US2677778A - Linear cathode - Google Patents

Description

May 4 1954 w. R. BAKER ETAL 2,577,778

LINEAR cATHoDE Filed March 51, 1952 INVENTORS. WILL/AM R. BAKER NOE BY DAVID A VA ,6am/MMO ATTORNEY.

Patented May 4, 1954 LINEAR CATHODE William R. Baker, Berkeley, and David A. Vance, Lafayette, Calif., assignors to the United States of America as represented by the United States Atomic Energy Commission Application March 31, 1952, Serial No. 279,544

3 Claims.

The present invention relates to improvements in dispenser type thermionic cathodes and in particular to a simple, economical and readily replaceable cathode and method of making same.

Dispenser type cathodes commonly comprise a porous metal either impregnated with or surrounding an activating material which slowly volatilizes when heated and escapes to the surface of the metal to reduce the work function thereof and provide a Very advantageous electron emission surface. Extensive research within recent years has shown that numerous metals may be employed in combination with a variety of activating materials to provide good emission surfaces and there are a large number of issued patents covering the results of this research. There remains, however, for this rather basic knowledge to be put to a practical advantage by the provision of a simple and economically feasible cathode of the dispenser type, for although undoubtedly the dispenser type cathodes illustrated in the art are operable, few are designed for economy of manufacture, ease of construction, simplicity, or replaceability, which of course is of major importance when considering large scale use and standardization.

It is therefore an object of the present invention to provide an improved dispenser type cathode which is rugged and efiicient yet simple and easily constructed.

It is a further object of this invention to provide an improved method of manufacture of dispenser type cathodes.

It is another object of the present invention to provide an improved directly heated dispenser type cathode which is easily disassembled.

Many other objects and advantages of the invention will become apparent to those skilled in the art from the following description o-f the invention taken together with the accompanying drawing wherein:

Figure 1 is a longitudinal cross sectional view of a preferred embodiment of the invention taken on the center line thereof;

Fig. 2 is a cross sectional view of the embodiment taken at line 2--2 on Fig. 1; and

Fig. 3 is a perspective view of the base element partially in section.

Considering one preferred embodiment of the invention as illustrated in the drawing and referring thereto, it will be seen that there is provided an elongated base member I I which is preferably formed of an electrically conducting material, such as for example tantalum. Base member l! includes an elongated rectangular plate portion I2 and a number of upright portions extending from the surface thereof to form a rectangular trough i3. These upright portions may consist of sides It and ends I5 which may either be integrally formed with plate portion I2, as shown, or may comprise separate elements joined together and to plate portion I2 to form therewith a substantially rectangular open-sided box extending less than the length of plate I2 and at a distance from each end thereof.

Within trough I3 there is disposed a quantity of activating material Il which is disposed evenly along the bottom of trough I3. Atop trough It and in closing relation thereto is a trough cover or bar I8 formed of sintered tungsten or other porous metal which reacts with activating material Il to form a good electron emitting surface. Bai' i8 has the same length as the internal length of trough i3 and the same width as the external Width of trough I3, and has a boss I9 upon one face thereof of the same dimensions as the interior length and width of trough i3. Bar I3 is assembled with base element II by slipping boss I9 into trough I3 from the top thereof which entirely closes trough I3 and, as boss It has a lesser depth than trough I3, there remains a closed chamber or reservoir within trough I3 which contains the above-noted activating material II.

Bar I8 preferably has the opposite face 2i! thereof from boss I9 formed with a slight concavity, with the curvature being transversely of bar i8. The concave surface 26 of bar I8 is the electron emitting surface of the cathode and by virtue of this curvature the electrons emitted therefrom are substantially focused in a line parallel to the surface 2li and at a distance dependent upon the radius of curvature thereof, as noted in more detail below.

