US2978605A - Gaseous arc discharge device - Google Patents

Description

April 4, 1961 J. M. LAFFERTY 2,978,605

GASEOUS ARC DISCHARGE DEVICE Filed Oct. 17, 1957 //7 ve mor James M. Lofferfy,

His Afforneyl GASEOUS ARC DISCHARGE DEVICE James M. Laiferty, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed Oct. 17, 1957, Ser. No. 690,851

12 Claims. (Cl. 313247) The present invention relates to gaseous electric arc discharge devices. More particularly, the invention relates to improved instant-start, high-temperature thyratron devices.

Thyratron gaseous arc discharge devices generally comprise a cathode, an anode and a control electrode or grid. The control electrode is arranged to provide almost complete shielding between cathode and anode so that a small negative potential applied to the control electrode prevents a gaseous discharge between cathode and anode even when a high potential is applied therebetween. There is, however, for each value of anode-cathode potential, a value of control electrode potential which just allows the gaseous atmosphere to become ionized. Once this occurs an arc discharge is established between cathode and anode and the control electrode loses control. The control electrode becomes effective again only if the anode-cathode potential is decreased to the point where the arc is extinguished. While sustaining an arc discharge, thyratrons exhibit only a few volts potential difference between anode and cathode. Because of this, thyratrons are quite useful as switching devices and the like.

In such applications it is of great importance that the breakdown characteristics of the thyratron devices remain constant and stable throughout the life of the tube. It may also be important that the characteristic be independent of temperature and that the device operate at high temperatures.

Accordingly, it is an object of the present invention to provide thyratron gaseous arc discharge devices which maintain a stable breakdown characteristic throughout the device life.

Another object of the invention is to provide thyratron gaseous arc discharge devices which operate stably over a wide range of temperatures.

A further object of the invention is to provide instantstart thyratron gaseous arc discharge devices.

Still another object of the invention is to provide thyratron gaseous arc discharge devices having improved heat dissipation characteristics.

In accord with the present invention, I provide thyratron gaseous electric discharge devices having cylindrical symmetry with concentric anode, control electrode and axial cold arc type cathode. The control electrode and cathode are supported by support members which are insulatingly stacked with the anode cylinder and hermetically sealed to form the device envelope. The interior of the envelope is filled with a high purity noble gas. The cathode utilized is. an instant-start arc typecause only metal and ceramic are utilized in its construc- 2,978,605 Patented Apr. 4, 1961 vention itself, together with further objects and advantages thereof, may best be understood by referring to the following detailed description taken in connection with the appended drawing in which:

Fig. l is a vertical cross-sectional view of a device constructed in accord with the present invention,

Fig. 2 is a vertical view "of a portion of one type cathode which may be utilized in the device of Fig. 1, and

Fig. 3 illustrates another suitable cathode.

In Fig. 1, a thyratron gaseous are electric discharge device constructed in accord with the present invention comprises a cylindrical metallic anode member -1, a concentric cylindrical mesh control electrode 2 concentric within anode 1 and of somewhat greater length, and an axial cold arc type cathode 3 along the axis of the cylinders comprising the anode and control electrode. Control electrode cylinder 2 is supported at both ends by annular metallic support members 4 which have an inside diameter less than the inside diameter of anode cylinder l and substantially the same diameter as control electrode cylinder 2. Cathode 3 is supported at either end by a metallic end wall member 5 which has substantially the same diameter as the outside diameter of anode cylinder 1. All of the metallic members supporting the cathode and control electrode are separated from one another and from the anode cylinder by a plurality of annular insulating ceramic spacers 6 which form hermetic seals with the metallic members to enclose the entire structure and form an hermetically sealed envelope. In operation, the end walls 5 provide a cathode terminal, the exterior of cylinder 1 provides an anode terminal, and the support rings 4 each provide control electrode terminals. The entire assembly is filled with a highly purified stable noble gas.

