US2871391A - Electron tube structure - Google Patents

Description

Jan. 27, 1959 o. H. SCHADEV ELECTRON TUBE STRUCTURE Filed Sept. '15, 1955 `I N V EN TOR. 07m Saw/:of

: Mam/L nited ELECTRON TUBE STRUCTUREl Otto H. Schade, West Caldwell, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application September 15, 1955, Serial No. 534,448

4 Claims. (Cl. 313-238) This invention relates to an electron tube structure of voltage peak, that is, when the anode is negative with respect to the cathode, the anode temporarily becomes a cathode. When the electrostatic field density between the anode and cathodel is relatively high, ofthe order of say 100,000 volts per centimeter (such as when the tube is operated at inverse peak voltages of over, say 30,000 volts), cold or field emission takes place. Electrons are pulled from the anode and are accelerated toward the cathode and toward metal members on which the cathode is supported. Some of the electrons miss the metal members and bombard the envelope. This electron bombardment eventually results in a rupture of the envelope. In previous rectifier tubes, such as tubes of the type wherein the cathode is supported within a tubular anode by a pair of parallel wires, the relatively small diameter of the wires gives rise to relatively high electrostatic field densities. Some of the electrons from the anode are attracted into the space between the cathode support wires (the support wires acting as a grid) and are accelerated by the electrostatic field between the wires to a portion of the glass envelope resulting in increased bombardment of the envelope.

Accordingly, it is an object of the invention to provide an improved rectifier tube structure which is economical of manufacture and wherein the destruction of the envelope, by electron bombardment from the anode of the tube, is substantially eliminated.

in high voltage rectifier tubes and electrostatic stresses between the anode and cathode often result in the cathode being torn from its support. The cathode is usually mounted in the center of a tubular anode. 1f the cathode is moved out of this center position during tube operation, such as when the tube is subjected to mechanical shock, the electrostatic stresses on opposite sides of the cathode are no longer balanced and the cathode is attracted by the anode with a force such that it is physically moved to the anode. In order to minimize electrostatic forces on the cathode, previous tubes have resorted to the use of hemispherical anodes and bow-shaped cathodes with the attendant increased cost and complexity of the tube. The bow-shaped cathode was subjected to relatively high stresses since the electrostatic force urging it vto the anode was directed to only one side of the cathode. The use of a hemispherical, as distinguished from a tubular, anode also resulted in an exposure of a relatively large area of the tube envelope to electron lbombardment by the aforementioned field emission from the anode.

It is a further object of the invention to provide an Patentimproved rectifier tube having a structure in which the cathode isv maintained substantially free of unbalanced and a tubular cathode support in coaxial relation andr with the cathode support having a portion thereof terminating an appreciable distance within the anode. A cathode is mounted on the end of the cathode support within the anode. The coaxial anode and cathode support arrangement provides a relatively strong mechanical support for the cathode; this better enables the cathode to withstand momentary unbalanced electrostatic stresses such as those which would be caused by mechanical shock during tube operation. In one embodiment of the invention, the tubular cathode support is provided with a skirt which has a radial extent from the tube axis at least as great as that of the inside extent of the anode and which is axially spaced a distance from the anode about twice that between the anode and the cathode support. The cathode support and the anode are preferably in theform of cylinders having a ratio between their diameters of the order of two to one for providing a minimum electrostatic field concentration at any point in the space within the anode. The inner, cathode support cylinder physically intercepts most of the electrons from the anode duringl the periods of inversevoltage. The electrons which escape from the space within the anode are intercepted by the skirt. The electrostatic held, between the anode and the tubular portion and skirt of the cathode support, focuses these electrons onto the skirt. Thus the tube envelope is preserved from electron bombardment during periods of inverse voltage.

