US3214632A

US3214632A - Low noise electron gun

Info

Publication number: US3214632A
Application number: US296632A
Authority: US
Inventors: Ward A Harman
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1963-07-22
Filing date: 1963-07-22
Publication date: 1965-10-26
Anticipated expiration: 1982-10-26

Description

Oct. 26, 1965 w. A. HARMAN LOW NOISE ELECTRON GUN Filed July 22, 1963 MINI; U

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WARD A. HARMAN EW/z, M

ATTORNEY United States Patent Office 3,214,632 Patented Oct. 26, 1965 3,214,632 LOW NOISE ELECTRON GUN Ward A. Harman, Los Altos Hills, Calif., assignor to General Electric Company, a corporation of New York Filed July 22, 1963, Ser. No. 296,632 7 Claims. (Cl. 315--14) This invention relates to an electron gun for producing and projecting a beam of electrons. The invention relates more particularly to an electron gun for producing an electron beam of high electron current density with relatively low noise. The invention is especially useful in a high power, low noise traveling wave tube for providing extended dynamic range.

In order to extend the usefulness of electron beam devices, considerable effort has been expended to reduce the undesirable noise attendant the production and projection of the electron beam. Theories have been advanced in explanation of noise generators and noise reduction in electron beams. See, for example, Noise in Electron Devices by L. D. Smullin and H. A. Haus, John Wiley and Sons, New York, 1959. However, the generation and the transformation of beam noise in the immediate vicinity of the cathode are not well explained by existing theories. Therefore, the production and projection of low noise electron beams thus far remains, to a considerable extent, an empirical art.

Among the features now believed to be necessary to achieve low noise is that of providing a region immediately following the region of electron emission wherein low axial drift velocity of the electrons is maintained. Low noise electron guns including the foregoing feature have been constructed, for example, as described by M. R. Currie and D. C. Forster in New Mechanism of Noise Reduction in Electron Beams, Journal of Applied Physics, vol. 30, pages 94-103, January 1959. The electron gun therein described, while capable of producing a low noise beam, employs an annular cathode which, because of its limited effective electron emission area, is not well adapted to the production of high power beams. It is desirable to produce and project an electron beam of high power with low noise.

It is therefore an object of the invention to provide an improved electron gun.

It is a specific object of the invention to produce a high power electron beam with low noise.

It is another object of the invention to provide a high perveance electron gun structure which provides an extended region of low electron velocity immediately following the region of emission.

These and other objects of the invention are accomplished by an electron gun of the magnetron type having a large emissive area from which large electron currents can be drawn at relatively low emission densities and having special features for providing an extended low velocity drift region immediately following the region of emission from the cathode for achieving minimum noise in the beam. The electrode shapes are also designed to give smooth electron flow by minimizing the presence of strong perturbing electric or magnetic fields.

Briefly, the electron gun of the invention comprises a cathode of circular cross section having an emissive surface along a portion thereof for electron emission predominantly transverse of the axis of the cathode and of the beam path. This emissive portion of the cathode is preferably tapered in the direction of the electron path for imparting an axial drift velocity to the emitted electrons.

An anode is positioned in spaced coaxial relationship to the cathode for extracting electrons therefrom, the anode having an internal bore surface at least coextensive with the emissive surface of the cathode, this internal bore preferably being formed with a taper.

The foregoing cathode-anode structure is immersed in a relatively strong magnetic field which is substantially parallel to the longitudinal axis of the cathode with the result that the extracted electrons are caused to describe circumferentially directed paths around the cathode.

A plurality of accelerating electrodes are positioned forward of the cathode-anode structure along the path of the beam. The combined action of the magnetic field and the electric fields of the accelerating electrodes, plus the influence of the cathode taper, causes the extracted electrons to move in spiral paths of decreasing radii toward the acelerating electrodes whereby a dense hollow electron beam is formed and projected, for example, along the interaction structure of a traveling wave tube.

As a feature of the invention the cathode of the present electron gun is provided with an extension which projects forward of the emissive portion whereby an extended low velocity drift region is simply achieved with a minimum of perturbing forces in this critical region. An electron beam of high power with low noise is thus achieved.

The invention is described more specifically with reference to the sole accompanying drawing which is a longitudinal cross section view of an embodiment of an electron gun structure according to the invention as employed in a traveling wave tube.

As illustrated in the figure, the electron gun structure of the invention includes a cathode 10, a cathode end hat electrode 11, an anode 12, a first accelerating electrode 13, a second accelerating electrode 14, and a drift tube 15.

The electron gun is illustrated as employed in a travel ing wave tube which includes, in addition to the gun structure, a slow Wave structure, illustrated as a helix 16, an input coupler 17 for introducing R.F. energy to the helix, an output coupler 18 for extracting R.F. energy from the helix, and a collector electrode 19 for capturing the electrons of the projected beam. Thus the electron gun and the collector electrode define a path through the interaction structure for an electron beam. (A coaxial line 20 is provided for transmitting R.F. energy to the input coupler 17. Similarly, a coaxial line 21 is provided for transmitting R.F. energy from the output coupler 18.) The electron gun structure, the slow wave structure and the collector electrode are enclosed in an evacuated envelope 22 which may be formed of a material such as glass.

