US2452075A - Velocity modulation electron discharge tube - Google Patents

Description

Oct. 26, 1948.

c. G. SMITH VELOCITY MODULATION ELECTRON DISCHARGE TUBE Filed Dec. 18, 1941 Patented Oct. 26, 1948 VELOCITY MODULATION ELECTRON DISCHARGE TUBE Charles G. Smith, Medford, Mass, assignor to Raytheon Manufacturing Company, Newton, Mass, a corporation of Delaware Application December 18, 1941, Serial No. 423,493

15 Claims. 1

This invention relates to an electron velocity modulation discharge tube, and more particularly to the utilization in such a tube of a cathode which is capable of supplying large peak values of current. Heretofore such electron velocity modulation tubes have had difiiculties associated with the cathode. The cathode used has been limited in its ability to supply large peak values of current. Also its life has been short, particularly when subjected to increased loads and severe ion bombardment.

An object of this invention is to produce an electron velocity modulation tube in which a limited amount of thermionic emission is produced, and in which a large part of the emission from the cathode is supplied by causing electrons to return to the cathode with suificient energy to release secondary electrons.

Another object is to produce a cathode adapted for the foregoing use which shall have long life and relatively high efiiciency.

The foregoing and other objects of this invention will be best understood from the following description of exemplifications thereof, reference being had to the accompanying drawing, where- Fig. 1 is a vertical cross-section of a tube embodying my invention;

Fig. 2 is an enlarged top View of the cathode illustrated in Fig. 1;

Fig. 3 is a top View of an alternative form of cathode; and

Fig. 4 is a cross-section taken along line 4-4 of Fig. 3.

In the arrangement as shown in Fig. 1, the tube consists of an envelope I of some suitable material, such as glass, containing a cathode Z, a focusing electrode 3, an electron velocity modulating grid structure 4, and an electrode 5.

In accordance with my invention, I construct the cathode so as to supply a limited amount of thermionic emission, and also to supply relatively large amounts of secondary emission which may carry whatever peak current the tube is called upon to deliver. For this purpose the cathode may be constructed in a satisfactory form, as shown, for example, in Figs. 1 and 2. This cathode consists of an outer cylinder 6 of a metal, such as tantalum, nickel or the like. A short distance below the top of the cylinder 6 is supported a transverse wall member I, leaving an upper relatively shallow cup. In this cup is inserted a ribbon 8 of some suitable refractory metal, such as tantalum, molybdenum or tungsten. The material which I prefer to use is tantalum. The ribbon B is coated with electronemitting oxides, and then rolled up into a tight spiral and inserted snugly Within the shallow cup in the top of the cylinder 6. The coating on the ribbon 8 may initially consist of a mixture of barium-strontium carbonates in a suitable binder, and baked in air at a temperature of about 400 F., so as to produce a strongly adherent coating consisting largely of the oxides of barium and strontium. The coating is scraped off the exposed surfaces of the ribbon 8, leaving coating 9 between the adjacent turns of the spiral. Thus the upper edges of the ribbon 8 are substantially bare of the coating while the coating material itself is exposed to the discharge space between the adjacent turns of the spiral. I prefer to construct the cathode so that the major portion of the surface thus exposed is substantially bare tantalum, and a minor portion of the surface thus exposed comprises electron-emitting oxides. The transverse wall memher I forms a lower chamber within the cylinder 6 in which is inserted a heater spiral Ill of some suitable refractory wire, such as tungsten. The spiral Iil has one end II thereof fastened to the inner wall of the cylinder 6, and the other end connected to a central lead I2. The cylinder 6 is provided with a lower transverse shield I3 having a central opening through which the lead I2 projects. The .entire cathode structure is supported by a pair of conducting standards it sealed in a stem I5 formed in the envelope i. The lead I2 is likewise sealed through said stem The cathode structure, as described above, possesses the property of being an excellent secondary electron emitter, particularly from the scraped tantalum surfaces, as well as a good thermionic emitter from the exposed oxide surfaces. The tantalum has a tendency to reduce the barium oxide liberating small amounts of barium on the surface of the coating which tends to give excellent electron emission. Also the barium so liberated tends to coat the bare surfaces of the tantalum, making them excellent secondary electron emitters. Even without any barium coating, tantalum in itself is a good seconda'ry emitter.

