US2976451A - Electric discharge device - Google Patents

Description

March 21, 1961 G. H. REILING ETAL 2,976,451

ELECTRIC DISCHARGE DEVICE Filed June s, 1959 2 Sheets-Sheet 1 Cs Rb K Na Li ALKAL/ ME TAL-MERCUR) POOL EXCITATION POTENTIALS INVENTORSZ GILBERT H. REILING, ANDREW O. JENSEN BY/P THEIR ATTORNEY.

March 21, 1961 Filed June 8, 1959 ARC DROP IN VOLTS 0 Q G. H. REILING ETAL 2,976,451

ELECTRIC DISCHARGE DEVICE 2 Sheets-Sheet 2 FIG.3.

WHOLLY MERCURY POOL ALKALI METAL-MERCURY POOL TUBE I I I I 60 I00 I40 I80 WALL TEMPERATURE IN DEGREES CENTIGRADE ARC DROP IN VOLTS ALKALI METAL-MERCURY POOL TUBE I l I I I 40 80 I20 I60 ANODE CURRENT IN AMPERES INVENTORSZ GILBERT H. REILING, ANDREW O. JENSEN,

g m ATTORNEY.

ELECTRIC DISCHARGE DEVICE Gilbert H. Reiiing, Schenectady, and Andrew 0. Jensen,

Guilderland, N.Y., assignors to General Electric Company, a corporation of New York Filed June 8, 1959, Ser. No. 818,874 5 Claims. (Cl. 313-171) -peres has been near the ionization potential of mercury or approximately volts, with the arc drop increasing in devices adapted for higher currents. This voltage drop has been considered necessary to maintain the electron emission from the mercury cathode spot and to replenish the positive ions which are ordinarily lost in neutralizing space charge normally occurring in the device.

Although the voltage drop of mercury cathode ignitrons is not generally considered substantially large and presents no substantial problems for many applications, it has been long considered desirable to provide an ignitron-type electric discharge device adapted for lower voltage drop than encountered in ignitrons, thus to obtain higher operating efliciency and higher operating tempera tures. Higher operating efiiciency is not only generally desirable in all devices of this type but would enable wider applications of ignitron-type devices; While higher operating temperatures are desirable in that they would obviate the need for liquid cooling generally required for mercury pool ignitron operation and would make feasi ble air cooling or other relatively low capacity cooling means in applications Which previously required Water cooling. Additionally, at the higher currents of the mentioned range there is a tendency in mercury pool tubes for the arc to run free or move rapidly and erratically over the surface of the cathode. For various reasons, including the desire to minimize the arc drop and obtain better control of vapor pressure in the device, it is generally desirable to stabilize the are or anchor the arc spot to a restricted area of the pool. Heretofore it has not been possible to obtain the desired spot anchoring at high currents.

In addition to various redesigns of the anode-cathode geometries of the device, the efforts previously made to reduce arc losses have been directed toward substituting cathode materials having lower ionization potentials for the mercury cathode or the confinement of the discharge by magnetic means. By some of these means reduction in :arc losses have been realized; however, other difficulties have rendered these means commercially infeasible. For example, some of the ionizable materials sought to be substituted for the mercury have been found 'too expensive to be employed in any appreciable quantity in a commercially feasible device, have been found so :reactive chemically. as to be incompatible with known United States Patent 0 2,976,451 Patented Mar. 21,

tube envelope and seal materials and also so reactive as to constitute substantial health hazards in normal manufacturing operations and handling. Further, some materials sought to be substituted for mercury required preheating to enable ionization and initial operation or start ing of the device. Devices requiring pre-heating as well as any requiring auxiliary equipment, such as magnetic field producing means, have been found generally undesirable for the obvious reason that they add considerably to the effort and cost of manufacturing and maintenance. In connection with the anchoring of the cathode spot it has been found that at high currents, in the order of amperes or more, elements placed in the device for serving as an anchor point have become dewetted of mercury and thus rendered incapable of serving effectively as an anchor. 1

Accordingly, the primary object of the present inven' tion is to provide a new and improved cold cathode arc discharge device adapted for low arc losses.

