US2512538A - Electric discharge device - Google Patents

Description

June 20, 1950 w. R. BAKE 2,512,538

ELECTRIC DISCHARGE DEVICE Filed July 26. 1949 l l v lu AIIIIIIAII'IIIIIIIIIIIII INVENTOR. WILL/AM R. BAKER A r romvsx Patented June 20, 1950 UNITED STATES PATENT OFFICE ELECTRIC DISCHARGE DEVICE William R. Baker; Berkeley, Calif., assignor to the United Statesof America as represented by the United States Atomic Energy Commission Application July 26, 1949, Serial No. 106,913 7 V 22 Claims. (Cl. 250-275) This invention relates to electric discharge depower rating is obtainable only at the expense of reducing the life of the apparatus. While electric discharge devices having ratings of the order of three kilowatts of continuous power and two to three megawatts of pulsed power have been developed, even these seemingly large power ratings have proven grossly insufficient for the demands of industry. Inherent limitation in conventional structure resulting from maximum possible size of cathode-emitting surface, extensive insulation requirements, and complicated grid construction have, prior to the present invention, served to prevent the necessary increase in power rating of conventional gaseous electric discharge devices. Various other types of devices, such as for example spark gaps, have been utilized in this connection; however, none has proven capable of satisfying the desired requirements, such as for example high power rating, variable voltage characteristics, precise control of ignition time, and small size.

It is therefore an object of the present invention to provide a new and improved heavy duty gaseous electric discharge device of relatively small size capable of passing greatly increased amounts of continuous power.

vide such an electric discharge device having the characteristic of precisely controllable ignition time.

It is yet a further object of this invention to provide a relatively small electric discharge device capable of passing thousands of amperes of pulsed current.

Another object isto provide a new and improved cathode construction for use in combination with electric discharge apparatus adapted for heavy-duty usage.

A still further object of this invention is to provide an electric discharge device capable of withstandin reversal of plate voltage within a short time of firing;

' The manner in which the above objects are realized may be best understood by reference to the following description and accompanying drawing in which Figure 1 is a longitudinal sectional view of the device taken on the median plane; Fig. 2 is a plan, view of the cathode; and Fig. 3 is a plan view of the grid structure.

With reference to the drawing, it is to be noted Y that in a preferred construction a cylindrical base unit I is provided to contain and support the It is another object of this invention to provide 2 a new and improved heavy duty electric discharge apparatus of relatively small size capable of passing greatly increased amounts of pulsed power.

It is another object of this invention to provide jection 4 about the exterior of the base unit I, and

is adapted to slip fit about a shoulder Suponthis annular projection thereby preventing .lateral' motion of the ring 3. V

The grid structure 2 has the form of a cylinder of slightly larger diameter than the cylindrical base unit I and has a circumferential rim 1 formed on the innendiarneter of the cylinder and-pro.- jecting downward from a vertical Wall thereof.

.In assembly of the device, the grid structure 2 rests upon the insulating ring 3 with therim .l in contact with the insulating ring 3 about the top inner circumference thereof, thereby preventing lateral motion of the grid 2 with respect to the insulating ring 3. With the grid structure 2, insulating ring 3, and base unit I assembled as 'set forth above, the base unit extends into the grid cylinder although not in contact therewith and thus entirely masks the insulating ring :3

from the interior of the base unit as illustrated in Fig. 1. The insulator ring 3 is thereby shielded from heat radiation by the cathode and also from stray electrons.

The grid structure 2 has a double-stepped portion formed about the top of the outer periphery whereby two horizontal annular levels are provided thereabout. A second, insulating ring 8 rests upon the lower of these annular levels and abuts the vertical portion of the first step, and is separated from the top of the grid structure by the widthof the top annular level As the inner diameter of the ring 8 is substantially the same as the diameter of the vertical portion of the lower step, the ring 8 is restrained from anylateral motion with respect to the grid structure 2 when assembled therewith.

Atop the grid structure Land adjacent-thereto.

of increased size of cathode-emitting surface.

