US2567674A - Velocity modulated electron discharge device - Google Patents

Description

m- M, 1951 E. G. LINDER 2,567,674

VELOCITY MODULATED ELECTRON DISCHARGE DEVICE Filed Nov. 8, 1949 26' hum mg; l I I INVEONTOR Zmdez Patented Sept. 11, 1951 UNITED STATE s" 2 ,567,674-

vELoeITyMonULArEn ELECTRON DISCHARGE DEVICE Ernest Gustaf Linder, Princeton N. J assigr'ior. to Radio'Corporation'of America; acoiiioratioiz" of Delaware Application November 8, 1949, Serial N 02 1265080 Claims: (01. 315 -5) The present invention relates to-improvements in electron discharge devicesuseful as microwave oscillators and more particularly to improvements in such devices involving-the velocity modulation of large-current electronbeams. 4 H i In my co-pending application Serial No. 68,605

filed December 31; 1948, there is shown an improved velocity modulated inductive-outputdis charge device of the reflex type. As is known, this is a type of device in which an electron gun projects a beam of electrons through an alternating RF field within a cavity resonator directed along the electric lines of force. In the first half of each of its cycles this'RF field accelerates any electrons which are instantaneously passing through the resonator and in the second half it decelerates those whichare then passing through it. The velocity modulated electrons then move; into a drift space against a retarding 'field pro,- vided by a reflector electrode} each'groupofaccelera'te'd electrons will penetrate "this field more deeply than the respective dcelerated 1 group; which follows it so that by'the time that they both have come to'a halt andhavesta'rted heartaward the exit aperture of the resonator th'e for mer will have lost its placeinwhestreamiana will tend to rejoin it at a pointne'ar meager: All of this occurs with such timingfthatfl) g "od bunching of the beam will have'been 'attaihediii' the time it takes an averageelectror'i'to' gtbaclf to the exit aperture and (2) thefelect'fons' c'ofliprising each bunch will pass backtnroughwne resonator one RFfperi'o'd"laterthan they projected throughit from theguni Finally the bunches of electrons are retarded'in the resonator with the result that they give up D; 'C. energyto the RF field therein.

The improved velocity modulated device "shown" in my above-mentioned application has an e1'e'c"'- tron gun which is capable of concentrating elec-" trons from a large-area'cathode into a 'beamof very great current density because it is adapted to overcome the electronspace chargeof the beam, and, since the gun itself can onl'y'oppos'e the space charge in a regionbetween the cathode of the gun and theentrance aperturet'of'the'- resonator, this device-is provided with a means? for preventing the'beam from spreadingwhile i'tis in said drift spa'ce despite thefact that-the space charge therein is very great due-to the high electron density and-to'electrondecelera tion. The means in question comprises a ba-r magnet which is mounted along an extension of the beam axis with one of its -poles adjacent' the reflector electrode so that the :fringin'g field are -parallelto;each other and-totheaxisof the drift'space in the region thereof and :are sub-'- stantially parallel to the convergent paths oftheelectrons in-the-region between thecathode and the entrance point of the beam into the drift,

space. This arrangement has the advantagethatr it is capable of assistingin the-process of concentrating the electrons into a densebeam the focusingregion of the electron -gun in'add" tion to preventing the electrons frorfispread'ingI in the drift space. 7

However "I have found' thatTdichar'ge'devics; of this type entail 'two'di'sadvantage" that T it diificult to cause the fluk'lins' of that ngmgin id to closelyi'c'oincid'with the mteiiaeatconvrgfit' paths of the electrons fin-the fgun, and that it often diflic'ult 'to' obtaina sufficie'ntly stfong'mag? netic field'due to the fact thatthe fiuxicircuit i11 clu'des" aflo'ng high reluctance air gap;

Accordingly; it Tran" object" of th'; presentv in: vention to devise; 'app'ropfiateiihprovef reflex velocity modul'ated' discharge ae type which has magnetic means for prevent 'g ericessive spread of a veryhigh}derisity'electroiif beam within the driftsp'ace "fcrincreas'ingfth' flux density of an axial magnetic field which is'; producedin the drift space, region.

