US2612613A - Electric discharge device - Google Patents

Description

Sept. 30, 1952 H. C.'STEI NER 2,612,613

ELECTRIC DISCHARGE DEVICE Filed Feb. 25, 1950 O0 0 o o 00 o o J Inventor":

Harry CSteiner,

by/flnzsz. 0 M

. His Attorney.

Patented Sept. 30, 1952 ELECIRIC; DISCHARGE: neuron Harry G. Steiner, Scotia, N, Y;, assignor to General Electric Company, a'corppration of- New- York-- Applica ion February 25, 1950, Serial No. 146,175}.

6 Claims.

My, I invention relates to. electric discharge devices and has. special referenceto, devices of the typewhich employes. a. pool-type, cathode, such as .a cathode constituted of mercury.

Control electrodes, or grids as. they are. commonlyrcalled, have been employed-in electric discharge. deviceshavin pool-type. cathodes to electnQStaticaHy. control the starting; of the anode current-in each loadcycle. Single-anode devices oi this typenhave; found wide and successful use, particularly in industrial applications, as in rec.- t-ifieation. and" inversion,,. where. the very high emission -capacity of :the mercury. pool commonly employed as the cathode meets heavy..duty. and short-circuit current. requirements. Arc backs are still alimiting condition.ln-theusualinstallation of-zsuch devices-;.-and hence provision must be made, tor rapid'deionization of the-arc plasma at the-"beginning; of an inverse cycle whenthe device, anode is: ne ative. in potential with respect tonthe-acathode;

*Wh-ile. theogrid,aperturesmay be made relatively small inorderthata greater degree of control beexercised-both during theload and inverse cycles, especially: at. high temperatures, the grid openings musthe sufiiciently, large. to pass the maximum currents. required. Accordingly, the usual'method of hastening deionization-.means for. cooling :the. are plasmafiecannot. be neglected, and; so,-. devices or this type: are usually encased in-a zwaterjacketedelongated metallic cylinder at cathode potential, which discharges thefiions and condenses thecathodevapor. At the samev time, itsis desirable tobeable to initiatethe conduc tion cycle. when-the-device is at low. temperatures without, providing, preheating means or. using excessive. anode-"voltages...

finch-difficulty in starting devices of lthis nature ooours-duetolack of ionization at. thelow cathode vapor pressure'prevailing when the tube is cold. 'Whenthe ambient tem peratureis very low the. internal vapor pressurebefore. operation approaches; that-. degree of vacuum associated with conventional vacuum tubes and the .mean

freepath of the-ions necessary toestablish' conduction-is relatively long. The passage. through the grid. openings; maybe-so.- constricted that the cathodevapordifiusestoo slowly up through, the gridtoreplenish theion supply. Even .at highly positiveanode potentialsrelatively few electrons reach the. anode, although an .arc may be estabe lishedbetween the .grid. and cathode; spot.

This proble'ni is, amplified as the-current rating, and. hence, the siz e o f;,- the device, is.- increased, due-yto. the usually longer are path. and the increased difiiculty of deionization;

x111 18.38112 object 0flmy;:in'vention to provide a pooltype; cathode dischargezadevice of the nature desoribed'in"whichr conduction may be easily established: at low temperatures. ti I be. best. iun derstood;.b-y re erenc to It is another, object of my invention to prqvifde an improved discharge device" ofthe. nature describedfwhiohmay be successfully operated ovei' awide, rangeofvapor pressures; I g

My, nventio i ad tag u c porated in an electric, discharge device-oithe type h-aving a pool-:tyhe. Cathode. and a'd'eionizii'1g"meta11lc envelope surface surroundingithe discharge path; A control electrode or gridf'having'a bottomd-i 'sk' liken rti n extendi t nsv rs y a ross e dischareelpath is. prov ded with axp r l relatively. ar onenine's e rpi ne ph of the bottom portion- Adfiitionalsmalle f, con-ventionallfgrid openings areprovidedin sufficient number to, permit passage; of themak-imum required. curr nt. h v ar ene he loneninss permit. easy. startin fifthe' oondno nnyc erat thelow vapor pressures encountered atlow tom-'- per'ature, starting without loss or control at higher operating. temperatures. This advantage is" due to th proximity :of the a t ro gh" he peripheral'gridjopenings to the cond-ucting;wallv or. envelope surface, which quickly deionizes the nearby. aroplasma of th discharge nathrthro sh the 1arge..pier pheral sr 'dlo ni ss h f a ond cti n cyc e-y uch" a sa fang men IIOtlaQYGESEIY- eiIect the maximum opera t perature and" the. 1-current'capacity; of th ow. va or pressu e s c erodiwhere: low amb en temp. .v