The present invention overcomes this limitation by utilizing a novel and particularly advantageous arrangement of fins wherein the spacing between any fin and its largest neighboring fin is always is situated the anode l l which is spaced from-thegrid 2 by the insulating ring'8. The anode H is also cylindrical, preferably formed. of ametal having good heat conductivity, and like the grid 2 has three different outer diameters joined by two annular levels; however, unlike the grid 2, the smallest diameter of the. anode l l is the lowest, thus producing an inverted step periphery. The anode l I is supported by the ring 8, the top of which is in contact with the outer annular level of the anode, and the anode is secured from lateral motion by circumferential contact-between the inner periphery of the rin 8 and the vertical portion separating the two annular levels on the anode ll.

As has been noted above, a metal having'a high free copper is preferable to-regular copper which disintegrates in such an atmosphere.

The entire assembly set forth above is made gas-tight by the insertion of ring gaskets [2 be- "tween the insulator ring 3 and the baseunit l and grid 2 and between the insulator ring 8 and the grid 2 and anode I I. The apparatus may be maintained in assembled position by means of clamps or other suitable mechanical means and may be surrounded-by appropriate insulating-material such as a glass cover; however, for certain applications it has been found advantageousto utilize only the effect of the low internal pressure to maintain the apparatus in assembled position and to forego the advantages: of exterior insulation for the resulting ease of disassembly of the apparatus.

Within the-base unit I there is situated an improved cup-shaped cathode 2| substantially concentric with the base unit. The cath0de-emitting surface comprises a'multiplicity of fins arranged longitudinally within the cathode cup, as illustrated' in Fig. 2. These fins are attached to the inner surface of the cathode wall and extend radially inward from the wall. This particular embodimentutilizes three sets'of fins of differing size, an illustrative example of each size of fin .being' numbered 22, 23, and 24 respectively in order of size from the largest to the smallest.

:Maximum cathode emission islimited in partiby :the area of cathode-emitting surface, and. thus it is advantageous to provide as large a surface as substantially equal to the first-mentioned fin depth. Fins of three different sizes are utilized in the'illustrated embodiment, the largest fins 22 having-a radial depth of twice the radial depth .of the intermediate fins 23 and four times the radial depth of the small fins 24. The fins are arranged so that a fin of intermediate depth is between and equidistant from two large fins, and a small fin is between and equidistant from a large fin and an intermediatefin. .Thus'the sequence of fins about the innercircumference of the cathode cup is large fin 22, small finl l, intermediate fin 23,. smallfin 2,4, largefin 22, etc. The distance between adjacent large fins 22 substantiallyequal to their radial depth, the distance between each. v23i of intermediate depth and. the adjacent ilarge fin 22 is substantially equal to'the radial. depth of rtheintermediate fin 23 andthe distancebetweenlthe small fin Hand the adjacent fin, either largev or intermediate, is substantially equal ,to the radial depth of the small fin .24. If the radial depth. of the largest fins be maintained substantially less than the radius of the cylinder, approximately ,4; the radius or less then ,t'he radial depth of the fins will be substantially equal to. their separation along the cathode cylinder. With the inner surface of the cylinder being-electronemissive the utilization ofone set of electroniemissive finsin accordance with the above principles will substantially-triple the cathode emission. surface, two sets of fins willmake substantially four times the emission surface and three sets of fins as illus- 'trated will make-substantiallyifive times the oathode em'msion surface that would be available without the fins. Thus the use of. any number of sets of straight fins in accordance with :the

principles of the invention, and under the cir- 'cumstances set forth above will result in the cathode emission surface being equalto the area of the area of the surface upon which the fins are'mounted multiplied by a factor of two plus the number of sets of fins. It is to be noted that the above increase in cathode emission surface; is accomplished without the deleterious effects of space charge, and thus-all ofthe surface is useful.

' It will of course be appreciated that the cathode may have any of a variety of configurations other than that ofthehollow cylinderillustrated and also that there is no limitation upon the number-cf sets of fins that-may be utilized. It

may be further noted that fins of othershape or configuration'maybe utilized; however, in such case the above-noted relationship between the cathode-emission surface with and without fins must of course be alteredto fit the circumstances.