It is aflflther'f'bbjcfibf thjpisfit to devise iriiprovernent'sfin ai'reflex'velocity n ce? ulated discharge device 'oftli'tpe which nas'mag: netic" mea'ns'ffor preventing xcessive-spread Qf 1 very high density electron beam'withm the drift space for' increasing theflniial magnetic" 'filld' which is produced in theregiop ofithedriftfspace and for shielding the electronlgunf'frgintheinagi netic field to the endthatits'performanceicanf be closely controlled and predetermined It is a'further objectofftheiare's'efitinvention to devise improvementsiiiia reflekvelocityim ulated discharge device offth type which has magnetic means for preventing,exeesswespre d of a very highdensity'elefctiofibeamwitliinthe: drift space to provide additional means rqrpre: venting such spread; p g 7 Other object's'ffeatujres and advantagesl'of present inventionwill beapparent to tho's'ieskil; d in the art .frornkthe' followingdeta iled 'des cripti of the invention-and from-the drawing -in which:

Figure 1. represents a cross-sectional view of one; illustrative embodiment of the invention; and

Figure 2 represents;- a-similar view of another; embodiment V The device shown in Fig; -11 comprises; -at;- its; rightrendsas shownrin=the3drawing ahigh-Icurrent:

emerging from Said poleincludes gq wh ion-trapelectroniiguni: of-az -typeoriginally *dis-a- 3 closed in my above-mentioned co-pending application and also described in co-pending applications Serial No. 122,516 filed October 20, 1949 and Serial No. 124,810 filed November 1, 1949 which are assigned to the same assignee as the present application. This gun includes a cathode which has the shape of a small segment of a sphere and is supported inside one end of a first envelope portion H with its concave side facing toward the other end thereof. The cathode is provided with a heater I 2, which may be an one of a number of suitable types, which is supplied with heater current through a pair of leads l3 and carries an emissive coating I4 on its concave side. The coating l4 faces toward an ion trapping chamber herein to be designated as ionization chamber l5 through which a conical beam of electrons represented at [6 is to be directed. To

the end that this beam may enter the chamber [5 through one of its sides while at the same time that-side can serve as an electrostatic shield, its wall I! is formed with an electron-permeable central portion, e. g., a convex grid I8 of woven wire mesh or similar open structure through which large numbers of electrons may freely pass. In order that equipotential surfaces to be established'between the concave coating 14 and the convex grid I8 will he -spherically concentric with said coating the grid-is formed in the shape of a segment of a sphere which will be concentric with the coating, or at least nearly so according to an adjustment which will be explained below. In the particular Fig. 1 type of embodiment shown hereinfor illustrative purposes the output end of the ionization chamber 15 is closed by surfaces of a portion of a low-reluctance magnetic circuit element 19. Element l9 may be thick enough, as in the present example,-so that the cavity resonator can be formed as a hollow space machined into it on a lathe. To permit electrons to g'ofrom the chamber'lfi into and through the interaction gap' of the resonator!!! a small central orifice 2| is formed in the element l9 so as to 'be concentric with the axis of the conical beam [6 and near to its apex. Because of the thickness of the element I9 the orifice 2| should sufficiently increase in diameter in the direction of the oathode todefine a conical space which will clear the perimeter of the conical beam 1 6.

Due to the curvatureof the emissive coating [4 and to the accelerating field provided between it and the convex grid I8, the emitted electrons will tend to converge toward the common center of curvature of these elements, i. e., toward the center of the orifice 2|. As is known, ordinarily the sharpness of focus of an electron beam is adversely affectedby the mutual repulsion of its electrons; i. e., by space charge effects, particularly if, as is likel for the gun shown herein, it is a high-density, low-velocity beam. However, as is already explained in the above-mentioned co-pending applications, it is possible to cause ent'rapment of positive ions within the initially defocused beam so as to neutralize its electron space charge effects to permit precise focusing with a sharp apex atthe center of curvature of the cathode. The requirementsfor positive ion entrapment are a region free of sweeping electrostatic fields (which may be designated a fieldfree region) in which cumulative entrapment of ions can occur, and gas molecules in the paths of the moving electrons to provide the ions. The

-i'elatively very small n umber of air molecules comprisingthe residual gas which remains aftof ions because, as will be seen, ions will be trapped at a higher rate than they will be lost. One factor responsible for this is that a relatively considerable number of the gas molecules will be ionized by the very large electron current provided by the type of cathode shown herein.

