US2937304A - Electric-discharge device and cathode - Google Patents

Description

May 17, 1960 Filed Sept. 25, 1957 FIG.

FIGZ

s. GOLDBERG ETA!- 2,937,304

ELECTRIC-DISCHARGE DEVICE AND CATHODE 2 Sheets-Sheet 1 mm: I

I I y I I H: I i 2" I i! i l! 4K" U INVENTORS ij SEYMOUR GOLDBERG DAVID S. GOLDSTEIN By KENNETH J. GERMESHAUSEN A TTORNE YS May 17, 1960 s. GOLDBERG ETAL ELECTRIC-DISCHARGE DEVICE AND CATHODE 2 Sheets-Sheet 2 Filed Sept. 25, 1957 INVENTORS SEYMOUR GOLDBERG DAVID S. GOLDSTEIN BY KENNETH J. GERMESHAl/SEN ATTORNEYS U d Sta P ten ELECTRIC-DISCHARGE DEVICE AND CATHODE Seymour Goldberg, Lexington, David S. Goldstein, Dorchester, and Kenneth J. Germeshausen, Newton Center, Mass., assignors to Edgerton, Germeshausen & Grier, Inc., Boston, Mass, a corporation of Massachusetts Application September 25, 1957, Serial No. 686,193

2 Claims. (Cl. 313-217) The present invention relates to electric-discharge devices and cathodes therefor, and, more particularly, to improved cathode structures for use in gaseous-discharge tubes and the like. i

As is explained in co-pending application, Serial No. 580,551, filed April 25, 1956, by Kenneth J. Germeshausen and Seymour Goldberg, for Electric-Discharge Device And Cathode, various approaches have been proposed for solving the problem, particularly associated with highpower gaseous-discharge devices and the like, such as hydrogen thyratrons that switch, pulse or key high-voltage circuits, of providing a cathode of small physical dimensions that, nonetheless, has a large electron-emissive surface area. Several types of cathode configurations have beenevolved to try to accomplish this end, including the utilization of a plurality of parallel cathode-element electron-emitting surfaces disposed within a cup.

Since, however, all portions of such cathode elements do not actually effectively emit electrons toward the anode of the discharge device at the instant that a sudden electrical discharge is initiated in the switching or keying circuits, before mentioned, and is applied to the discharge device, most of the electrons being drawn from the portion of the cathode nearest to the anode, improved operation has been obtained with cathode elements of limited length, the emitting surfaces of which are more uniformly utilized. As is further explained in the said co-pending application, a preferred substantially optimum length L of cathode element has been selected that represents a compromise between the provision of va-large electronemissive surface area and the effecting of more uniform utilization of the cathode emitting surface.

anode is given substantially by the following equation:

where R is the specific resistance at the electron-emitting surface, having a nominal value of four ohm-centimeters squared; i is the total current discharged cumulatively to the anode in the dischargedevice; E is theelectrical field in the plasma between the anode and the cathode, having a nominal value of fourteen volts per centimeter; V, is the potential drop across the cathode sheath;

That length 7 L, measured in the direction between the cathode and r" ce ternal coatings, or preferably, as described in the said copending application, comprising a base member carrying a plurality of spaced vanes each having electron-emitting surfaces extending along a direction away from the base member. The cathode vanes may either be indirectly heated by the base member, or they may be formed into a continuous vane configuration, rather than separate vane segments, in order to become adapted for the passage of heating current therethrough so as to operate as a directly or self-heated cathode, as in co-pending application, Serial No. 660,592, filed May 21, 1957, by the said Seymour Goldberg, for Electric-Discharge Device and Cathode,

It has been further discovered that in certain types of gaseous-discharge-tube configurations, more particularly where the tips or free edges of the cathode vane segments are very closely positioned to an anode or other electrode in order, for example, to provide for a relatively low starting voltage, the electron-emissive coating near the said tips or free edges becomes sputtered off. The sputtered particles, moreover, may deposit upon the adjacent anode or other electrode, giving rise to secondary emission therefrom or inverse-current phenomena, as well as modifying the starting and other characteristics of the tube. These secondary electrons are drawn to the cathode vane tips and strike the tips with an energy equal to the applied inverse voltage.

In the case where the vane tips are coated, these electrons may cause either excessive heating of the coating in this region or could result in chemical decomposition of the coating. Either ofthese result in bursts of released gas generally leading to breakdown. These difliculties are avoided, as later explained, by not coating the vane tips.

An object of the present invention, accordingly, is to provide both a new and improved cathode structure and a new and improved discharge device that shall not be subject to these disadvantages.

Other and further objects will be explained hereinafter and will be more particularly pointed out in the appended claims.

The invention will now be described in connection with the accompanying drawing, Fig. 1 of which is a perspective view of a cathode of the separate vane-segment type, above described, embodying the present invention;

Fig. 2 is a similar view of the previously describedcontinuous-vane type of cathode, also incorporating the invention; I

Fig. 3 is a view similar to Fig. 1 of a modified construction; illustrating further details; and

Fig. 4 is a longitudinal section of an electric-discharge devicecontaining the cathode of Fig. 1.