Although anode cylinder 1, control electrode support members 4 and cathode support members 5 may be fabricated of a highly conductive material such as copper, they are preferably fabricated of titanium due to the unique gettering characteristics of titanium. I have found that, in noble gas thyratron gaseous arc discharge devices, the reproducibility of breakdown parameters throughout the life of the devices is markedly dependent upon the purity of the gaseous atmosphere utilized. Thus, if a substantial quantity of a gaseous impurity such as CO H O, H 0 and N is present within the noble gas atmosphere, the breakdown characteristics tend to change as the tube ages so that a higher or lower negative potential is required to inhibit breakdown at a given anodecathode voltage depending upon the particular impurity present. Accordingly, utilizing titanium electrodes, a highly purified stable noble gas atmosphere which may comprise any single noble gas such as helium, argon, neon, krypton, xenon or mixtures thereof, but which preferably comprises xenon, may be obtained by mechanically fitting various parts of the thyratron device together as shown in Fig. l of the drawing and sealing the ceramic members to the titanium members in an atmosphere of the noble gas with which the tube is to be charged at a suitable pressure. This process for the formation of noble gas gaseous electric discharge devices is disclosed and claimed in my copending application Serial No. 690,849, filed concurrently herewith and assigned to the present assignee, now U.S.. Patent No. 2,957,741. V I

When the devices of the present invention are so formed, the titanium is heated to a temperature at which it bonds to the titanium-matching ceramic members 6. At these temperatures (of the order of 700 C. to 1100" C.) titanium is an excellent getter for gases such as CO H O, H N and Thus, While the tube is being formed, the titanium members which comprise the tube eifectively remove all impurity gaseous constituents therefrom leaving a highly purified noble gas, preferably xenon, which results in the maintenance of stable breakdown conditions throughout the life of the device.

In a gaseous arc, as used in devices of the invention, and as opposed to a gaseous glow discharge, the potential difference between anode and cathode is dependent only upon the gas utilized. In most applications wherein thyratron devices are used, namely in switching applications and the like, it is of the greatest importance that the potential difference across the thyratron be a minimum, leaving the major portion of the available potential for the load device to be operated thereby. Accordingly, the devices of the present invention, to great advantage, preferably are filled with an atmosphere of xenon (arc voltage=8 v.), although for certain applications argon (16 v.), neon (2.1 v.), krypton (14 v.) or helium (25 v.) may be used.

Titanium-matching annular sealing members 6 are composed of a refractory ceramic, the coefficient of thermal expansion of which is a close match for that of titanium, and which may be suitably bonded at high temperatures to form hermetic seals with titanium anode cylinder 1, control electrode support members 4 and cathode support disks 5. Such a ceramic may be a sintered agglomerate of silicon oxide, magnesium oxide, and aluminum oxide denominated as Forsterite. One such Forsterite ceramic and the method of preparation thereof is disclosed and claimed in the copending application of A. G. Pincus, Serial No. 546,215, filed November 10, 1955, and assigned to the assignee of the present invention.

Cylindrical mesh control grid 2 may conveniently be formed of stainless steel, molybdenum, tungsten or any material which does not react strongly with titanium. Preferably the electrode is made from stainless steel, since at the temperatures at which the tube may be formed (approximately 700 C. to 1100" C.) stainless steel and titanium form a strong bond with one-another, and no further means is necessary to secure control electrode cylinder 2 and control electrode support members 4 together. Conveniently cylinder 2 may be approximately 20-20 per inch mesh.

