US2810853A - Electron discharge apparatus - Google Patents

Description

Oct. 22, 1957 A. H. w. BECK ELECTRON DISCHARGE APPARATUS 2 Sheets-Sheet 1 Filed NOV. 29, 1951 Y Inventor AJ-LW B EC K Attorney Oct. 22, 1957 A. H. w. BECK ELECTRON DISCHARGE APPARATUS 2 Sheets-Sheet 2 Filed NOV. 29, 1951 Inventor HW. BEC K B AAM am Unite States Patent ELncrnoN DISCHARGE APPARATUS Arnold Hugh William Beck, London, England, assigner to International Standard Electric Corporation, NewI York, N. Y., a corporation of Delaware Application November 29, 1951, SerialNo. 258,819

Claims priority, .applicationY Great Britain December 1, 1950 Claims. (Cl.` S15-3.5)

The present invention relates to noise suppression'. in electron discharge apparatus.V The. invention is concerned with means whereby shot noise fluctuations in the current emitted by a thermionic cathode may beprevented from causing appreciable. noise voltages inthe circuits associated with the electronbeam.`

In present day receivers for use, at; centimetric wave-` lengths with low level input signals it is usual. to feed the signals direct to a crystal detector or frequency--y changer stage. This is due to thev fact that existing thermionic arnpliliersA are uoisier than silicon crystals. To be; useful in front of a frequency changer' or detector, the amplifier` must, have sufficientl gain so that the noise power contributed by the; amplifier output circuit and detector input may be neglectedk in comparison with the amplified signal; for this,V purpose again o db maybev considered sufficient, the total noise of the receivingv system may then be considered as that due to theY amplifier input circuit alone.. It followsV therefore, that the' use of a thermionic amplifier in front of the detector stage of a receiver becomes advantageous provided the gain of the amplifier stage can be made greater than about l()V db and provided the input noise is not worse. than that of the silicon crystal heretofore used. With present day*I electron velocity modulation tubes, such as the travelling wave tube, it is a comparatively simple matter to arrange for suiiicient gain, but the input noise of these tubes is normally very high. The mainV cause of the noise generated bythe discharge device is the random rate of emission of. and the Maxwellian distribution of ve-i locities among the electrons emitted from a thermionicI cathode. We refer to the noise from these effects'as shot-noise. The remainder of the noise is dueto thermal noise uctuations in the input circuit. We' are notl corr-V cerned with these'in this speciiication.

1n triodes and the like having inter-electrode spacirlgs'Y small compared to the electron transit angle, shot noise is considerably reduced by space charge effects. We define the electron transit angle as 211- times the ratio of the electron transit time at any frequency of operation to a period of oscillation at thatV frequency. For the purposes of the present specification we dene a space charge limited electron beam as one in which only a small fraction of the electrons emitted by the thermionic cathode` is drawn off, the remainder returning to the cathode thusV forming a potential minimum in front of the cathodei It' may be taken that a ratio of total cathode emission current density to initial beam current density of 3:1' is sufficient to produce complete space charge kcurrent limitation, i. e. any further increase in. saturatedY emission will produce only a negligible change in beam current and we shall assume in the present, specification,A unless otherwise stated, that we are concerned with a cathode total emission current density of the order. of 10. amps/cm..2 at temperatures of the order of. l,l0() K. These figures are representative of good modern oxide cathodes.

ICC

Asstatedv above, it is well known that, at least at low frequencies, shot noise is reduced by the effect of space charge; in fact, if in a diode the cathode emission be increased,- the accelerating anode voltage remaining constant, the anode current remains practically unaltered. A transient increase in cathode emission causes a transient increase of anode current which, it can be shown, is due, to a major extent, to electron velocity modulation in the region between the space charge induced potential minimum and the anode. Shot noise, for all practical purposes, canbe considered as due entirely to velocity modulation effects in this region between the potential minimum and anode. We shall also show that at the potential minimum (to be strictly correct, just beyond this, so that electrons are moving only in one direction), fluctuations in the conduction current are negligible, there being present merely a velocity fluctuation; a bunching effect converts the electron velocity uctuations into conduction current variations as the beam progresses beyond the minimum. Space charge effects can reduce the conduction current at one given plane in a manner familiar from the theory of electron velocity modulation devices.

