US2409715A - Electric arc device - Google Patents

Description

Oct. 22, 1946. cf M, g K HAL 2,409,715

ELECTRIYC ARC DEVICE Filed Jan. 27, 1942 mvsN'rdR an. 6246/: E a. F. manor-r ATTORNEY WWWM- v Patented @ct. 22, 1946 ELECTRIC ARC DEVICE Charles M. Slack, Glen Ridge, and Edward G. F. Arnott, Upper Montclair, N. J., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application January 27, 1942, Serial No. 428,352

This invention relates to electric are devices and particularly to such devices having a liquid cathode which operates with a stationary starter or make-alive device, and constitutes a pulser as well as a means for rectifying, controlling or converting high voltage, and is directed to improvements which have been introduced for the purpose of overcoming difficulties encountered in electric vapor-arc devices of the prior art.

The invention contemplates use of a vaporizable reconstructing liquid cathode, constituting and often designated a pool cathode. Inasmuch as the pool is more often a liquid than a solid, and as mercury is commonly employed for the purpose, the pool will be hereinafter designated as mercury, but it is to be understood that such designation is merely a matter of convenience and not to be construed as limiting, as the pool may be of known substitutes of which various amalgams, as well as lead, cadmium, caesium, tin, etc., are examples.

Analogous devices of the prior art have shown ability to pass relatively high current at low voltage, but to employ high potential with either high or low current, particularly at high frequency, has heretofore eluded adequate solution. The invention herein set forth is directed to meet the present-day need of a pulser which will effectively, and with long life, function with high voltage, say 50,000 volts more or less.

Operation of devices of the general character under consideration depends upon use of a makealive electrode, commonly referred to as a starter or ignitor, to form a cathode spot. When the cathode spot is formed, if the voltage between cathode and anode is sufficient, the main discharge will start. However, because of the ionization present in the device, the possibility of loss of control is considerably increased and has proved to be a serious handicap in prior art devices. Study of the involved situation makes it clear that tendency towards loss of control in part depends directly on the gas or vapor. density in the device; that lack of control increases as the vapor density increases; that these factors of gas and vapor density affect the mean free path of electron travel; and that loss of control is more prevalent as the mean free path of electrons is shortened.

The mean free path of an electron in a gas, as herein meant, is the average distance which an electron travels under the influence of an electric field impressed on the gas between impacts with gas molecules (or atoms) If the potential difference through which the electron passes is suffi- 11 Claims. (Cl. 25027.5)

ciently great to bring the electron up to the ionizing speed before impact, ionization will take place, the collision in this case resulting in more electrons and positive ions being given off from the gas molecule, the positive ions passing to the cathode and releasing further electrons, this process being cumulative and sustaining gaseous conduction. The electrical conduction through the gas increases with its pressure and the mean free path of an electron becomes progressively shorter with increases in pressure.

In view of the above dissertation, it will now be recognized that an object of the present invention is to provide a pool type of electric arc device which will withstand high voltages effectively and without loss of control.

A further object of the invention is to provide a pool type of electric arc device which will not only withstand high voltages effectively without loss of controhbut which also will pass high currents.

A still further object of the invention is to provide 3, p001 type of electric arc device which will withstand high voltage, will pass high current, and yet which will have a rapid recovery time sufficient to enable the 'device to be used with higher frequencies than can be utilized in presentday analogous devices.

Another object is to provide a device with cathode-anode spacing relatively short and arranged to eliminate long paths of electron travel.

Yet another object of the invention is to maintain low vapor pressure during operation.

Still another object is to ignite the device by low energies as compared with energies required by present-day standard form of ignitors.

Additional objects include reduction of distortion of the electric field by the ignitor; to maintain uniform electric field between anode and cathode; to coat all parts with glass which are subject to high gradients; to secure simplicity of construction and operation; to prevent accumulation of mercury on undesirable portions of the envelope; and to obtain other advantages and results as will appear to those skilled in the art as the description progresses, both by direct statement thereof and by implication from the context.