The features which I desire to, protect arev pointed out wi h p t u a i y; amended. cl ims. The in n on' f t ge her safes} meta withfurther ob .ot and adv n a es tth iiinay e l wifij ,t h r win in .wn 'oh' i'e-j is new al' V ew, or is h g8 d so iption taken i conn c n Wi device embodying my nvention; Fig; 2- is a hottom. view otithe .o trnl. e' Ero o n; Fi 1'; Big; .3. is a'uott n; v ewiof-i' g the rid.'emn oyedimFig-l1;. and. 51a tional' View off a portion of "anotherudisoharge device/embodying my invention. j

Rei rrin now toi liis- I h hDW n Sichargle. device I embodyin in e io on two metal oylnidersz'and o spa d'f s j awator j cket between them, comprise an; 1%?- ated. conduc e env ope 'io idis harse dev ce- Inlet. andv outlet connections t r .oooling o are respectively plaooda the. bo tom'and the outer cyl nder a in ic ted; a 41am: sn oe b twe n. th nn r. and outer... cylind and. is c1Q$e.d..-at .the, endsofthe cy1ind). closure r s, 6 r. o her. suitable. meansn transverse header 8.

end of the envelope in contact with the lower This cathode may comprise a body of mercury or some other material, such as cadmium, which is capable of analogous use. A cathode current terminal is provided by a terminal block l0, secured to the underside of the lower header 8. A starting electrode II, which, as illustrated, is of the immersionignitor type, is insulatingly supported from the lower header. An external terminal 52 for the starting electrode extends through the lower header 8 and is insulated therefrom. A conventional holding anode I3 is similarly insulatingly supported from the lower header with its external terminal I4 extending therethrough.

-;Within the envelope at its upper end is an anode [5 in the form of a graphite cylinder mounted on a rod-like conducting stem It. A lead-in arrangementcomprising a flanged metal cylinder IT, a glass sleeve I8, and a metal cap [9 serves to insulatingly support the anode and its stem from the upper head 1. An anode terminal block 20 is secured to the upper surface of the metal cap I9 and ,a flexible copper cable 2! is suitablyconnected to the terminal block to serve as a means for conducting current to the anode.

,An apertured graphite control electrode or grid substantially surrounds the anode [5 to thus extend across the path of any discharge are between thecathode 9 and the anode l5. This grid may suitably takethe form of a basket having a flat bottom 23 extending transversely of the cylindrical casing and intersecting any straight-line path between the grid and anode. The grid is, of

course, insulated and spaced from the anode and the envelope in order that it may function as an electrostatic control electrode, but its periphery is relatively close to the envelope where deionizationtime is shortest. Ihe cylindrical side portion 24 or flange of the grid is integral with the grid bottom 23 and is connected at its upper end to'the upper transverse header 7 by a plurality of'cylindrical insulating bushings 25. A grid terminal bushing 26 is also connected to the grid and extends through the upper header to provide an external connection for applying the grid potential.

A splash bafiie 2! is mounted above the cathode between the cathode and the face of the anode to prevent any droplets of mercury from splashing onto the grid or anode. The bafiie may suitably take the form of a disk so positioned as to b ock any straight-line path between the active surface of the cathode and the anode.

,As shown in Figs. 1 and 2, the grid is plurally I perforated or'apertured to allow a .number of arcdischarge paths therethrough in order that the initiation of the conduction cycle during each period when the anode is positive with respect to-the cathode may becontrolled by the grid potential. In accordance with my invention and asfurther shown by Fig. 2 the bottom 23 ofthe gridielectrode is provided with a plurality of relatively large' openings, 28 near its periphery. These openings may-suitably be circular perforations for ease of manufacture. Since the splash baffle. 21 blocks. any straight-line arc path betweenthe cathode and the anode, the shortest arc paths to the anode extend around the edge of the baffle and through the large peripheral gridiopenings 28. Despite the relatively large size of the openings, however, arc-backs through theselarge grid openings are minimized due to the proximity of the grid openings to the inner casing orenvelope 2, which eifects rapid deionization of the arc plasma after the conduction period. The deionization processis itself conventionalthe device-casing 2 is at cathode potential so that it discharges the ions, and it serves to cool and condense the cathode vapor.

