US2456900A - Spark-gap device and electrode therefor - Google Patents

Description

1948. H. J. SULLIVAN ET AL 2,456,900

SPARK-GAP DEVICE AND ELECTRODE THEREFOR Filed Sept. 2', 1944 INVENTORS H J'- SULA /l//]/V E 61. FT fie/V077.

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ATTORN EY Patented Dec. 21, 1948 l} NI TE D S TATES PATENT OF F'l'CeE SPARK-GAP. DEVICE AND ELECTRODE THEREFOR 'Application september 2, 1944, SerialNo.-552,i14

3 Claims. -(Cl. 250-275) The present invention relates to spark-gap devices of the type now commonly used in connection with pulsing of electrical energy and has particular reference to an anode electrddeitherefor.

Devices of this general type are most frequently employed as a modulator to generate energy pulses and are subjected to high voltage. It has been found that the breakdown voltage of the device tends to decrease during the life of the device. Such variation in breakdown voltage naturally causes trouble in operation ofthe device and is a detriment to the useful life of the device. Investigation has proven that the diifh culty is due mostly to the nature of the sputtered deposit which is formed on the surface of the anode. Presumably the sputtered material arrives at the anode as finely divided particles which gradually cover the surface. Instead of forming a smooth deposit the surface is slightly rough having numerous small projections. Such proiections cause the voltage gradient to be higher at such points than it would be for a smooth surface and this high voltage gradient in turn results in a reduction in the breakdown'voltage the device.

It is accordingly an object of the present invention to provide a spark-gapdevice in which the breakdown voltage is maintained substantially constant during the useful life of such device.

Another object of the present invention is the provision of a spark-gap device provided with, an anode having a roughened surface and an increase in gas pressure over that required with an anode of smooth surface, for the purpose of maintaining the breakdown voltage of the device substantially constant during the entire useful life of the device.

Still further objects of, the present invention will become obvious to those skilled in the art by reference to the accompanying drawing wherein:

Figure 1 is a side view partly in cross-section of a spark-gap device constructed in accordance with the present invention;

Figure 2 is a side view of one form which the anode as used in the device of Fig. 1 may take;

Figure 3 is a side view of another form which the anode may take;

Figure 4 is a side View of still another form which the anode may take.

Referring now to the drawing in detail, the device as shown in Fig. 1 comprises a vitreous envelope 5 closed at each end by metallic end caps 6 and 1, formed of a metal such as Kovar or the like having substantially the same .00- eflicientxof expansion as that of the vltreous'envelope' 5, thus maintaining an hermetic seal therebetween. One of the end capsli rigidly supports ananode-electrode 8 of iron or cast steel which coaxially projects into a cylindrical cathode Q of suitable metal, such as aluminum or the like, and supported by the other end cap l.

It will be noted that the anode of the device is provided with an elliptical end it at its inner extremity and likewise has what might be termed an elliptical surface ll. of substantially the same radius of curvature adjacent the inner extremity of the cylindrical cathode 9, for the purpose of preventing concentration of the arc discharge due to a high potentialgradient at anypoint on the anode surface. vice thus far described is disclosed more in detail in the prior patent of Clarence E. Dawley and Edward G. F. Arnott, No. 2,411,241, granted November 19, 1946, and assigned to the same assignee as the present invention. Consequently it is believed unnecessary to herein further detail such construction since reference can be had to this copending application therefor.

Heretofore such device has been filled with a suitable ionizable medium, such as a mixture of about 80% hydrogen and 20% argon at a pro"- sure of approximately 680 min, prior to sealingoff at an exhaust tip 13, which together with the electrode spacing, establishes a desired initial breakdown voltage for the device. Moreover, the anode 8 has heretofore been of relatively smooth surface, which, as previously mentioned, accumulates sputtered particles roughening the surface and causing a variation in the breakdown voltage of the device with shortening of its useful life.

Paradoxically enough, it has been found that by deliberately roughening the anode surface as occurs during operation under previous conditions and slightly increasing the gas pressure, this variation in breakdown voltage during life of the device is substantially decreased. The anode is accordingly roughened in any suitable manner, such as by sandor shot-blasting, by threading as shown in Figs. 1, 3 and i, or by knurling as shown in Fig. 2. It is only essential that sufficient area of the surface be initially roughened to sustain the discharge until the balance of the smooth surface is roughened during operation by the sputtered material.