Referring to Fig. 4, an electric-discharge device, illustrated'as of the rectifier type, comprises a sealed preferably ceramic-walled envelope 21 bounded at its top by a cup-sh aped'anode 1, and containing a cathode structure 3, illustrated as of inverted cup-shaped form and provided along its preferably substantially planar top 'surface, with: an aperture for exposing the internal structure and, that is disposed near and substantially parallel to the substantially planar bottom surface of the anode. The envelope 21 may be filled with an ionizable medium such as hydrogen gas.' The flanges 1' and 3' of the respective anode and cathode structures 1 and 3 are sealed 7 to the envelope walls by, for example, a titanium-hydride- 3 shown disposed close to the envelope walls to provide long arc-discharge paths therebetween.

Within the cathode-structure cup 3 is disposed a base member 11 supporting or carrying upon its upper surface a plurality' of thin substantially planarcon'ducting vane segments 13, 15, 17, 19, that may be in the form of annular concentric cylinders, as more particularly illustrated in Fig. 1, and as described in the said co-pending application, Serial No. 580,551. Juxtaposed vane segments 13, 15, 17, 19 are thus spaced from one another preferably uniformly, and substantially parallel to one another. The vane segments 13, 15, 17 19 extend upward from the base member .11 along the direction from the cathode 3 to the anode 1, substantially perpendicular or normal to the lower surface of the anode 1. While five vane segments 13, 15, 17 19 are shown, it is to be understood that a larger or smaller number may be employed and that the vane surfaces may also assume other configurations than cylinders, including the parallel rectangular configuration, 13', 15, 17', etc. within the single aperture cathode cup 3 of Fig. 3.

The lateral flat surfaces of the thin vane segments 13, 15, 17 19 are coated with electron-emissive material and the vertical length or height of the vanes preferably conforms substantially to the dimension L of equation (1) herein, as before explained. The cathode may be heated by means of a heater winding 2, shown of round strips in Fig. 3 and of fiat strips in Fig. 4, disposed in the base 11, as described in the said co-pending application, Serial No. 580,551. Struts 41, 43 may serve not only to assist in supporting the cathode-vane assembly in position, but to connect by sealed terminal posts 45 to conductors outside the base 35 of the envelope for con nection to a source of heater current. If a continuous cathode-vane surface is employed, Fig. 2, having, for example, reversely curving spiral or other vane segments 23, 25, 27 29, substantially parallel to one another in regions, the heater current will be passed between the inner free edge 2" of the vane segment 23 and the outer free edge 2' of the vane segment 29 through conductors 43, 41, as described in the said co-pending application, Serial No. 660,592.

Investigation has shown that when the length or height of the cathode vane segments is adjusted substantially to the optimum value L of equation (1), highly uniform and etficient emission can be achieved even though the electron-emissive coating 4 upon the vane segments 13, 15, 17 19 of Figs. 1 and 4, or 13', 15', 17', etc. of Fig. 3, or 23, 25, 27 29-of Fig. 2, terminates short of the tips or upper, free edges thereof opposite the adjacent anode or other electrode 1. This fortuitous circumstance, accordingly, has permitted the obviating of the sputtering difiicultly, above mentioned, by permitting the regions 6 near the vane tips or free edges to be void of electron-emissive-coating material 4. These void regions cannot, however, have any arbitrary extent. They must be limited to a narrow strip 6 adjacent the vane tips or upper edges, as otherwise loss in electron emission and alteration in operating characteristics result.

With cathode-vane surfaces of nickel, say of the order of 0.02 inch in thickness and with spacings of the order of l to 2 millimeters, and of the order of one-quarter of an inch in height L, a very large effective electronemitting surface area can be achieved with the void strip 6 of height restricted to from substantially one-sixteenth to one-eighth of an inch down from the upper edges of the vane segments. Preferred electron-emissive coating layers 4 comprise barium, strontium and calcium oxide compounds.

The cathode of the present invention is, of course, useful in other than theparticular hydrogen discharge rectifier device before referred to. It may, as but a further illustration, be employed in gaseous thyratrons, as described 'in the said co-pending applications. Further modifications will occur to those skilled in the extend 4 all such are considered to fall within the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. An electric-discharge device having a plurality of electrodes including an anode and a cathode spaced from one another along a predetermined direction within an ionizable medium, the cathode comprising a plurality of spaced vanes having electron-emitting surface coatings extending substantially along the predetermined direction, and the length L of the vane, measured along the said predetermined direction, being given substantially by the following equation:

dV, dx

is the potential gradient in the cathode sheath, and P is the total perimeter of the cathode in a plane substantially parallel to the surface of the anode, the coatings terminatitritg short of the free edges of the vanes opposite th ano e.

2. An electric-discharge device having a plurality of electrodes including an anode and a cathode spaced from one another along a predetermined direction Within an ionizable medium, the cathode comprising a continuous vane surface shaped to provide a plurality of juxtaposed spaced vane segments having electron-emitting coatings extending substantially along the predetermined direction, and the length L of the vane, measured along the said predetermined direction, being given substantially by the following equation:

where R is the .specific resistance at the electron-emitting surface, i is the total current discharged cumulatively to the anode in the discharge device, E is the electrical field in the plasma between the anode and the cathode, V is the potential drop across the cathode sheath,

dV, dx

is the potential gradient in the cathode sheath, and P is the total perimeter of the cathode in a plane substantially parallel to the surface of the anode, the coatings terminating short of the edges of the vane segments opposite the anode. 1

References Cited in the file of this patent UNITED STATES PATENTS

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