US2936396A - Low noise electron gun - Google Patents

Description

May 10, 1960 M. R. cuRRlE 2,936,396 1 Low NOISE: ELEcTRoN GUN Filed Jan. 8, 1958 2 sheets-sheet 1 47' 172 Wi/vrai MaY 10, 1960 M. R. cuRRlE 2,936,396

LOW NOISE ELECTRON GUN Filed Jan. 8, 1958 y 2 Altmann@ 2 sheets-sheet 2 Athe velectrons in the stream.

from the emitting surface.

y'Low-NOISE ELEcrRoN. GUN i aired States Patent O Malcolm R. carrie, newly Hills; Calif., as'signrio:

Hughes Aircraft Company, Culverl City, Calif., a cor- *poration of Delaware Application January s, 195s, serial No. 707,719 9 claims. (cl.- 315-3.s) 'l This invention relates to electron beam tubes and more particularly to devices. y. Y h,

lnelectron discharge devices, especially in amplifiers. such as travelingwavetubes lbackward-wave tubes or klystrons, limitationson application and usefulness areg the maximum range of the lradarsystem is `dc'eterinined by thenoise figureof the tube. 'I'hu's, itis obviously advantageous to. employ tubes havingthe lowest possibleA noise figure. Many other advantages of a low'noise ele o l tron beam device will readily suggestVV themselves to one skilled in the art. The measure ofthis no isiness will throughout this specification be designated, as :is Couventional, by noise figure and is defined as the signal-toi 2,936,396 Patented May 1f),

ice

theoretical noiseigure inthe rangel of f5 V to'6V- decibelsk which has been `approached in practice fand'has been', considered generally to Ibe jA the bestthat could ever-.be

attained (see;r 'Proceedings of the; AL RJE., vol.y 43 pages 981-;991, August 1955, ;-Minimum Noise Figure of MicrowavefAnlplifiers', by* H. A. Hausy and N. Robinson).

It is stresse'dfth'at in all'prior artlow noise'gunselecr trons are emitted 'in theydirection Akof the beam. Sub'- sequent to the initial diode region the noise in the beam has in various ways'beend'ecreased, as discussed above.

Two noisev sources in such a beam are shot current and velocity fluctuations. The velocity uctuations Yoccur because the, electrons are emitted with a Maxwell- Boltzmann velocity distribution having shot current noise associated with each of the many different velocity incre-r ments. This givesrise toJuctuationsin the mean Velocity of the beam. For the cathode-anode configuration lof the prior art, the velocity distribution of the emittedelectrons may be referred to as a half-Maxwellian distribution. In other words, electrons arey emitted having velocitiesin the-forward direction ranging fromV zero Ato positive infinity' with the` peak of the distribution lying displaced from; zero rand somewhere in between. The theoretical minimum noise produced in electron vstreams of the' general-type provided in .traveling-wave tubes",-V

backWard-wave tubes and kly'strons, is proportional to the average forward rvelocity component of the electrons at noise ratio at -the input of theldevice "divided by the 1 signal-tonoise ratio at the output.

Low noise electronA guns have heretofore usuallycon-'- sisted essentially ofV a diodelregion','that is", Van emitting cathode and accelerating anode,follow ed either by a series of accelerating anodes or a series of driftv tubes v.operated at different potentials. In' either case, the pur-y pose of the region beyond the .first anode is to minimize and de amplify the noise inthe beam.' In the former type; this is accomplished as gradually as .possible bya change in potential as a function of distance from the cathode.

f In the latter type,l it is accomplished byl sudden velocity jumps which tend to minimize velocity fluctuations among In both cases, the initial diode region .is designed with considerable effort toward launching the beam in a uniform flow from the forward end of the emitting cathode. The resultingbeam has a moreor 'less uniform current density across its widthf It hasl previously been `consideredl to be* essential, in accordance with the theoretical models utilized, to launch the. electrons as smoothly and evenly forward as possible The' use" of a distorted potential profile, a nonuniform current density'or side emission has been regarded as 'highly harmful to the noise figure'of tubes, particularly those employing relatively long electron beams. Ithas also been considered essential, according tothe theoretical models used, that Y. a space charge limited type of emission be utilized in order to smooth out current tiuctuations in the beam near the cathode. In fact, all accepted prior art theoretical models have assumed a' one-dimensional Fry-Langmuir reduction in velocity fluctuations." These models have the beam entrance and is proportionalto the deviation betweenthe root-meanfsquare quantity of vthe. forward velocity component and ,the average forward velocity component. In fact, the theoretical minimumnoise gurev isproportional'toY the: product of these two-quantities.` l

Itmayebe seen that in priorfartelectrongunsthe source of noise associatedwith velocity-fluctuations',in'gthe beam Y is thus-essentially. fixed yby the.halffMaxwellianvelocityf distributionwhich results from thebeambeing. emitted in the forward direction.'

A prime object of the present inventionis to-provide an exceedingly low noise electron gun.' ,q I

VAnother object. is to providega Ylow noise'electron gun for use in abeam type amplifier device havinga noise figure'considerably below what has been considered to be ledr to the prediction of a generally accepted minimum the theoretical minimum.