To deliberately roughen the anode surface without some provision to the contrary would naturally change the breakdown voltage of the device, as above noted. However, by increasing The construction of the dethe pressure of the gaseous medium to about 800 mm., the device with a deliberately roughened anode surface will have a breakdown voltage over substantially the same range as that of a device with a smooth surfaced anode and a gas pressure of 680 mm., all other factors, such as electrode spacing, etc., remaining the same. Moreover, the breakdown voltage of the spark-gap device with the roughened surface anode and slightly higher gas pressure will remain substantially constant throughout its useful life, whereas that of the devices heretofore employing smooth surfaced anodes varies during operation, thus shortening the useful life thereof.

In Fig. 4 a modification is shown wherein the anode 8 differs from that shown in Fig. 1 merely in the provision of a recess M in the end of the anode which projects into the cathode. Such recess will serve to prevent building-up of sputtered material at the extreme end of the anode.

It thus becomes obvious to those skilled in the art that a spark-gap device is herein provided of the type normally employed in connection with the pulsing of electrical energy and wherein the 1 range of breakdown voltage of the device remains substantially constant during its entire useful life. Moreover, while several embodiments of the present invention have been shown and described, it is to be understood that still further embodiments thereof may be made without departing from the spirit and scope of the appended claims.

We claim:

1. A spark-gap device comprising a vitreous envelope provided with metallic end closure members hermetically sealed thereto, a cathode supported by one of said closure members and an anode supported by the other of said closure members in telescopic relation with said cathode, an ionizable medium in said envelope for supporting a discharge between said electrodes upon the application of a potential thereto, said anode being provided with an end devoid of an abrupt change in radius of curvature at its inner extremity and having a similar surface of curvature adjacent the inner extremity of said cathode, said anode having a roughened surface to offset the effect of sputtered particles accumulated thereon from said surrounding cathode, and the pressure of said ionizable medium being higher than that normally employed for the same applied potential with an anode of smooth surface.

2. A spark-gap device comprising a vitreous envelope provided with metallic end closure members hermetically sealed thereto, a cathode supported by one of said closure members and Number an anode supported by the other of said closure members in telescopic relation with said cathode, an ionizable medium in said envelope for supporting a discharge between said electrodes upon the application of a potential thereto comprising a mixture of approximately hydrogen and 20% argon, said anode being provided with an end devoid of an abrupt change in radius of curvature at its inner extremity and having a similar surface of curvature adjacent the inner extremity of said cathode, said anode having a roughened surface to off-set the effect of sputtered particles accumulated thereon from said surrounding cathode, and the pressure of said ionizable medium being higher than that normally employed for the same applied potential with an anode of smooth surface.

3. A spark-gap device comprising a vitreous envelope provided with .metallic end closure members hermetically sealed thereto, a cathode supported by one of said closure members and an anode supported by the other of said closure members in telescopic relation with said cathode, an ionizable medium in said envelope for supporting a discharge between said electrodes upon the application of a potential thereto comprising a mixture of approximately 80% hydrogen and 20% argon, said anode being provided with an end devoid of an abrupt change in radius of curvature at its inner extremity and having a similar surface of curvature adjacent the inner extremity of said cathode, said anode having a roughened surface to off-set the effect of sputtered particles accumulated thereon from said surrounding cathode, and the pressure of said ionizable medium being at least 15% higher than that normally employed for the same applied potential with an anode of smooth surface.

HAROLD J. SULLIVAN. EDWARD G. F. ARNOTT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Name Date 1,565,659 Lilienfeld Dec. 15, 1925 1,603,279 Gray Oct. 19, 1926 ,787,699 Morrison Jan. 6, 1931 1,903,996 Gray Apr. 18, 1933 1,992,852 Bader Feb. 26, 1935 2,366,459 Rosa Jan. 2, 1945 2,404,658 Robinson, Jr July 23, 1946 2,422,324 V/atrous, Jr June 17, 1947 2,422,659 Depew et al June 24, 1947

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