US3465205A - Vacuum gap devices with metal ionizable species evolving trigger assemblies - Google Patents

Description

2 Sheets-Sheet l PUL SE J. M- LAFFERTY ES WITH METAL IONIZAELE SPECIES EVOLVING TRIGGER ASSEMBLIES Sept. 2, 1969 VACUUM GAP DEVIC Filed Feb. 12, 1968 r0 LIME SOURCE [7'7 ver'r to r-: dd mes A4. LaFFer-lry, by @P Hie Attorney.

Sept. 2, 1969 J. M. LAFFERTY 3, VACUUM GAP DEVICES WITH METAL IONIZAELE SPECIES EVOLVING TRIGGER ASSEMBLIES 2 Sheets-Sheet 2 Filed Feb. 12, 1 968 SOURCE M {5 L PULSE Inventor".-

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0 m 5 w v m k w m J Q. n0 W M w 4 M v v \\V\ x M United States Patent 3,465,205 VACUUM GAP DEVICES WITH METAL IONIZABLE SPECIES EVOLVING TRIGGER ASSEMBLIES James M. Laiferty, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Continuation-impart of application Ser. No. 516,942, Dec. 28, 1965. This application Feb. 12, 1968, Ser. No. 704,935

Int. Cl. H013 11/04, 13/48 US. Cl. 315-330 10 Claims ABSTRACT OF THE DISCLOSURE The present invention is a continuation-in-part of Ser. No. 516,942, and now abandoned, filed Dec. 28, 1965 and assigned to the present assignee. The present invention relates to improved vacuum gap devices of the triggered or triggerable gap type and more particularly to such devices as are adaptable to operate at high ambient temperatures of at high duty cycles which result in high quiescent operation temperatures.

Vacuum gap devices including vacuum switches and fixed gap devices have recently become more technically significant than they have been for many years. One reason for the increased significance "and commercial utilization of such gap devices has been my discovery, disclosed and claimed in US. Patent No. 3,087,092, issued Apr. 23, 1963, and entitled Gas Generating Switching Tube, of means for reproducibly and rapidly firing a device which holds off high voltages under normal or quiescent conditions. In accord with the invention disclosed in the aforementioned patent, I provide means within the device envelope, generally constituting a trigger gap and associated trigger electrodes, for propelling a highly ionized electron-ion plasma into a main or primary discharge gap upon the receipt of a low voltage electrical pulse across the trigger gap. This socalled triggering of the vacuum gap results in near instantaneous (within less than a microsecond) and reproducible breakdown of such primary gaps, thus avoiding the instabilities inherent in vacuum gaps of the prior art.

While triggerable vacuum gap devices, as disclosed in my aforementioned patent, are suitable in most instances for the purposes described above, there is a need for improved devices for operation under conditions of high temperature of the trigger electrode as, for example during heavy duty cycles, as for example 125,000 amperes offset peak current with an arc-drop of approximately 150 volts, when the electrode temperature are substantially above that of normal ambient.

Accordingly, it is an object of the present invention to provide triggerable vacuum gap devices suitable for operation under high temperature conditions therein.

Another object of the present invention is to provide triggerable vacuum gap devices in which the quiescent pressure within the devices does not rise above 10* mm. of Hg during repeated arcing conditions.

3,465,205 Patented Sept. 2, 1969 Still another object of the present invention is to provide triggerable vacuum gap devices wherein no gaseous material is deliberately included for operation of the trigger gap thereof.

In accord with the present invention, I provide vacuum gap devices including a pair of primary electrodes defining therebetween a vacuum gap. These electrodes are located within an evacu'able envelope, which envelope is composed partially of metal and at least in part of a high voltage insulating material, so as to provide electrical isolation between the primary electrodes to prevent short-circuiting of the gap defined thereby. At least one of said primary arc-electrodes has associated therewith a trigger electrode assembly including a trigger gap. The trigger gas is between two electrically isolated metallic members, preferably thin films over the surface of an insulating dielectric. The material of which the trigger electrode and gap-adjacent surface :members are fabricated is chosen to be a metal or metal alloy which has a high vapor pressure and low thermal conductivity. -Its work function is not sufiiciently low enough to reduce the hold-off voltage of the main gap and it is chosen to have a boiling point between 1500 C. and 2800 C. and preferably between 1500 C. and 2300" C. These metallic electrode members comprise the source charged electrical carriers which are injected into the primary gap upon pulsing of the trigger gap to cause a breakdown thereof. Since the materials chosen are all stable at relatively elevated temperatures the device is well suited for operation at high temperatures, particularly those obtained under high duty cycle operation and during fabrication bakeout.