The assembly of base element il and bar 5: is surrounded in part by a thin sheet or foil of material 2l which has substantially the same length as trough I3 and which extends across the bottom of base element I I and along the sides of trough I3 and bar It up to and flush with the top surface Z of bar I8. Sheet ZI may be forme-d of an insulating material to` confine electrical currents upon base element II or may alternatively be formed of a metal, such as molybdenum; and 'acts as a heat shield to contain heat within trough i3. Exterior to sheet or shield 2l there is provided a second shield or cover 22 which lee shield 2l has substantially the same length as trough I3 and which covers shield 2I in contiguous relation thereto. In addition to extending around the bottom and sides of trough I3 and the sides of bar I8, cover 22 has its edges disposed upon the edges of surface 20 of bar I8 in clamping relation thereto whereby base element I I, bar I8 and shield 2I are held together in assembled relation.

As noted above, plate portion I2 of base element II extends beyond trough I3 at both ends thereof and these extensions 23 may be employed as connecting lugs for the connection of electrical leads 24 to the cathode. Joinder of electrical leads 24 to lugs 23 may be accomplished by any suitable means such as soldering or by bolts 26 extending through holes 21 in lugs 23 and engaging nuts on the bottom thereof, as shown, to force electrical leads 24 into good electrical contact with lugs 23. A further function of lugs 23 may be in the mechanical mounting of the cathode in a tube or discharge device and lugs 23 may be formed into desired conguration to facilitate such use.

There has been described above the major elements of one embodiment of the invention and a brief description of the operation thereof now follows. A power supply 28 is connected between electrical leads 24 and which contact opposite ends of base element II at lugs 23 thereof to cause a heating current to flow directly through base element Il. This current flow causes base element Il, bar I8 and enclosed activating material Il to heat up and to thereby vaporize activating material Il. The heating current is controlled so that the temperature of activating material Il is raised to the point where it very slowly vaporizes. This vapor is contained by the bottom, sides and ends of trough I3 but escapes by migrating through the porous structure of bar II. A continual movement of vapor through bar I8 is maintained and there is consequently produced upon surface 20 of bar I8 a monoatomic layer of activating material which is being continually replaced. This monoatomic layer of activating material I'I, such as for example barium or strontium or both, upon the surface of bar i8, formed of sintered tungsten for example, materially lowers the work function of the surface and there is consequently formed an electron emissive surface having very good emission properties.

Shield 2I acts to prevent heat transfer away from trough i3 and bar I8 and may have a polished inner surface to facilitate the accomplishment of this function. Outer shield or cover Z2 also reflects heat back into trough I3 and bar I8 and, in addition, serves to clamp the assembly together. Cover 22 may have the further function of dening or limiting the extent of the electron emissive surface and for this purpose may be formed in whole or in part of a metal which does not have its electron emission properties materially affected by activating material. By this construction the leak or iiow of 'vaporized activating material upon the portion of cover 22 contacting surface 2i! does not extend the electron emission surface beyond the exposed portion of surface 23 of bar I8. While numerous materials are suitable for use as cover it has been found that molybdenum is particularly adaptable in this respect. It will, of course, be appreciated that various materials may be used in the above-described embodiment of the invention and that the examples are in no way limiting, and it should also be noted at this point that the inclusion of shield 2I is optional and is not absolutely necessary, although advantageous. It is further noted that the described cathode is adapted to be employed in combination with means to remove or attract electrons from the electron emission surface thereof and in this respect potential supply means may be connected between the cathode and one or more other electrodes as of a vacuum tube or other device employing thermionic cathodes.