The cathode-emission surface-of the present invention ;'ispreferably either oxide-coated or formed of a suitable cathode material, and indirect heating is utilized to make this surface electron-emissive. In order to properly utilize the large cathode-emission surface, it is of .importance that a large amount of heat be available. Heat in the necessary quantity is provided in the illustrated embodiment by a heater unit 3| which is coiled about the exterior of the cathode cup 2| and is substantially in contact therewith. This heater unit may be advantageously composed of a stainless steel sheath 32 having a chrome nickel heating wire 34 passing through the center of the sheath 32 and insulating material 33 such as magnesium oxide, within the sheath 32 and about the heater wire 34, thereby preventing electrical contact between the heater wire 34 and sheath 32 ove the length of both. One end of the sheath 32 is closed and the heater wire 34 is electrically connected to the sheath at this end, the other end of the sheath being open with the wire extending therefrom. In assembly, the closed end of the heating unit 3| is positioned at the top of the cathode cup 2| and the heating unit 3| wound about the exterior of the cathode cup from top to bottom. The open end of the heating unit is extended through the bottom of the base unit I and the sheath is secured to the base unit by suitable means such as brazing. A grounded source of heating current is connected to the heater Wire 34 exterior to the discharge device whereby a heating current flows through the heating wire 3d to the closed end of the sheath 32 and thence through the sheath to the grounded base unit. It is to be noted that numerous advantages accrue from the use of this type of heater unit. First a very large amount of heat is available without danger of burning out the heater unit, secondly the heater unit has a very long life, and thirdly the problem of sealing the heating unit leads is minimized.

Provision is made to fully utilize the heat available from the cathode heater 3| by supplyin a heat shield 26 which is located within the base unit I about the cathode cup 2| and heater unit 3|. The heat shield depicted in Fig. 1 is formed of multiple layers of stainless steel spaced apart and situated about the inside of the base unit I. The inner surfaces of these layers are polished in order to reflect heat toward the center of the base unit I. It will of course be appreciated that innumerable types of heat shields could be utilized 'in this connection and that the one shown and described is only presented by way of illustration.

The grid structure 2 as illustrated in Figs. 1 and 3 is relatively deep in a vertical dimension and has vertical slots 4| formed therein. These slots may be formed directly in the grid 2 or as illustrated a slot structure comprising equivalent bars 42 defining equivalent slots 4| may be inserted in a composite grid structure. This slot structure may be advantageously formed of a durable material such as tungsten.

Considering the grid structure it is to be noted that the discharge device of the present invention preferably has a voltage rating of the order of tens of thousands of Volts and that the grid must be capable of preventing electrical breakdown between the anode and cathode at these voltages. In this respect the depth-to-width ratio of the grid slots is of importance, and it has been found to be advisable with this order of magnitude of voltage rating to limit straight grid slots to a depth-to-width ratio of 3 to 1 or more in order to prevent electrical breakdown between the anode and cathodeas a result of the voltage differential existing therebetween, and also to limit the slot width to approximately the same dimension as the anode-grid spacing in order to preclude undesirable ionization. With such a ratio, the anode is adequately electrically masked from the cathode to prevent a sufficient number of electrons from passing into the gridanode region to ionize the gas therein. It will of course be appreciated that the grid slots could be other than straight as depicted and that in such. case the grid slot depth-to-width ratio could be reduced. However, in the interest of minimizing the length of the electron path from the cathode to anode for reasons set forthbelow, it has been found advantageous to utilize straight grid slots with the required depth-towidth ratio.

It will be appreciated that the problems of insulation in the present device are of appre ciable magnitude. In order to overcome these problems the grid-anode spacing is maintained lessthan the mean-free path of electrons in hydrogen at the pressure inside the device, thereby substantially eliminating the possibility of ionization in this space by stray electrons. Also, all of the metal-insulator contacts are in field-free regions, thus preventing electrical breakdown across the insulators.

The base unit grid structure 2, and anode may be cooled by means of water flowing through pipes secured about these units as illustrated. All of the above elements are formed of metal having a high coefficient of heat conductivity and thus very large amounts of heat may be dissipated within the device without damage to the elements thereof as the heat rapidly passes through the elements and may be carried away by the cooling water.