' In the example shown herein and in order to permit the resonator 20 to be formed by a lathe operation the wall on its output side is formed by attaching a circular disc 22 to the corresponding side of the element l9, e. g., b inserting and silver soldering the disc into an appropriate recess therein. The disc has a small central opening which is aligned With the orifice 2| and space therefrom to form the capacitive interaction gap of the resonant cavity 20. To the left of the opening in disc 22, as shown in the drawing, there is a reflector electrode 23 which is surrounded by a second envelope portion24 which has the form of a shallow dome and is sealed to the left surface of the element l9. Thus a vacuum-tight enclosure is provided if appropriate glass-to-metal seals are used between the element 19, and each of the envelope portions II and 24 respectively. The reflector electrode 23 is carried on support wires 25 which are welded to it and extend radially outward to points where they are sealed to the envelope portion 24. A lead wire '28 is sealed through the envelope portion 24 and connected to the electrode 23 to afford means for connecting it to an external circuit.

A bar magnet 21 ,is mounted along an extension of the axis of symmetry of the device with one of its poles adjacent the reflector to provide an axial magnetic field through the drift space for preventing the beam from spreading therein.

It will be seen that in the arrangement shown in Fig. 1 the region in which electrons from cathode ID are focused into a beam, i. e., the region inside of the chamber 15 plus that between the grid and the cathode is shielded from the magnetic field of the magnet 21 by using a low-reluctance magnetic circuit to determine the paths to be followed by flux which it provides.

' It is possible but not necessary to form the magnetic circuit with a portion 28 as shown herein Which completely surrounds the drift space and encloses the second envelope portion 24. All that is essential is that it be appropriately formed to guide the magnetic field to extend axially through the drift space and to concentrate it therein and that it have such low reluctance that only negligible fringing fields will extend into the electron focusing region.

The portion 28 in the form in which it is shown herein for illustrative purposes corresponds to a figure of revolution of an L-shaped section which includes a circular base 29 and a cylindrical wall 30. The bar magnet 21 has external screw threads which engage a threaded hole through the center of the circular base 29 to the end that the flux density in the drift space may be adjusted by rotating the magnet. The previously-mentioned magnetic circuit element l9 has a circular periphery of the same diameter as the portion 28 and it is mounted closely adjacent thereto so that no significant air-gap remains between them.

In some embodiments of this invention the portion 28, the element I9 and the disc 22 may all be of low-reluctance magnetic material(s), such as soft iron, so that a line of flux originating near the center of the magnet 21 will follow a single path through-the portion 28, as represented by the dotted line 3| and two branch paths, as represented by the dotted lines 3| and 3|, one.

5. through the diseZ-Z and theother through .aportionsof the element I Q incIuding a :tapered wall portion- 32 thereof-which bounded on one side -by the -conicallyflaring walls of the orifice 2| and on the other-side by a conically curved .in- -ter-ior wall of the cavity-ill.

-It-maysometimes be advantageous to seta up a carefully limited fringing magnetiafield in-a portionof the space-inside ofthe chamber near to the apex of the beam. Tothisend there is;.pro

vided an arrangement of adjustable, auxiliary, parallel magnetic circuits which willbe .described 'ixr-detail below. This arrangement comprisesin the present example two-disc-shaped, low-reluc- =-tance, auxiliary-magnetic-circuit-elements .433

-and 34 each of which. of suchdiameter that when inserted within the envelope'portion. I its periphery is in close contact with the inside wall thereof, andeach of which has a-sufliciently large central opening, for the "point atw-hich thelele- --ment is located along the gun :axis, to allow all of the conical beam- |=6 to-passthrough it. If .desired, the magnetic coupling between either -(or both) of the elements 33 and-34 andan external ring element to be described-below can be in- "creased by increasing their diameters and-sealing them through the envelope. Or it may -be increased by thickening the entire disc orat least its peripheral portion to lower the reluctance of the path through-the wall of-the envelope i. be-

tween said peripheral portion and said ring .element. As shown Fig. l-theabove-mentioned ring element 35 has an internally threaded rim at its "left end to engage external threads formed around-the outside of the'cylindrical wall :30 so "thatby rotating the-ring element it may bemoved along the axis of the device in eitherdirection. At'itsright end it has an inwardly facing-cylindrical surface 36 which serves as a pole piecefor .engaging either of the elements 33 on indivilines of force will be set up as'represented-at 43 which will similarly extend from the element'33 into the drift space.