The electrons emitted from the upper surface of the cathode 2 are preferably concentrated into a beam by means of the focusing electrode 3. This electrode is in the form of a ring, and is supported by a conducting standard It likewise sealed in the stem I5.

The electrode 4 is preferably in the form of a hollow annular ring having a portion cut out Of its inner central region to provide openings through the center of the electrode 4 through which the electron beam may pass. Grids H and [8 of relatively fine wire and wide spacing may be placed across the openings, although these grids maybe omitted if desired. The proper potential may be impressed upon the electrode 4 by means of a lead-in conductor I9 sealed through a wall of the envelope I. Oscillations which are generated in the grid electrode -4 may be supplied to a suitable external circuit. through a coupling loop 20, one end of which is fastened to the inner wall of the grid electrode4 and the other end of which is connected to alead 2i. Surrounding the lead 2| is a pipe 22 fastened to an opening in the outside of the grid electrode 4. The pipe 22 is sealed through the .wall of the envelope I, and the conductor 2! passes through a suitable seal 23 within the pipe 22. The seal23 is preferably at a node of voltage, --so-as to reduce the losses at this point. The pipe '22 and lead =2I form a concentrictrans-mission line which readily passes the ultra high frequencies generated by the tube described.

The electrode 5 functions to refiectelectrons "back through the electron velocity modulating grid *4. "It is preferably curved so as to direct the "reflected electrons toward the opening through said grid 4, and is likewise supported on-'a-lead24 sealed through the' wall of the envelope I. 1

Income instances I prefer to use coils 25 and "26 surrounding the tube for the purpose of setting up a longitudinal *magnetic field --which assists in focusing the electron beam issuing from the cathode '2.

The tube describedabovev may be connectedin some suitablecircuit such as' illustrated, for example, inFig. 1. Inthis circuitthe heating fila- "ment I is supplied with heating current from the secondary winding 2 of aheating transformer 128 having .a primary winding '29 which may'be connected to some suitable source of alternating current. A sourceoi biasing potential "39 maylbe connected, between the focusing electrode 3 and the cathode? with such a polarity as to, make. theelectrodeifi. slightly negative with respectto the cathode. If desired, however, the electrode 3 could .be conneciteddirectly tothe cathode2. A high voltage-sourcefil, which may be for example, ofithe order -.of 1000 volts-is connected between the cathode? and the gridfll. The electrode .5. connected through a variable biasingmoltage. arrangement 32 to the .cathodel.

The arrangement 32 may-consist ofa battery across whichcis .connecteda voltage-dividingv resistance provided witnan adjustable tap. .The tap is connected to the lead '24eand the proper terminal .of the battery is connected to .the

Loathodellead i l. so .as'to make the electrode slightly negative .withrespect to the cathode. ,If

.desired, the electrode iicouldbe connected .di-

.rectly to thecathodefZ. In, operation the, magni- .t.ude..of the voltage bias oncdmay. beadjusted .to give,.a.,maximum outputfor the device. .The coils and 26 are connected inseries and :may

.besupplied with currentnfrom a-suitable source ..33 through a control switch. 34.

In starting thev operationof the device, the

. cathode is heated to atemperatureat which .sufiicient thermionic emission occurs .toinitiate the operation, such emission ordinarily. being much JESS i. then that required to supply either the normal current requirements or the p ak he initiated therein.

voltage difierences to the edges I7 and I8 which vaccelerate .some of the electrons and decelerate :speeds, -and fall-upon the outersurfaces of the electrode 4, which .in this way acts .asawollector of =theseeelectrens.

. structure f4.

i! and I8aof'thegridi4. "The thermal agitation of the electron atmosphere which is thus created within the grid 4 sets up variations in voltage in said grid structure which causes oscillations to These oscillations impart others. The velocity differences thus imparted to the electrons cause faster electrons to catch up with slower electrons and produce bunching.