It is another object to providea low noiseVV electron gunV in whichthe improvement'in Vnoise reduction Vis achieved in the cathode-anode region of "the velectron beam device.`

It isJ another objectv to provide a low noise electron gun which provides in the forward direction avelocity dis' tribution of electrnsfother than the conventionalv half- Maxwellian'distribution. f

' .It is still another object to provide a low noise/electron gun in which the average forward velocity,'componentofl the electrons in the' stream at` the initial cathode planeV is considerably belowwhat may be obtained ina conven-r tional electron gun. i, Y

vIt is another object to providea low noise electron gun inI which the deviationA between rootmjean-square quantity of the forward velocity and averageforward velocity is' substantially decreased below what is possible to achieve present invention are achieved in a particular emb ment by providing an arrangementv including `a sideemissiv cathode immersed in a relatively strong'vinagnetit:V field j which is coincident with the desired path of vthe electroni beam. The cathode has a frontend and aside eniissivel region lying contiguously rearwardly therefrom. i: :Disev 3 posed further along the path and spaced from the cathode is an accelerating electrode having a potential only slightly greater than that of the cathode.` Disposed contiguously transversely to and abeam` of the emissive region of. the

cathode and extending both behind and in frontthereof is a potential proiile-shapingv electrode having .an inclined surface toward the emitting region or segmentof the cathode and diverging away fromthe path'at a predetermined angle. tained at a direct current potential which is more positive with respect to the cathode than is the accelerating electrode. This arrangement of electrodesv and relative potentials produces a potential field having a gradient near the edge of the emitting region and having a pronounced radial component tending to pull electrons toward the profile-shaping electrode. However, the strong axial magnetic eld and the small forward electric gradi.- ent pulls a major portion of the emitting electrons toward the accelerating electrode by the influence of the crossed lield effects.

It is to be noted that the electrons emitted from the emissive Vside of the cathode have a full Maxwellian distribution in the direction of the magnetic field ranging from minus infinity to plus infinity with the peak at zero velocity. Thus, it may be seen that considering only this distribution the average forward velocity component is-substantially zero and is thus indeed decreased from that which exists `in the conventional half-Maxwellian distribution. It remains, however, to also decrease the deviation between the root-mean-square quantity of the forward velocity and the average forward velocity. This is accomplished by providing a forward component of the electric iield by the accelerating electrode which causes a predetermined fraction of the emitted electrons having a negative forward velocity component to be reected forwardly-.in a manner such that their rearward component becomes a forward component of equal magnitude. Those emitted electrons having a forward `velocity component' algebraically less than this predetermined magnitude are deliected and precluded from entering the beam by the action of the longitudinal magnetic field. After this reflection is accomplished, the maximum of the velocity distribution is still substantially at zero with the average being displaced somewhat therefrom in the positive direction. However, the average velocity is 'still considerably below the average `for a conventional half- Maxwellian distribution and the deviation between rootmean-square quantity of the forward velocity components and the average velocity is also considerably decreased.

The novel features of this invention, as well as the invention itself, both as to its organization and method of operation, will be better understood from the following description taken in conjunction with the accompanying drawings in which like reference numerals refer to like parts and in which:

Figure l is a schematic view partially in section of an electron beam device constructed in accordance with the present invention;

Fig. 2 is a perspective view of a generalized embodiment of the low noise electron gun of the present invention;

Fig. 3 is aview of a sheet beam-forming electron gun' embodiment; i

Fig. 4 is a View of a generalized cylindrical embodiment of the `electron gun of the present invention;

Fig, 5 is a view, partially in section, of a modifiedl cylindrical version of the electron gun of the present invention;

Fig. 6 is a sectional view of a hollow cylindrical beamforming embodiment of the electron gun of the present invention;

Fig. 7 is a diagram showing in greater detail the geometrical relationships of certain features of the electron gun of the present invention;

Fig. S is a pictorial graph plotting relative currentl This prole-shaping electrode is maiu-` density across the front end surface of a cathode of an electron gun of the present invention;

Fig. 9 is a pictorial graph plotting relative electron current density across the front end surface of a cathode of a hollow electron beam gun of the present invention;

Fig. 10 is a sectional view of a modilied cylindrical version of the electron gun of the present invention;

Fig. 11 is a graph plotting the electron velocity distribution of a prior art electron gun; and

Fig. 12 is a graph of the Maxwell-Boltzmann velocity distribution of electrons associated with the electron gun of the present invention.

Referring now to the drawings with more particularity, Fig. 1 illustrates an embodiment of the present invention combining a low noise electron gun with a particular electron beam amplifying device, the particular example chosen being a backward-wave traveling-wave tube amplilier 13, a glass envelope 14 includes an enlarged diameter portion 16 at -its left-hand extremity connected to an elongated portion 18 having a lesser diameter. Sealed to the righthand extremity of the elongated portion 18 is a collector electrode 20. Disposed along substantially the length of the elongated portion 18 is a slow-wave structure 22 which is terminated on either end by a matching ferrule 24. Coupled to the input or righthand end of slow-wave structure 22 is a coaxial transmission line 26, the inner connector 28 thereof having a helical extension V30 which is coiled about envelope 14 in a manner to provide a contrawound helical transducer for launching traveling waves onto slow-wave structure 22. Similarly, an output coaxial transmission line 32 is coupled to the output end of slow-wave structure 22 in a manner to couple slow-wave signals olf from slowwave structure 22 and transmit them to a utilization device, not shown.