Another object of the present invention is to provide a new and improved cold cathode 'arc discharge device of the ignitron type adapted for greater operating eificiency than ignitrons incorporating a wholly mercury pool cathode and thus eiiective for adapting ignitron-type devices for wider applications.

Another object of the present invention is to providea new and improved cold cathode arc discharge device adaped for affording low arc losses by means of a contained ioniza-ble medium and requiring no auxiliary equipment, such as pre-heating means and magnetic field producing means, for low arc loss operation Another object of the present invention is to provide a new and improved cold cathode arc discharge device incorporating a liquid cathode pool, the major constituent of which can comprise mercury for afiordin-g, in addition to the desired low arc losses, all of the advantages and minimizing other undesirable effects of arc instability.

Another object of the present invention is to provide a new and improved cold cathode arc discharge device of the ignitron type including means for suppressing the internal pressure of the device and thus adapting the device for higher operating temperatures.

Another object of the present invention is to provide an improved cold cathode arc discharge device adapted for withstanding higher inverse voltages without'arc back and which may be satisfactorily cooled by air or other means not having the substantial cooling capacity of water-cooling means.

Still another object of the present invention is topro vide a new and improved cold cathode arc discharge device which is commercially feasible from the standpoints of function and cost and presents no substantial difficulties in the way of health hazards in manufacture or handling of materials employed therein.

Further objects and advantages of the invention will become apparent as the following description proceeds and the features of novelty which characterize the invention will be pointed out with particularity in the claims annexed to and forming part of this specification.

In carrying out the objects of the present invention there is provided a cold cathode arc discharge device of the ignitron type including an envelope having an insulative wall section electrically separating a cathode.

However, when operating temperature is attained, excited metastable mercury atoms ionize the alkali metal atoms and the ionized alkali metal neutralizes the effects of the space charge in the device to increase the current conducting capacity of flue device. Lower arc drop follows since a greater voltage drop across the device for ionizing mercury atoms to effect neutralization of the space charge is not required. Additionally, lowering of the arc drop results from a lowering of the anode work function by condensation of alkali metal on the anode. Also disposed in the tube is a member of high refractory low sputtering rate material which protrudes from the surface of the cathode and is adapted for being wetted by the mercury for serving as an anchor for the cathode spot. In order to avoid de-wetting of the member at high currents the alkali metal serves to maintain the member substantially oxide free.

For a better understanding of the invention reference may be had to the accompanying drawing in which:

Figure l is a sectional view of an electric discharge device incorporating a form of the present invention;

Figure 2 is a chart illustrating the excitation potentials of cathode materials employable in the structure of Figure 1;

Figure 3 is a comparative illustration of the arc-drop versus temperature curves of a wholly mercury pool tube and a tube constructed according to the present invention; and

Figure 4 is a comparative illustration of the arc-drop versus current of a wholly mercury pool tube and a tube constructed according to the present invention.

Referring to the drawing, there is shown in Figure 1 a cold cathode arc discharge device embodying a form of the present invention. The device comprises an envelope 1 including an insulative bulb Z constituting an insulative wall section of the device and which can be advantageously formed of glass or ceramic. The upper endof the envelope includes an external portion 3 of an anode generally designated 4 which is supported centrally in the envelope and includes an enlarged portion bearing atransversely extending planar active surface 5. The anode 4 is formed preferably of any suitable high refractory metal. Additionally, the mentioned external portion 3 can serve effectively as an anode connector. A sealing assembly 6 bonded to both the anode and the bulb 2 hermetically seals the anode to the bulb and, for reasons which will be brought out in detail hereinafter, the seal afforded by the assembly 6, as well as all other seals in the device, is selected so as to be substantially oxide-free at least on the inner surfaces thereof.

Closing the lower end of the bulb 2 to complete the envelope is a metal header 7 carrying a cathode lead 8. The header 7 can also be advantageously formed of a high refractory metal and is sealed to the lower end of the bulb by a sealing ring 9 adapted also for presenting a substantially oxide-free inner surface.

Contained in the bottom of the envelope is a liquidpool-type electrode 10 comprising mercury and an alkali metal. The percentage composition of the cathode 10 and the function thereof in the operation of the device will be brought out in greater detail hereinafter.