The novel features believed characteristic of the present invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may be more readily understood by reference to the appended drawing in which:

FIGURE 1 is a vertical cross-sectional view of the fixed triggerable vacuum gap device constructed in accord with the present invention;

FIGURE 2 is a vertical cross-sectional view of one trigger electrode assembly which may be utilized in the device of FIGURE 1,

FIGURE 3 illustrates, in vertical cross-sectional view, an altenrative trigger assembly which may also be used in the device of FIGURE 1,

FIGURE 4, is a vertical cross-sectional view of an alternative embodiment wherein both primary arc-electrode has a trigger,

FIGURE 5 illustrates, in vertical cross-section a vacuum switch constructed in accord with the invention,

In FIGURE 1 of the drawing, a gap discharge device constructed in accord with the present invention is illustrated in vertical cross-sectional view. The device of FIGURE 1 includes a gas impervious insulating envelope which is composed of a lower flanged end wall assembly 1, a cylindrical sidewall member 2, and an upper end wall member 3 which serves as one primary terminal for high voltage connection. The other primary voltage connection is made to apertured disc 5 which serves also as a cathode support member. Lower end wall assembly 1 includes an integral, inwardly-depending member 4, which constitutes a portion of a trigger electrode assembly for the device. A pair of primary arc- electrodes 6 and 7 are supported in spacedapart relation within envelope 100 to define a primary gap 8.

Cathode arc-electrode 6 is a hollow cylindrical body having an apertured inwardly-depending closed end. The aperture in cathode electrode 6 is tapered at the interior depending portion thereof to provide a bore in the end of electrode 6 having an exterior cylindrical portion 9 and an interior conical portion 10. As used herein the terms interior, exterior, inner and outer are with reference to the center of the device. Arc-electrode 6 is fitted over the apertured central portion of support member over the inwardly-depending member 4. The inwardly depending end of member 4 is capped with a metallic disc cap 11 which is slightly larger in diameter than the diameter of protruding member 4. A conducting lead 12 is welded, brazed, or otherwise firmly and conductively secured to cap 11 and passes through a bored aperture in end-wall assembly 2 to the exterior of envelope 100. The side of cap 11 in contact with the inwardly depending end of member 4 is hermetically sealed thereto by conventional metal-to-insulator sealing techniques, so as to maintain an hermetic seal for envelope 100. Cathode arc-electrode 6 is supported within envelope 100 upon the inner periphery of the annular aperture in support member 5 which rests upon the annular upwardly flanged edge of end-wall assembly 1. Member 5 is bonded to end-wall assembly 1 and to one end of cylindrical sidewall member 2, so as to form hermetic seals therewith. End-wall member 3 is similarly bonded to the opposite end of cylindrical sidewall member 2. Anode electrode 7 is suspended within the envelope 1 by means of an anode electrode support member 13 which is passed through a central aperture in endwall member 3 and hermetically sealed thereto by welding, brazing or other suitable techniques.