The advantage of the large grid openings 28 is realized at low vapor pressures when the mean free path of the ions is relatively long and the arc voltage needed to produce the required ionization in conventional tubes is relatively high. Since the large openings minimize constriction of the arc path by the grid the electrons initiating conduction are able to pass through the grid openings in greater numbers to reach the anode. The cathode vapor is also able to difiuse more quickly into the regionbetween the anode and grid and thus replenish the available ion supply. This advantage is especially significant in high current tubes of relatively large volume where the arc paths are longer. It has been observed that without the presence of the large peripheral grid openings employed according to my invention, the arc path may be so constricted that at extremely low vapor pressures there are insuflicient ions to neutralize the spacecharge and for an instant the current is abruptly reduced as in a high vacuum discharge device with resultant voltage surges produced in the associated circuit. I v I In larger devices having greater volume and longer are paths, the necessity of preventing arc constriction increases and it is likely that a substantial thickness of the grid face is required to provide mechanical strength and resistance to deterioration. Accordingly, the diameter of the peripheral grid openings should also be keptrelatively large withrespect to the thickness of the grid, i. e.,the distance through it. This ratio should bein the neighborhood of 4:1 or'greater to permit most of the electrons to pass through i Were these center openings 29 also to be largeas the peripheral openings, arc-backs would more frequently occur at high temperatures since the arc plasma in the center portions of the discharge device requires a greater deionization time at the end of the conducting cycle due to its distance from the cooled conductive sidewall 2.

In the embodiment of my invention shown in Figs. 1 and 2, I have also shown a plurality of grid openings 30 positioned between the central openings 29 and the largeperipheral openings 28, these intermediate openings 30 also being intermediate in size. By utilizing such a graduation in size of the grid openings from the center of the grid bottom to its periphery to compensate roughly forthe deionization time of'the arc plasma from the center of the discharge device to its sidewalls, the operating characteristics are further improved.

A plurality of grid openings 3| are also provided in the cylindrical sidewall of the grid to increase the short-circuit'capacity andalso to permit greater heat dissipation. from'thejanode mtnesieewans ore tlieidevicer fI hese opnings are acent the sdewallsmfi thestubeabutztheir size i'seiincreasing ss Eiinportant with-l the; crease in tli are-paths lirough than Accordinglwwhile flie size mf tlie openings fil could he?- variously regulated fihave no't wand iti' necessary td db 7 so sinee gnc openings ifs th-b'o'ttom f aceiof me iifng ftowards thepenter-of the grid bottom so that greater electros-tatic--control rnay be eierted at this apex," while the larger and more open portion of the grid opening is nearest the periphery where the deionization time is less. Various configurations and combinations of larger and smaller openings may be employed without departing from the spirit of my invention, depending upon the requirements of the device as to minimum vapor pressure and current requirements.

In Fig. 4, I have shown my invention as embodied in a single anode, multi-grid discharge device of the type having a pool-type cathode. In this device, the anode 35 is surrounded by a successive plurality of grids; in this case, an

inner grid 36, an intermediate grid 81, and an outer grid 38. The envelope 39 of the device is preferably a double-walled metallic cylinder of the type shown in Fig. 1 and which acts in the same manner to deionize the arc plasma at the end of each conduction cycle. Each of the grids is provided with a plurality of central openings 40 which operate in a conventional manner. In accordance with -my invention, a plurality of larger peripheral or near peripheral openings 4|, which function as to the large openings 28 shown and described for Figs. 1 and 2, is provided in the bottom portion of each grid. While those portions" of the arc path defined by the large peripheral openings in the inner and intermediate grids 35 and 31 are not subject to such rapid deionization as are the arc path portions at the peripheral openings in the outer grid 38, the inner and intermediate peripheral grid openings should also be relatively large in order not to constrict the arc paths and cancel the advantages obtained by utilizin the large peripheral openings in the outer grid. The large peripheral openings in the respective grids are preferably aligned so far as practical to further decrease arc constriction and permit electrons to reach the anode to begin'each conduction cycle. Since without the large openings nearthe grid bottom peripheries the minimum vapor pressure at which conduction could start would be relatively high, due to the constriction of the arc paths caused by the plurality of grids, my invention is very advantageously employed in multi-grid devices of this nature.