" Thebunching usually occurs after the electrons in bunched-condition. These electronspreferably have such a time phase'relati-on 'thatthey enter the electrode 4 when the grid i'8- is negative and'the'grid'il is p,os itive. Under these conditions the electrons :are accelerated and have imparted to them an additional velocity to :that which they possessed upon enteringtheelectrode In thus accelerating the" electrons, the oscillating electrode structure '4 :imparts energy thereto. The electrons thus accelerated leave the electrode 4,.pass the grid I! at relatively high speedsand travel to the cathode witlrsuficient energy to impingevupon. the upper surface of the cathode and liberate secondary electrons,

particularly from the bare tantalum surfaces, as 'described above. "The 'bunching action is substantially completed-by the-time the electrons reach .the cathode. T-hereforesecondary electrons in increased numbers are-liberated from thecathode surface ,each time a. bunch of primary' (electrons collide i with said cathodenas describedabove. .It. will be seen, therefore,- that the secondary .electrons .are likewise emitted .in bunches, and will proceed toward the electrode-4 under. the action of thevoltage impressed on said electrode. The bunched secondaryelectrons will enter the-structure .ofthe electrode .4 when the grid I8 is negative and the grid ll .is'positive. Under these conditions the electrons which are travelling at relatively high speeds-moveagainst the action of the; electrostatic field createdbetween the grids H and I8, and thus'give-iup energy-tothe electrode structure 4. This energy .is of proper phase and fre uencyto. reiniorcathe initial oscillations and cause thecclevicetogencrate -.-susta-ined oscillations sat considerable amplitude. The electrons which thus impart energy to;theelectrode-structure l. mayleave said structure through the grid I8 with relatively-low ,Inorder that .the time phase relationships-described above-shall occur, the-.=elec.trodes'should be .spacedirrelative to each other in a;;predeter :minedrelationship. The, distance between the emitting surfaceof the :cathodehand the grid i? should be equal to whereN -is an. integer and MD is the dis-tancebetween electronbunches. This distance D maybe calculated approximately from the values of voltreceived a filament 31.

age impressed on the tube and the frequency at which the device is intended to be operated. The distance between the reflecting surface of the electrode 5 and the grid 18 should be where N is an odd integer. If the actual spacing in a device differs somewhat from these relationships, the efiect will still be produced butto a reduced extent. However, as long as the action described occurs to a substantial degree, the operation of the device will be improved to that extent. The relationships as described above, whereby amplified operation of the device is secured, may

be affected by the voltage impressed on the electrode 5. This affords a convenient way of modulating the oscillations generated.v For this purpose a modulating voltage source 40 may be connected in series with the electrode 5, preferably through a coupling transformer 4|.

Under some conditions where it is difficult to set the system oscillating, the voltage of the electrode 5 may be varied through a relatively wide range, enabling a condition to be reached which may be more favorable to the initiation of such oscillations. Once oscillations have been started, the voltage of the electrode 5 may be restored to any desired value.

Instead of using a cathode structure, as illustrated in Figs. 1 and 2, a simplified cathode arrangement can be used advantageously in my invention. An example of a simplified form of cathode is illustrated in Figs. 3 and 4. This cathode consists of a disk 35 of a suitable secondary electron-emitting material. For example, this material may be a magnesium silver alloy which when placed within the tube and subjected to activating operations therein becomes an excel lent secondary electron emitter. The disk 35 is provided with a central opening 36 in which is This filament 31 is a good thermionic electron emitter, and may be of the thoriated tungsten or of the oxide-coated type. The ends of the filament 31 are supported on leads 38. The disk 35 may be supported from one of the leads 38 by a supporting wire 39. When the cathode illustrated in Figs. 3 and 4 is substituted for the cathode 2 in Fig. 1, the operation is substantially that as described above in connection with said Fig. 1.

In the arrangement as shown at Fig. l, the electrode 5 may also be made of material, such as that described in connection with Figs. 3 and 4, which is a good secondary electron emitter. Under these conditions, the parameters of'the device are so selected that electrons are permitted to fall upon the surface of the electrode 5 with sufficient energ to liberate increased secondary electrons. These emitted electrons proceed from the electrode 5 toward the electrode 4, substantially as described in connection with the electrons reflected from the electrode 5. Liberation of secondary electrons from the electrode 5 likewise may increase the amount of power which may be derived from an arrangement such 'as herein described.