A metallic arc shield 14, having a semi-cylindrical shape with a flared open end to prevent arcing, is suspended from anode electrode support member 13 and extends past gap 8 between arc- electrodes 6 and 7. Shield 14 is utilized to preclude metal sputtered or evaporated from arc- electrodes 6 and 7 from completely coating the inner surface of cylindrical sidewall member 2 and thus destroying the insulating characteristics thereof. The cylindrical portion 15 of inwardly protruding member 4 is coated with a thin layer 16 of a metal suitable for operation in a trigger electrode assembly, as is more fully described hereinafter. The surface of conical aperture in arc-electrode 6 is similarly coated with layer 21 of the same metal as layer 16, unless the electrode is constructed of a readily vaporizable metal such as copper or beryllium, in which case the coating with a film of a readily vaporizable metal is unnecessary. After layer 16 has been formed, a groove 20 is scored around the circumference of cylindrical portion so as to remove the metallic coating therefrom and expose the insulating ceramic. Layer 16 is then divided into two gapadjacent surface members 18 and 19 which are electrically a portion of the trigger cathode and trigger anode respectively. The position of groove 20 is chosen so that when cathode arc-electrode 6 is positioned over member 5 the junction between cylindrical bore 9 and the tapered bore 10 is slightly below the lower edge of groove 20. Typically, groove 20 is of 0.005 in width and thick enough to penetrate approximately 0.005 into the ceramic of member 15 and may break down at a voltage of 500 volts or less.

Envelope members 1 and 2 may be fabricated from any gas impervious, non-conducting dielectric which may be hermetically sealed to a metal electrode. Generally, any gas impervious ceramic may be utilized such as, for example, Coors V-200 or American Lava-T164. Alternatively, an aluminum oxide or forsterite ceramic body may be used. It is to be understood, however, that although these specific materials have been enumerated, any gas-impervious ceramic or glass which may be hermetically sealed to metal members may also be utilized.

Arc- electrodes 6 and 7 are fabricated from a metal or alloy that is substantially free from all gaseous impurities or impurities which upon decomposition may produce gases. This metal or alloy should contain less than one part in 10 atomic parts of all gases and gasforming constituents and should be operative, under arcing conditions, to keep the pressure within the device always at a pressure of less than 10 mm. of Hg. Such materials may readily be formed by a special zone-re fining process as, for example, that which is set forth in US. Patent No. 3,234,351 to M. H. Hebb, issued Feb. 8, 1966. Arc- electrodes 6 and 7 are preferably fabricated from copper, beryllium or alloys thereof. Electrode support members 5 and 13 need not meet the stringent criteria set forth with respect to the arc- electrodes 6 and 7, since they are not brought into contact with an electric arc and therefore are not a potential source of vacuum-spoiling gases.

The specific embodiment disclosed in my abovementioned US. Patent No. 3,087,092, rely upon the storage of an active gas as, for example, hydrogen, within a chemical compound as, for example, titanium hydride, closely associated with a metal-to-ceramic interface breakdown gap to cause the thermal production of a large quantity of gaseous electron-ion plasma and the injection of this plasma into the primary gap, which is under high electric stress to cause the breakdown thereof. While this approach is entirely satisfactory for applications in which repetitive arcing and excessive heating do not occur, ditficulties may be encountered when repetitive arcing, due to a heavy duty cycle, or when other extraneous conditions cause a high ambient temperature to exist within the device when the device is in a non-conducting state. These difiiculties are due to the fact that those materials as, for example, titanium, which are suitable for chemically combining with active gases as, for example, hydrogen, and maintaining the active gas in a bound condition so as to support a hard vacuum in the absence of a trigger arc and, subsequently, to release the same when heated by the temperature of the footpoint of such a trigger arc, may evolve gas at such temperatures that heavy duty-cycle operation or high temperature operation of the device may cause a sufiicient quiescent pressure within the device as to very severely limit the hold-off voltage of the primary gap. Thus, for example, at an equilibrium temperature of 250 C., the vapor pressure of hydrogen in equilibrium with titanium hydride is approximately 10- mm. of Hg. Obviously, this vacuum is not sufficiently hard as to holdoff, with the high dielectric strength of a vacuum, high voltages of tens of thousands of volts. This is a necessary objective in devices in accord with the present invention which operate at temperatures of up to 500 C. and have readily operated to hold off voltages of 50,000 volts, and higher.