For proper operation of the traveling wave tube and of the electron gun of the invention, a magnetic field directed along the axis of the tube is required. For this purpose an electromagnet 23 is illustrated, it being understood that other well-known types of magnet structures can be used. The axial magnetic field provides a force for focusing the extracted electrons into a hollow cylindrical beam 9 along the tube axis.

A power supply, illustrated as a battery 24, provides appropriate operating potentials for the various elements of the electron gun and for the collector electrode, it being understood that each terminal of the battery 24 is connected to a similarly numbered terminal of the tube. Typical operating voltages are shown. Further features and details of operation of the traveling wave tube are well-known and will not be repeated herein, the present invention being directed to an improved electron gun.

The electron gun of the invention will now be discussed in greater detail. The cathode 10, as illustrated, is circular in cross section to its longitudinal axis. The cathode 10 includes a frusto-conical portion 25, that is, portion 25 is tapered in the direction of projection of the electron beam. Along this frusto-conical portion an emissive surface 26, formed of any suitable well-known emissive material, is provided. (A heater element 27 is supplied with a current through a pair of terminals 28 to maintain the emissive surface 26 at the appropriate operating temperature.

The anode 12 is provided for extracting electrons from the emissive surface 26. The ring shaped anode 12 is positioned coaxial of the cathode and it is formed with an internal bore surface which is at least coextensive with the emissive surface 26. The internal bore of the anode 12 is preferably formed with a taper. (It is noted that an anode formed of a series of similar disk shaped electrodes to which successively higher potentials are applied will provide an electric field similar to that provided by the illustrated ring shaped anode.)

The anode 12 is operated at an electron extracting potential with respect to the cathode 10 to thus form an electron extraction region between the emissive surface 26 and the anode 12. In the absence of a magnetic field, the extracted electrons would strike the anode 12. However, the electron gun structure is immersed in the longitudinal or axial magnetic field of the magnet 23. By the combined influence of the electric and magnetic fields, the region between the cathode 10 and the anode 12 is constituted a magnetron space in which the extracted electrons describe circumferentially directed paths about the cathode. This type of an electron extracting structure provides a relatively large emissive surface as compared, for example, to an annularly shaped cathode of similar diameter. Thus large electron currents can be extracted at relatively low emission densities.

It has been found that beam noise is reduced by providing an extended low velocity drift region immediately following the region of electron emission. To provide this low velocity drift region the cathode 10 is formed with an extension 30 which projects forward of the emissive surface. The extension 30 maintains the drift region between the extraction region and the first accelerating electrode 13 at a lower potential than would obtain in the absence of the extension 30. The extension 30 also aids in establishing a smooth acceleration within the low velocity region.

-Additionally the first accelerating electrode 13 is operated at a relatively low potential, typically below the potential of the anode 12. Thus under the combined influence of the electric and magnetic fields, the extracted electrons move in spiral paths of decreasing radii toward the first accelerating electrode 13, it being noted that the taper of the emissive portion of the cathode aids in imparting axial velocity to the electrons. The cathode taper also is an aid to avoiding space charge instability, the nature of which is not well understood, which can otherwise occur in the electron emission and extraction region under high magnetic field conditions. To this end, it is also desirable to avoid large length-to-diameter ratios for the emissive surface.

After the electrons traverse the low velocity drift region between the extraction region and the first accelerating electrode, they come within the influence of the electric fields of the second accelerating electrode 14 and the drift tube 15 by which the electrons constituting the beam are accelerated and projected through the helix to the collector 19.

It is noted that the extension is preferably formed with a conically tapered end portion 31. It has been found that this tapered end portion materially aids in minimizing disturbances to the electron beam in its transition from the low velocity drift region to the region of more rapid acceleration by the electric fields of the second accelerating electrode 14 and the drift tube 15. (A discussion of the design and operation of accelerating regions, such as between the first accelerating electrode 13 and the drift tube 15, for minimum excitation of noise may be found in the aforementioned publication Noise in Electron Devices) What has thus been described is an improved electron gun of the magnetron type which provides a large emission surface for high power capability and which provides an extended low velocity drift region for minimization of noise.

Thus, while the principles of the invention have now been made clear in an illustrative embodiment there will be obvious to those skilled in the art many modifications in structure, arrangement, proportions, the elements, materials, and components used in the practice of the invention which are specifically adapted for specific environments and operating requirements, without departing from those principles. The appended claims are therefore intended to cover and embrace any such modifications, within the limits only of the true spirit and scope of the invention.