Associated with the electrode 10 is a starter electrode generally designated 11 which, as seen in Figure 1, can comprise a dielectric starter including a dielectric element 12 partially immersed in the cathode and supported by a conductive lead 13 extending through the side Wall of the bulb 2. The lead 13 is also sealed in the device by a sealing ring 14 adapted for presenting a substantially oxide-free inner surface. The starter 11 can be one of various generally available dielectric starters adapted for pool tube firing. Alternatively, a band starter, or one in which an electrode surrounds the external wall of the envelope, or a resistance ignitor, both of which types are widely known, may be employed in place of the dielectric starter 11.

Secured on the inner surface of the header 7 and protruding centrally from the surface of the cathode 10 is a cathode spot anchoring membr 15. The member 15 is formed of a metal which is high-refractory in nature and has a low sputtering rate such, for example, as molybdenum, zirconium, tungsten, vanadium, niobium, and columbium. Of these materials, molybdenum has been found particularly highly satisfactory as a spot anchoring member.

As pointed out above, the cathode 10 comprises mercury and a quantity of alkali metal. The alkali metal is preferably selected from the group of alkali metals having first ionization potentials below the first excited metastable level 4.7 e.v. of mercury. As seen in Figure 2, cesium, rubidium, and potassium constitute alkali metals having ionization potentials below the first excited metastable level 4.7 e.v. of mercury. The alkali metals sodium and lithium, as seen in Figure 2, have first ionization potentials below the excited metastable level 5.4 e.v. of mercury. These alkali metals are employable in the cathode 10. However, sodium and lithium have relatively low vapor pressures and in some devices the use of rubidium, potassium and cesium appears preferable. The advantages of employing a pool cathode including both mercury and an alkali metal and the operational relationships of the ionization potentials of the alkali metals and the excited metastable levels of mercury will now be brought out in detail.

During initial operation of the present device, it functions in a manner similar to that of the conventional ignitron incorporating a wholly mercury pool cathode. That is, the starter 1 1 initiates an arc to the surface of the pool to establish a cathode spot and the electrons traveling from the cathode toward the anode surface effect ionization and metastable excitation of mercury molecules in the region between the cathode and anode, thus to supplement the current flow toward the anode by providing positive ions effective for neutralizing the space charge which normally results in the device. The space charge neutralization has the desirable effect of increasing the electron flow or current carrying capacity of the device. However, in the conventional tube the voltage drop across the device in the low current range of approximately 10 to amperes is near the ionization potential of the mercury, this voltage drop increasing as the current is increased. This voltage drop is required to maintain the electron emission from the cathode spot and to replenish the positive ions lost from the discharge.

In the presently disclosed device and, as pointed out above, the operation is initially the same as in the conventional wholly mercury cathode arc device. However, soon after starting the long-lived metastable mercury atoms will have become excited at least to the metastable level of 4.7 e.v. and if either rubidium, potassium or cesium are used will effect ionization of the alkali metal atoms and the cathode 10 by means of what is currently understood to be a two-stage collision process, 'If sodium or lithium are used the ionization by the metastable mercury atoms will occur when the latter have become excited to the 5.4 e.v. level. The thus ionized alkali metal has the desirable effect of neutralizing the space charge and results in increased effici encypwhich adapts the device for applications where devices of less efficiency would be unacceptable.

Additionally, the work function of the cathode pool is not believed to be afiected by the presence therein of the alkali metal. However, in operation the alkali metal in the device is believed to be condensed on the active surface 5 of the anode at least to the extent of a monolayer which is understood to have a lowering elfect on the work function of the anode. These factors are also believed effective in decreasing the voltage drop across the device.

Further advantage of the presence of the alkali metal in the cathode material is believed to result from the depressing effect thereof on the vapor pressure of the mercury. With the vapor pressure thus depressed the device is better adapted for withstanding inverse voltages and less reliance is placed on cooling of the device to require satisfactory operation. That is, in wholly mercury pool tubes substantial cooling, such as by a circulating liquid coolant, is required to insure a vapor pressure in the device at which the tube would operate as eflectively as possible without substantial danger of undesirable arc back. The present device can operate cooled, for example, only by circulating air and still be elfective for withstanding substantial inverse currents or avoiding arc back.

