US2909704A - Electron gun - Google Patents

Description

` R. w. PETER 2,909,704

ELECTRON GUN Oct. 20, 1959 Filed Oct. 9, 1953 2 Sheets-Sheet 1 INI/E NTO R.

W PETER POU- BWM

R. w. PETER Oct. 20, 1959 ELECTRON GUN 2 sheets-sheet 2 3 'l ||||||l1||||1||||| Il lllllllnllllllllll Filed Oct. 9, 1953 INI/Emme. /PULF W PETER www ORNE Y `following expression:

[,lnitelV StatesPatent Oice ELECTRON GUN Rolf Walter- Peter, Cranbury, NJ., `assignor to Radio Corporation of America, a corporation of Delaware VApplication Getober 9, 1953,` Serial No. 305,064

1s'c1aims. (ci. sis- 15) The present invention relates to `electron guns for generating electron beams in beam 'tubes suchV as traveling wave tubes and klystrons. a

The principal object of the invention is to provide a low-noise electron gun.

Another object of lthe invention is to provide an improved electron gun capable of producing a divergent elec'- tron beam with minimum collection of electrons by the gun electrodes.

The noise theories of Rack, Effect of VSpace Charge and Transit Time on the Shot Noise in Diodes, Bell System Technical Journal (17), October 1938, pp. 592-619, show that the noise fluctuations of an electron beamunder space-charge-limited conditions leaving the potential minimum consist essentially of velocity fluctuations only. These theories have been verified experimentally for small transit angles. For this case Rack showed that Athe actual multi-valued velocity electron stream can be replaced by an equivalent single-valued stream which yields the same final noise formula. The equivalent single-valued noise velocity v, has an R.M.S. value where:lv

I=beam current n=e/m=electron charge/mass K=Bo1tzman constant Tczca-thode temperature a Af=frequency bandwidth.

Pierce, Traveling-Wave Tubes, D. Van Nostrand Co.,

Inc., 1950, applied this concept 4to noise factor computa- `tions of- `traveling Wave tubes. a 1 i From the assumption that the beam leaves the virtual cathode free of random density modulation but with random velocity, one has `to conclude that, by analogy to the well-known bunching phenomena in klystrons, there exist periodical density and velocity fluctuation maxima and minima along the beam. 'Such standing noise waves in an electron beam have been observed experimentallynby transit-angle beams.

Computing the noise factor F of a traveling wave tube onv the base 'of the standing noise wave theory yields the Fauna-70% gf "(2) where:

'v,=maximum' velocity lluctuationamplitudegin the drift- .ingbeam Y v Y i 2,909,704 Patented oct. 20,1959

vv=velocity iluctuation` amplitude at the virtual cathode,

see (1) v C K 1*/3 P Y iereesgam parameter (4) optimum Watkins, Traveling-Wave-Tube Noise Figure, Proc. IRE,l vol. 40, No. l, January 1952, pp. 65-70, has computed. function f for different parameter values. His

For small `space charge and helix Yloss and for position.

curves show that f increases with circuit loss but is prac-- :tically unaffected by increased space charge. a

The important factors in'Equation 2 are yr and C. The gain parameter (C) can be increased toa certain extent by using a smaller D.C. beam Iimpedance (V/Io) and a larger circuit impedance (K). The noise reduction factor (r) can be improved greatly by propergun design. l/r becomes a factor of merit for a low-noise gun.. u

Watkins, supra, suggested the incorporation in au yelectron gun of several elongated drift tubes separatedV by short accelerating and decelerating gaps suitably correlated in space with the velocity and current maxima and minima of the standing noise wave along the beam -path to reduce the amplitude .of the noise uctuations and thereby yreduce the noise reduction Vfactor (r).

In accordance with one feature of the present invention, I produced, independently of Watkins and others, substantial reductions in the noise reduction factor (r) by use of a three-region electron gun incorporating three or more spaced apertured accelerating electrodes of short axial length as compared to elongated drift tubes such as those used by Watkins. This three-region gun comprises a cathode surrounded by a focusing electrode near cathode potential, a first apertured accelerating electrode at a low positive potential relativeV to the cathode, a second apertured accelerating electrode ata somewhat higher positive potential located substantially at a velocity maximum (current densityl minimum) of the standing noise wave in the beam, and a` third apertured accelerating electrode at a high positive potential, which may be the final beam velocity. The adjustable acceleration of ythe beam between the iirst and second acceleratingv electrodes followed by the great acceleration distributed over the relatively Vlong distance between the second and third accelerating electrodes causesa substantial reduction in the amplitude of the noise current and velocity liuctuations in the beam.