Referring to the accompanying drawing in which like numerals of reference indicate similar parts throughout the several views;

Figurel is an elevation of an electric are device constructed in accordance with the present invention; V

Figure 2 is a central vertical-sectionthereof with the scale of drawing enlarged over Fig. 1; and

Figure 3 is a cross-sectional view on line III III of Fig. 2.

In the specific emboliment of the invention illustrated in said drawing the reference numeral l designates a metallic cathode cup next the bottom of the device and numeral 1 i designates a similar anode cup next the top of the device, the hollows of the cups being directed toward each other. The said cups are preferably composed of a nickel-cobalt-iron alloy sold under the trade name Kovar. The rims of the cups are sealed into the opposite ends of a cylindrical glass or other suitable insulative body portion 12 intervening between the said cups and forming therewith a sealed and evacuated chamber [3. By preference, said body portion I2 comprises borosilicate glass for obtaining a permanent joint with the said cups which will not readily crack or leak in use.

The middle portions of-both cups are spun or otherwise fabricated to form outwardly projecting hollow necks l4, l5, of which the neck I5 in the anode or upper cup constitutes an evacuating opening which is finally sealed by a bead I6 likewise preferably of borosilicate glass.

Cathode cup [0 has a starter electrode I'I projecting upwardly and axially through said neck l4. As here shown, the said neck is centrally disposed with respect to the cup, thereby locating the axis of the starter electrode coincident with the axis of the chamber and thus equidistant in all radial drections with respect to the side wall of the cup. The cathode cup is partially filled with the mercury or other pool material is, preferably leaving approximately half the depth of the side wall of the cup exposed above the mercury surface level.

Electrode I! may conveniently comprise a wire or rod of suitable metal such as tungsten or other conductive or partially conductive material, said wire being sealed by a glass or other bead l9 to neck M of the cathode cup. The structure contemplates glass-coating all of that portion of the wire or rod I! below the mercury level, an upward extension of glass in continuation of the glass of bead l9 being shown for the purpose.

An insulative cap or shield 2|, preferably of glass, is carried at the upper end of the wire or rod. This cap is downwardly facing, and by pref erance is dome-shaped and hollow, for providing an area for receiving mercury which sputters from the cathode spot and to return that sputtered mercury by gravity to the pool. By virtue of the dome-shape of cap, the same presents an outward and downwardly directed rim and said rim is brought into close proximity to the mercury surface level, but leaves a gap 22 between the rim and the surface of the mercury.

Spacing and positioning of the rim of the cap with respect to the surface level of the mercury as well as spacing with respect to the meniscus of the mercury next the electrode, and with respect to the exposed wall of the cathode cup are important for several reasons of which one is avoiding loss of control in operation, and another is obtaining sharp and accurately timed valve action. The cathode spot forms generally at or near the meniscus, and mercury sputtered thereby travels in nearly straight lines except as gravity may be effective to deflect it downward. Accordingly, the cap is arranged to intercept any mercury sputtered in directions above the juncture of the glass body portion and cathode cup.

In fact, we prefer to position and arrange said cap so sputtered mercury passing beneath the rim of the cap will strike the exposed part of the cathode cup well below the lower edge of the glass. In other words, an imaginary line from the meniscus next the starter which extends outwardly so as to be tangent at the under edge of the rim of the cap and projected therebeyond, will terminate at the envelope wall on the cathode cup portion thereof below the glass part. Considering such imaginary line the hypotenuse of a right triangle of Which the other less are the line of mercury surface and the height of intersection with the side wall above the mercury, calculation of proper spacing of the cap from the mercury is obtained by solving a simple ratio, the radius of the cap being known. The said cap is preferably substantially semi-spherical and as small as practical for accomplishing its purpose, reducing to a minimum any distortion of the electric field between the pool and the anode. According to the present showing, projection of the cap above the pool is not substantially greater than projection of the cathode cup thereabove.