The alkali metal also has the desirable effect of maintaining the surface of the spot anchoring post 15 substantially clean or oxide-free. In prior devices, when current approached, for example, 100 amperes anchoring elements have become oxidized and'de-wetted of mercury causing the meniscus of the mercury to be convex or to turn down at the point of juncture with the post 15-; and, thus, at high currents the post was rendered less effective for anchoring a spot. In the present invention the alkali metal maintains the post 15 clean or substantially oxide free, thus to cause the meniscus to be concave in the manner illustrated in Figure 1 or, in other words, to avoid any substantial de-wetting of the member 15 and thus to insure elfectiveness of the member 15 in anchoring the cathode spot at high currents as well as relatively low currents. In addition to attributing to the low arc drop, the anchoring of the spot has the deisrable effect, by avoiding free running of the are over the surface of the pool, of reducing evaporation and vapor blast, known as the Tanberg efiect, from the cathode spot. Additionally, better control of the vapor pressures is possible, resulting in ability of the device to withstand higher applied inverse voltages and avoidance of the need for de-ionization surfaces such as grids and bafiles.

Still further, the presence of alkali metal with mercury in the atmosphere of the device does not cause undesirable low electrical resistance across the internal insulative surface of the envelope, as would be expected if only alkali metals were employed. This advantage desirably obviates the need for external heaters and other means to prevent unwanted electrical conduction across the envelope wall.

The above-described functional advantages can be attained over substantially wide percentage composition ranges of mercury and alkali metal. For example, decreases in arc drop have been obtained with mixtures of mercury and rubidium where the rubidium has constituted up to about 80% of the mixture. However, in constructing a commercially feasible device it is deemed desirable to utilize as much mercury as possible relative to the amount of alkali metal employed because of the relative ease of initiating an arc to mercury and the ability of mercury readily to supply a copious quantity of electrons for high current purposes. Additionally, mercury is more readily available and less expensive than the alkali metals; and the alkali metals, because of their high chemical reactivity, can, when used in substantial amounts, present substantial problems in connection, for example, with handling and the provision of compatible seals. Further, it has been found that mixtures containing substantial quantities of alkali metal relative to mercury have a tendency to solidify which precludes starting of a device without first pre-heating. While it is recognized that the presently disclosed device can be supplied with PIC-heating means if for any reason it is desirable to provide a device viously known to be obtainable with the use'of a wholly mercury pool cathode and also without introducing prob lems or requiring auxiliary equipment not heretofore required with wholly mercury pool cathodes. These desiderata are obtainable when the pool 10 comprises about from .02% to about 20% of an alkali metal and the remainder being mercury. Alkali metals selected from the group including cesium, rubidium and potassium and combinations thereof, including alloys and mixtures are considered particularly advantageous. These particular alkali metals, as seen in Figure 2, have ionization potentials lying below the excited metastable levels 4.7 e.v. and 5.4 e.v. of the mercury and, thus, are readily ionizable by the metastable atoms of the mercury to provide the above-described ionization effect for neutralizign the space charge in the device, The other alkali metals sodium and lithium are employable in some applications and, as seen in Figure 2,' have ionization potentials below the excited metastablelevel, 5.4 e.v. of the mercury and are also ionizable by the metastable atoms of the mercury to provide the mentioned ionization effect for neutralizing the space charge. Illustrated, for example, in Figure 3 and indicated. by the solid line is an arc-drop versus temperature .curve of a device constructed according to the present invention and including a liquid cathode pool comprising 1.8% alkali metal, which in this particular case' was rubidium, and the remainder mercury. As shown ,by

this curve an arc drop of approximately only 5.4'volts was attained at a temperature extending over a substantially wide high temperature range. tive purposes the dash line'is provided in Figure'3'to illustrate the fact that in wholly mercury pool tubes the lowest arc drop is substantially higher than the presently considered type of device and is attainable only when the tube is operating at a substantially lower tem-' perature than that at which the lowest arc drop is experienced in the presently disclosed type of device.