An inductive coupling loop 4| is provided within the cavity 2|] for withdrawing radio frequency energy therefrom. The free end of this loop: ex- I tends through a vacuum-tight seal .42 to the exterior of the device and it co-operates with a hollow conductor 43 to afiord a coaxial output line terminating in a fixture 44.

If desired, the disc 22 may be made of nonmagnetic material so that a line of flux which starts along the path 3| will follow only the branch path 3|" rather than bothof theibranch paths 3| and 3l"-with the result that theaxial flux through the drift space will be concentrated more closely about the tube axis. On the :other hand it may be made of such increased diameter as directly to engage the magnetic circuit portion 28 so that, if it is made of magnetic material and the element -|9 is made of non-magnetic material (such as copper), said line of flux will follow only the branch path 3| with the result that the axial field will extend through the drift space but not through the capacitive gap of re- 'sonator 20.

Obviously, if the magnetic circuit- 5 ,(either with .or. without :the auxiliary-magnetic circuits) .is to operate ascindicated;above,;i the components of the device which are not parts; of :the magnetic circuit (with the -optional; exception of. the reflector electrode 23) should be non-- zmagnetic.

' 'In a preferred-circuit arrangement -foropera- .tionof the device of Fig. 1' the cavity lfl (and with it-thechamber l5) ,isgrounded, e, g,, grounding the magnetic circuit portion p28 a .source of cathode heater energy 45 is connected between the leads IS; the cathode ,lll'is' connected .toanadjustable source of negative potential 46 and the reflector electrode :23 is connected tea source .of negative potential which preferably should be adjustable to values slightly greater .than'those afiordedby the source 46.

' In the operation .of the device ofFig. 1,, electrons emitted from the coating M are acceler vated in the region between this coating andthe grid Hi to attain velocities which may be,; for example, of the order of 300 volts. This wilLcarry them into the chamber l5 wherein they will drift 'in directions which tend to be convergent. At the very start of operation the. electron space charge will prevent the beam from immediately coming .to a sharp focus at the orifice 2|. However, thesomewhat defocusedelectron beampassing through the chamber I5 .will causeionization of the residual gas moleculestherein. and ma few microseconds will entrap enough of the positive .ions to neutralize its .space'cha rgeso that it will come to a sharp .focus. The resulting conical cloud of entrapped ions will be relativelystatic whereas the electrons will becarried along .by their kinetic energies beyond their point of convergence and therefore will shoot through the orifice 2| in a, very dense stream. Space charge neutralization of the conical beam will be sustained even though electrons are constantly entering one side of the chamber |i5 and leaving through the other, since the average :number thereof which areinside it at any instant .wilhbe relatively constant and will have a total charge substantially equal to the relatively static ion cloud. Due to the negative potential of therefiector electrode 23 some .of the ,ionswill be drawn from the apex of the conical beam I6 through .the orifice 2| into the driftspace. However, this will not prevent sustained neutralization since, due to the smallnessof the orifice and .to the mass of the ions, ions .will'not escape faster than they can be replaced by new ionization and entrapmentand this will be true even if the envelope 1 I is .under .very hard vacuum.

-On the other hand, sincethe ions which escape through the orifice 2| are relatively yerylfewjby comparison with the electrons whichare. carried therethrough by their kinetic energies and since the potential of the-reflector electrode willeset up a sweepingfieldfl i. e., a field which will influence electrons and ions to move inopposite directions, space charge neutralization will abruptly cease as soon as theelectrons emerge from the chamber |5.

As a result once it is inside the drift space the dense electron stream will strongly tend tospread apart radially. However, the strongaxialmag- -netic field provided by-magnet 21 will translate some of the force-which tends toact radially on an electron into orbitally acting force whereby, due to its axial drift the electron will traverse a substantially spiral-path. Assuming the presence of radio frequency energy in the cavity '20 (which may have been initiated in aknown -manner by a shot noise eak or some other. transient) "radio'frequency fields will be present'acro'ss the capacitive interaction gap of the resonator.