There has been set forth above the details of a single preferred embodiment of the invention and attention is invited to the ease of assembly and disassembly thereof. In fabrication of the cathode, base element II is formed either from a single plate of stock with trough I3 milled therein or may be built up of plates joined together by suitable means. Bar I8 likewise may be milled from one piece, or boss I9 may be separately formed and joined thereto. In assembly trough I3 is filled to a predetermined level with activating material which may be in powdered form, and bar I8 is placed upon trough I3 with boss I9 thereon mating with trough I3 and extending into trough i3 to contact activating material Il. Shield 2|, which may be precut and folded, is then slipped on to the troughbar combination; or alternatively a sheet of material may be folded about the trough-bar combination to form shield 2I and such may be either pre-cut or cut-off after assembly. The final step consists of' placing cover 22 upon the combination and this may be accomplished by pre-cutting and preforming cover 22 and slipping it onto the combination from the end and then pressing the turned edges and back of cover 22 into close contact with base element I I and bar I8 to clamp same together; or may alternatively be accomplished by folding a sheet of cover material about trough I3 and bar d8, cutting off the ends thereof above surface 2G and then crimping these ends over the edges of surface Z to tighten cover` 22 about the combination and hold same in assembled relation. It will be appreciated from the foregoing that assembly and disassembly of the cathode structure is a simple operation accomplished with a minimum of equipment and time, and therefore very advantageous economically. Furthermore, the elements of the invention are relatively few, quite simple and rugged, and comparatively inexpensive, all of which resuits in a practical dispenser type cathode readily adapted for use in a variety of tubes and devices.

The particular embodiment illustrated is adapted for use in multiple about the circumference of a circle with a plurality of elongated anodes disposed one adjacent each cathode, and as electrons leave the electron emissive surface substantially normal thereto the radius of curvature of the surface is calculated to focus the electrons at the adjacent anode, or alternatively it may be considered that the electrostatic field about the cathode surface'converges away from the surface and thereby draws the emitted electrons together at a focus some distance from the surface as determined by the radius of curvature thereof. The use of a curved cathode surface is not limited to this suggested application and, in fact, the entire invention is not to be limited by the illustrative examples set out above, and thus attention is directed to the following claims for a delineation of the true scope of the invention.

What is claimed is:

1. A dispenser type cathode comprising a metal trough having electrical connections extending from the ends thereof and adapted to be connected across a current supply whereby current ows through said trough heating same, a trough cover mating with said trough in sealing relation thereto and deiining an enclosed reservoir therein, said trough cover being formed of a porous metal to admit vapor flow therethrough, volatile activating material disposed in the enclosed reservoir in said trough to be vaporized by the heat from said trough to pass through said trough cover, a heat shield disposed about said trough, and a cover sheet disposed about said trough and a portion of said trough cover in clamping relation thereto whereby said trough, cover, and heat shield are maintained in position.

2. A dispenser type cathode comprising a metallic trough having an open top and a bottom member extending beyond the ends thereof forming electrical connectors, a current source connected across said connectors whereby an electrical current iiows through said trough and heats it, a bar of porous metal having a boss thereon mating with said trough opening and in assembly with said trough dening an enclosed reservoir therein, a volatile activating material disposed Within said enclosed trough reservoir to be vaporized by the heat of said trough to pass through said porous metal bar, a metal foil disposed about and contiguous with said trough to reflect heat from said trough back into said trough, and a cover disposed about said trough and bar in clamping relation thereto holding said trough and bar together, said cover extending over less than the entire outer surface of said bar defining an electron emissive surface thereon, the outer surface of said porous bar with the vaporized activating material forming an electron emissive surface and said cover formed of a material substantially unaffected by vaporized activating material.

3. A dispenser type cathode as defined in claim 2 further defined by said trough being formed of tantalum, said porous bar being formed of sintered tungsten, said activating material comprising barium and strontium carbonate and said metal foil and cover being formed of molybdenum.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,121,589 Espe June 21, 1938 2,131,204 Waldschmidt Sept. 27, 1938 2,459,841 Rouse Jan. 25, 1949 2,488,716 Elenbaas 1 Nov. 22, 1949 2,543,728 Lemmens et al Feb. 27, 1951 2,640,949 Cook June 2, 1953 OTHER REFERENCES Philips Technical Review, vol. 11, No. 12, pp. 341-350, June 1950. See Fig. 1(b).

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