Of course it is to be understood that'thisinvention is not limited to the particular detailsas described above, as many equivalents will suggest themselves to those skilled in the art. For example, the electron velocity modulating grid! may take a wide variety of forms, including variousshaped hollow chambers as well as straight device comprising-a cathode having an active cylindrical 'electrodesi Also devices of this kind may be made to function as amplifiers and detectors as well-as oscillators. Likewisefithe modu lating efiect exerted by the variation of voltage on-the electrode 5 maybeutilized where electrons are not returnedtothe cathode but are permitted to pass back through the grid' l and fall upon its outer surface adjacent the grid 11. In this case the-cathode need not bemade a secondary electron emitter. It will be seen, therefore, that the various features of my invention as described above are capable of a wide variety of embodiments as many other equivalents will suggest themselves to those skilled in the art.

' What'is claimed is::. 1. An electron velocity modulationdischarge devicecomprising a cathode having a substantial thermionicemission less' than that required to carry the peak current of said device, an electron velocity modulating grid interposed in the "path of electrons emitted from said cathode for impartingvelocity modulation to electronspassing through said grid, andelectrode means interposed in the path of the electrons passing through said grid for reflecting said electrons back onto said cathode with sumcient energy and in sufi'icient numbers to liberate secondary electrons in sufficient' numbers 'toproduce a total electron emission capable of carrying said peak current.

2; An electron velocity modulation discharge device comprising a plurality of electrodes including a cathode having an active surface which liberates secondary electrons readily, an electron velocity modulating electrode'structure comprising a pair of-electrodeelem'ents through which an electron path-from said cathode" passes, the electrons-emitted from said cathode formingan electron stream along said path, said electrode elements being connected to each other" by a cav ity resonator having a suflicie'ntly high resonant frequency to'impai't'bunchihg to 'the electron strea'mlin said path; and electrode means for directing electrons onto said cathode with suflicient energy" and in sufficient numbers to'liber a'te secondary electrons which form part of said electron stream.

3. An electron velocity modulation discharge device comprising a plurality of electrodes including a cathode having an active surface which liberates secondary electrons readily, an electron velocity modulating electrode structure comprising a pair of electrode elements through which an electron path from said cathode passes, the electrons emitted fromsaid cathode forming an electron stream along said path, said electrode elements being connected to each other by a cavity resonator having a sufficiently high resonant frequency 'to impart. bunching to. the electron stream? in said path, and electrode means for reflecting said electron stream after it has passed through said electrode structure back onto-said c'athode'with-suflicient energy to liberate second-- ary electrons which then form part of saidx'electron'stream."

4." An electron velocity modulation discharge surface which liberates secondary electrons readily, an electron "velocity modulating electrode structure comprising a'pair of electrode elements through which anelectron path from said cathode passes, the electrons emitted from said cathodeforming an electron stream along said path, said electrode elements being'conn'ected toeach other by a cavity resonator-having a sufiicientl'y high resonant frequency to impart bunching'to mnduiatingzelectrodestructureicomprisingga .pair

ofitelectrodeeelementsrthroughnwhiciran electron patlm from saido. cathode r; passeslgsaid electrode elements: beingc connected; toeeach: other; by; a cavity resonator? havinglz as sufilcientlyy high resonant frequency to impart bimchingg to the elcctronf-streami in.- saidixpa-the whereby-- the elecbl'DlBi. ofii saidn streamuare concentrated. into bunchesstravellingg through said; devicena-l predetermined-distance. aparla andtelectrodeimeans for reflecting saidlelectrom streamiaitenithas; passed tlmoughisaid'selectrode:stnuctwrerbacke onto: said cathode; saidi cathodes being. spaced: from: said electrode .-.struct1irea aa-distanlcee-whichi is substan tiaiiy-r wherevzN issan integemandsD -is'zatlre-:distancerb.etweemelectrombhnches:

62. Am electrom velocity." modulation 1 discharge devic-eiconrprisinga catlmde; anrxelect-ron velocity modulatingjelectrod-erstructure comprising; a pair of? electrode elements-:ethroughwwhichan \electron pathfrom: saidcathodes. passes, said. electrodeelements being; connectedzto-eachother by-acavity 1 resonator.howing:-asuflicientlmhigbresonantirequeney to'impar-t zbunching-tmtheelectronstream inqsaictpath; wherebysthe electron-sci saidstream are*concentratectintmbunchesrtravellingtthrough said devicera; pnedetermi-ncded-istance. apart,l.and

an. additionah electrode; on -.the opposite. side. of said electrode; structure from. said cathode. and adapted; to reflect said... electronrstream. back through. said. electrodes-tructure, said.ca.t1iode.he-. ingz; spaced: from said v electrodastructure a dis: tance which is substantially whereiN (is ant'integen. andr-D lathe-distance. be+

tweem-electromhun ches-;; said "additional electrode being; spacedvifrornesaids electrodesstructureesub, stantiall-yc whereiNi i's1an cddintezer.