Disposed within the enlarged portion of the envelope 14 is an electron gun 34 which comprises in this particular example a cylindrical cathode 36 having a substantially planar front end surface 38 at its righthand end and an axially short emissive ring segment 39 on the exterior Vcylindrical surface of the cathode 36 immediately to the rear of the front end surface 38. Cathode 36 is heated in the conventional manner by a heating electrode 40. A potential profile-shaping electrode 42 is disposed about the cathode 36 in a manner Vto substantialy surround the emissive ring segment 39 and to extend both fore and aft of the emissive segment. The potential profileshaping electrode 42, along with other elements of the gun 34, may be supported by a setl of dielectric rods 44 which maintain axial alignment and mechanical rigidity.

Disposed further along the axis of tube 12 to the right of the potential profile-shaping electrode 42 may be placed a series of accelerating electrodes maintained usually at progressively more positive potentials. Throughout this description, although only three or less such electrodes are shown for purposes of simplification, it is to be understood that a greater number may be used in a particular practical application. The first accelerating electrode 46 is an annular member having an opening 48 o-f suiiicient diameter to permit passage of an electron stream emitted by the cathode 36. Spaced further along the tube 12 are a second and a third accelerating electrode 50 and 51 which have a geometry similar to that of the electrode 46. The three accelerating electrodes are also supported by the dielectric rods 44. The cathode 36 `is maintained at a negative potential by a source of Vpotential 52, the negative terminal of which may be connected to the cathode 36. At an Yappropriate tap, the heating electrode 4t! is also connected to `the source 52. rI`he lirst accelerating electrode 46 is maintained at a positive vpotential with respect to the cathode by a connection to an appropriate tap, las shown, to the source 52. The profile-shaping electrode 42 is connected to the source 52 at a next more positive tap. The second and third accelerating electrodes '5,5 'and sgi "are maintained, respectively. next .more positive; by appropriate vtaps also tov the source 52. TheslQwfL wavje xstrucnure 22 maybe maintained next more positi ve,' vas shown, by connecting it to ground potential .through anisolatingresistor 54.` l I @Surrounding substantially lthe entire llength 'of the traveling-wavev tube 12 is a focusingmagnet 56 which maygbe a solenoid as shown and which is energized Jby' the source ofpotential 58,. The-magnet'extends over` the electrongun 3.4 and` immerses ,itina strong axial constrainingvmagnetic iield so thatelfectlrons which would tend to be drawnradially'outward toward the prolef shaping 'electrode 42 Aare insteadconstrained to iow in the. axial direction along the length ofthe traveling-wave tube 12and within ther slow-wave structure 22. VThe velectrons are eventually intercepted by thecollector electrode which dissipates their kinetic energy 'and vcon-,- ducts themv to ground through al source'of potential-60.V Thev sourcev 60 is connected with itsv positive terminalto the collector 20 so that secondarily, emitted electronsY will not drift backwardly into.the .interaction region ofthe slow-wave structure 22 Aand causeVY added shot noisei orV `otherdeleterious effects'. f f

' electron, stream 68.. The cathode62- isz'maintainedat areference potential of Yzero volts. .Atproiile-shaping electrode, surface. 70 is disposed `transversely from: the path and extends axially both behind and in front Yof the emissive segment. 66. The surface v70 ,is,inclined, in a.

manner to be exposed .towardthe emissive segment 66 'iandfdiverges alongthedirection Aofthefelectron stream 68forming an acute angle Vtherewith. .Theprofile-sliap' ing electrode surface 70. is ,maintained ata direct current potential V2 of a`few.volts positive withrespecttothei cathode 62 whichis designated asbein'gof potential V0.3

Disposed further along the path ofthe electron streamv 68 from. the profile-shaping electrode, surface 70 'isla'n accelerating electrode 72 which is placed adjacentto the. electron stream and maintained at a'direct currentpoten` tial V1 which is a few volts positive and algebraically intel-mediate V0' and V2. Thev entire structure of Fig; 2'is immersed 'ina strong axial-magnetic field H which Yis` parallel to the electron stream 68.

Referring now to Fig. V3, thereis shown aA sheet-beam forming electron gun in which a cathode 74 has a side emissive vsurface 476whicl1 is included withinl a short axial emissive segment 78 of the cathode 74, segmentv78 lying a'djacently rearwardv of the front end thereof. A potentialprofile-shaping electrode 80 is disposed symmetrically aboveandjbelowwhe emissivesegment78 and has'inclined potential prole-shaping surfaces .82 whichareex-n posed vtoward the emissive segment78and. diverge along the path o-f the electron stream 84. The surfaces 82 are contiguous to the cathode :74 andiextend hothfore and aft of theemissive segment78. 1 Disposed axially to the right, in Fig. 3, of profile-shaping electrode 80 is an ac-` next more. positive with respectto the cathode. Again,

the entire configuration of Fig. 3 is immersed in a strong axial magnetic field designatedy z Referring'tof Fig. .4, there is. shownV ar generalized cylindrical embodimentroffthe-low noise electron.gun of the`v presen@ invention.. :A cylindrical cathode, isi axially aligned with the Path-Ofrece@ strewn` 9.0 anais main: tained at, a .direct currentreferencepotential.V0 equal to.