The reduction in noise in the three-region gun is'enhanced greatly, in accordance with a second feature of the invention, bythe use of a beam that is initiallydivergent near the cathode. f t' In accordance with this second feature of theinvention, l position a focusing ring in a plane behindthe emissive surface of the cathode, in which case the beam'can be madeto diverge initially between'the cathode and the first `accelerating electrode with a low potential on the focusing ring. All of :the accelerating electrodes are shaped and located substantially the same as they would beto produce parallel flow.. Experiments have shown that :with suitable electrode arrangements divergentrow isfproduced with the potential of. the focusingr'ing'zero or even negativerelative tothe cathode,'which substantially eliminates collection of beam electrons by the focusing electrode. v Moreover, this electron gun arrangement hasadvantages over 3 produced by making the focusing ring more negative, and convergent ilow can be produced by making it still more negative.

,I discovered experimentally that the initially divergent 'electron ow which Vcan be produced in the vimproved gun design described above reduced the noise fluctuations inthe beam over the reduction produced with parallel flow. Another advantage of this particular improved gun structure lies in the fact that it has a higher perveance than conventional gun structures having the focusing ring in the usual location at the emissive surface of the cathode.

The present invention was described in my paper entitled Low-Noise Traveling-Wave Amplier, RCA Rei yiew,vol. )HIL No. 3, pp.-34468, published October 13, 1952. f

In the annexed drawing:-

F`ig. 1 is a schematic view4 showing a three-region parallel-liowgun structure embodying my'invention; Figs. 2 and 3 are graphs showing one set of possible D.C. voltage, noise velocity and noise current distributions along the beam path in Fig. l; l

Fig. 4 is a graph showing the variation of the noise reduction factor r as a function of the relative voltage, Vc/VB, for several relative drift space lengths, LC/LB, for the three-regionl parallel-flow gun of Fig. 1;

Figs. 5 and 6 are schematic views comparing the electric eld patterns in a parallel-ilow gun and an improved gun operating under divergent-flow conditions, respectively;

Fig. 7 is a longitudinal sectional view of an electron gun structure embodying the present invention;

Fig. 8 is a graph showingrthe relation between noise l reduction factor r and rst accelerating electrode potential, VB, for a three-region gun as shown in Fig. 7 operatingv under divergent flow conditions, for the specialcase Where VB and VC are equal; and p Y y Fig. 9 is a graph showing the relation between focusing electrode potential VA and rst accelerating electrode potential VB, for several values of beam current, for the improved gun structure of Fig. 7. e f K Referring to the drawing,lFig. 1 shows a parallel-flow type electrongun comprising a cathode 1, a focusing ring A surrounding theV cathode with its inner edge-in the plane of the emitting surface 3, and four apertured accelerating electrodes B, C, D and E.V In operation, the various electrodes are connected by separate leads to-a and A C. velocity maximum occur in the plane of this.

electrode, as shown in Figs. l and 3. Under these conditions, when the beam is accelerated between C and D to high Velocity the A.C. velocity Wave is greatly reduced in the region C--D with little increase in the A.C. current density wave, with the result that the beam leaves the gun and enters the interaction region of the tube, I

e.g., the helix in a traveling wave tube, with substantially reduced .noise fluctuations in the beam.

The three-region electron gun arrangement of Fig. 1 is very llexible in that the potential of each of the ele'ctrodes can be independently adjusted, for a given electrode spacing, forA optimum noise reduction. Fig. 4 shows the calculated variation of noise reduction factor r, normalized as rVD/VB, with the relative voltage Vc/VB, for several values of relative drift length LC/LB. It can be seen that for a particular relative length there yisV an optimum value of relative voltage to obtain the lowestk noise reduction factor r.

As stated above, the noise reduction obtained by a three-region gm can be enhanced considerably by use of a gun structure that will produce a beam/that is divergent inthe region near the cathode, instead of having parallel flow. Figs. 5 vand 6 show a parallel-flow gun source of direct current voltages (as shown in Fig. 7) to maintain the electrodes at suitable direct-current potentials, as indicated in Fig.v 2. The focusing ring A is maintained at zero potential, as shown, relativeto the cathode; electrode B is made 10 to 50 volts positive;

electrode C is made somewhat more positive than electrode B; and electrode D is maintained at a high positive potential, which may be several hundred volts or more. Thus the potential of electrode D is more than four times the potential of electrode B. Electrode E may be at the same potential as electrode D, or somewhat different, or

may be omitted.