In the sole figure of the drawing there is illustrated a high voltage rectifier tube of thetype similar to that known commercially as the 3B2. The tube comprises a glass envelope 1 having at one end thereof a base 2. Within the envelope referred to are mounted a tubular anode 3 and a cathode 4. The anode 3 is closed at one end 5. This end 5 is fixed to a tubular access terminal 6 for supporting the anode 3 and for providing electrical access thereto. The other end 7 of the anode 3 is open. Adjacent to its open end' 7, the anode 3 has outwardly ared edges 8 to prevent high electrostatic field concentration at this end. The cathode 4 is supported at one end thereof by a Wire 9 which is positioned along the axis of the tube andl at the other end thereof by a second wire 10 which is fixed to a relatively heavy, generally U-shaped, wire loop 11. The wire loop 11 is in turn fixed at -its ends to a cathode support cylinder llZ which is concentric with the anode 3. The wire loop 11 acts as a low mu grid; it lowers the space potential adjacent to the cathode 4 since the loop is electrically connected to the cathode. This reduces the magnitude of the electrostatic forces of the attraction between the cathode 4 and the anode 3 acting directly on the cathode. The cathode support cylinder is supported by a metallic shield or skirt 13 which protects fromkelectron bombardment the portion of the glass envelope 1 adjacent to the base 2. The skirt 13 has a radial extent from the tube axis at least as great as that of the inside radial extent of the anode 3- and is axially spaced a distance from the anode about twice that between the cathode support and the anode. The tubular cathode support 12 and the central wire 9 are connected to appropriate ones of prongs 14 mounted on thebase 2. The cathode 4 is operated at a predetermined reference po-v tential, and the anode 3 is adapted to have an alternating current potential applied to it. In operation, the potential difference between the anode and cathode are Patented Jan. 27, 1959 often of the order of 70,000 volts in the tubes of the aforementioned type 3B2.

The coaxial arrangement of the cathode support and anode provides a relatively strong support for the cathodel 4 to better enable it to withstand mechanical shock during tube operation Without being torn from its sunport by unbalanced electrostatic stresses. Since the cathode support is in the form of a cylinder, the free or unsupported end of it does not bend appreciably even when the tube is subjected to shock. Thus the cathode is maintained along the anode axis where the radial electrostatic forces on the cathode are equal in all directions,

The anode cylinder 3 and cathode support cylinder 12 have a ratio between their diameters of the order of about two to one in order to at the same time (a) prevent bombardment of the envelope by electrons from the anoder during periods of inverse voltage and (b) avoid field emission from the anode so that no appreciable inverse current flow (that is, current from the 'anode to the cathode) occurs within the tube. As the diameter of the cathode support cylinder is decreased the electrostatic field density about any place on the surface of this cylinder is increased. This is so because, as is known, for two concentric cylinders the eld density around the inner cylinder increases with decreasing inner cylinder diameter. Consequently, in order to provide a minimum electrostatic field density it would seein desirable to use as large a support cylinder diameter as could be mechanically accommodated within the tube. A large diameter support cylinder is also desirable since greater numbers of electrons, traveling from the anode to the cathode, are then intercepted by the support cylinder. Then, too, the cathode is provided with a greater mechanical stability with an increased support cylinder diameter. However, as the cathode support cylinder diameter is increased, and the anode cylinder diameter remains constant, the space between the anode and the support cylinder is decreased; increased field emission from the anode to the support cylinder then takes place due to the close adjacency of the two members. A condition is soon reached in which the field emission is so great that appreciable inverse current flow occurs within the tube, and the tube ceases to act as a rectifier since it passes current in both directions. Thus, a compromise in the ratio of anode diametei' to support cylinder diameter must be had in order to eiiect an optimum relation between electrostatic field density and field emission. This optimum relation exists in a ratio between the inside diameter of the anode and the outside diameter of the cathode support cylinder ot the order of about 2 to l, that is, when the `outside diameter of the support cylinder is between 40 percent and 60 percent of the inside diameter of the anode. For example, in a tube of the aforedescribed type operated at 70,000 volts peak-to-peak voltage and having an anode cylinder with an inside diameter of 1%6 inch and a cathode support cylinder with an outside diameter of 1%2 inch, an appreciable direct current output was obtained without inverse current ow and without electron bombardment of the envelope. Ori the other hand, when the same lfyf; inch inside diameter anode was used with a 1%@ inch diameter support cylinder, the envelope glowed under electron bombardment; and when the same anode was used with a 1/2 inch diameter support cylinder, no appreciable direct current output was obtained due to high field emission from the anode to the cathode. The actual sizes of the support cylinder and the anode may be scaled up or down for correspondingly higher or lower voltage operation as long as a .ratio between the diameters is of the order of 2 to 1.