It is desirable to keep the cathode pool as near room temperature as practical, and in any event at or below the temperature of the glass portion !2 of the envelope which in turn should be at or below the temperature of the anode. Exemplary of means for the purpose fins 23 are shown radiating from and attached to the exterior of the cathode cup. Other cooling or radiator means may be employed, however.

The tube constituting the present invention may be ignited by very low energies, less than one milliampere at one to two thousand volts being sufficient. This is in contrast to many amperes required with the standard form of ignitor. There is no actual flow of current but a static field is formed between the dielectric covered rod and the mercury pool which causes a tiny cathode spot to form on the mercury adjacent to the ignitor which is suflicient to allow the arc to form passing large currents with low tube drop.

It has been found that the introduction of the ignitor through the pool does not increase appreciably the energy requirements for igniting yet greatly increases the voltage standing ability. v f desired, the thickness of the dielectric coating below the mercury may be increased, thus lowering the electrical capacity as well as decreasing the energy of firing and at the same time decreasing the likelihood of failure due to electrical puncture. The thickness of the dielectric at the point of emergence from the mercury plays the predominant roll in determining the voltage requirements for initiating the arc.

Emphasis is given to the fact that present-day pool devices of analogous type to the present invention, while capable of passing large currents, suffer partial or complete loss of control when attempt is made to operate them at high voltages. The device as constructed in accordance with the foregoing description will, however, withstand high voltage and operate successfully with a long life and even at relatively high frequencies as compared with other present-day pool devices. For instance, tests have demonstrated adequacy of the present invention to function in the range of 1000 cycles and amp. peak with applied potentials of 60,000 volts, to 2,000 cycles, 40 amp. peak and 30,000 volts. Structural features lending themselves to this accomplishment include the limited projection of the ignitor with presence of and non-interferring character of the shield or cap, reducing distortion of the electric field between pool cathode and anode; the described spacing of cap edge from the mercury surface, to obtain sharply synchronized valve action; the short distance between pool cathode and anode, eliminating long paths over which an are back or glow discharge might form; prevention of undue rise in temperature of the pool cathode, maintaining low vapor density and limitation on mean free path of electrons; avoidance of mercury deposit on the glass body of the envelope, preventing formation of adverse conditions or discharge thereby; protection of metal parts, such as the starter electrode and the edges of the metal cups by glass to avoid their being subjected to high gradients and consequent break-down or possible incipient discharges originating at either the anode or cathode end of the device; avoidance of sharp corners which might produce points of possible incipient discharge; and other features of construction and operation.

Since the various details of construction and steps involved in the method, as well as the precise relation and functioning of parts are subject to variation and change without departing from the inventive concept or scope of the invention, it is intended that all matter contained in the specification or illustrated in the drawing, shall be interpreted as exemplary and not in a limiting sense. It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein shown and described and all statements of the scope of the invention herein set forth as a matter of language which might be said to fall therebetween.

We claim:

1. An electric arc device comprising an envelope having an anode and cathode, said cathode providing a pool, an ignitor electrode protruding through the pool, and a shield around said electrode, said shield having a lower edge parallel to the pool surface in spaced proximity to an underlying portion of said pool surface thereby forming a gap the length whereof parallels the pool surface and the width whereof is normal to the pool surface said lower edge of said shield constituting a limitation to exposure of the electrode thereunder.

2. An electric arc device comprising an envelope having an anode and cathode at opposite end portions thereof, said cathode providing a pool, an ignitor electrode protrudin through the pool, and a downwardly directed hollow shield around said electrode, said shield having a lower edge above and in spaced proximity to a portion of the pool surface with the spacing from said surface providing a continuous, even and open gap at the entire lower edge of the shield, the entire lower edge of the shield constituting a peripheral limita tion to exposure of the electrode thereunder.

3. An electric arc device comprising an envelope having an anode at one end portion thereof and having a metallic cup at the opposite end thereof and said envelope providing an insulative portion between said cup and said anode, a cathode pool partially filling said cup, an ignitor electrode projecting into said cup from beneath said pool, a dielectric interposed between said electrode and the material forming said pool, and a shield supported in its entirety by said ignitor electrode and situated in its entirety above said cathode pool.