-Fig. l3 shows the noise standing wavealong the beam path due to initial velocity fluctuation originating atrthe cathode 1. The solid line represents the velocity fluctuation amplitude of the noise, or the A.C. velocity wave,

and the dotted line represents the current density iluctuation or the A.C. current density wave. As shown, the A.C. velocity wave has a maximum at the cathode emitting surface 3 and rapidlyrdropsl to zero close. to the cathode, while the A.C. -current density-wave increases toa maximum. Then,the A.C. velocity wave-increases up to electrode B. ln the region B-C,'where the beam drifts-in alow accelerating eld, the A.C. velocity wave increases somewhat while the A.C. current density wave decreasesto zero (beam completely de-bunched); The potental'of-electrode C is adjusted, for its location along the'beam path, so that the-A.C. current density minimum and an improved electron gun operated under divergent ilowl conditions, respectively, for comparison. Fig. 5 shows a conventional Pierce-type parallel-flow gun hav- -ing a conical annularfocusing electrode A starting at the edge of the emissive surface 3 of the cathode 1 and extending outwardly and forward at an angle Vof 67.5 degreeswwith the gun axis," and an accelerating electrode B spaced along the beam axis from Vthe cathode 1. Electrode A is at cathode (zero) potential. Severalequipotential surfaces have been shown in dotted lines for the condition where electrode B is at 26 volts. It is impossible to produce stable divergent flow With'the electrode arrangement shovm in Fig. 5. If any'other potential than zero is applied to electrode A the electron flow is greatly disturbed nearthe edge of the emissive surface of the cathode. If a positive potential is applied to electrode'A, to try to produce divergent ow, electron current is collected at A, which produces additional noise. Fig. 6 shows an improved electron gun, in accordance with one feature of the present invention, having a conical annular focusing electrode A starting ata transverse plane spaced substantially behind'the emissive surface 3 of the cathode 1, and a irstaccelerating electrode B With substantially the same shape andrin the same position as electrode B in the parallel flow gun of Fig. -5. In the improved gun the cathode 1 protrudes a substantial distance into the space between the focusing electrode Aand therst accelerating electrode B. VvSeveral equipotential surfaces S are shown in dotted lines for the same electrode potentials as in Fig. 5. 'It will be noted that, with the focusing electrode A at cathode potential, rtlfle'heam diverges sharply in the region near the cathode surface. The electric field pattern in the region between the cathode and electrode B is Vsuch that the initially vdivergent beam is converted into a substantially parallel flow beam of largercross section than the cathode near lelectrode B. A focusing magnetic field maybe provided axial to the beam to assist the electric eld'fin confining `the beam and preventing collection of electrons at the Vof flanged rrings. 9 and 1'1, within a metal shell =13. At

the end nearest the cathode ,1, the shell 13 hasan inward reduction. velectrode B is of the order of the distance, LB, between angelS which serves asthe 'apertured focusing electrodefA of Ithe gun. 'IIhis :flange Lis 'truncated-cone-like in shape with the aperture surrounding the cathode 1 and spaced substantially behind the emissive :surface 3 .ther'eof, with the cone surface extending outwardly and forwardly, as shown.V Coaxially mounted in spaced relation on the axis of the cathode are three apertured'discs 17, 19 and Z1, which serve as the accelerating electrodes B, C and D of Fig. 1. The diameters ofthe `apertures in electrodes B, C Vand D are-substantially greater than Y 'the diameter of the cathode to avoid interception of beam electrons, which would .introduce` noise.

As in lFig. 6, the cathode 1 protrudes .asubstantial distance into the space between the focusing electrode A 'and the irst accelerating electrode B. Disc 17 is curved in cross section with its convex side .facing the cathode '1, as shown,

yto conform to the shape of the equipotential surface in that region. Preferably, but not necessarily, `the spacing i4between electrodes B and `C is greater than that between the cathode 1 and electrode B, as shown. These spac- Vings, which are designated LC and LB in Fig. l, are not very critical because `the vpotentials of the various electo cause a velocity maximum of the noise standing wave to occur near electrode C, to produce optimum noise Preferably the diameter of the aperture in thecathode and that electrode. l

Fig.V 7 also includes .a fourth apertured disc 23, which serves as an additional accelerating electrode E. This `electrode may be maintained at` the potential at which the beam is to be utilized: In a helix traveling wave tube this lwould be the helix potential. Electrode D may be at the same potential as electrode E, or somewhat different. For example, .if electrode D is at a higher poten- `tial lthan electrode E it yprevents 'positive ions from traveling back towards the cathode.V For some purposes,

`electrodeE may lbe omitted and the beam utilized at `the potential of electrode D.

