US2526054A - Electrode assembly for very highfrequency electron discharge devices - Google Patents

Description

G. w. WARREN 2,526,054 ELECTRODE ASSEMBLY FOR VERY HIGH-FREQUENCY ELECTRON DISCHARGE DEVICES Filed Nov. '7, 1946 INVENTOR 'ea/frey W Warren- Patented Get. 17, 1950 V UNITED STATES ELECTRODE ASSEMBLY FOR VERY HIGH- FREQUENCY ELECTRON DISCHARGE n1:-

VICES GeoffreyW. Warren, Eastcote, England, assignor to M-O Valve Company, Ltd.,London, England, a limited company of Great Britain Application November 7,1946, Serial No. 708,277 I In Great Britain March 13, 1940 Section 1, Public Law 690, August 8, 1946 iatent expires March 13, 1960 directly heated thoriated tungsten cathodes of high electron density may be employed with comparatively low heating current.

' Other objects and advantages will be'apparent from the following description of the invention, pointed out in particularity in the appended claims and taken in connection with the accompanying drawing in which Fig. 1 is an elevational View partly in axial section of a cathode structure and a cooperating electrode constructed in accordance with the invention; Fig. 2 is a crosssection in the plane along the lines 2-2 of Fig.1; Fig. 3 is an elevational view partly in axial section of a modification of the structure shown'in Fig."

1; Fig. 4 is a cross-section in the plane along the line 4-4 of Fig. 3.

In vacuum tubes intended for use at-very high frequencies the electron transit time is an important factor. These tubes may have a number of electrodes or they may be simple two-element devices. In any case, the electron transit time from the cathode to the next cooperating electrode is the most important factor from a structural point of view. The transit time from the cooperating electrode, which is usually in the form of a grid, to other electrodes can more easily be controlled and can be made less than the transit time betweencathode and grid by I adjusting the anode voltage or screen grid volt- The invention herein disclosed ,is particularly concerned with thetransit time between'theelectron emitting cathode and the next cooperating electrode which may be an anode or-a grid, de-v pending upon the particular function for which ing, only the cathode'and the next cooperating heated by practicable currents.

electrode will be illustrated and described. It is to be understood that a vacuum tube incorporating the basic features of the invention may have a number of other electrodes besides the cathode and the next cooperating electrode.

Considering a two-element tube, the transit time T of the electrons from a plane cathode to a plane anode is given by'the well knownf or in which it is the anode-cathode distance measured in ems. and i the density of spacecharge limited current flowing to the anodein amperes per square cms. Accordingly, for a given current and given (1, T decreases as the current density increases. practically obtainable depends on the nature of the cathode. For an oxide coated cathode it is about 150 ma./cm. but for a thoriated tungsten cathode it may be as great. as 2000 ma./cm. If, therefore, it is important to have the smallest possible transit time, thoriated tungsten cathodes are preferable to oxide coated cathodes.

But in order that the transit time should be small, d must be small. Thus, in certain vacuum tubes d is sometimes as small as 0.01 cm., and is much less than the radius of curvature of the cathode. (This is the justification for discussing only the case of plane cathode and anode.) It is necessary then that the cathode surface should be smooth on the scale of d. Thusgif the cathode were a wire helix, the gaps or depressions between the turns would cause'variations in the effective value of d over the cathode which are known to be undesirable. Smooth oxide coated cathodes of large diameter are easily obtainable in the form of indirectly heated cylinders; but, so far as it is known, thoriated tungsten cathodes have always been "directly heated wires. However, a wire 1 mm. in diameter'would require an impracticably large current to heat it. If, therefore, the theoretical advantage of thoriated tungsten in respect of transit timeis to be utilized, some method must be-found of makingcathodes of thoriated. tungsten that havea diameter at 5 least of the order of 1 mm., are substantially V smooth on a scale of 0.1 mm., and yet can be According to the invention, this problem is solved by winding a helix of thoriated tungsten wire, nearly close-woundfgr'inding the surface I of the helix until it becomes substantially a .smooth cylinder and presenting the groundsurface to the adjacent cooperating electrode. If, as has been more usual in the pastjthe cathode The maximum current density is inside the anode, the outer surface of the helix must be ground. But if, as is now known to be advantageous in vacuum tubes of the type specified, the cathode is outside the anode, then the inner surface of the helix must be ground.

If the helix were really close wound and consecutive turns touched each other, the current required to heat the cathode would be as great as if the helix were a continuous cylinder. There is no difiiculty in winding the helix so that there are gaps between the turns large enough to electrically insulate adjacent turns yet not large compared with the distance between the ground surface and the adjacent electrode; then the gaps will not detract substantially from the effective smoothness of the ground surface.