Accordingly, in accord with the present invention, active gas storing materials are not utilized but, rather, I rely upon metal films, which are part of the trigger anode and trigger cathode and are immediately adjacent the trigger gap, for the provison of the ionized species which constitute the electron-ion plasma which causes breakdown of the primary gap. Not all metals are suitable for this function, however. The material which is utilized must not have a low work function so that the presence thereof does not compromise the ability of the primary gap to hold off high voltages. Preferably, the materials utilized should all have a work function for the clean metal of no longer than that of beryllium. Since work function is a parameter which is subject to different measurements, due to the activity of the metal being measured and the effects of adsorbed and reacted gases (principally oxygen) at the surface of the metal and the effects thereof upon measurements, many different values have been reported. While it would be convenient to set forth a definite value of minimum work functions such as approximately 4 ev., this is difiicult since, for beryllium, reported values range from 3.17 to 4.02 and, for aluminum, reported values range from 2.98 to 436. Accordingly, although, accord ing to the most recent data, the materials with which the invention may be practiced have work functions measured to be approximately 4 ev. or higher, they will be described herein as having work functions of clean metal surfaces at least as high as that of beryllium.

The boiling point of the material chosen is of great importance. The boiling point relates to vapor pressure and it is desirable to have a material having a sufficiently high vapor pressure that heating under the initial trigger arcing conditions is suflicient to cause the arcing area. On the other hand, the boiling point should not be so low, nor should the vapor pressure be so high, that the material is so violently removed from the area of the trigger are that the trigger gap rapidly deteriorates or, when the device is baked out in normal vacuum processing, that the material is deposited on substantially all of the portions of the interior walls of the device. For this reason the materials I use are chosen to have a boiling point of from 1500 C. to 2800 C., and preferably from l500 C. to 2300 C.

In accord with the foregoing considerations, I am able to construct devices in accord with the present invention utilizing, as the metal which supplies the ionized species, those materials having boiling points in excess of 1500" C. but not in excess of 2800 C. Elemental metals which satisfy these boiling point and work function criteria are beryllium, tin, aluminum, copper, and lead. It is preferable that the metals used be the same as that used for the primary arc-electrodes. Accordingly, I prefer to use copper, beryllium, or alloys thereof. Since it is not possible to utilize materials whose boiling point is so high that they are considered refractory and would not supply a sufiicient number of ionized species to cause breakdown of the gap, the boiling point of the material utilized must not exceed 2800 C. Those materials having boiling points in excess of 2800 C. may be considered refractory for this purpose in that the amount of vapor which is evolved therefrom under arcing conditions is not considered, in the light of recent experience sufiicient to sustain a strong enough trigger pulse to cause predictable and repeated breakdown of the primary gap.

One preferred embodiment of the invention is illustrated in FIGURES 1 and 2 in that a thin metallic film 16 approximately 0.10" thick is deposited upon an insulating ceramic 15. The film is separated by gap 20, approximately 0.005" wide, and 0.015 deep into two gapadjacent metallic surface members 18 and 19 which are electrically a portion of trigger cathode and trigger anode, respectively. This has the advantage that the electric field due to the trigger configuration is very high at the trigger gap due to the unique characteristics of the metal-ceramic interfaces. Additionally, the small mass of the thin film of metal deposited upon the ceramic to form surface members 18 and 19 results in a small thermal inertia so that, with the first breakdown, the metal immediately adjacent the gap is rapidly raised in temperature to a value at which substantial emission of vaporized metal is possible. The thickness of film 16 is not critical, but is selected for optimum operating characteristics. The lower the thickness, the shorter its lifetime. The thicker the film, the more likely it is to become detached from member due to different thermal expansion. I find that thicknesses of approximately 0.005" to 0.015" are best, but other thicknesses, of the order of thousandths of an inch may be suitable under particular circumstances. In order to facilitate the continued evolution of metallic particles while a trigger arc is moved by magnetic forces up the annular space toward the primary gap, it is also preferable to coat the inner conical surface of cathode electrode 6 with a layer 21, preferably of the same material as is coated upon ceramic member 15, unless the arc-electrode 6 is made of a high vapor pressure metal, as suitable for film 16.