These-fields will act to accelerate some electrons and to decelerate others, i. e.,' to produce velocity modulation, whereby the electrons will become grouped together in bunches as they move into materially affect the operation of the device that the electrons comprising each bunch are moving with orbital components since this kind of movement will not have any substantial effect on the axial definition of the bunch. In fact, as will be readily understood from the foregoing,

it is this kind of movement that prevents the "frequency energy.

I have devised a method which may be supplemental or alternative to that of using a magnetic field for opposing the tendency of a beam to spread in the drift space. According to it a device of the type shown herein may be initially assembled or later adjusted so that the apex toward which the electrons tend to be convergent will be located somewhat beyond the orifice 2| and therefore inside of either the resonator interaction gap or the drift space. If this is done the electrons will be travelling in convergent directions when they enter the region in which their space charge ceasg to be neutralized. Therefore for the first part of the drift period the kinetic energies of the electrons will be acting in directions to oppose spreading. This will reduce the likelihood that excessive spreading will occur before the two-way trip of the electrons is completed. To permit the device of Fig. 1 to be adjusted for this type of operation a means is provided for axially moving the cathode. This means comprises a Sylphon bellows 49 which is sealed to the envelope II at one end and to the cathode ID at the other to preserve the continuity of the vacuum-tight envelope of the tube, and

an adjusting screw 59 which is in threaded engagement with a cap 5| for the right end of the sylphon. At its opposite end the screw 50 is i formed to include a flange which is axially captive in a socket carried on the back of the cathode l in such a manner that the screw may be freely rotated and, when rotated, will act to move the cathode backward or forward. It is obvious that there will be only one position for the cathode ID 'at which it will be precisely concentric with the grid |8. However in practice it will be found that even when the cathode is sufliciently moved from that position to attain a significant adjustment of the position of the beam apex, the defocusing effects which will be produced on the electrostatic lens existing between the cathode and the grid I8 (by their being slightly eccentric) will usually be negligible.

If desired, however, the cathode and the grid l8 may be made axially-adjustable as a unitary structure for example by interconnecting their peripheries with a plurality of stand-off insulators and by modifying thecylindrical side wall of the chamber so that it will include a Sylphon bellows corresponding to the bellows 49 In such the drift space and back out of it. It will not an arrangement the concentricity of the cathode and the grid will be preserved but their common center of curvature will be moveable axially.

Figure 2 shows another embodiment of the present invention, many of the parts of which are substantially identical to corresponding parts of the Fig. 1 embodiment and accordingly are similarly numbered. 7

This embodiment includes a magnetic circuit element 6|] which in most respects is identical to the element |9 of Fig. 1. It may be considered as an element I9 with an appropriate modifica tion for increasing the air gap traversed by magnetic lines of force such as the line 3|" whereby they may fringe toward or into a predetermined part of the focusing region near the apex of the conical electron beam. A cavity wall 55, which is made of non-magnetic material is added to take the place of magnetic material which is removed according to this modification, The purpose of showing such a modification is to illustrate how it is possible to vary the magnetic circuit so that a fringing magnetic field will extend to only a desired extent into the region in which the conical beam is formed. As shown for this embodiment the magnet 21 may be tapered on its end toward the cathode as a means of further concentrating the magnetic field about the axis of the drift space.

While the auxiliary magnetic circuits have'been omitted from Fig. 2 in order to simplify the drawing, they may be included, if desired, in anembodiment of this type.

The Fig. 2 embodiment shows a different type of large-area-cathode, high-current-density electron gun. This gun is of the type known as a Heil-gun and is described by one Adolph Greenbaum in an article entitled German Wartime Research and Development in Klystrons which is listed in the bibliography of Scientific and Industrial Reports, vol. IV, No. 9 (February 28, 1947) as P. B. 52348.