zvero. ,The cathodev 88 has ats'ideemissive vsurface 92 adjacent'itsrighthandfextremity which is oriented sub stantially `in afplane perperidicularfto` thefy path of the.-

stream` 90. Disposed `concentrically. about the emissive surface 92 is a potential profileshaping electrode ksur-V face 94 having thev form of a fru'st'oconical ysurface of revolution about the axisfof; the` stream v90. lThe deter-Y mining planes of the conical frustum lie respectively fore andaft of and parallel4 tothe side emitting surface 92. Disposed to thev rightin theiigure along: lthe path of the electron stream 90.is.a`n acceleratingpelectrode'96 having an inner circular aperture whichis 'contiguously concentric about the electronistream and which is perpendicularly vorienteclu/ ith respect thereto. The accelerating electrode. 96 is maintained at a direct current potential V1 which is slightly positive` with respectto the cathode, and the potential profile-shaping electrode 94 is maintained morepositive at a potential designated V2. Again, the entire. coniigurationis immersed in a strong axial constraining in agnetic eldH. v

Referring now to Fig. 5, there isA shown a different embodiment of a cylindrical beam gunY which. includes 'al cylindrical cathode 98 having a side emissive ring surface 100 lying within an axially short side emissive segment 102. A potential profile-shaping electrode 104 vis :dise posed perpendicularly tothe axis of the cathode 98 and has a coaxial frusto-conical aperture 106 disposed aboutV the cathode 98 and extending both forwardly and rearwardly of the side emissivesegment`102. The frustof.Y

conicalsurface of `aperture l106 is contiguous to, the 'side emissive segment 102 and` diverges in the"direction'fofA the'electron stream 107.. `The aperture lemaycomprise; also surfaces 108 'and 110, which are, in this' particular example, cylindrical surfaces concentric *withffthefa'iis t of the cathode .98 andwhichjointo the smaller and larger ends of the frusto-conicalsurface, respectively. Disposed to the right of the profile-shaping electrode 104 is, an

accelerating electrode 112 which has va circular vaperture 114 concentric with and disposed contiguouslyabout Vthe Although it is obvious that the electrony stream 107, emitted by the embodiment depicted in FigQ'S 'fis hollowY 1 duetothe side emission, Fig. 6 represents an embodiment of a cylindrical Ybeam forming electronj gun which produces two concentric hollow electron streams116andf' 117. An annular cathode 118 is maintained vat aidirectcurrent reference potential YY@V Aequal,102er@ volts and isi.

immersed in a strong axial constraining vmagnetic field H. Apotential prole-shapingY electrode V120.2hayivr1g` a,

substantially coplanarinner portion v1.2 .is Ydisposed iperpendicularly to the axis ofthe cathode 111,8.and i s axiz 1llyfTA oriented so as to extend both fore andaftofthe side;k

emissive segments 119 near the righthand Aend of ,theV

cathode 118. The prole-shaping electrode j120has a circular aperture which comprises a frusto-conical surfaceA of revolution 126 disposed concentrically about-andmadially contiguous to thecathode 118.V The profile-shapingY Y electrode portion 12,2 comprises anouter frusto-conicaL j surface of revolution which is concentricwith ,thezcathg ode 118 and, as previously mentioned, 'isvsubstantiallly vcoplanar with theremainder of profile-shapingA electrdderfr. f f:`

120.. The-frusto-concal surface of profile-shaping electrode 1 20diverges at arpredetermined angle in the direction of the. electron streams 116 and 117; and the frustoconical surfaceof the inner electrode,12 2 converges at the same predetermined' angle alongthe direction of the electron stream. Disposed, perpendicularly'to the electron stream-116 and spaced therealong to the right of the profile-shaping electrode'120 is a first accelerating electrode 128 which has an annular aperture 130 concentric with and adjacent radially to the electron stream 117. Disposed further `yet along the path of the electron stream is a second accelerating electrode 132 which is similarly orientedand has similar geometry with respect to the accelerating electrode 128. One or more additional accelerating electrodes may also be utilized here.

