US3377502A

US3377502A - Pierce-type electron gun with combined modulating and beam-forming electrode array

Info

Publication number: US3377502A
Application number: US541677A
Authority: US
Inventors: Amboss Kurt; William E Kramer
Original assignee: Hughes Aircraft Co
Current assignee: Raytheon Co
Priority date: 1966-04-11
Filing date: 1966-04-11
Publication date: 1968-04-09
Anticipated expiration: 1985-04-09

Description

Apnl 9, 1968 AMBQSS ET AL I 3,377,502

PIERCE'TYPE ELECTRON GUN W'ITH' CQMBINED MODULATING AND BEAM-FORMING ELECTRODE ARRAY Filed April 11, 1966 Kurt Amboss, William E. Kramer,

INVENTORS. v

ATTORNEY.

United States Patent PIERCE-TYPE ELECTRON GUN WITH COMBINED MODULATING AND BEAM-FORMING ELEC- TRODE ARRAY This invention relates to electron beam generation and modulation, and more particularly relates to a modulating electron gun of the Pierce-type in which the beam modulation is accomplished by means of a unique beam-forming electrode array.

Certain electron beam tubes such as klystrons and traveling wave tubes require high density electron beams which are well collimated over a distance many times larger than the transverse dimensions of the beam. Such beams maybe generated by means of electron guns of the Pierce-type, details of which are described in an article by J. R. Pierce, Rectilinear Electron Flow in Beams, Journal of Applied Physics, vol. 11 (1940), pages 548 554. Essentially, the Pierce-type gun comprises a cathode having a concavely shaped electron-emissive surface, an anode spaced from the cathode along the electron beam pat-h, and a beam-forming electrode longitudinally disposed between the cathode and the anode. The beamforming electrode has a surface extending outwardly from just beyond the cathode perimeter at angle of essentially 67 /2 with respect to the normal to the electron-emissive surface atthe cathode perimeter.

It is often desired to operate klystrons and traveling Wave tubes on a pulsed basis, and for such operation it is necessary to incorporate a modulating means into the Pierce-type electron gun without impairing the desired density and collimation characteristics of the generated beam. In one such modulating arrangement the relative potential between the anode and the cathode is Varied on a pulsed basis. However, since the required change in potential between a level at which the beam is cut oil and a typical operating potential when the beam is present is in the vicinity of 5000 volts, such a modulating arrangement requires the generation of pulses having magnitudes of around 5000 volts. Elaborate and expensive power supplies must therefore be employed, especially when pulse durations as short as 2-20 microseconds are desired.

In another beam modulating scheme which has been employed in a Pierce-type gun, the modulating potential .is applied to a grid interposed between the anode and the beam-forming electrode. Although the required change in grid voltage between the desired opera-ting potential and the beam cut-ofl? level is about two orders of magnitude less than that required for anode-cathode modulation, a positively biased grid not only causes defocusing of the beam, but it also absorbs a substantial number of the beam electrons.

Attempts have been made to modulate the electron beam of a Pierce-type gun by varying the potential of the beam-forming electrode relative to that of the cathode. Such attempts have not been efiective, however, because during operation of the gun, .on account of the close proximity between the cathode and the beam-forming electrode and the high temperatures present in the vicinity of these electrodes, some of the electron-emissive material from the cathode deposits upon the adjacent side of the beam-forming electrode. Consequently, when the beam-forming electrode is made negative with respect to the cathode in order to cut off the beam, the resultant electric field between the beam-forming electrode and the cathode causes electrons to be emitted from't-he beamforming electrode toward the cathode. Thus, an undesired electron current is generated, resulting in higher power dissipation and lower operating efficiency.

Accordingly, it is an object of the present invention to provide a Pierce-type electron gun incorporating means for modulating the generated electron beam, and which electron gun has relatively low power requirements and high operating efiiciency.

It is a further object of the present invention to provide a modulating Pierce-type electron gun in which the beam defocusing and electron absorbing effects of grid modulation are eliminated, while at the same time the change in modulating voltage required for a beam variation between typical operating beam currents and beam cut-oil is an order of magnitude less than the modulating voltage change which would be required if the modulating voltage were applied between the cathode and the anode.

It is a still further object of the present invention to provide a Pierce-type electron gun which is capable of efiiciently genera-ting high density, well col-limated electron beam pulses of durations of the order of 220 microseconds.

In accordance with the foregoing objects, an electron gun according to the present invention includes a cathode having a concave electron-emissive surface facing a predetermined longitudinal direction and an anode spaced from the cathode along the predetermined direction. The anode has an electron beam aperture which is aligned with at least a portion of the electron-emissive surface. An electron beam-forming electrode array is spaced from the cathode at a location radially outwardly of the cath; ode and longitudinally substantially between the cathode and the anode to establish a potential profile in the region between the cathode and the anode causing electrons emitted from the electronemissive surface to converge radially inwardly asthey travel longitudinally toward the anode. The beam-forming electrode array includes a pair of spaced electrode segments defining a gap-containing frusto-conical surface extending outwardly from the electron-emissive surface at an angle of essentially 67 with respect to the normal to the electron-emissive surface at the perimeter thereof. The beam-forming electrode segment nearer the'cathode is maintained at the same electrical potential as the cathode, while a modulating voltage is applied to the beam-forming electrode segment more remote from th cathode.