Like an ion-trap gun, the Heil-gun comprises a large-area concave emissive surface as shown at 6| in Fig. 2. The emission from this surface is drawn in a dense stream through a funnellike accelerating electrode 62 which under typical operating conditions for this gun would be above the potential of the cathode by an amount, such as one thousand volts, which is greater than a characteristic potential diiferencebetween the cathode I0 and the grid I8 of the embodiment of Fig. 1. The electrons passing through the funnel-like electrode 62 do not move toward a cross-over point along straight-line convergent paths as represented at H; in Fig. 1 but instead toward a region of parallel travel along' convergent curved paths as represented at 63 and gun will not be acting in divergent directions .the electrons are in the gun. For this reason an embodiment of the type shown in Fig. 2 also requires a means for preventing excessive spread of the beam after it leaves the electron gun and, as in Fig. 1, this means may utilize either an axial magnetic field or (though with less effect) kinetic energies of the electrons which are made to act 9v in" convergent directions --in the beginning of the drift space by movingth'e cathodeaxially forward.

While-certain specific embodiments have been illustrated and described; it will be understood that-various changes and: modifications may be made therein without departing from the spirit and scope of the invention.

I claim:

1-.An -electrondischarge device comprising: a cavity resonator having closely-spaced, opposed entrance and exitaperturesforming a capacitive interaction gap through which electrons may be projected alongan. axis; an envelope portion surrounding adrift space: which extends along said axis-onthe exit side of said gap; an electron gum-including a cathode having an emissive surfaceof relatively largearea. withrespect. to the size-of either of said apertures for supplying a large difi'use. electron current in a first region between the cathode and the entrance side of saidagap, means located. in said first region for focusing the-electrons toproduce a great concentrationthereof. about said axis at a point near tosaid gap, the. focusing means including means for accelerating the-electrons from said cathode toward said gapwhereby said concentration of electrons will pass thru said entrance aperture into a second region including the. gap and said drift space; means acting in said second region for opposing the mutual repulsion of the electrons of said concentration to prevent spreading thereof to a cross-sectional size substantially greater than said exit aperture; means for restricting the effectiveness of said last-mentioned means to a limited zone including at least said drift space and at most said drift space, said interaction gap, and a part of said first region near to said entrance aperture; and means positioned in said envelope portion for reflecting said concentration of electrons back along said drift space andback through said gap after it has been initially projected therethrough'.

2."An electron discharge device as in claim 1' inwhich: the means for opposing mutual repulsion includes means for establishing a magnetic field surrounding and'parallel with said axis and extending thru said limited 'zone; and'said means for restricting the effectiveness of'themeans for opposing mutual" repulsion includes a low-reluc tance shield for guiding said magnetic field to concentrate it about said 'axi's'ih said zone and to "shield atleas't par'tof said first region from the' field'.

3. Arr electron discharge device as in claim 2 inwhicli' said means for "establishing a magnetic field 'includesa magnetic circuit-portion which has concentricsymmetry about said axis and extends alon'g'a portion thereof on the exit side of" said" gap, said' circuit portion being tapered downward toward the-gap' to a poleof' relatively small diameter which defines oneend of said limited zone.

4; electron discharge device as in claim- I in which: the means for opposing' mutual repulsion-includesmeans for establishing amagnetic field=surr-oundingandparallel with said axisand extendingthru said limited zone; andsaid'means for restrictingthe effectiveness of the means for opposing mutualrepulsion-includes a low-reluctance shield for guiding said:- magentic field to concentrate itabout-said axisin said zone and to shield atleast a predetermined part of said firstreg-ion from said field; said. shieldextending traI-isversely to and" across :said axis between the drift. space and said electron gunand having an opening which is concentric with said axis and is of? proper size and shape to permit said electrons to pass thru in moving from said cathode to said drift space.

5. An electron discharge device comprising: a, cavity-resonator having closely-spaced, opposed entrance andexit apertures. forming a capacitive interaction gap through which electrons may be projected along an axis; an envelope-portion surrounding a drift space which extends along said axis on the exit side of said ap; an electron gun including a cathode having an emissive sur-. face of relatively large area with respect to the size of either of said apertures for supplying a large diffuse electron current in a first region between-the cathode and the entranceside. of the: gap, means located insaid. first region for acceleratingythe electrons along. paths which are sharply convergent toward said. axis so that except for their mutual repulsion they would crossover at a point thereon on the far side of said entrance aperture from saidcathode; and means in said envelopeportion'for refiecting back alon said-drift path and back through said gap electronswhich have been-projected therethrough from the cathode 6; An electron discharge device as in claim. 1 in which saidmeans foraccelerating the electrons is adopted toaccelerate them with sufficient convergence and at high enough velocities to effectivelyoppose. space charge effects tending to preventattainmen't of an intense concentration.