As is common with analyses of this type we replace the real electron beam with variable velocities by a single beam with an A. C. velocity equivalent to the mean square value of the variable velocities.

The space charge induced potential minimum is so close to the cathode that, in most cases, it would bel practically an impossibility to place a grid at the potential minimum. Our analysis of the eect of transit time and shot noise in space charge limited beams shows, however, that a very considerable improvement in noise can be obtained we place a grid within an electron transit-angle of one radian from the potential minimum.

According to one aspect of the' present invention, therefore, there is provided an electron velocity modulation apparatus comprising arrangements for projecting an electron beam from an electron gun through a space in which the electrons of the beam may be velocity modulated by` interaction with the electro-magnetic field therein, the said gun having a cathode and a grid at the same H. F. potential closely spaced thereto defining between them a region in which electron beam current is fully space charge limited, the cathode-grid spacing and the beam accelerating potential being such that the electron transit angle' between the' space charge induced potential minimum and the grid is not greater than one radian at the highest frequency of the said electro-magnetic field.

Although with any given electron velocity modulation tube, the external circuit may be adjusted to provide for a` Wide range of variation in electron transit times in different regions of the electron beam path, it will be realisedV that the present invention necessitates the provision ofV a tube having an electron gun with special characteristics. Accordingly, they invention provides an electron velocity modulation tube adapted for the amplification ofV electromagnetic waves comprising an electron gun having a grid closely adjacent to the cathode defining therewith a reg'ion shielded from external electromagnetic Yfields of the frequency of th'e Wave to be amplified, the spacing between cathode and grid being such that when the tube current and potentials are adjusted forV the said amplification and so that the electron current inthe saidV region 'is fully spacerchargel limited, the elec-tron transit angle between the space charge potential minimum and the said grid is not greater than one radian at the highest frequency of thersaid electromagnetic waves.

The invention will now be more fully described with reference to the accompanying drawings, in which:

Fig. 1 illustrates. the variation of potential in a space 'charge' limited planar diode.

' potential minimum and anode, electrons are Fig. 2villustrates the effect of increase in cathode .emiS-M sion current -on the anode current of such a diode,

Fig. 3 illustrates diagrammatically an embodiment of the present invention arranged to elucidate. our analysis, and

Fig. 4 shows diagrammatically an embodiment in whichy 5 anode asVa, the potential varying with distance between K'and A in the manner indicated bythe'curve 1 a potential minimum being formed at the planeMdistant d, from the cathode and d2 from the,anode,;;the'potential at this minimum being indicated as -L/m; At the cathode,

electrons are emitted at randominstants ofjtimeand with a Maxwellian distribution of velocities. In lthe region (a) between K `and M electrons which aregemittedwith insufficient velocity to pass beyond the potential minimum are returned to the cathode, so that in this regionelectrons asrqssafr discussed' inv the. region. (b') .l

.moreelectrons than appropriate to the value of the -potential minimum, for theywill have passed the minimum while the retarding potentialwas less thanit now is. During this time interval, therefore, there Vwill always be more electrons inthe region (b') than will be present dur-Y ing any time interval after the slowestelectron emitted immediately before the'step function has reached the anode.V Theseelectrons are subject to a greater acceleration than they hadbefore, and hence the anode current, being the product of the number of electrons reachingY the anode per second and their final velocity, will increase beyond -ther'value which it will 'finally attain. Thus', a

L positive pulse of current will occur at the anode. To some extent this will be ott-set by a negative-going pulse at the cathode due to the greater number and velocity of the reiiected electrons, a similar argument obtaining for these electrons in the region (a') as for those we have just However,..in practice, the distance d1 isnisuallyYinuchI .smaller than d2, while the are movingin both directions. In the region (12'), between 20 screening effect of the space vcharge at the potential mini-v Y moving 1in one direction towards the anode and are accelerated.v The position of the space charge minimum and the mean value of .-Vm depends upon the cathode temperature and the ratio of total cathode emission Is to anode ,current Ia. The values of Va and d2 for any given', anode current are interdependent, so that if one be giventhe other can have but one Yvalue in order to maintain the said anode current. Thus, if Va be maintained constant by means of an external battery, andthe cathode emission 3 0 bev increased e. g. by a sudden change in Workfunction, the. anode current tends to remainconstant. VAn additional number of electrons, however, are emitted by the cathode, which causes the potentialminimum to be de; pressedto a lower value. of thepotential minimum is shifted towards the anode,t'

but to'such a small extent that its changein position can be neglected forjour purposes,-.the potentialdistribution beingaltred as indicated bythe dotted curveZ in Fig.y 1;