The improved electric discharge device of the present invention is sealed gas-tight as noted above and is filled with an ionizable atmosphere at low pressure. The embodiment illustrated utilizes hydrogen which is circulated through connections 5| and 52. In connection with the hydrogen circulation, there may be utilized a hydrogen filter, regulator, and valve (not shown) of a type well known in the art. The hydrogen atmosphere is maintained at a few hundred microns pressure which may be varied considerably without materially affecting the operation or characteristics of the device. It will of course be appreciated that a sealed system may be utilized instead of the circulating hydrogen system depicted and described above and in such case it is advantageous to utilize a gas reservoir Within the device. Also many difierent types of atmospheres may be utilized; however, advantage lies in employing a gas having a low atomic numbar, as the deionization time is thereby min- 1 volts.

imized, and it is necessary to employ an atmosphere which will not deleteriously affect the cathode emission surface.

In operation, the anode is maintained at a positive potential relative to the base unit and cathode which may be advantageously grounded. Inthis respect it may be noted that the apparatus satisfactorily operates with an anode voltage of 20,000 With the base unit I grounded the heater wire 34 is connected to a source of electric current which utilizes a ground return throughthe base unit via the connection between the heater and base unit I. With the heater unit energized and a high positive potential applied to the anode II, the grid electrically masks, the cathodefrom theanode sothat an insufiicientnumbcr of electrons enter. the anode-grid space to ionizer. the gas therein and cause electrical breakdown. This masking efiect is a result of the grid structure and not because of any particular potential impressed thereon.

Electric discharge is initiated by a positive trigger voltage applied to the grid: by any suitable means (not shown). This positive voltage pulse applied to the grid 2v causes electrons to be attracted to the grid. A large amount of electrons is available at the cathode as a consequence of the oversized cathode-emitting surface and the application of a positive trigger voltage to the grid attracts a large quantity of electrons from the cathode toward the grid. These electrons subsequently pass through the grid slots 4| into the anode-grid space, ionizing the gas therein and causing electrical breakdown and discharge of the device. A suitable load (not shown) is connected between the anode and the cathode, thus providing a return path for the electric discharge.

It is to be noted at this point that electrons traveling from the cathode to anode are not constrained to negotiate tortuous paths as is customary in conventional gaseous electric discharge devices known in the art. To the contrary, the electron path from the cathode contains only a slight bend in order to pass directly through the grid slots to the anode.

Great difficulty has heretofore been encountered in firing discharge devices precisely with regard to time. In practical terms this may be stated as a variation between cycles of trigger voltage of the time that discharge is initiated in the discharge device. This phenomenon is depicted on an oscilloscope screen as a waver or jitter of the discharge line and is thus generally referred to as jitter time. Thus the term jitter time as employed herein is taken to mean the variation of the voltage necessary to initiate discharge of the device, expressed as time. It will of course be apparent that the time Voltage relationship is obtained from the trigger voltage wave form.

As noted above, the minimizing of jitter time is highly advantageous. The present invention decreases jitter time to a remarkable degree. This is strikingly apparent when considered in the light of the power rating of the device. Owing .in part to the oversized cathode-emission surface and in part to the novel grid construction and placement, the present discharge device is capable of operating with a jitter time of less than 0.05 microsecond.

Another important consideration is the ability of the device to withstand rapid reversals of plate voltage. The repetition rate of conventional discharge devices is limited by the tendency for electrical breakdown to occur between the anode and no consequence owing to the rugged structure of these elements.

The present invention has been disclosed in only one particular embodiment; however, it will be apparent to those skilled in the art that numerous modifications and variations are possible within the spirit and scope of the invention and thus it is not intended to limit the invention to the. details shown except as defined in the following claims.

What. is claimed is:

l. A gaseous discharge device comprising a cathode, an anode at a positive potential with respect thereto, an ionizable atmosphere therebetween, and a grid having transverse openings therethrough positioned between said anode and cathode, said grid being electrically insulated from said anode and cathode and being-separated from said anode by less than the meanfree path of electrons in said atmosphere, the openings in said grid having a depth-to-width ratio of approximately three to one, whereby said cathode is electrically masked from said anode, said grid being adapted to receive a pulse of positive voltage whereby the masking action of said grid is overcome.