'IiAi-r elctronrrrvelocitx modulation discharge device scomprisingia-:ca'thoderhawing;an activessurxfaceewhichcliberates:.secondarweleetronstreadil35;

anielectronevelccity?modulatingcclectrodeestruc turez comprising.7;a; ofv. electrodes elements throuetm anrnelectrom: patlmfizomvsaid cathode passes, said electrode elementsabeing '8 .beingspaced from saidelectrode structure-9a dis tance which is substantially,

where N is an integeirand D is the distance between electron bunches.

8. An electron velocity modulation discharge device comprising acathode having an active surface which. liberates secondary electrons readily, an electron velocity modulating, electrode structure comprising a pair of electrodeelements through which. an electron path from said cathode passes, said electrode elements being connectedcto eachnother by a; cavity resonator havingiasuflicientlyhigh resonant frequency to impart=bunchingv to the electronstream in said path, .whereby the. electrons of 4 said strean-rare concentrated: into: bunches travelling. through said device-.ai predetermined distance apart. and an. additional. electrodev on the opposite sideof said electrodeistructurefrom said cathodeand adapted toe reflect. said. electron. stream back through saidelectrode structure with suficient energwtm liberate secondary electrons whichthen formpart .of .said electron. stream. .saidcathode beingespaced. from.said..electrodestructureadis: fiance-Which issubstant-iall-yv where N is aninteger' and D is "the distance between electron'bunches; said additional electrode being spaced from 'said electrod'estructure sub= stantiallyf' where N is an odd integer;

9": In: an. electrical system, an electron: velocity modulation discharge device comprising a: phirality ofielectrodesincluding: a--cathode,1 an electhe electron-stream in saidi path and anadditionazlxelectrod on the'opposite'side. of 'saidelectroderrstructure from said cathode and adapted to reflect 'saidi'electron stream back through said electrode structure, whereby said device is adapted. to operate with. high frequency currents in saidrelectrodestructure, and meansv connected connected .ato each-i otherrbyea cavity? resonator havingga; sufifcientIm high: resonantc-zfreqyencyto impart-.rxbunchingg tortheeelectromstream inesa-id pat wherebrth'e: electronsof':said..stream are concentratedr into" bunches: travelling, through sairtrdevicecaa predeterminedudistanceeapara. and electrode means.' for: reflecting said? electron stream-after: it 1hasinassedathmugknsaidlelectrode structurerback: onto .-said'-.- cathode witha. sufiicient enesgmtodiberatesecondaryselectrons whichthen dorm: 281312; of? saidielectnon s streamg, said; cathode 'tosaid": additional:- electrode for varying the voltage on-said =additional--.electrode with respect to zsaid icathode' for varying said high .frequency currents;-

10. In'an:electricalsystem, an electron velocity modulationwdischargedevice-comprisinga'plurality oi -electrodes:in'cludinga cathode having an active; surface" Whichliberates secondary elece t'ronsreadily; an electron-velocity modulating electrode'.structure comprising a pair. of electrode elementsthrough which: an electron path .from saidcathode passes;=said electrode elements being connected-to eachother by a cavity resonator having a suificiently high resonant-frequency to impartbunching. to the. electron stream. in said path and an additional. electrodeon theopposite side ofsaidelectrode structure from .saidcathode and adapted to: reflect said.- electron stream back through said electrode. structure onto said cathodelwitli. sufiici'entenergy to. liberate secondelectrons..which thcn.form part of said electron stream, whereby said device is adapted to operate with high frequency currents in said electrode structure, and means connected to said additional electrode for varying the voltage on said additional electrode with respect to said cathode for varying said high frequency currents.