While the elemental metals set forth herein as satisfying the criteria enumerated hereinbefore are well suited for incorporation into devices constructed in accord with the present invention, the list of suitable materials is not limited to these elemental metals. Thus, for example, another desirable characteristic of such metallic coatings in the trigger arcing area is that of a low-thermal conductivity, which results in a high rate of erosion of the electrode material and the generation of vapor for the trigger pulse. This is so because, with a low thermal conductivity, heat is not dissipated away from the arcing area. Under identical conditions, a material having a low thermal conductivity is hotter in the vicinity of the arc footpoint than the material having a high-thermal conductivity. Thus, alloys between the materials enumerated hereinbefore or alloys of these materials with other substantially inert materials, which alloys satisfy the work function, boiling point, and other criteria enumerated hereinbefore and are essentially dominated by the characteristics of the listed material, are, in many instances, preferable to the elemental metals.

In fabricating the device in accord with FIGURE 1 the arc-electrode bodies are prepared and suitably shaped from highly-purified copper, or other suitable high-vapor pressure materials, as is set forth hereinabove. Electrode support 13 is fastened to arc-electrode 7 as, for example, by welding or brazing. Shield member 14 is fastened to electrode support member 13 in a similar fashion. Arcelectrode 6 is fastened to electrode support member 5 by a suitable Welding or brazing step. Trigger electrode lead 12 is sealed within trigger cathode disc 11, which is sealed to the inwardly depending end of protruding portion 15 by an appropriate metal-to-ceramic sealing technique, and a layer 16 of the chosen vapor supplying metal or alloy is deposited over the surface of cylindrical portion 15 thereof. Groove 20 is cut in the peripheral surface thereof through the metal film to provide two metallic, gap-adjacent trigger cathode and anode surface members 18 and 19, respectively, with two metal-ceramic interfaces, separated by an interposed ceramic gap. The ceramic body 15 is then inserted into the cavity within cathode arc-electrode 6 which is sealed to electrode support member 5 so as to provide mechanical and electrical contact therewith.

As a final step in assembly, the ceramic envelope is assembled around the members 4 and 5 and hermetically sealed thereto. The envelope is evacuated to a pressure of 10- mm. of Hg under out-gassing conditions while baking at approximately 400 to 600 C. for four or five hours. Tubulation 22 is then sealed off to complete fabrication.

In operation, a high voltage is connected in circuit with arc- electrodes 6 and 7. Due to the high vacuum of the order of 10" torr or better, and the high dielectric strength of vacuum, extremely high voltages as high as 100,000 volts may be held off without spurious breakdown. When it is desired to render the primary gap conductive a pulse of, for example, 500 volts at 200 to 1000 amperes for approximately 1 to microseconds is applied across the trigger gap. The trigger gap breaks down and metallic species are evolved therefrom and ionized, providing a trigger arc, which is propelled by its own created magnetic field out of the trigger and into the main gap. A large quantity of ionized particles is discharged into the primary gap. Simultaneously, the trigger arc establishes on the arc-cathode a cathode spot. The main gap is thus caused to break down, all within the order of a microsecond after excitation of the trigger assembly.

FIGURE 3 of the drawing illustrates an alternative cathode arc-electrode structure, suitable for the fabrication of devices in accord with the present invention. This cathode arc-electrode structure may be substituted for the cathode arc-electrode structure 6 in FIGURE 1 of the drawing and is the functional equivalent thereof, with added advantages. In FIGURE 3, a cathode arcelectrode 6, composed of the same material and having the same purity as that which is described with respect to the arc-electrodes in FIGURE 1, includes a cylindrical metallic body, preferably of copper, beryllium, or an alloy containing a major portion thereof, having a central cylindrical cavity therein into which is inserted a trigger electrode assembly 23, which is conveniently formed of a refractory cylinder 24 having a regular, stepped, right-circular cylindrical radially exterior surface 25 and a radially interior surface which has a right-circular, cylindrical portion 26 at the outwardly-depending end thereof and which changes at annular shoulder 27 approximately half its longitudinal dimension into a curvilinear surface 28 which constitutes an axisymmetric convergent-divergent nozzle. This shape is characterized by a cylinder of revolution which, moving inwardly, (toward the center of the device), begins to converge or decrease in diameter from a larger value at shoulder 27 to a smaller value, which latter value is the minimum dimension 29 of the trigger electrode 24, (the trigger cathode) in FIG- URE 3 of the drawing, and from which the interior diameter regularly and curvilinearly increases to the value of the exterior diameter thereof. A trigger anode assembly 30 is inserted into the right cylindrical portion of the interior bore of trigger electrode assembly 23 and constitutes an annular ceramic washer 15 having surface coating 16 of a thin layer of one of the trigger electrode materials set forth hereinbefore with respect to the embodiment of FIGURE 2 and an annular grooved gap 20 which pierces the same and penetrates into the ceramic body 15 to establish a narrow (approximately 0.005") trigger gap thereat, separating a pair of gap-adjacent metallic surface members 18 and 19, which are electrically a portion of trigger cathode 24 and trigger anode 30, respectively.