2. A gaseous discharge device comprising a. cathode, an anode at a positive potential with respect thereto, an atmosphere of low pressure hydro-gen therebetween, a control electrode disposed between said anode and cathode and parallel to said anode, said electrode having transverse openings therethrough and being separated from said anode by less than the mean-free path. of electrons in said atmosphere, the transverse openings in said control electrode having a depth-towidth ratio of approximately three to one, whereby said cathode is electrically masked from said anode.

3. In a gaseous discharge device, the combination comprising a cathode, an anode at a high potential with respect thereto, a low pressure ionizable atmosphere therebetween, a grid parallel to said anode between said anode and cathode and electrically insulated therefrom, said grid being separated from said anode by less than the mean-free path of electrons in said atmosphere, said grid having transverse openings theret'hrough of e, depth-to-widtli ratio of approximately three to one, whereby said cathode is electrically masked from said anode.

4. In a gaseous discharge device, the combination comprising an anode, a cathode, and a control electrode disposed therebetween, a low pressure hydrogen atmosphere between said anode, cathode, and control electrode, and said anode and control electrode being formed of oxygenfree copper, whereby said anode and control electrode have the property of good electrical and thermal conductivity without suffering destruction by said atmosphere.

5. A gaseous discharge, device comprising a cathode at substantially ground potential, an anode at a positive potential, an ionizable atmosphere therebetween, and a control electrode disposed between said anode and cathode and insulated therefrom whereby the potential thereof is electrostatically determined, said electrode having transverse apertures therein for the passage of electric discharge therethrough, said apertures having a depth-towidth ratio in excess of three to one, whereby discharge of said device is prevented, said grid being adapted to receive a positive voltage pulse, whereby said device discharges.

6. In a gaseous discharge device the cornbina'tion comprising an anode and a cathode in close proximity and having a large potential difference therebetween, and a slotted grid interposed between said anode and cathode, said grid being parallel to said anode and adjacent thereto,

and said grid slots having a depth-to-width ratio of at least three to one, whereby said anode is electrically masked from said cathode.

7. In a gaseous discharge device, the combination comprising an anode and a grid disposed in parallel relationship in a low pressure ionizable atmosphere, said grid and anode being separated by 'a distance less than the mean-free path of electrons in said atmosphere at said low pressure and more than the maximum distance for field emission between the anode and grid.

8. A gaseous discharge device as set forth in claim 7 further characterized by said atmosphere being helium.

9. A gaseous discharge device as set forth in claim 7 further characterized by said atmosphere being hydrogen, and said anode and grid being composed of oxygen-free copper.

10. In a gaseous discharge device, the combination comprising a fiat metallic anode, and a fiat metallic grid parallel to said anode and separated therefrom by less than the mean-free path of electrons in the atmosphere therebetween, both said anode and said grid being formed of a metal having a high coefficient of heat conductivity and being provided with external means for dissipating heat, whereby inverse discharge therebetween causes no damage to the anode or the grid.

11. In a gaseous discharge device, the combination of a thick metallic element having an opening therethrough, and a tungsten grid situated within said opening and secured therein, said combination comprising a control electrode of said gaseous discharge device.

12. In a gaseous discharge device, a control electrode comprising a thick metallic element having a plurality of transverse openings therein for the passage of an electric discharge therethrough, said openings having a depth-to-width ratio of at least three to one.

13. In a gaseous discharge device, the combination comprising an anode and a grid adjacent thereto, said anode and said grid having large volumes and being formed of copper metal, whereby the removal of power dissipated internal to said device is facilitated.

14. A gaseous discharge device as set forth in claim 13 further characterized by said copper metal being oxygen-free copper metal.

15. A gaseous discharge device comprising the combination of a cathode-emission surface including a hollow cylinder having a plurality of sets of fins longitudinally disposed upon the inner wall thereof, each of said sets of fins being of a different radial depth and being arranged with the circumferential distance between adjacent fins at least equal to the radial depth of the smaller, an anode disposed adjacent an open end of said cylinder, and a grid having slots of large depth-to-width ratio therein for the passage of electric discharge therethrough interposed between said anode and cathode, whereby said anode is electrically masked from said cathode v10 the cathode emission surface is substantially free of blocking by space charge.