4. An electric arc device comprising an envelope having opposed upper and lower metallic cups the edges whereof are directed toward each other and having an intervening insulative portion sealed to and separating said edges and cups, a

5 cathode pool partially filling the lower one of said cups an ignitor electrode projecting upwardly through said pool and above the same a distance less than projection of the said lower cup above the pool, and a shield around said electrode open toward and in spaced proximity to and above the pool a distance materially less than spacing of the lower edge of said insulative portion of the envelope from the pool. 7

5. An electric arc device comprising an envelope having opposed upper and lower metallic cups the edges whereof are directed toward each other and having an intervening insulative portion sealed to and separating said edges and cups, a cathode pool partially filling the lower one of said cups, an ignitor electrode projecting upwardly through said pool and above the same, and a hollow shield around said ignitor electrode opposed to and having its lower edge in spaced proximity to a portion of the pool thereby forming a continuous gap around the ignitor electrode between the shield and cathode pool, the space between the pool and anode being entirely unobstructed at the sides of and above said shield, and said shield intervening in all straight-line directions between said electrode and the said insulative portion of the envelope.

6. An electric arc device comprising an envelope having an anode and a pool cathode spaced from the anode, said envelope having an insulating cylinder as part thereof in the area opposite the space between the anode and cathode, an ignitor electrode protruding through said pool, and shield means intervening in all straight-line directions between said ignitor electrode and said insulating cylinderenabling said device to Withstand high Voltage effectivelyand without loss of control.

'7. An electric arc device comprising an envelope having an anode and a pool cathode opposed to each other, an ignitor electrode protruding through said ool, and shield means supported by and overlying said electrode and having a free edge spaced radially therefrom and spaced from the pool cathode providing a continuous peripheral gap between said edge and cathode open to the electrode enabling said device to withstand high voltage effectively and without loss of control.

8. An electric arc device comprising an envelope having an anode and a pool cathode opposed to each other, an ignitor electrode protruding through said pool, insulation on said electrode extending upwardly thereon above the pool surface with part of the electrode exposed thereabove,

and shield means supported by and overlying said electrode and horizontally opposed to all exposed area of said electrode enabling said device to withstand high voltage effectively without loss of control and to pass high currents.

9. An electric arc device comprising an envelope having an anode and a pool cathode opposed to each other, an ignitor electrode protruding through said pool, insulation on said electrode extending upwardly thereon above the pool surface with part of the electrode exposed therebeyond, and shield means supported by and overlying said electrode and having a free edge spaced radially therefrom and spaced from the pool cathode closer thereto than the spacing of the exposed part of electrode from the pool cathode 7 whereby all straight-line directions radiating from the exposed part of the electrode and passing under said free edge are prevented from sloping upwardly outwardly thereby enabling said device to withstand high voltage effectively without loss of control and pass high currents and promoting rapid recovery time enabling the device to be used with frequencies of 60 cycles and over.

10. An electric arc device comprising an envelope having an anode at one end thereof and a metallic cathode cup at the opposite end thereof below the anode, a pool cathode in said cup extending partially of the height of the sides of said cup, an ignitor centrally disposed with respect to the pool surface, and a cup-like downwardly open shield entirely above the pool cathode, said shield having a rim interposed in the straight line path from the center of said pool surface to the upper edge of the sides of said cathode cup for preventing sputtering of the pool material from adjacent the electrode going above the said sides of the cathode cup.

11. An electric arc device comprising an envelope having an anode at one end thereof and a metallic cathode cup at the opposite end thereof below the anode, a pool cathode in said cup extending partially of the height of the sides of said cup, an ignitor protruding through said pool, and a cup-like downwardly open shield entirely above the pool cathode, said shield intercepting all straight line directions of sputtering between the point of protrusion of the ignitor through the pool and all points above the upper edge portion of said cathode cup.

CHARLES M. SLACK. EDWARD G. F. ARNOTT.

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