Ceramic disc 15 fits into the radially central aperture within trigger cathode cylinder 24 and rests upon shoulder 27, which is the intersection between the cylindrical and curvilinear portions of the interior surface of trigger cathode cylinder 24. Trigger anode electrode 31 having a first larger diameter portion 32 and a second smaller diameter portion 33 is bonded to the inwardly-depending surface of ceramic disc 15 to make mechanical and electrical contact with coating 19 thereof, and is hermetically sealed thereto. The reduced diameter portion 33 of trigger anode electrode 31 extends through a central aperture in disc 15 and terminates in a trigger electrode lead 12. The peripheral surface of the larger portion 32 of trigger electrode 31 is coated with a coating 34 of the same metal of metallic alloys as constitutes layer 16. A similar coating 35 is bonded to the curvilinear inner surface 28 of trigger cathode electrode insert 24.

The advantage of the embodiment of FIGURE 3 of the invention over the trigger electrode assembly, illustrated in FIGURE 2 of the drawing, lies in the protection afforded the trigger gap 20 by the over-extending portion of the curvilinear inner surface of cylindrical cathode body 24. Since gap 20 is located under the shadow of the radially inwardly-depending portion 29 of this curvilinear surface, any metallic material tending to condense from the main electrode discharge upon the trigger gap cannot see the trigger gap. Similarly, and of greater importance, under conditions of high voltage breakdown, any tendency for a cathode spot associated with the main gap discharge to be established at the trigger gap or anywhere within the cavity below the minimal diameter portion 29 of member 24 and the closely abutting edge of trigger anode electrode 31 upon the initiation of a first breakdown is obviated, since energy considerations prevent the establishment of a cathode spot anywhere within the lower cavity portion of the trigger electrode assembly.

The operation of the device of FIGURE 1, utilizing either the trigger electrode of FIGURE 2 of FIGURE 3, is essentially as follows: The triggering pulse is applied between cathode arc-electrode connection and trigger anode lead 12. Upon initiation of a trigger pulse, which, in the instance of a metallic vapor phenomenon, may be accomplished with a voltage of approximately a few hundred to several thousand volts, for example, breakdown occurs across the annular gap 20 and a cloud of metallic vapor is vaporized from the gap-adjacent trigger cathode and anode surface members 18 and 19, which have a moderate work function and a relatively high-vapor pressure, so that a substantial amount of metallic vapor is released upon reasonable heating. This metallic vapor is ionized and supports the metal-vapor are which causes further evaporation of metallic vapor and the creation of an electron-ion plasma which is rapidly propagated up through the throat of the trigger electrode assembly and injected into the primary gap 8 establishing a cathode spot on arc-electrode 6 and causing the breakdown of gap 8.

In accord with the present invention, breakdown of a high voltage vacuum gap is achieved repetitively and reliably with a minimum of jitter by the application of a voltage pulse which is no higher than that which is required to break down a gaseous loaded trigger gap, but which may require a higher energy input which may be obtained with an increased trigger current. The additional advantage gained by the present invention is that this device may be operated at heavy duty cycles as, for example, at currents of 125,000 amperes offset peak current at an arc drop of approximately volts and at a high ambient temperature of, for example, 400 to 500 without in any way atfecting the quiescent vacuum within the device and, consequently, the breakdown characteristics thereof.