The various elements of the. gun structure are supportedV as a unit within the tube envelope 25 by means `of three longitudinal glass rods 27, only one of which is 25 and connected to suitable points on a direct-current jsupply source 35, as shown.v o Fig. 8 shows the variation vof noise-reduction factor `1`With'the potential VCV of the second accelerating electrode C, between the limits 0 and S0 volts, for several values of the ratio R of the beam diameter to the cathode diameter, for the special case where VC=VB and VD=500 volts, in a three-region gun of the improved type as in Fig. 7, operated with divergent ilow. A comparison of the value of noise reduction factor in Fig. 8 for a given value of R l with the corresponding value for R=1 shows the improvement in noise reduction produced by use of a divergent-flow gun structure.

Fig. 9 shows the relation between focusing electrode potential'VA and first accelerating electrode potential VB, between the limits +20 and 8O volts for VA and 0 and 90 volts for VB, for several values of beam current, I0. The heavy line P drawn from the point VA=VB=0 (cathode potential) represents parallel-flow operation.

VFor agiven value of VB, V'when VA is below this heavy line, divergent flow is produced, and when `VA is above this line, convergent iow is produced. It can be seen that divergent flow can be produced with the improved gun of the invention over most of the operating region with the focusing electrode Al negative with respect to -the cathode, which prevents collection of electrons by the focusing electrode.

With the improved arrangement of the focusing elec- 'trode land cathode Vshown in YItigs and 7 operated under divergent-flow conditions, the electrode B can 'be omitted, `provided the potential of electrode Cis adjusted to -cause a velocity maximumof the noise standing wave to occur near electrode C.

Experimental results on traveling wave tubes utilizing the three-region divergent-ow'electron gun ofthe present Vinvention' yielded consistently lowf noise factors.` For example, a Ytube using the improved gun structure of Fig. 7 and operated at `3000 megacycles and`helix voltageof 500 volts, with 400 megacycle bandwidth, yielded a gain of about 20 db at a best noise factor of only 8 db.

What is claimed is: o I

1. A low-noise electron gun comprising a cathode electrode and three apertured -accelerating electrodes spaced along a given axis with the apertures of said accelerating electrodes alignedwithsaid cathode electrode, at least the first two-of said accelerating electrodes yhaving an axial length which is short compared to the distances between said three electrodes, the diameter of the Vapertures in said accelerating electrodes being substantially greater than the diameter'of said cathode electrode, the distance between the cathode electrode and the first accelerating electrode being smaller than the distances between adjacent accelerating electrodes, a directcurrent voltage source connected to said electrodes for applying low positive `potentials to the first two accelerating electrodes and a high positive potential to the third accelerating electrode, relative to said cathode electrode, the potential of the secondaccelerating electrode being of the same order as the potential of the first accelerating electrode, the potential of the third accelerating electrode being more than four times the potential of 'the first' accelerating electrode.

2. A low-noise electron gun as-in claim l, further comprising a fourth apertured electrode spaced. along said Vaxis beyond said third accelerating electrode, and means connected to said fourth electrode for applying thereto a high lpositive potential somewhat below that of said third accelerating electrode. y

3. A llow-noise electron gunV comprising a cathode electrode and vthree apertured `accelerating electrodes spaced along a given axis with the apertures of said accelerating electrodes aligned with `said cathode'electrode, at least the Vfirst two of Vsaid accelerating electrodes having an axial length which is short compared to the distances between `said three electrodes, `the diameter of the apertures'in said accelerating electrodes being substantially greater than the diameter of saidcathode electrode,

the distance between the cathode electrode and the first accelerating electrode being smaller Vthan the distances between adjacent accelerating electrodes, separateV leads connected to said electrodes, and a direct-current supply source connected to said leads for maintaining the first accelerating electrode at a low positive potential of l0 to 50 volts, the second accelerating electrode at somewhat higher potential than the iirst accelerating electrode, and the -third accelerating electrode at a high positive potential of at least several hundred volts, relative to the cathode electrode.

