traversing the space be- 1 273523 Langmuir J l 23 1913 tween said cathode and aode and including a thin coat- 3 44 Birdsan 5 1958 ing of refractory, conductive material along portons of 10 %916,659 Sege 8, 1959 said cup tending to be in electromagnetic fields of high

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