7'. An electron discharge device as in claim 1 in which said means for focusing includes electrodes for providing an electrostatic electron optic adjacent the cathode for starting the electrons along sharply convergent paths and an electron-permeable faraday cage between the optic. and said gap for providingan ion-trapping field-free region thru which the -convergent electrons may drift whereby their negative space charge willbe neutralized in the cage and: an intense concentration will be attained-even if the electrons drift thru at relatively low velocities.

8. Anelectron -discharge device comprising: at least a portion of an oscillatory radio frequency circuit including two opposed conductive walls respectively having entrance and exit apertures forming a. capacitive interaction gap through which electrons maybe-projected along an axis; an envelope portion surrounding a drift space which extends along said axis on the-exit side of said gap; an electron gun including a cathode having anemissive surface of relatively large area with respect to the size of either of said apertures for supplying a'largediffuse electron current-in a first'region between the cathode and the entrance sideof said gap-means located in said first region forfocusing the electrons to produce argreat concentration-thereof about said axis ata point near to said gap, the focusing means including means for accelerating the electrons fromsaidcathode toward said gap whereby said-concentration of electrons will pass thru said entranceaperture 'into a second region including the' gap and-said drift space; means acting in said second region for opposing the mutual repulsion of the-electrons of said concentration to prevent spreading thereof to a cross-sectional size substantially greater than said exit aperture; means for restricting the effectiveness of said'last-mentioned'means to a limited zone including at least said drift space and at most said drift space, said interaction gap,-and apart of said first'regionanear to said entranceaperture;

and means positioned in said envelope portion for reflecting said concentration of electrons back along said drift spaces and back through said gap after it hascbeen initially projected therethrough.

9. A velocity modulated electron discharge device of the reflex type comprising; a cavity resonator including opposed conductive walls each having an electron-permeable opening the perimeter of which surrounds the drift axis of the device and which is spaced from the other opening to define the interaction gap of the device; an electron gun for projecting a dense concentration of electrons through said gap including a cathode having a large emissive surface, a substantially field-free ionization chamber having a large electron-permeable grid on one of its sides towardsaid cathode and a small orifice in predetermined alignment therewith on its opposite side and means for projecting electrons from said cathode along convergent paths through said grid into said ionization chamber whereby the space charge of the electrons will be neutralized by ion entrapment within the chamber and the electrons will be formed into a beam with a sharp apex directed out of the chamber through said orifice; means for establishing a magnetic field surrounding and parallel with said axis and extending through said gap and a drift space on the far side thereof from said cathode, said last-mentioned means including a magnetic circuit element which extends in all radial directions transversely to and across said axis, said circuit element having a conductive surface on ,one

side which forms said opposite side of said ionization chamber and an opening concentric with said axis which comprises said small orifice.

10. A velocity modulated electron discharge device of the reflex type comprising: a cavity resonator including opposed conductive walls each having an electron-permeable opening the perimeter of which surrounds the drift axis of the device and, which is spaced from the other opening to define the interaction gapof the device; an electron gun for projecting a dense concentration of electrons through said gap including a cathode having a large emissive surface, a substantially field-free ionization chamber having a large electron-permeable grid on one of its sides toward said cathode and a small orifice in predetermined alignment therewith on its opposite side and means for projecting electrons from said cathode along convergent paths through said grid into said ionization chamber whereby the space charge of the electrons will be neutralized by ion entrapment within'the chamber and the electrons will be formed into a beam with -a sharp apex directed out of the chamber through said orifice; means for establishing a magnetic field surrounding and parallel with said axis and extending through said gap and a drift space on the far side thereof from said cathode, said last-mentioned means including a magnetic circuit element which extends in all radial directions transversely to and across said axis, said circuit element having a conductive surface on one side which forms said oppo- .site side of said ionization chamber and an opening concentric with said axis which comprises said small orifice and a conductive surface on its opposite side which forms one of the walls of said cavity resonator so that said opening constitutes one of said electron-permeable openings of said cavity resonator.