11. In an electrical system, an electron Velocity modulation discharge device comprising a cathode, an electron velocity modulating electrode structure comprising a pair of electrode elements through which an electron path from said cathode passes, said electrode elements being connected to each other by a cavity resonator having a sufiiciently high resonant frequency to impart bunching to the electron stream in said path, whereby the electrons of said stream are concentrated into bunches travelling through said device a predetermined distance apart, and an additional electrode on the opposite side of said electrode structure from said cathode and adapted to reflect said electron stream back through said electrode structure, whereby said device is adapted to operate with high frequency currents in said electrode structure, said cathode being spaced from said electrode structure a distance which is substantially where N is an integer and D is the distance between electron bunches, said additional electrode being spaced from said electrode structure substantially where N is an odd integer, and means connected to said additional electrode for varying the voltage on said additional electrode with respect to said cathode for varying said high frequency currents.

12. In an electrical system, an electron velocity modulation discharge device comprising a cathode, an electron velocity modulating electrode structure comprisin 'a pair of electrode elements through which an electron path from said cathode passes, said electrode elements being connected to each other by a cavity resonator having a sufficiently high resonant frequency to impart bunching to the electron stream in said path, whereby the electrons of said stream are concentrated into bunches travelling through said device a, predetermined distance apart, and an additional electrode on the opposite side of said electrode structure from said cathode and adapted to reflect said electron stream back through said electrode structure, whereby said device is adapted to operate with high frequency currents in said electrode structure, said cathode being spaced from said electrode structure a distance which is substantially where N is an integer and D is the distance between electron bunches, said additional electrode being spaced from said electrode structure substantially where N is an odd integer, and means connected to said electrodes for varying the relative potentials on said electrodes with respect to said cathode for altering said relationships and thereby varying said high frequency currents.

13. An electron velocity modulation discharge device comprising a cathode, said cathode having a surface portion of a good thermionic emissive material and another surface portion of a good secondary electron-emissive material, an electron velocity modulating electrode structure comprising a pair of electrode elements through which an electron path from said cathode passes, said electrode elements being connected to each other by cavity resonator having a sufficiently high resonant frequency to impart bunching to the electron stream in said path, and electrode means facing said cathode for reflecting electrons emitted from said cathode back onto said cathode with sufiicient energy and in sufficient numbers to liberate secondary electrons from the secondary electron-emissive portion of the cathode, which electrons form part of said electron stream.

14. An electron velocity modulation discharge device comprising a cathode having an active surface which liberates secondary electrons readily, and having a substantial thermionic emission less than that required to carry the peak current of said device, an electron velocity modulating electrode structure comprising a pair of electrode elements through which an electron path from said cathode passes, said electrode elements being connected to each other by a cavity resonator having a sufficiently high resonant frequency to impart bunching to the electron stream in said path, and electrode means on the opposite side of said electrode structure from said cathode for reflecting said electron stream after it has passed through said electrode structure back onto said cathode with sufiicient energy to liberate secondary electrons in sufficient numbers to produce a total electron emission capable of carrying said peak current.

15. An electron velocity modulation discharge device comprising a. cathode, said cathode having a surface portion of a good thermionic emissive material and another surface portion of a good secondary electron-emissive material, an electron velocity modulating electrode structure comprising a pair of electrode elements through which an electron path from said cathode passes, said electrode elements being connected to each other by a cavity resonator having a sufficiently high resonant frequency to impart bunching to the electron stream in said path, and electrode means on the opposite side of said electrode structure from said cathode for reflecting said electron stream after it has passed through said electrode structure back onto said cathode with suiiicient energy to liberate secondary electrons from said latter portion, which electrons form part of said electron stream.

CHARLES G. SMITH.

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

UNITED STATES PATENTS Number Name Date 1,650,232 Pickard Nov. 22, 1927 2,117,636 Tjoflat May 1'7, 1938 2,173,208 Lecorguillier Sept. 19, 1939 2,244,318 Skellett June 3, 1941 2,259,690 Hansen et a1 Oct. 21, 1941 2,263,032 Farnswort-h Nov. 18, 1941 2,278,210 Morton Mar. 31, 1942 2,295,396 George Sept. 8, 1942

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