Additional objects, advantages and characteristic features of the present invention will become readily apparent from the following detailed description .of a preferred embodiment of the invention when considered in conjunction with the accompanying drawing in which the sole figure is a schematic longitudinal sectional view illustratng an electron gun in accordance with the principles of the invention.

Referring with greater particularity to the figure, an electron gun according to the invention may be seen to include a tubular cathode 10 having a concave end surface 12 which is provided with a coating of electron-emissive material such as barium oxide, for example. The cathode 10 is heated in a conventional manner by means of a filament 14 energized from a source of potential 16. An

anode 18 having a cylindrical aperture 20 through which the generated electron beam passes is coaxially mounted with respect to the cathode a preselected distance away from the electron-emissive surface 12. The crosssectional area of the aperture is less than the area of the electron-emissive surface 12. The anode 18 is maintained at a suitable potential, such as +5000 volts, with respect to the cathode 10 by means of a source of potential 22.

Disposed about the electron beam path longitudinally substantially between the cathode 10 and the anode 18 is a beam-forming, or focusing, electrode array consisting of a first beam-forming electrode 24 and a second beamforming electrode 26 which also functions as a modulating electrode, the

electrodes

24 and 26 being separated by a preselected spacing. The first beam-forming electrode 24 is of an annular configuration and is coaxially disposed about the cathode 10 in a manner leaving an annular space between the outer lateral surface of the cathode 10 and the inner lateral surface of the electrode 24. A portion 28 of the beam-forming electrode 24 extends longitudinally beyond the electron-emissive surface 12 in the direction of the anode 18. The extending portion 28 of the beam-forming electrode 24 is provided with a frusto-conical aperture defined by a surface 30 which, in accordance with well-known design criterial for Piercetype guns, is disposed at an angle of 67 /2" with respect to the normal to the emissive surface 12 at the perimeter of the surface 12. The first beam-forming electrode 24 is maintained at the same electrical potential as the cathode 10 by means of a lead 32 connected between the cathode 10 and the electrode 24.

The second beam-forming electrode 26 is also of an annular configuration and is coaxially disposed with respect to the cathode 10 longitudinally substantially between the first beam-forming electrode 24 and the anode 18. The inner lateral surface of the beam-forming electrode 26 defines a first portion 34 aligned with the frustoconical surface 30 in .a manner forming what would be a continuous overall frusto-conical surface except for a gap 36 between the

electrodes

24 and 26. The width of the gap 36, i.e. the spacing between the

electrodes

24 and 26, must be at least equal to a distance which ensures that voltage breakdown will not occur between the

electrodes

24 and 26. An electrode spacing in inches equal to .003 times the maximum potential difference in kilovolts between the electrodes is usually sufficient to prevent voltage breakdown. The inner lateral surface of the beam-forming electrode 26 which is longitudinally more remote from the cathode 12 than the surface 34 defines a radially inwardly projecting annular ridge 38 which causes equipotential lines in the cathode-anode region to lie more nearly parallel to the cathode surface 12 over a larger portion of the cathode-anode region than in the absence of the ridge 38, thereby providing a more homogenous electron beam.

The second beam-forming electrode 26 may be biased for quiescent operation in which the electron beam is cut off by means of a voltage source 40 providing a potential relative to the cathode 10 of 500 volts, for example. A modulating voltage source 42 is connected in series with the biasing source 40 in order to furnish voltage pulses during those intervals of time when it is desired to generate the electron beam. The pulses provided by the source 42 may be of a magnitude of +500 volts and may have a duration of 2-20 microseconds, for example.

In the operation of the electron gun of the present invention, under quiescent conditions the modulating pulse source 52 provides an output of zero volts so that the potential source 40 biases the beam-forming electrode 26 at a potential sufficiently negative with respect to the cathode 10 to prevent the flow of electrons between the cathode 10 and the anode 18. When it is desired to generate an electron beam pulse, the modulating source 42 is triggered to provide an output pulse which essentially cancels the negative bias from the source 40 so as to bring the beam-forming electrode 26 to essentially cathode potential. An electron beam is then able to flow from cathode 10 toward the anode 18, with the potential profile in the anode'cathode region being such as to cause electrons emitted from the surface 12 to converge radially inwardly as they travel longitudinally toward the anode 18 in accordance with well-known Pierce conditions.