Cathode 3 is a cold arc type cathode. As used herein the phrase cold arc type cathode means a cathode having low heat capacity for the volume it occupies, low thermal conductivity along its length, low spectral emissivity and good thermionic emitting characteristics. When these conditions are satisfied it is not necessary that the cathode be heated by an external source, such as by the passage of an electric current therethrough, in order to cause the formation of an are between the cathode and an anode. This is because, in a cold arc type cathode, the process by which an arc is established is one of the ejection of secondary electrons and the bombardment of the cathode by positive ions which gradually raise its temperature without external energy being supplied therethrough. Thus, when the cathode has a low heat capacity, only a small amount of heat need be added to a particular spot to raise it to a temperature at which thermionic emission occurs. Similarly, when the cathode possesses low thermal conductivity it is possible for one spot upon the cathode to be raised to a temperature at which thermionic emission may occur without dissipating heat rapidly throughout the entire cathode. Likewise, when the cathode possesses low spectral emissivity it is similarly easy to build up the temperature in one spot thereon without the loss of heat energy by radiation. Finally, when the cathode material possesses good thermionic emitting characteristics the cathode temperature at which an arc is established need not be excessively high.

. One such cathode structure satisfying these conditions comprises a doubly coiled helix of tungsten wire formed into a third loosely wound helix 8 and coated with a mixed carbonate of barium, calcium and strontium for the attainment of good thermionic emitting characteristics. In Fig. 2 the composite doubly-coiled helix, from which the large loosely-wound helix 8 shown in Fig. 1 is formed, is illustrated in detail. No claim is made to the novelty of this cathode structure per so since it is disclosed and claimed in U.S. Patents Nos. 2,306,925 to Aicher and 2,774,918 to Lennners. This cathode, however, possesses the characteristics necessary for an are between cathode and anode in the present device. Thus, since the cathode is formed of a triple coil of fine wire rather than a solid rod, it possesses low heat capacity. Since the coil is not a single conductor and is fabricated from fine tungsten wire, the electrical conductivity of which is relatively low, the thermal conductivity of the coil along its length is low. Additionally, heat conduction does not occur along the wire but must occur through the carbonate coating thereupon which coating presents a high impedance to the passage of heat from one coil to an adjacent coil. The carbonate upon the surface of the tungsten wire possesses good thermionic emitting characteristics and low spectral emissivity. Accordingly, all the characteristics for a cold arc type cathode are satisfied. Cathode leads 7 support cathode 3 in place and may be heavy rods of the same materials comprising cathode 3.

Other structures may similarly satisfy the necessary criteria for the cathode utilizing the present invention. Thus, for example, as illustrated in Fig. 3, the cathode may comprise a conducting base material 10 about which there is woven a finely divided mass of metallic wool fibers 11 coated with a suitable thermionic emitting substance. This cathode likewise, has low heat capacity, poor thermal conductivity, low spectral emissivity and good thermionic emitting characteristics. Such a material, in mass, is disclosed and claimed in my copending application Serial No. 444,939, filed July 22, 1954, and assigned to the present assignee.

Since the cathodes utilized in devices of the present invention are cold arc-type cathodes which require no heating, the devices of the present invention are instant starting and may be utilized in a number of switching applications in which conventional thyratron gaseous arc discharge devices, which require a finite time for cathodes to become incandescent, may not be used.

If the cathodes utilized in the present invention are operated at the gas pressures conventionally utilized in thyratron devices, for example, 0.05 to 0.1 mm., the cold arc type cathodes do not possess long-life characteristics. I have found, however, that long life may be obtained by operating the devices at a pressure of approximately 1 to- 3 mm. of mercury pressure. For optimum results, this requires that the control electrodeanode spacing be reduced because of the reduced mean free path of ions at this increased pressure. In general, for optimum operation in the devices of my invention, the product of gas pressure, in millimeters, and control electrode-anode spacing, in centimeters, should be approximately equal to 2.5 for xenon, 3.5 for helium, 3.5 for neon, 1.0 for argon, and 2.25 for krypton.

A further advantage of devices constructed in accord with the present invention is derived from the fact that anode cylinder 1 comprises an integral part of the envelope of the discharge device. Thus, anode 1 may readily be liquid cooled or air cooled, facilitating the operation of the device at high current densities and at high temperatures, resulting in a greatly increased efliciency. Additionally, since no soft glass parts are utilized in these devices, which comprise only ceramic and a a metal, the devices may be operated at temperatures up to 500 C. with no deleterious eifects and maintain their operating characteristics stable over this range of tem peratures.