Referring now to Fig. 7, there is shown in more detail the dimensions and relationships of a low noise electron gun constructed in accordance with the invention, The cathode 134 is disposedl along the axis of the electron stream 136 and has a side emissive ring portion 138 which is substantially planar and lies substantially perpendicularly to the axis of theelectron stream 136. The emissive ring 138 lies substantially immediately to the rear of the righthand end, as seen in the figure, of the cathode 134. The cathode 134 has a diameter, or transverse dimension, D and is maintained at a direct current potential V equal to zero. A potential profile-shaping electrode 140 is disposed about and lies between a transverse plane 142 which is aft of the emitting ring portion 138 by a distance a and a transverse plane 144 which is forward of the emitting ring portion 138 by a distance designated b. The surface of the profile-shaping electrode exposed toward the emissive ring portion 138 is inclined at an angle IB from the axis of the cathode 134. The edge of the proiile-shaping electrode 140, designated by the plane 142, is spaced from the cathode 134 by a distance designated e. Spaced forwardly from the plane 144 by a distance c is an accelerating electrode 146 which is disposed symmetrically above andbelow, or around, the electron stream 136 and is maintained at a direct current potential V1. A potential profile-shaping electrode 140 is maintained at a direct current reference potential of V2. Some typical ranges for the dimensions and voltages for optimum noise reduction in a practical embodiment constructed substantially in accordance with the geometry of Fig. 7 are:

a-Betwcen .O inch and .200 inch; b-Between .005 inch and .060 inch; c-Between .2O inch and .100 inch; d-Between .O05 inch and .20() inch; e-Between .003 inch and .O70 inch; -Between 30 and 75;

Vo-Zero volts;

VZ-Between plus 3 and plus 20 volts; Vl--At or below V2.

` These dimensions and voltage ranges are not given as designating the limits for their respective parameters, but are merely given as empirical numbers which have proven to be especially successful in achieving noise reduction in electron guns constructed n accordance with the present invention.

Referring now to Fig. 8, there is shown a pictorial graph of relative electron current density on the horizontal scale versus position across the electron stream 150 of a typical side emitting cathode 152 as shown by the curve 148. The curve is seen to be at a maximum oli the edge of the cathode and decreases to a minimum in either direction from the positioning maximum.

Fig. 9 shows a pictorial plot similar to that of Fig. 8 in which a curve 154 plots relative electron current density horizontally versus radial position across the electron stream 156 of an annular beam forming cathode 158. The profile-shaping electrodes are not shown in either of Figs.. A8 or 9, :but the -electron Vcurrent density plots shown exist by virtue of a potential profile-shaping electrode surface such as shown in the other figures which is symmetrically disposed about thev side emitting portions of the cathode schematically shown in Figs. 8 and 9, and by virtue of the unique potential distribution among the various electron gun electrodes.V Again, in Fig. 9 it is seen that the electron current density is greatest just off the edges of the cathode 158 and decreases to' a minimum in either radial direction therefrom.

Referring to Fig. 10, there is shown an embodiment of the present invention utilizing a negative grid probe 160 coaxially disposed within a hollow side emissive cathode 162. Emissive material 164 is disposed on the outer cylindrical surface of cathode 162 near the front end or righthand end, as seen in the figure. A profileshaping electrode 166 is disposed about the emissive material 164 similarly to the manner depicted in the previous figures. The grid probe 160 is maintained at a direct current potential which is negative with respect to that of the cathode 162, as indicated by the connection of a battery 168 connected with its negative terminal to the probe 160 and its positive terminal to the cathode 162. The negative potential on the probe 160 aids in preventing the migration 'of the emissive material 164 from its position on the cylindrical surface of the cathode 162* to the front end, planar surface to thus preclude all but a substantially pure side emission from the cathode 162.

In the operation of a low noise electron beam device of the present invention, of which the backward-wave amplifier of Fig. 1 is described as an example, microwave energy is delivered to the traveling-wave tube 12 along rthe coaxial transmission line 26. From the helical transducer 30 a traveling wave is launched upon slow-4 wave structure 22 and is caused to interact therealong in a well-known manner with an electron beam projected from the electron gun 34 along the helix 22. The microwave energy in the form of the traveling-wave havingA experienced interaction with the electron stream normally in a manner to achieve amplification is then coupled of of the helix 22 by the helical transducer' at the lefthand end, in Fig. 1, of lthe traveling-wave tube 13 and transmitted via the transmission line 32 to a utilization device, not shown.

Referring to the operation of the low noise side emission electron gun 34, the cathode 36 is heated by current flowing through its heater coil causing thermal' emission of electrons from the side emissive area 39. The surrounding profile-shaping electrode 42 being maintained at a positive potential tends to cause electrons to be pulled radially outwardly toward it. However, the

strong magnetic constraining field maintained by the solenoid 56 and the electric field from the slightly positive accelerating electrode 46 cause a predetermined proportion of the emitted electrons to spiral and travelV forwardly along the beam path toward the collector end. of the tube 13. It is to be noted that the inclined surface of the profile-shaping electrode is maintained at a higher potential than the first accelerating electrode 46 in order. to enhance the side emission from the emissive surface In order to describe in more detail the operation of the side emitting cathode of the present invention, refer-4 ence is now made to Figs. 1l and l2 of the drawing.` Fig. ll is a plot of the conventional Maxwell-Boltzmann distribution Vof electrons emitted in the direction of the electron stream of a prior art electron gun. The relative' amountof current in an infinitesimal increment of velocity is plotted on the ordinate, versus magnitude of electron velocity on the abscissa. It is seen that the average velocity lies well to the right of the origin and is desigf nated as V on the figure. It is also indicated on this figure that the deviation between the root-mean-square quantity of the velocity and the average velocity 17 is of a relatively large magnitude. e .Referring to `Fig. ,12, vun `the other hand, `there is resents thel average velocity i7.