Although some electron-emissive material from the cathode surface 12 may deposit upon adjacent surfaces of the beam-forming electrode 24 during operation of the electron gun, since the electrode 24 is always maintained at the same potential as the cathode, current flow from the focusing electrode 24 back to the cathode 10 is substantially prevented, thereby eliminating undesired current and improving operating efiiciency. Moreover, since the focusing electrode 26 affords the desired beam modulation, the need for a grid is eliminated, thereby avoiding geam defocusing and electron absorbing effects inherent in grid operation. At the same time, the magnitude of the modulating pulses required for operation of the electron gun of the present invention is an order of magnitude less than that which would be required if the modulating voltage were applied between the cathode and the anode.

Although the present invention has been shown and described with reference to a particular embodiment, nevertheless various changes and modifications obvious to a person skilled in the art to which the invention pertains are deemed to lie within the spirit, scope and contemplation of the invention as set forth in the appended claims.

What is claimed is:

1. An electron gun comprising: a cathode having a concave electron-emissive surface facing a predetermined longitudinal direction, an anode spaced from said cathode along said predetermined direction and having an electron beam aperture aligned with at least a portion of said electron-emissive surface, electron beam focusing means spaced from said cathode at a location radially outwardly of said cathode and longitudinally substantially between said cathode and said anode for establishing a potential profile in the region between said cathode and said anode causing electrons emitted from said electr-on-emissive surface to converge radially inwardly as they travel longitudinally toward said anode, said focusing means including a pair of spaced electrode segments defining a gap-containing frusto-conical surface extending outwardly from said electron-emissive surface at an angle of essentially 67 /2 with respect to the normal to said electron-emissive surface at the perimeter thereof, means for maintaining the focusing electrode segment nearer said cathode at the same electrical potential as said cathode, and means for applying a modulating voltage to the focusing electrode segment more remote from said cathode.

2. An electron gun comprising: a cathode having a concave electron-emissive surface facing a predetermined longitudinal direction, an anode spaced from said cathode along said predetermined direction and having an electron beam aperture aligned with at least a portion of said electron-emissive surface, a first annular beam-forming electrode spaced from said cathode at a location radially outwardly of said cathode, said first beam-forming electrode defining and inner frusto-conical surface disposed adjacent said electron-emissive surface longitudinally between said electron-emissive surface and said anode, said frusto-conical surface extending outwardly from said electron-emissive surface at an angle of essentially 67 /2 with respect to the normal to said electronemissive surface at the perimeter thereof, a second annular beam-forming electrode spaced from said first beamforming electrode and substantially disposed radially outwardly of said first beam-forming electrode longitudinally between said first beam-forming electrode and said anode, said second beam-forming electrode defining an inner frusto-conical surface aligned with said frusto-conical surface of said first beam-forming electrode, means for maintaining said first beam-forming electrode and said cathode at the same electrical potential, and means for applying to said second beam-forming electrode an electrical potential variable between a first level of potential sufficient to prevent the flow of electrons between said electron-emissive surface and said anode and a second level of potential essentially equal to the potential of said cathode.

3. An electron gun according to claim 2 wherein the spacing in inches between said first and second beamforming electrodes is not less than .003 times the differ- 6 ence inkilovolts between said first and second levels of potential.

4. An electron gun according to claim 2 wherein said second beam-forming electrode further defines a radially inwardly projecting annular ridge in a portion longitudinally more remote from said cathode than the portion of said second beam-forming electrode which defines its said frusto-conical surface.

No references cited.

RODNEY D. BENNETT, Primary Examiner. J. G. BAXTER, Assistant Examiner.

Claims

1. AN ELECTRON GUN COMPRISING: A CATHODE HAVING A CONCAVE ELECTRON-EMISSIVE SURFACE FACING A PREDETERMINED LONGITUDINAL DIRECTION, AN ANODE SPACED FROM SAID CATHODE ALONG SAID PREDETERMINED DIRECTION AND HAVING AN ELECTRON BEAM APERTURE ALIGNED WITH AT LEAST A PORTION OF SAID ELECTRON-EMISSIVE SURFACE, ELECTRON BEAM FOCUSING MEANS SPACED FROM SAID CATHODE AT A LOCATION RADIALLY OUTWARDLY OF SAID CATHODE AND LONGITUDINALLY SUBSTANTIALLY BETWEEN SAID CATHODE AND SAID ANODE FOR ESTABLISHING A POTENTIAL PROFILE IN THE REGION BETWEEN SAID CATHODE AND SAID ANODE CAUSING ELECTRONS EMITTED FROM SAID ELECTRON-EMISSIVE SURFACE TO CONVERGE RADIALLY INWARDLY AS THEY TRAVEL LONGITUDINALLY TOWARD SAID ANODE, SAID FOCUSING MEANS INCLUDING A PAIR OF SPACED ELECTRODE SEGMENTS DEFINING A GAP-CONTAINING FRUSTO-CONICAL SURFACE EXTENDING OUTWARDLY FROM SAID ELECTRON-EMISSIVE SURFACE AT AN