In Fig-2 we show'cur'ves relating current I and time 40 for the cathode emission current and the anode currentl when, dueto any cause, the cathode emission is suddenly changed from Is to Is-i-Als as shown by the curve 3. We

c'an consider the effect of this qualitatively'as follows. First the potential of the minimum is made more negative; it follows that there is an increase in the number ofelectrons which have insufficient initial velocity to vreach the anode.l Had the potential minimum not Vbeen made more negative, the increase in emission would have led to a proportionate increase in the number of electrons reaching Y the anode.

. Instead of this, however, under the conditionsV where Is la, ,the depressionl of the potential minimum is s uch that a greater fraction of the .total number of electrons emitted from the cathode is reflected bythe po tential minimum; hence ,the increasedemission causes a net decrease in the total number of electrons reaching thel anode. These electronshowever,V since the potential difference between M and A is increased, areY subject to a greater Vacceleration than before, so that the anode cur-v rent remains substantially" unchanged but for a transient.60

pulse. It maybe shown that the proportionate change. of anode currents' Ala, is related tothe proportionate change of emission currents AIS by the equation (Le) "I 3M" .A If Ie z-) ZeU/'a-i- Vm) Is where Boltzmanns constant, 1.38 l0 Li3 ljoules/V Tis the! cathode temperature in degrees Kelvin, and "e is the electronic charge, 1.5 9 l019 Coulomb. f

"In Fig.. 2 the variation of anode current Ia isl indicated 70 Vby the curve 4. The anode curreiitpulse V5 shownl in Fig. 2 is to beexpected from general principles, but the mediatej- 'factors involved warrant. further discussion. Dpring the time Ywhile they potential minimum is changing its value, at any instantin the region ;(b) `there will be r `At the same time the position 3 5 "i transit effect therefore isa'positive pulse such as 5 shown in'Fig. 2. From the above discussion it will be appreciated 25 thatthe major causeof the transient uctuation of anode current due to the sudden increase in cathode emission is a velocity modulation effect in the region (b')., Asshown above, general considerations lead us to enpectuctuation noise in a diode to be considerably fact, well known inV short electronv beams. :In a short electronbeaml in lthe absence of space charge, the mean upon :the mean cur-'rent-'Imamps emitted by a Vthermionic cathodefwithinthe lfrequency band VAf`cycles/sec. is given (in th'absenceof space-charge) by l Y l l izzelff-- (2)y I-f the emission bespace charge limited, 'l however, it 'is'. known that the'mean square fluctuation current is reduced to 2 .y Y

' 1=2e10r2af (3)1 where* is'a smoothing factor, lying between O and l 45 depending upon the space charge, but having a value of about 0.1 for Imost triodesv at vfrequencies up to about mc./ s. In what follows We shall introduce a factor 10, a function of transit 4angle 0, applicable for longer electronl` beams. y Y Y From a-quaiitative consideration of the stepfunction. increasein cathode-emission represented in Fig. .2, weV have concluded that'the anode current pulse 5 is due toV avelocity variation effect rather than a number variation effect. We have analysed. the case represented in Fig. 2 Iand'find that we can represent the anode current iluctuation Vby means of an, equation similar to Equation 3 but in which I2 is given by the expression f Non-mier Y- 1e va+vm meanv square conduction current just beyond the spaceV charge. induced potential minimum has Ybeen smoothed by `a factor' of the order of T4. Wegconclude, therefore, that the effect of the space charge minimum is to elimi-l nate virtually-all V,conduction current variations Aat the f. minimum;leaving only electron velocity modulation which `produces by drift action the'conduction current variation eventually observed at the Vanode plane.