For the purpose of this application, I have used the terms grid and control electrode synonomously and intend that they be so interpreted. 1

While the present invention has been described by reference to particular embodiments thereof, it will be understood that numerous moi-22ers Iengtrsand; voltage fdi op 'required or Pats use arr withont actdally departingifromifthe he;

venticn I", therefore aintzzin the is'appended claims to covenal l' suc equivalentivariartl'onsms coins tr ue spirit an'eb scapes ofa-ithe fbiegeri disclosurez wag claim as newi'andidesirezto lfletter si atent of 'til'e Unitedj s'tates =1.;Ain*. lectric 'diseharge iievice.. comprisiirgr-ai single; 5m .1 anode and a. I pool-type 1 cathode theanode -tofiprovid di'scharge paths fe conduction 'erebetween means-lion dei6fli2ifig th$ spacei sai discharge ecure bya -c *a-cros's said discharge; path with near said wall surfaces; caresses-seine provided with a plurality of openings to define discharge paths therethrough, said openings near the periphery of said surface being substantially larger than those near its center in order to facilitate initiation of conduction in paths near said deionizing means.

2., An electric discharge device comprising a single main anode and a pool-type cathode spaced from the anode, deionizing means comprising metal wall surfaces laterallysurrounding the discharge path between the anode and the cathode, said metal wall surfaces being in conductive relationship with said cathode, and

a control grid having a face extending transversely across said discharge path with its periphery near said wall surfaces, said face of said grid having a plurality of perforations to provide arc passages therethrough, the perforations near the periphery of said face being substantially larger than the perforations near the center of said face in order to facilitate initiation of conduction in paths near said deionizing means.

3. An electric discharge device comprising a single main anode and a pool-type cathode spaced from the anode, deionizing means comprising metal wall surfaces laterally surrounding the discharge path between the anode and the cathode, said metal wall surfaces being in conductive relationship with said cathode, and a control grid having a face extending transversely across said discharge path with its periphery near said wall surfaces, said face of said grid having a plurality of perforations to provide arc passages therethrough, the perforations being progressively smaller from the periphery of said face to its center in order to facilitate initiation of conduction near said deionizing means.

4. An electric discharge device comprising a single main anode and a pool-t pe cathode spaced from the anode, deionizing means comprisingmetal wall surfaces laterally surrounding the discharge path between the anode and the cathode, said metal wall surfaces being in conductive relationship with said cathode, and a control grid. having a face extending transversely across said discharge path with its periphery near said wall surfaces, said face of said grid having a plurality of large openings near said periphery to provide arc passages there through, and relatively the center portion of said face being imperforate in order to facilitate initiation of conduction near said deionizing means.

5. In an electric discharge device employing mercury as an ioniz'able medium and compri..-v ing a generally cylindrical elongated metal envelopevgand an anodeqand cathode within the Y plurally perforated disk-like graphite portion exenvelope at opposite ends thereof, a-.control electrode interposed in the discharge path between said anode and said cathode, said electrode having a disk-liked portion extending transversely of said envelope withits periphery near said envelope, said Portion having a plurality ;of apertures with at least one ofsaid apertures near its= periphery being substantially larger than cooling said envelope to facilitate deionization 20 2,500,153

of said space, and a control electrode having a tending transversely of said discharge path'with 'its' periphery near saidzenvelope, said portion having relatively large perforations near its periphery; the diameters of said perforations being at least four times the thickness or said disk-like portion; and having substantially smaller; perforations near its center portion;

HARRY C. STEINER;

REFERENCES CITED The following references are of record in the file of this patent:- 'i' a i UNITED s'rAT s PA'I'ENTs v Date Number Name 2,459,827 Marti- Jam 25, 1949 2,490,087 Pakala' Dec. 6, 1949 Cork Mar. 14, 1950 2,501,308 i -Boyer Mar. 21, 1950

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