Two embodiments of the invention will now be described by way of example, with reference to the accompanying drawing.

In each figure, 5 is a cathode in the form of a helix of thoriated tungsten wire which in practice may be 0.4 mm. in diameter; 6 is a cylindrical cooperating electrode; .1 is the ground surface of the helical wire presented to the electrode 6,; '8 indicates the gaps between the turns of the helix whose width is app oximately equal to the .distance between the surface I and the electrode 6.

In Fig. 1 one lead 9 to the helix is returned inside the helix; in Fig. 3 it projects outwardly from the helix. The other lead !9 (not shown in Fig. 3) is parallel to the axis of the helix.

The embodiment shown in Fig. 3 differs only in that :the helical cathode 5 is 'wound around the cooperating electrode 6. In this case the .cooperating electrode 6 which, as previously .explained, may be an anode or a grid, is inside the cathode .5. In order to obtain the advantages in this structure, the inside surface .of the oathiode 5 is :smoothly ground to present .an even surface to the outer periphery of .the electrode :6. The distance between the two cooperating surfaces is preferably not greater than the distance between adjacent turns of the helically wound wire. This distance is referred to as the gap and is indicated by reference character .8.

The cross-sections illustrated Figs. 2 and 4 show the uniform cylindrical spacing .of the electrodes and the arrangement of the lead wires.

"The uniform surface presented by the ground side .of the cathode :5 is practically equivalent to .a cathode of solid cylinder yet the heating ,current is not more than required by the length of wire :of which theicathode is wound. In this manner thoriated tungsten .as well'as other high electron density materials may be used for a directly heated cathode :of high emitting strength.

'What'I claim is:

'1. An electron discharge device having two electrodes, .one of said electrodes comprising a cathode for supplying electrons, said cathode comprising .a helix haying a :plurality of relatively closely wourrd turns .of wire, adjacent ones of said turns being :spaced from each other a wire being adjacent said-smooth surface "of said :other of said electrodes and spaced therefrom zbysa distance less than -.a diameter of said wire.

2. An electrondischarge device having a pluerality of electrodes, including a cathode, said --cathode comprising -'a tungsten wire wound in the form of a helix, said wire having a flat active surface on one side of said helix, adjacent turns of said helix being relatively closely spaced to cause said active surface .to form substantially continuous tubular surface along the entire of said one side of said helix, another of said electrodes comprising a tubular member disposed adjacent said cathode, with said fiat active surface facing said tubular electrode and spaced therefrom by a distance less than the diameter of said wire for reducing the transit time of electrons from said active surface whereby said device is adapted for operation at very high frequencies.

3. An electrode assembly including a cathode and an additional electrode adjacent said cathode, said cathode comprising a helix of a plurality of wound turns of wire, the distance between adjacent ones of said turns being substantially equal to the spacing between said cathode and additional electrode, said spacing being less than a diameter of said wire, a portion of said wire having a flat side, said portion comprising one side ,of said helix, said side of said helix facing said additional electrode and comprising a substantially smooth active surface.

4. ,An electron discharge device having a plurality of electrodes including a cathode, said cathode comprising a helix formed by a plurality of turns of wire, said wire having a flat surface on the outer side of said helix, adjacent turns of said helix being spaced by a distance less than the diameter of said wire for providing a substantially continuous and smooth tubular surface on said outer side of said helix, and a tubular electrode surrounding said cathode and having a uniformly smooth inner surface facing said fiat surface of said helix and spaced from said fiat surface by a distance less than a diameter of said wire for good operation of said device at very high frequencies.

5.. A directly heated cathode comprising a helix of thoriated tungsten wire, saidhelix having inner and outer walls, said wire being semicircular in cross section whereby one side of said wire is flat and the opposite side is rounded, the trans of said helix being disposed with said flat side of said wire parallel to the axis of said helix and forming .oneiof said walls, adjacent ones of said turns being spaced to provide insulation 'therebetween, the spacing between said turns being smaller than a diameter of said wire, whereby said flat side .of said wire forms a substantially smooth and continuous tubular surface for increased current density thereon.

GEOFFREY WARREN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number 'Name Date 1,889,612 "Perrot-t Nov. 29, 1932 1,969,496 Beardon Aug. 7, 1934 2359,2523 Clark May 23, 1939 224443752 Wolff June 10, 1941 2,256,297 Smith et'al Sept. 16, 1941 '2 i-l6 298 Fisk Feb. 25, 1947 REFERENCES Designing Thoriated Tungsten Filaments ;by H. J. Dailey inJan. 1948 issue of f-Electronics (pages 107 through 109),.

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