In order to analysethe transit time elects of shot noise in V electron beams applicableV to electron velocity modula-w I tion apparatus, we have used Llewellyns electronic equations, Ywhich are` applicable to theY analysis of conditions between infinite planar electrodes for any degree of space reduced Vbythe presence of space. charge, and this is, in

value ofv the tiuctuationcomponent superimposed Y charge limitation and in which A. C. variations of potential, conduction current and electron velocity are assumed to be superimposed upon average or D. C. values of these quantities. We take the initial plane of emission as that of the plane M of Fig. l and consider the case where there is zero H. F. potential difference between the two planes, the initial conduction current variation being negligible and the initial electron velocity variation being given by la known expression previously used in diode noise analysis.

In Fig. 3 we show very diagrammatically a two resonator klystron arrangement embodying the present invention. An evacuated envelope, indicated by the dotted line 6, to which is sealed a pair of resonators 7 and 8 joined by a drift tube 9, encloses an electron gun comprising a cathode 10, heating arrangements for the cathode being indicated at 11, and a closely spaced grid 12 parallel to the emitting surface of the cathode. In practice other electron gun electrodes following the grid may be desirlable but are omitted from the drawing to avoid confusion. The electron beam, after traversing the resonators 7 and 8, is lcollected by the collector electrode 13. Resonators 7 and 8 are provided with the usual input and output wave feed arrangements indicated by the coupling loop and coaxial line attachments 14 and 15 respectively. The grid 12 is shown connected to the cathode 10 by means of a capacitance 16, which will normally be intrinsic to the mounting of the grid, and which is such that grid 12 is maintained at cathode potential at 'all frequencies within the band-width of resonator 7. Grid 12 should be so constructed that it imposes upon the electron beam a unipotential surface orthogonal to the electron trajectories and forms with the emitting surface of cathode 10, so far las is possible, a region bounded by a pair of planes which in the analysis may be considered to be of infinite extent. It is here pointed out that although our analysis applies strictly only to the infinite planar electrode case, the separation between cathode and grid is so small at microwave frequencies that in the present invention these electrodes, provided they be parallel to one another, may be curved without detriment to the performance of the system.

Cathode is connected to the negative pole of potential source 17, the positive pole of which is connected to the collector electrode 13. Grid 12 is shown also connected to the negative pole of source 17 through decoupling resistance 18 and biasing potential source 19, while the wall of resonator 7 is shown connected to a tapping point on the potential source 17. For the purposes of the following analysis in Fig. 3 we have indicated `at (a) the plane of the potential minimum in front of the cathode 10. This plane, together with plane (b) coinciding with the grid 12, deiines a region (l) in which the current is completely space'charge limited. A region (2) is defined between the boundary planes (b) and (c), the latter plane being 1ocated at the centre of the inter-action gap 20 of resonator 7. It is assumed, for present purposes, that the interaction gap 20 is bounded by grids 21 and 22 which ensure that the iield within resonator 7 in the absence of an input signal at 14 may be influenced only by the conduction current of the electron beam. Y

It may be as well to point out at this stage that in previous analyses of shot noise in high requency diodes, the cathode and collector electrodes have both been within the boundary of the resonator. In the present case a'll the electrodes affecting the beam before it enters the interaction gap 2@ are maintained at the same high frequency potential, while, at the space charge minimum the total current is equal to the conduction current. It follows, therefore, that we are concerned only with conduction current variations and electron velocity uctuations. We have found that the latter are proportional to the distance between the planes (a) and (b) if the transit time involved is short and that, as has been shown above, at the plane (a) the conduction current variations are negligible. Hence, if plane (b) be very close to plane (a),