4. A low-noise electron gun as in claim 1, further including an apertured focusing electrode surrounding said cathode electrode and main-tained by said direct current supply source at a potential near that of said cathode electrode. i

5. A low-noise electron gun comprising means includ- -ing a cathode for producing along a given axis an electron beam having a noise standing Wave with noise veloc-` ity and current density maxima and minima spaced along said axis, first and second apertured accelerating electrodes of relatively short axial length as compared to the distance therebetween spaced along said axis from said cathode, the distance between the cathode and the rst accelerating electrode being smaller than kthe distance between adjacent accelerating electrodes, means sepaat a noise velocity maximum of said noise standing Wave, -a third apertured accelerating electrode spaced a substantial distance along said axis beyond said secondelectrode, and means connected to said cathode and said Vthird accelerating electrode to maintain the latter at a high 'positive potential relative to said cathode, the diameter of the apertures in said accelerating electrodes being substantially greater than the diameter of said cathode.

6. An electron gun as in claim 5, further comprising an apertured focusing electrode surrounding said cathode.

7. An electron gun as inclaim 6, furtherV comprising means connected between said focusing electrode and said cathode for maintaining said focusing electrode at a potential near that of said cathode.

8..An electron gun as in claim 5, further including an apertured focusing electrode surrounding saidcathode, said cathode protruding a substantial distance into the space between said focusing electrode and said rst accelerating electrode to prevent collection of beam electrons by said focusing electrode when the latter is operated at .potentials equal to or higher than said cathode.

9.,An electron gun as in claim 8, further including means for maintaining said focusing electrode at a potential near that of said cathode, to cause said beam Yto diverge initially in the region between said cathode and said rst accelerating electrode.

1,0.,An electron gun as in claim 9, further including means for establishing a focusing magnetic field along said beam axis.,

11. An electron gun for projecting a variable-flow electron beam along a predetermined axis comprising a cathode having an emissive surface coaxial and transverse l to ksaid axis, Vat least one apertured accelerating electrode coaxially mounted on saidaxis in spaced relation to said cathode, and anrapertured focusing electrode-coaxially surrounding said cathode and having a ,truncated-conrelike surface extending outwardly and forwardly from its aperture in the direction vof electron travel from said cathode said cathode protruding a substantial distance vinto 4the space between said focusing electrode and said first acceleratingelectrode whereby said beam can be caused either to be convergent, to have parallelYV flow, or

to be divergent, as desired, in the region between said cathode and said accelerating electrode, merely by the ,application to said'focusing electrode of suitable potentials relative to said cathode. t Y ,12. Anelectron gun as in claimV 11, further including lmeans for establishing a focusing magnetic eld along said beam axis.

,vex side facing said cathode.

another. apertured 13. An electron gun as inclaimll, wherein said accelerating Velectrode is curyed in cross section with its, con- 14. An electron gun as in claim 11, .wherein the diameter of the aperture in said accelerating electrode yis-of the order of the distance between said cathode and said accelerating electrode. z f.

15. An electron gun as in cla' 11, including at least three apertured accelerating electrodes coaxially mounted on said axis in spaced relation to each other and said cathode, and separate leads connectedto said cathode and rsaid focusing andaccelerating electrodes for apply ing suitable potentials thereto.

16. An electron gun for projecting a low-noise electron beam along a given axis comprising a cathode havingl an emissive surface coaxial and transverse to said axis, an apertured focusing electrode coaxially surrounding said cathode and having a truncated-cone-like surface extending outwardly and forwardly from its aperture in the direction of electron travel from said cathode, first and second apertured accelerating electrodes, of relativelyy short axial length as compared to the distance therebetween coaxially mounted on said axis in spaced relation to each otherY and said cathode, said cathode protruding a substantial distance into the. space betweensaid focusing electrode and said first acceleratingA electrode, and a direct current supply Ysource connectedxto lsaid cathode and said electrodes to maintain. saidfocusing electrode at a potential near that of said cathode, said rst accelerating electrode at a low positive potential and said second accelerating electrode at a high positive potential, relative to said cathode.'

17. An electron gun as in claim l16, further including accelerating electrode coaxially mounted on said axis between said cathode and said first accelerating electrode and maintainedV by said sourcev at Va low positive potential. relative to said'cathode.

18. A low-noise electron gun as in claim 1, wherein the ratio of the potentials of the rst and second accelerating electrodes for optimum noise reduction lfactor for various electrode spacings is determined approximately by the minima of theY curves shown in Figure 4 ofthe drawings. i

References Cited in thefile of this patent UNITED STATES PATENTS 2,110,911 Y Knoll et al. Mar. 15, 1938 2,581,243 Dodds Jan. 1,Y 1952 2,636,948 Pierce Apr. 28,Y 1953 2,771,568 Steigerwald Nov. l 20, 195,6

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