Additionally, in Figure 4 the curves illustrate arc-.

drop versus anode current of the same devices. The

dash line represents the operation of the wholly mercury pool device and, as illustrated, the current of this device appears to change relatively sharply which is generally considered characteristic of this type of device. 'For, example, at 40 amperes the arc drop of this device is approximately 11 volts compared with 5.6 volts for the device of the present invention, the curve of which is illustrated by a solid line. Comparable low arc drops are obtainable when comparably small amounts of alkali metals or combinations of such metals are included in 1 a cathode pool, the remainder of which is mercury.

Additionally, it has been found that mixtures of the allower than those attainable in wholly mercury cathode 7 devices. Additionally, the alkali metal serves to de press the mercury vapor pressure to adapt the device For comparai for withstanding substantially greater inverse voltages and higher operating temperatures to enable cooling, for example, by air. Further, the alkali metal present in the tube atmosphere does not significantly decrease the electrical resistance of the insulative wall surfaces between the opposed cathode and anode. Still further, in such small amounts use of the alkali metal does not become prohibitively expensive or present any substantial problems regarding handling and the provision of compatible seals and envelope materials. When more than about alkali metal is employed, the arc tends to anchor on the juncture of the glass wall and pool edge which has the undesirable effect of rendering the are less stable and damaging the envelope. Additionally, higher amounts of alkali metal have a solidifying effect on the pool and thus necessitate pre-heating means to enable starting. Where a solid pool or the use of heating means are not objectionable, the higher amounts of alkali metal are employable for obtaining the various advantages described above.

While a specific embodiment of the invention has been shown and described, it is not desired that the invention be limited to the particular form shown and described, and it is intended by the appended claims to cover all modifications within the spirit and scope of the invention.

What is claimed as new and desired to secure by Letters Patent of the United States is:

1. A cold cathode arc discharge device comprising an envelope, an anode in said envelope, a liquid cathode in said envelope in spaced insulated relation with respect to said anode, said cathode consisting of alkali metal in an amount between about .02% to about 20% and the remainder mercury, and means for initiating an arc discharge between said anode and cathode.

2. A cold cathode arc discharge device comprising an envelope, an anode in said envelope, a cathode in said envelope, said cathode having an active portion consisting of mercury and an alkali metal, a high refractory metal member protruding from the surface of said cathode adapted for being wetted by said mercury, thereby to anchor a cathode spot in said device, said alkali metal being effective for minimizing de-wetting of said member during operation of said device, thus to insure continued anchoring of said spot, and means for initiat- 8 ing an arc discharge between said anode and cathode.

3. A cold cathode arc discharge device comprising an envelope, an anode in said envelope, a cathode in said envelope, said cathode having an active portion c0nsisting of a pool of mercury and alkali metal with said alkali metal consisting from about 02% to about of said active portion of said cathode, a post of high refractory low sputter rate metal protruding from the surface of said pool and adapted for being wetted by said mercury thereby to anchor a cathode spot in said device, said alkali metal being effective for maintaining said post substantially oxide-free, thereby to minimize de-wetting of said post during operation of said device, and means for initiating an arc discharge between said anode and cathode.

4. A cold cathode arc discharge device according to claim 2, wherein said envelope includes an insulative wall section separating said anode and cathode, said post comprises molybdenum, said cathode having an active portion consisting of between about .02% to about 20% alkali metal and the remainder mercury, and said alkali metal is additionally eifective for being ionized to neutralize any space charge in said device and to lower the work function of said anode, thereby to increase the current capacity of said device.

5. A cold cathode arc discharge device comprising an envelope, an anode in said envelope, a cathode in said envelope in spaced insulated relation with respect to said anode, said cathode consisting of at least one alkali metal selected from the group consisting of cesium, rubidium and potassium in an amount between about 02% to about 20% and the remainder mercury, and means for initiating an arc discharge between said anode and cathode.

References Cited in the file of this patent UNITED STATES PATENTS 1,757,605 Ulrey May 6, 1930 2,152,201 Miles Mar. 28, 1939 2,500,153 Cork et al Mar. 14, 1950 2,541,842 Teare Feb. 13, 1951 2,650,319 Middleton Aug. 25, 1953 2,651,737 Marshall Sept. 8, 1953

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