To further insure that electrons emitted from the anode cylinder 3 during periods of inverse voltage are prevented from bombarding the envelope, the configuration of the anode cylinder and rcathode support cylinder 12 structure is chosen such that while, optically, one can see the inside of the anode from the glass envelope, yet electrically, electrons from the inside of the anode cannot see the envelope due to frefraction or deflection of the electrons by the space potential which is indicated by equipotential lines 15. The relatively few electrons which escape from the space between the anode and the cylindrical portion of the cathode support are intercepted by the skirt t3. Interception of the electrons by the skirt is effected by virtue of its relatively large radial extent and by its relatively close axial spacing from the ancde. 'the electrostatic lield established in the space between the anode and the skirt defiects to the skirt electrons (indicated by line 16) which would otherwise have a path ot travel through the space between the anode and skirt and on to the envelope. While it is 'desirable to have the anand skirt as close to each other as possible in order to establish as dense an electrostatic detiecting field as possible, the anode and skirt must be spaced far enough from each other to prevent appreciable iield emission from taking place between these two members and to prevent excessive dielectric stress of the glass envelope, the latter occurring when adjacent portions of the space adjacent to the envelope are maintained at high potential differences. Since the spacing between the anode cylinder 3 and the cathode support cylinder 12 represents the closest practical spacing between members for the voltages at which the tube is adapted to operate, the spacing between the anode and skirt is at least as great as that between the cylindrical members 3 and l2. Actually, since the re-entrant edge S of the open end 7 of the anode has a relatively small radius of curvature, the axial spacing between the anode and skirt is preferably greater than that between the two cylinders in order to insure that no appreciable field emission takes place. A spacing between the anode and skirt of about twice that between the cylindrical members 3 and 12 has proven optimum.

From the foregoing it will be apparent that the invention provides an improved rectier tube wherein the envelope is adapted to be maintained substantially free of electron bombardment, and the cathode is adapted to be maintained substantially free of electrostatic stresses even though subjected to mechanical shock.

What is claimed is: l

l. An elongated rectilier tube comprising an envelope containing a hollow cylindrical anode closed at one end only; an elongated cathode mounted within said anode', and a support structure supporting said cathode within said anode; said support structure including a hollow cylindrical member coaxial with said anode and extending 'into the open end of said anode for a distance equal to about half of 'the inside axial extent of said anode, a U-shaped metallic loop fixed at the ends thereof to the end of said cylindrical member within said anode and extending toward the closed end of said anode, andl a wire disposed coaxially within said cylindrical member and extending beyond the end of said member within said anode and spaced from said loop; said cathode being iixed at one end thereof to the end of said wire extending beyond said member and at the other end thereof to a portion of said loop intermediate said loop ends; whereby said loop is adapted to reduce, during tube operation, the magnitude of the electrostatic force of attraction between said anode and said cathode and acting directly on said cathode.

2. A rectifier tube including an envelope containing a hollow cylindrical anode closed at one end; a cathode mount comprising a hollow cylindrical member, a U- shaped metallic loop tixed at the ends thereof to one end of said cylindrical member and extending away from said member, a wire having a portion thereof disposed coaxially within said cylindrical member and extending beyond said one end of said member and spaced from said loop, and a cathode iixed at one end thereof to the end of said wire portion extending beyond said member and at the other end thereof to a portion of said loop intermediate said loop ends; whereby said loop is adapted to reduce,

during tube operation, the magnitude of the electrostatic force of attraction between said anode and said cathode and acting directly on said cathode.

3. A rectifier tube comprising an envelope containing a hollow cylindrical anode closed at one end, a hollow cylindrically tubular cathode support having a common axis with said anode and extending into the other end of said anode for a distance equal to about half of the inside axial extent of said anode, the outside diameter of said cathode support being between 40 percent and 60 percent of the inside diameter of said anode, a metallic skirt xed to said cathode support and having a radial extent from the common anode and cathode support axis at least as great as that of the radial extent of said anode and spaced axially a distance from said other end of said anode about twice the radial distance between said anode and said cathode support, and a cathode mounted on the end of said cathode support within said anode.

4. A rectifier tube including an envelope containing a hollow cylindrical anode closed at one end; a cathode mount comprising a hollow cylindrical member extending concentrically to within said anode, a rst cathode connector and support means fixed to the end of said hollow cylindrical member within said anode and extending away from said member toward the said closed end of said anode, a second cathode connector and support means comprising a wire having a portion thereof disposed coaxially Within said cylindrical member and extending beyond the said end of said member within said anode, and a cathode connected and supported between said irst and said second cathode connector and support means.

References Cited in the le of this patent UNITED STATES PATENTS 1,647,238 Manthorne Nov. 1, 1927 2,332,428 Atlee et al Oct. 19, 1943 2,397,982 Salzberg Apr. 9, 1946 2,656,479 Brown Oct. 20, 1953 2,719,935 Muller Oct. 4, 1955

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