While the invention has been described hereinbefore with respect to certain embodiments thereof, many changes and modifications will immediately occur to those skilled in the art without departing from the spirit of the present invention. Accordingly, I intend by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the present invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A thyratron gaseous electric discharge device comprising a metallic anode cylinder member; a mesh control electrode cylinder concentric with said anode cylinder and interior thereof said control electrode being longer than said anode cylinder; a pair of metallic annular support members supporting said control electrode; a cold arc-type cathode extending axially along the center of said cylinder said cathode exhibiting characteristics of low heat capacity and low thermal conductivity as compared with metals and adapted to serve as the footpoint of an arc between anode and cathode in the absence of resistance heating thereof; a pair of metallic disk-shaped cathode support members supporting said cathode and forming end walls for said device; a plurality of insulating ceramic members interposed between said end wall members and said control electrode support members and between said control electrode support members and said anode member and hermetically sealed thereto to form an hermetically sealed envelope; and a highly purified stable noble gas atmosphere within said envelope.

2. The device of claim 1 in which said metallic members are of titanium.

3. A thyratron gaseous electric discharge device comprising a metallic anode cylinder member; a mesh control electrode cylinder concentric with said anode cylinder and interior thereof said control electrode being longer than said anode cylinder; a pair of metallic annular support members supporting said control electrode; a cold arc-type cathode extending axially along the center of said cylinder; a pair of metallic disk-shaped cathode support members supporting said cathode and forming end walls for said device; a plurality of insulating ceramic members interposed between said end wall members and said control electrode support members and between said control electrode support members and said anode memher and hermetically sealed thereto to form an hermetically sealed envelope; and a highly purified atmosphere of xenon within said envelope.

4. The device of claim 3 in which said metallic members are of titanium.

5. A thyratron gaseous electric discharge device comprising a metallic anode cylinder member; a mesh control electrode cylinder concentric with said anode cylinder and interior thereof said electrode being longer than said anode cylinder; a pair of metallic annular support members supporting said control electrode; a cold arctype cathode extending axially along the center of said cylinder; a pair of metallic disk-shaped cathode support members supporting said cathode and forming end walls for said device; a plurality of insulating ceramic members interposed between said end .wall members and said control electrode support members and between said control electrode support members and said anode member and hermetically sealed thereto to form an hermetically sealed envelope; and a highly purified xenon gas atmosphere within said envelope, the product of gas pressure in millimeters and the radial spacing between anode and control electrode cylinders in centimeters being approximately equal to 2.5.

6. The device of claim 5 in which said metallic members are of titanium.

prising in stacked array a metallac cylindrical anode member; apair of annular metallic grid support members in spaced relation with respective ends of said anode member, the inside diameter of said grid support members being substantially less than the inside diameter of said anode member; a pair of metallic disk-shaped end wall members in spaced relation with respective outer ends of said grid support members; a plurality of insulating ceramic spacers interposed between each pair of adjacent metallic members and forming hermetic seals therewith to form an hermetically sealed envelope; a cylindrical metallic mesh grid electrode extending between said grid support members and concentric within said anode member; an electron emissive cold arc-type cathode member extending centrally along the axis of said cylindrical grid electrode and between said end wall member said cathode exhibiting characteristics of low heat capacity and low thermal conductivity as compared with metals and adapted to serve as the footpoint of an arc between anode and cathode in the absence of resistance heating thereof; and a highly purified noble gas atmosphere within said envelope.