Although the invention has been described hereinbefore with respect to one embodiment in which both arc-electrodes are fixed to define a fixed gap and only one of such arc-electrodes has a trigger assembly associated therewith, in other embodiments of the invention as, for example, illustrated in FIGURE 4, wherein like elements to those of FIGURE 2 are identified by like reference numerals may be adapted for use with alternating current voltages by the utilization of a trigger assembly in connection with each arc-electrode.

Likewise, a vacuum switch such as is illustrated in FIGURE 5, wherein like elements to those of FIGURE 2 are identified by like reference numerals, may utilize a fixed arc-electrode and a movable arc-electrode, rendered so by its attachment to the envelope by a Sylphon bellows 36.

Additionally, for extremely heavy duty cylcle operation it may be preferable to utilize the arc-electrode geometry set forth in my US. Patents 3,356,893 and 3,356,894.

While the invention has been disclosed herein with rerespect to particular embodiment thereof, many modifications and changes will occur to those skilled in the art. Accordingly by the appended claims I intended to cover all such modifications and changes as fall within the true spirit and scope of the invention.

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

1. A triggerable vacuum gap device comprising:

(a) an evacuable envelope at least a part of which is composed of an insulating dielectric to provide two electrically isolated portions thereof and evacuated to a pressure of the order of 10- mm of mercury or less;

(b) a pair of primary arc-electrodes disposed in electrically insulated relationship within said envelope and defining therebetween a primary gap,

(b at least one of said arc-electrodes having associated therewith a trigger electrode assembly including a trigger anode, a trigger cathode, and a trigger gap electrically separating said trigger anode and cathode,

(c) said trigger gap being bounded by a pair of gapadjacent thin metallic surface members electrically a part of said trigger cathode and said trigger anode, respectively,

(c said metallic surface members comprising a material selected from the group consisting of metals and metal alloys having a clean metal surface Work function at least as high as that of beryllium, a boiling temperature in the range of 1500 C. to 2800 C., and serving as a source of vaporized metallic species upon the establishment of an arc across said trigger gap;

(e) means for connecting said primary arc-electrodes in circuit with a high voltage;

(f) means for supplying a low voltage pulse to said trigger electrode assembly to cause electrical breakdown of said trigger gap, the evaporation and ionization of metallic species from the metallic surface members adjacent said trigger gap and the injection of a highly ionized plasma into said primary gap and the formation of a cathode spot to cause the electrical breakdown of said primary gap.

2. The device of claim 1 wherein said gap-adjacent metallic members are formed from the class consisting of copper, beryllium, aluminum, tin, lead, and alloys thereof.

3. The device of claim 1 wherein said primary arcelectrodes comprises material selected from the group consisting of copper, beryllium, and alloys thereof and the gap-adjacent metallic film members are of the same material.

4. The device of claim 1 wherein one of said primary arc-electrodes has an internal cavity with a trigger electrode assembly located therein.

5. The device of claim 1 where each of said primary arc-electrodes has an internal cavity with a trigger electrode assembly located therein.

6. The device of claim 1 wherein both of said primary arc-electrodes are fixed to provide a fixed gap there- 'between. i

7. The device of claim 1 wherein at least one of said arc-electrodes is movable to provide a variable primary gap.

8. The device of claim 1 wherein the trigger electrode assembly includes a ceramic body, a pair of metallic members covering the surface of said body and separated from one another by a narrow annular spacing to define a trigger gap, one of said members being electrically a part of said trigger cathode and the other of said members being a part of said trigger anode.

9. The device of claim 1 wherein said trigger assembly includes a hollow cylindrical trigger anode; a concentric trigger cathode; and insulating ceramic disc closing the space between said trigger electrodes; and a pair of concentric metallic members on the inwardly-depending surface of said ceramic disc and spaced apart to define a trigger gap, one each of said members being electrically a part of said trigger cathode and said trigger anode, respectively.

10. The device of claim 1 wherein said metallic surface members are chosen to have boiling points from 1500 C. to 2300 C.

References Cited UNITED STATES PATENTS 10/1946 Slack et a1. 313-233 X 4/1963 Lafferty 31533O U.S. c1. X.R.

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