What is claimed is:

1. A high perveance electron gun for producing and projecting a hollow beam of electrons in a forward direction along a predetermined path, comprising: a cathode of substantially circular cross section and including a first portion of decreasing cross section area in the direction of said path, said cathode having an emissive surface along at least a part of said first portion, said cathode having a second portion which is substantially non-emissive extending forward of said first portion; an anode in spaced coaxial relationship to said cathode and having an internal surface at least coextensive with said emissive surface of said cathode, the internal diameter of said anode decreasing in the direction of said path; means providing a potential difference between said anode and said cathode for extracting electrons from said emissive surface; means for providing a magnet field about said cathode in the direction of said path for causing said extracted electrons to describe circumferentially directed paths around said cathode; and means positioned along said path forward of said anode for accelerating said extracted electrons along said path.

2. A low noise electron gun for projecting a beam of electrons in a forward direction along a predetermined path comprising: a cathode having at least a portion thereof tapered in the direction of said path, said cathode having an emissive surface comprising at least a part of said tapered portion, a portion of said cathode extending beyond said emissive surface in said forward direction; means for providing a magnetic field about said cathode in the direction of said path; an anode surrounding a portion of said cathode, the forward edge of said anode being substantially aligned with the forward edge of said emissive surface; means for establishing a potential difference between said anode and said cathode for extracting electrons from said emissive surface; and at least one accelerating electrode positioned along said path forward of said anode for accelerating said electrons along said path.

3. A low noise electron gun for projecting a beam of electrons in a forward direction along a predetermined path, comprising: a cathode including a frusto-conical portion, said cathode having an emissive surface extending along at least a part of said frusto-conical portion; means for providing a magnetic field about said cathode in the direction of said path; an anode disposed coaxially about said cathode, said cathode having a substantially non-emissive portion extending forward of said emissive surface and forward of the forward edge of said anode; means for establishing a potential difference between said anode and said cathode for extracting electrons from said emissive surface; and at least one accelerating electrode positioned along said path forward of said anode.

4. A low noise high perveance electron gun for projecting a beam of electrons along a predetermined path, comprising: a cathode including a portion of decreasing cross section area in the direction of said path, said cahode having an emissive surface along at least a part of said portion; an anode spaced apart from and in electron extracting relationship to said emissive surface of said cathode, said cathode further including a substantially non-emissive extension forward of said emissive surface; means for establishing a potential difference between said anode and said cathode for extracting electrons from said emissive surface; means providing a magnetic field about said cathode substantially parallel to said path for causing said extracted electrons to describe circumferentially directed paths around said cathode; and electron accelerating means for directing extracted electrons in the region of said cathode along said path.

5. An electron gun for producing and projecting a beam of electrons, comprising: an elongated cathode of substantially circular cross section, said cathode having an emissive surface along a first portion thereof; an anode in spaced coaxial relationship to said cathode and having an internal surface at least coextensive with said emissive surface of said cathode; means providing an electric field between said cathode and said anode for extracting electrons from said emissive surface; means for immersing said cathode in a magnetic field substantially parallel to the longitudinal axis of said cathode whereby the combined influence of said electric and magnetic fields causes said extracted electrons to describe circumferentially directed paths around said cathode; and electron accelerating means for projecting said electrons from said circumferentially directed paths into a beam having a longitudinal axis substantially coincident with said longitudinal axis of said cathode, said cathode having a substantially non-emissive second portion projecting in the direction of said beam from said emissive first portion for decreasing the electron accelerating force in the region adjacent said second portion of said cathode.

6. A low noise high perveance electron gun for projecting a beam of electrons along a predetermined path, comprising: a cathode including a portion of decreasing cross section area in the direction of said path, said cathode having an emissive surface along at least a part of said portion; an anode spaced apart from and in electron extracting relationship to said emissive surface of said cathode, said cathode further including a substantially non-emissive extension forward of said emissive surface; means for establishing a potential difference between said anode and said cathode for extracting electrons from said emissive surface; means providing a magnetic field about said cathode substantially parallel to said path for causing said extracted electrons to describe circumferentially directed paths around said cathode; a plurality of accelerating electrodes positioned along said path forward of said cathode; means providing a potential difference between said cathode and each of said accelerating electrodes, the potential difference between said cathode and the accelerating electrode nearest said cathode being less than said potential difference between said anode and said cathode.

7. A low noise high perveance electron gun for projecting a beam of electrons along a predetermined path, comprising: a cathode including a portion of decreasing cross section area in the direction of said path, said cathode having an emissive surface along at least a part of said portion; an anode spaced apart from and in electron extracting relationship to said emissive surface of said cathode, said cathode further including a substantially non-emissive extension forward of said emissive surface, said extension being formed with a conically tapered end portion; means for establishing a potential difference between said anode and said cathode for extracting electrons from said emissive surface; means providing a magnetic field about said cathode substantially parallel to said path for causing said extracted electrons to describe circumferentially directed paths around said cathode; and electron accelerating means for directing extracted electrons in the region of said cathode along said path.

No references cited.

DAVID G. REDINBAUGH, Primary Examiner.