Vis considerably decreased f` plotted a full, nonconventional MaXwellLBoltzinann dis# tribution of electron velocity in the direction of thebeam; which is characteristic of the present invention. Plotted on theordinate again is a relative amount of currentv in a given velocity increment versus magnitude of velocityY inthe direction of the electron stream'on the abscissa.V It may be Vseen that since this direction is parallel to -theV emitting surface of the electrons Ythey may. have such aV distribution of velocities ranging from negative infinity to'v positive infinity with a peak yat the origin which also` rep- It may also be understood that a plot of electron velocity distribution in the transverse direction would be one similar to that depicted `by Fig.v l1.4 However, in thel present invention all ofthe energy thus represented is transmuted into rotationalenergy as the transverse component of velocity is caused to spiral about the direction of the strong magnetic lield and hence will not further be considered in the presentA description.

As discussed above in the introduction, the theoretical minimum noise figure is proportional .to the average' electron velocity, the deviation between root-mean-square ofthe forward velocities' of electrons traveling in the direction of the'beam and the average velocity in that'. direction and to the product of these two quantities. `It is indicated on Fig. 12 that the deviation quantity represented by the spacing between and frompthe similar deviation shown in Fig. 11. Y *In accordance with the present invention, it` may be' seen that" of the electrons represented by the distribution' of Fig. l2; all 'those having a positive vvelocity in the direction of' this stream will find their way into the stream. AIt may further beseen thatA electrons having a' l 'velocity algebraically less than a critical velocity v determinedby the magnitude ofthe forward directed electric eld of the first accelerating electrode will be lost from 'the beam because these electrons will spiralinto*A the cathode and be lost. However, electrons having a velocity in the forward direction algebraically'greater than v' will be pulled forwardly by theelectric field of the Afirst accelerating electrode and form a part of the beam. All the electrons having velocities between v and zero will obviously, when thus reflected forwardly, have Videntical velocities in the vopposite dir ectionas by a con-f servative fieldv reflection.

The resultant beam then has the. velocity distribution indicated bythe dotted line 1618i' .ofjFig l2. It may now be seen that the average velocity fin ythe forward direction is` considerably lower than` vhaif-Maxwellianfvelocitydistribution results from the beam being 'emitted in thefdirection of the stream while in .f the present invention it is avoided by emitting the electrons, predominantly transversely to the direction of the electron stream.' YBy utilizing such side emission, it is -possibleto grasp a greater fraction of the total number of electrons at low velocities.

An analysis of theV electron velocities in an electronl gun of the present invention has shown that the product 1 of the average velocity in the forward direction at the g cathode and the deviation between theroot-mean-square quantityof Vthe forward velocity and the average forward Y velocity at the cathode may; by adjusting the magnetic that of the conventional velocityf distribution depicted inn noise electron gun in an electron beam device .of such improved characterl asV to achieve a noise figure yvastly lower thanwhat has heretofore been considered their retically possible. Conventional theoretical models have Q been departed from toprovide a basically different struc-` ture with, among others, the attendant advantagesV dis-. cussed above. Y

What is claimed as new is: Y

l1. A low noise electron gun for providing an electron beam in a forward direction along a predetermined lineal path comprising: a side emissive cathode for emittingv electrons in substantially all outward velocities from said side emissive cathode; means yfor providing a `constraining magnetic ieldabout said cathode in said forward direction along said path; a profile-.shaping electrode disposed contiguously tosaid cathode at right an-;

gles therefrom Vwith respectto said path, y.said profileshaping electrode having a surface inclined toward said cathode and toward saidv forward direction, said surface forming an acute angle therewith; an accelerating elecv trode disposed ,adjacent to said path and spaced from said profile-shapingelectrode, said cathode being ata reference potential of zero volts, said accelerating electrode being at a potential `positive with respect tofsaid cath,

ode, said profile-shaping electrode being at af potential positive with respect to said accelerating electrode whereby electrons are emitted ifrom the side of said cathode` lying toward said profile-shaping electrode, thoseemib;

ted with a rearward velocity component ofk greater than a predetermined magnitude being precluded from entering ysaid vbeam while electrons havingfuafrearward veai locity component less than said predetermined magni tude are reflected into and constrained to flow alongfsaid.

beam. Y v

2. A low noise electron gun for projectinga' streantv I of electrons. in a forward ydirection lalong a rpredeterl mined path comprising: a side emissive cathode having an emissive surface lying within a longitudinally Vshort seg-- ment along s-aid path for emitting electrons having sub-V` stantially all outward velocities from the side of saidV emissive cathode; a Aprofile-shapingA electrode ydisposedl from said emissive segment transversely to said path and having a longitudinal length which is greater thanV the length of s-aid emissive segment and whichV extends both fore and-aft ofA said emissive segment, said profile-y shaping electrode having an inclined surfacerexposed 'to-j; ward said axial segment which forms an acute angle with" said forward direction; means for providing aconstrain'- ing magnetic field about said cathode parallel to said predetermined path, and anvaccelerating electrode disposed.

also transversely from -said pathand forward of said profile-shaping electrode, said cathode being maintainedl at a direct current reference potential of zero volts, ,saidf f accelerating electrode being maintained at a direct cur? rent potential which is positive with respect to said cath-. ode and said profile-shaping electrode Vbeing maintained.

at ia direct current potential which is`r positivelwithrespect to that of said accelerating electrode whereby the: electrons emitted from said emissive segment of said side emissive cathode having a rearward velocity kcom.V ponent of more than a predetermined magnitude are pre-V cluded from entering said stream, while electrons hav-k` ing a rearward velocity component less than said'pre? ldetermined magnitude are reflected into said stream.