17. In a gaseous discharge device a cathode emission surface comprising a hollow cylinder having three sets of fins longitudinally disposed upon the inner wall thereof, the fins of the first set having twice the radial depth of the fins of the second set and four times the radial depth of the fins of the third set, there being twice as many fins in the third set as in the first and second sets, the fins'of the first set being separated by a distance equal to the radial depth of one of these fins, and the sets of fins being arranged alternately with-a fin of the second set equidistant between two fins of the first set, and a fin of the third set equidistant between each adjacent fin-of the first set and fin of the second set whereby the effective area of the cathode emission surface is subtantially equal to five times the surface of the cylinder wall.

18. In a gaseous discharge device, the combination comprising a cathode having a cathodeemission surface thereon, said cathode including a wall and a number of sets of fins of varying size secured thereto, said cathode-emission surface comprising said wall and said fins, and said sets of fins being symmetrically arranged upon said wall, whereby said cathode-emission surface is substantially equal to the area of said wall multiplied by a factor of two plus the number of sets of fins.

19. In a gaseous discharge device, the combination comprising a cathode having a cathodeemission surface and including a wall and a number of sets of fins, said cathode-emission surface comprisingthe surface of said wall and said sets of fins, the fins of each set having twice the radial depth of the fins oi the next smaller set and said sets of fins being symmetrically disposed upon said wall with the spacing between adjacent fins being equal to the radial depth of the smaller, whereby the effective cathode emission surface is equal to the area of said wall multiplid by a factor of two plus the number of sets of fins, and a heater unit in close proximity with said cathode, said heater unit including a metal sheath closed at one end, a heater wire internal to said sheath and insulated therefrom, said heater wire and said sheath being electrically connected at the closed end of said sheath, and said heater wire extending from the open end of said sheath, whereby an electric current path is provided through said heater wire and said sheath, said heater embracing said cathode, whereby said cathode is secured in position and heated by said heater.

20. A gaseous discharge device comprising a hollow cylindrical metal base unit, a cathode internal to said base unit, a heater unit about said cathode in contact therewith, said heater unit extending through a wall of said base unit and rigidly joined in metallic connection thereto, whereby said cathode is secured in position within said base unit.

21. In a gaseous discharge device a cathode heater comprising a metal sheath in the form of a tube closed at one end, a heater wire attached to the closed end of said sheath, passing through the center of said sheath and extending from the open end thereof, and insulating material filling said sheath about said heater wire whereby said heater wire is insulated from said sheath throughout its length.

22. A method of establishing an electric discharge in an ionizable atmosphere comprising the SQS'Df establishing "a: potential"- ctifferentiai in said ionizaibie atmosphere'produeing a quantity of freefelectron's adjacentthe'negative end of said potential idifierential,"establishing a region of substantially zerojp'otei'itial gradientgseparating said free eiectronsfrom said potentialdiffer- 'ential by said'i'egion'whereby said free electrons are 'electricallyimasked from said potential differential, and instantaneously imparting 'an appreciable unidirectionalvelocity tosaid free eleetrons toward sa-idregion'of zero potential gradient causing 'saidireeelectrons to pass therethrough into the influence of *said potential differential, thereby ionizing said atmosphere and establishing an electric" discharge.

' VVLLLIAM"R. BAKER.

12 REFERENCES crum The. following references are'of r'ecordiinth fiie of this --patent:

UNITED STATES "PATENTS Number Name Date 2,097,490 Kob'el Q. Nov. 2, 19.37 2,169,643 Holst et a1 Aug.15,1939 2,192,162 'Kniepkamp Feb.'27, 1940 10 2,292,382 Le Van .Aug.'11, 1942 2,399,003 Crapu'chettes Apr; 23,, 1946 2,399,752 McCullough ."May"'7, .1946 2,451,938 Garbuny et a1. 001.119., 1948 5 FOREIGN PATENTS Number Country Date 410,337 Great Britainr Maynl'l, .1934

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