8. The device of claim 7 in which said metallic members are of titanium.

9. A thyratron gaseous electric discharge device comprising a metallic anode cylinder member; a mesh control electrode cylinder concentric with said anode cylinder and interior thereof; a pair of metallic annular support members operatively connected with and supporting said control electrode; a cold arc-type cathode extending axially along the center of said cylinders; a pair of metallic disc-shaped cathode support members supporting said cathode and forming end walls for said devices; a pair of insulating ceramic members interposed between said end wall members and said control electrode support members; a second pair of insulating ceramic members interposed between said control electrode support members and said anode member and hermetically sealed thereto to form an hermetically sealed envelope; and a highly purified gaseous atmosphere selected from the group consisting of xenon, helium, neon, argon and krypton within said envelope, the product of gas pressure in millimeters of mercury and the radial spacing between said anode and said control electrode cylinders in centimeters being approximately equal to a number having a value of 2.5 when the gaseous atmosphere utilized is xenon, 3.5 when the gaseous atmosphere utilized is helium, 3.5 when the gaseous atmosphere utilized is neon,

1.0 when the gaseous atmosphere utilized is argon; and

2.25 when the gaseous atmosphere utilized is krypton.

10. A thyratron gaseous electric discharge device comprising a metallic anode cylinder member; a mesh control electrode cylinder concentric with and interior of said anode; a pair of metallic annular support members operatively connected with the ends of said control electrode; a cold arc-type cathode extending axially along the center of said cylinders and comprising a thermionically emissive active surface region adapted to serve as the footpoint of an electric are between said anode and said cathode in the absence of resistance heating thereof, said surface region exhibiting the characteristics of low heat capacity and low thermal conductivity as compared with metals; a pair of metallic disc-shaped cathode support members supporting said cathode and forming end-walls for said device; a plurality of insulating ceramic members interposed between said end-wall members and said control electrode support members and between said control electrode support members and said anode member and hermetically sealed thereto to form an hermetically sealed envelope; and a highly purified stable noble gas atmosphere within said envelope.

11. A thyratron gaseous electric discharge device comprising a metallic anode cylinder member, a mesh control electrode cylinder concentric with said anode and interior thereof; a pair of metallic anular support members'operatively connected with the ends of said control electrode; a cold arc-type cathode extending axially along the center of said cylinders; a pair of metallic disc-shaped cathode support members supporting said cathode andforming end-walls for said device; a plurality of insulating ceramic members interposed between said end-wall members and said control electrode support members and between said control electrode support members and said anode member and hermetically sealed thereto to form an hermetically sealed envelope; and a highly purified stable noble gas filling at a pressure of approximately l-3 mm. of mercury within said envelope.

12. A thyratron gaseous electric discharge device comprising a metallic anode cylinder member; a mesh control electrode cylinder concentric with said anode cylinder and interior thereof; a pair of metallic annular control electrode support members operatively connected with the ends of said control electrode; a cold arc-type cathode extending axially along the center of said cylinder and comprising a highly thermionically emissive surface region adapted to form the footpoint of an electric are between said cathode and said anode, said surface region exhibiting low thermal conductivity and low heat capacity as compared with metals; a pair of metallic disc-shaped cathode support members supporting said cathode and forming end-walls for said device; a plurality of insulating ceramic members interposed between said end-wall mem- .anode members and hermetically sealed thereto to form an hermetically sealed envelope; and a highly purified stable noble gas atmosphere at a pressure of approximately l-3 mm. of mercury within said envelope.

References Cited in the file of this patent UNlTED STATES PATENTS 515,465 Cottrell Feb. 27, 1894 1,401,510 Baumbauer Dec. 27, 1921 1,991,174 Rose Feb. 12, 1935 1,991,767 McCullough Feb. 19, 1935 2,025,565 Blau Dec. 24, 1935 2,099,531 Passarge Nov. 16, 1937 2,104,784 Wiegand Jan. 11, 1938 2,167,515 Katsch July 25, 1939 2,368,060 Wooten Ian. 23, 1945 2,497,911 Reilly Feb. 21, 1950 2,567,369 Edwards Sept. 11, 1951 2,613,336 Doolittle Oct. 2, 1952 2,731,578 McCullough June 17, 1956 2,870,364 Doolittle Jan. 20, 1959 2,910,607 McCullough Oct. 7, 1959 FOREIGN PATENTS 907,201 Germany Mar. 22, 1954

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