3. A backward-wave amplifier comprising; an elongated envelope disposed about a longitudinal axis; a; slow-wave structure within said envelope, said slow-Wave structure having an emitter end and a collectoi""end;-;y and'electric elds'in a manner to choosean optimum -75collector meansdisposed -at .the collector endqof-L saldi:

11 slow-wave structure; an electron gun housing coupled to the emitter end of said envelope and defining an enclosure for an electron gun assembly; input and output radio frequency coupling means contiguously disposed about said envelope at different points therealong; magnet means for providing a strong axial constraining magnetic field throughout the length of said electron gun assembly; and an electron gun assembly mounted within said electron gun enclosure, said assembly including a cylindrical cathode mounted along the longitudinal axis of said amplifier, the axis of said cathode coinciding with the axis of said amplifier and the end of said cathode which is closest the collector end of said slow-wave structure lying in a plane transverse to said axes; an emissive surface `disposed in a ring on the outer surface of said cathode, the ring of emissive material lying substantially in a plane transverse to the axis of said cathode and spaced at a predetermined distance from said collector end of said cathode, the potential of said cathode being at a reference of zero volts, electrons traveling from said emissive surface along the axis of said amplifier thereby providing a cylindrical, hollow stream of electrons passing along the length of said slow-Wave structure to the collector end thereof; a first, profileshaping, annular electrode disposed about and concentric with the ring of emissive material on said cathode,

the inner surface of said first electrode extending froma point which is further from the collector end on one side of said ring of emissive material to a point which is nearer the collector end on the other side of said ring, said inner surface defining a frusto-conical surface of revolution which diverges in the direction toward the collector end and whose axis is coincident with the longitudinal axis of said amplifier, said prole-shaping electrode being maintained at a potential of approximately five volts with respect to said cathode; a rst, planar anode spaced along the path of the stream of electrons at a pre-selected distance from said ring of emissive material and substantially parallel to the plane thereof, said first anode including an internal aperture, the surfaces defining said aperture being adjacent to and concentric with the hollow stream of electrons, said first anode being maintained at a potential of substantially four volts with respect to said cathode, and a plurality of further accelerating anodes mounted in successive planes transverse to the longitudinal axis of said amplifier and disposed successively further along the stream of electrons away from said cathode, each of said plurality of anodes having an internal surface defining an aperture extending about, adjacent to, and concentric with said hollow stream of electrons and each being maintained at a different potential, said potentials increasing incrementally in correspondence to the position of the anode along said stream of electrons.

v4. A low noise electron gun for producing a stream of electrons along a predetermined path comprising: means for providing a constraining magnetic field along said path; a side emissive cathode disposed at one end of said path; an accelerating electrode disposed along said path and spaced from said cathode; a profile shaping electrode disposed contiguous to and a beam from said cathode and having a surface inclined towards said cathode and said path at an angle with said path of 70 to 35 degrees and extending forwardly from said cathede .005 to .06@ inch, said cathode being maintained at a reference potential of zero volts, said accelerating electrode being maintained at a potential positive with respect to said cathode; said profile shaping electrode being maintained at a potential positive with respect to said accelerating electrode whereby electrons are emitted from the side of said side emissive cathode, those electrons emitted with a forward velocity component algebraically less than a predetermined magnitude being precluded from entering said stream While electrons havnga forward velocity Component greater than said PI@- determined magnitude are constrained to flow along said predetermined path.

5. A low noise sheet-beam electron gun for projecting a stream of electrons along a predetermined axial path comprising: a rectangular side emissive cathode having a planar front end surface disposed transversely to said path and having an axially short emissive segment disposed rearwardly Qf said front end surface; a profile shaping electrode disposed substantially parallel to said planar front end surface and contiguously about said cathode and having an axialthickness which is greater than the axial length of said axially short emissive section, said profile shaping electrode having an inclined planar surface exposed toward said axial segment and toward the direction of said path; an accelerating electrode disposed substantially parallel to said profile shaping electrode contiguously to said path and axially spaced from said profile shaping electrode along said path; and means for providing a relatively strong axial magnetic field throughout at least the region defined by the rear of said profile shaping electrode and the front of said accelerating electrode, said cathode being maintained at a direct current reference potential of zero, said acelerating electrode being maintained at a direct current potential positive with respect thereto and said profile shaping electrode being maintained at a direct current potential positive with respect to said accelerating electrode.