11. A velocity modulated electron discharge device of the reflex type comprising: a cavity resonator having an interaction gap through which electrons may be projected along an axis; an electron gun including a source supplying a large diffuse electron current, means for opposing the space charge of the electrons in a region between said source and the entrance of the electrons into said gap and for focusing them to produce a very great concentration thereof, and means for projecting said concentration of electrons through said gap; means for establishing a mag-' netic field surrounding and parallel with said axis and extending through said gap and through a drift space on the far side thereof from said source of electron current, said last-mentioned means including a magnetic circuit element which extends in all radial directions transversely to and across said axis and has a small orifice concentric therewith through which said concentration of electrons may pass from said gun into said gap; said magnetic circuit element being formed of low-reluctance material which is thick in the direction along said axis and said cavity being formed at least in part by walls of a hollow space formed within said element; and means positioned along said axis on the extremity of said drift space farthest from said source of electron current for reflecting said concentration of electrons back through said gap after it has been initially projected therethrough.

12. A velocity modulated electron discharge device of the reflex type comprising: a cavity resonator having an interaction gap through which electrons may be projected along an axis; an electron gun for projecting a dense concentration of electrons through said gap including a cathode having a large emissive surface, a substantially field-free ionization chamber having a large electron permeable grid on one of its sides toward said cathode and a small orifice in predetermined alignment therewith on its opposite side, and means for projecting electrons from said cathode along convergent paths through said ionization chamber whereby the space charge of the electrons will be neutralized by ion entrapment within the chamber and they will be formed into a beam with a sharp apex directed out of chamber through said orifice; means for establishing a magnetic field surrounding and parallel with said axis and extending through said gap including a magnetic circuit element which extends transversely to and across said axis and has an opening concentric therewith through which electrons may pass from said gun into said gap, said circuit element magnetically shielding at least a portion of the space within said ionization chamber; and means positioned within the region of said parallel magnetic field and along said axis on the opposite side of said gap from the position of said source of electron current for reflecting said concentration of electrons 13 along convergent paths to form a beam of very great concentration about said axis and to pro- J'ect the beam through said gap; means for establishing a magnetic field surrounding and parallel with said axis and extending through said gap and into a drift space on the opposite side thereof from said cathode, said last-mentioned means including at least one magnetic circuit element which extends transversely to and across said axis and has an opening concentric therewith through which electrons may pass from said cathode into said gap, said circuit element magnetically shielding at least a portion of the region in which the electrons are focused; and means positioned along said axis on the far side of said drift space from said cathode for reflecting said concentration of electrons back through said gap after it has been initially projected therethrough.

14. Apparatus for producing a high-current density electron beam comprising: a source providing a large difiuse supply of electrons, means for accelerating the electrons along paths which tend to converge at an apex, a hollow electron permeable shielding structure providing a substantially field-free region between said source and said apex and including a gaseous medium whereby in the operation of the apparatus positive ions of said medium are entrapped by electrons within said region to neutralize their space charge and prevent it from interfering with the convergence of the electrons toward said apex, and means providing a non-field-free region in a zone including portions of said paths near to said apex whereby no substantial number of positive ions will be entrapped by electrons in said zone and their unneutralized space charge will be free to exert mutually repulsive forces in opposition to the convergent kinetic energies of the electrons.

15. An electron gun, for use in a vacuum envelope containing a gaseous residuum, including a source having an .emissive area providing a large difiuse supply of electrons, means providing a field-free region bounded on a side nearest to said source by an electron-permeable area immediately adjacent said area and on an opposite side by a conductive wall containing an aperture which is relatively small with respect to said area, means for projecting said electrons through said electron-permeable wall in directions all of which tend to extend through said aperture and to be convergent at a point therebeyond, and means located beyond said aperture, on its side away from said source, and including an electrode and a terminal connected thereto and connectable to a source of potential for establishing an ion-sweeping electric field in the region beyond said aperture wherein the electrodes tend to converge at an apex, whereby in the operation of the gun electrons will move over convergent paths through the field-free region due to neutralization of their negative space charge by the entrapment of positive ions therein and they will follow paths, in said region beyond said aperture, the directions of which will be the resultant of their convergent kinetic energies and the mutual repulsion of their unneutralized space charge.

ERNEST GUSTAF LINDER.

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

UNITED STATES PATENTS Number Name Date 2,278,210 Morton Mar. 31, 1942 2,489,298 Lafierty Nov. 29, 1949 Ekstrand et al Jan. 17, 1950

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