6. A low noise electron gun for projecting a stream of electrons along an axial path in a predetermined direction comprising: a cylindrical cathode having an end portion substantially perpendicular to said axial path and an axially short emissive segment on the cylindrical surface of said cathode rearwardly from said end portion of said cathode; a profile shaping electrode disposed concentrically about said emissive segment and extending fore and aft thereof along said path, said profile shaping electrode comprising a frusta-conical internal surface of revolution concentric about said axial path and being exposed toward said emitting segment and diverging toward said direction; an accelerating electrode comprising a transverse sheet having an aperture concentric with said path, disposed transversely thereto, and spaced from said profile shaping electrode along said path; and means for providing an axial constraining magnetic field through at least the region defined by the rear of said profile shaping electrode and the front of said accelerating electrode, said cathode being maintained at a direct current reference potential of zero, said accelerating electrode being maintained at a direct current reference potential positive With respect to that of said cathode and said profile-shaping electrode being maintained at a direct current reference potential positive with respect to that of said accelerating electrode.

7. A low noise electron gun for producing a hollow cylindrical stream of electrons along an axial path in a predetermined direction comprising: a profile shaping electrode having a frusto-conical internal surface of revolution concentric about said axial path and diverging in said direction at a predetermined angle, the conical frustum being limited by first and second planes perpendicular to said axial path, said profile shaping electrode further comprising a second conical surface lying substantially between said first and second planes radially within said frusto-conical surface and converging at said predetermined angle in said presdeterrnined direction, said frustoconical and said conical surfaces being spaced from each other by a predetermined annular spacing; a cylindrical annular side emitting cathode concentric about said axial path and having an axially short emissive segment comprising an emitting circle on the external surface of said cylindrical cathode and a concentric emitting circle on the inner surface of said cylindrical cathode, saidrcathode being disposed in the annular spacing between said frustoconical surface and saidY conicalA surface such thaty said emissive segment lies between said lirst and second v planes; an accelerating electrode disposed substantially ina plane perpendicular to said axial path and spaced axially therealong from said pro'le shaping electrode, said accelerating electrode having an, annular opening concentric about said axial path, said annular opening being disposed contiguously to both the inner and the outer surfaces of said hollow electron stream; and means for pro- Aviding'an axial constraining magnetic eld throughout the region -including said cathode and said accelerating electrode, said accelerating electrode being maintainedr at a direct current reference potential lof zero, said cathode being negative with respect thereto andr said profile shaping electrode being positive with respect thereto, whereby the electrons are emitted from said emitting segment of said cathode towardsaid frusto-conical and said conical surfaces and are in part constrained by said magnetic field to flow along said axial path in said predetermined direction.

8. A low noise electron gun for providing a cylindrical beam of electrons along an axial path in a predetermined direction comprising: a cylindrical side emissive cathode member having a ring shaped emitting portion and a substantially planar end, said emitting ring portion lying on the surface of said cylindrical cathode contiguously rearwardly of said planar end; a profile shaping electrode concentric with said cathodedisposed betweenrst and second planes transverse to said axial path, said first plane lying aft of said emitting ring portion by .001 to .550 inches and said second plane lying forwardly of said ,emitting ring portion by .001 to .100 inch and having a frusto-conical internal surface of revolution disposed symmetrically about said axial path at an angle of`40 to 50l degrees therewith and extending Vbetween said planes and having a minimum diameter at said first plane and the maximum diameter at said second plane thereby diverging along said axial path, the end of said frustoconical surface having said minimum diameter being radially spaced from said cathode by a distance of from .001 to .100 inch; an accelerating electrode lying substantially in a plane transverse to said path and spaced therealong from said profile shaping electrode and having a circular aperture contiguously concentric about said axial path; and means for providing an axial constraining magnetic field between said first plane and said accelerating electrode, said cathode being maintained at a direct current reference potential of zero, said accelerating electrode being maintained at a direct current potential of the order of 3 volts positive with respect thereto and said prole shaping electrode being mainode concentric therewith and extending into the region Y of said end of said hollow cylindrical cathode member; a profile shaping electrode concentric with said cathode disposed between iirst and second planes transverse to said axial path, said first plane lying aft of said emissive surface of said cylindrical cathode and said second plane lying forward thereof and having a frusto-conicalV surface of revolution disposed concentrically about said axial path and extending between said planes and diverging in the forward direction along said axial path; an accelerating electrode lying substantially in' a plane transverse to said path spaced therealong from said profile shaping electrode and having a circular aperture concentrically contiguous about said axial path; and means for providing an axial constraining magnetic eld throughout at least the region between said rst plane'and said accelerating electrode, said accelerating electrode being q maintained at a direct current reference potential ofy zero, said cathode being maintained negative with Yrespect thereto, said grid probe electrode beingmaintained negative with respect to said cathode and said profile shaping electrode being maintained positive with respect to said accelerating electrode.

References Cited in the file of this patent UNITED STATES PATENTS Y 2,632,130 Hull Mar. 17, 1953 2,652,512 Hollenberg Sept. k15, 1953 2,798,183 Sensiper July 2, 1957 2,801,361 Pierce July 30, 1957 2,811,663 Brewer et al. Oct. 29,1957 2,851,630 Birdsall Sept. 9, 1958 2,869,019 Watkins Jan. 13, 1959 OTHER REFERENCES Article by R. C. Knechtli and W. R. Beam,'pages 410 to 424, R. C. A. Review, vol. 17, September 1956.

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