6 the full eitects of the space charge smoothing of the shot noise are manifested at plane (b). Ideally, therefore, we should like 'to place the inter-action gap 20 very close to the space charge minimum, in which case We should expect to obtain ya shot-noise smoothing factor of the same order as is 'obtained at low frequencies. In general, this is physically impossible, but it is possible to place a grid 12 very close to the space charge minimum. Immediately to the right of the grid 12, i. e. to the right of the 'plane (b), therefore, we obtain the effect of space charge smoothing, and in the region (2) between planes (b) and (c) we need not maintain the voltages such as to conform to complete space charge limitation of the beam circuit, and, in fact, we ind that, in general, the noise at plane (c) notwithstanding an electrically long region (2), is very much less than it would be in the absence of the grid 12 whether or not there were complete space charge limitation between the planes (a) and (12); without the grid 12, if the region between planes (a) and (c) were not space charge limited, we could not expect much space charge smoothing; with grid 12 placed according to the invention there is imposed, at a plane intermediate the cathode and the utilisation plane, a region of zero A. C. potential which causes the effect of the large transit angle in 'the region (2) to become small and We tend to obtain at the plane (c) a noise condition not very much worse than that at plane (b) adjacent the space charge minimum.

Llewellyns rst order electronic equations upon which our analysis is based are given below in slightly modified form: Y

1:('Vb- Va) '1n-i" gaizrlveais qz.=(Vb-VQMn-ljqaan-l-vaan vb:(Vb'Va)llail-Qafl32lva 1sa where I is the total alternating current, q is the A. C. component of the conduction current, v is the A. C. component of the electron velocity and V is the alternatingv voltaoe at the plane (a) `or (b) as indicated by the suix. The suffixes to q and v denote similarly the planes to which these quantities apply; the as are coetiicients involving the D. C. conditions, degree of space charge and transit time; the coeicients will have diiferent values, therefore, according to whether we are considering the region (l) or the region (2) and when it is necessary to distinguish these we shal'l add an appropriate index to the coeicient a. The values of the coefficients in terms of transit angle 0 and the velocity u at the appropriate planes are simply related to the coeicients tabulated in Tables I and II of the paper Vacuum Tube Networks by F. B. Llewellyn and L. C. Peterson in Proceedings of the Institute of Radio Engineers, for March 1944, at pages 148 and 149, by transforming Llewellyns Equation 5 into the form of our Equation 5 above. In both the regions (1) and (2) indicated in Fig 3 the Vs vanish, and for region (1) we put the conduction current variation at the potential-minimum equal to zero, i. e. qa=0 for region (2.), so obtaining:

qu:(1 02H0--axiqbamLl-aaazvbl (7) Substituting for qs and v3 from (6), Qe=va(1-0l2)[(1'a1)a23a222iasa1a232l (8) According to the present invention grid 12 is placed close to the planeet the potential minimum, so thatpi, the

transit angle' of region (1), is small. If the D. C. current density be denoted by Io, evaluation of the coeicients for small gives us:v A

' this result might appear unreasonable until we remember that we still stipulate thatv the region between the -planes is `to remain spacecharge limited, which condition, for any given value of the direct current and transit angle between the p'lanes (a) and (b) fixes the relationship between'the other components at the two boundary planes.

. Thus, Vtheeiect is merely one of space charge de-bunching and'rebunching. p In theY region (2) the direct current is (1 -a1) times the'direct current In in the region 1), except to the right of grid 20 where it is reduced by the further factor (l-az), and if we assume that the degree of space charge is small,

and

. 92 eXP .7'92) where uc=2e/mV2,'Vz being the D. C. potential at plane (c). l Y

' `It is to be notedV that, except for special conditions of space charge which need not concern us here, the same values for the above coeicients a2 would have .been obtained hadwe assumed the degree of space charge arbitrary but made 02 large.

InfEquation 8 we substitute the above values for the coeicients a together with the substitution for va of the root mean square noise velocity variation at plane (a) as given by the following known expression:

4K7 e 1r 2: un I0 m 1 (See, for example, A. I. Rack. Shot Noise in Diodes, Bell System Technical Journal V17 (1938), page 592.)

` We thus obtain for the mean square noise conduction current at plane (c):

, gere-ae #ein For present purposes we may put u12=2b v. +v

ulg@

Vmay dene a frequency dependent smoothing factor T02 Y which, if a, and a2 0 is given by Y In order to .compare thisv result with the noise factor obtainable with usual present Vday velocity modulation Y Y l." I fr tube designsV we must evaluate the coeicients o'czi and usal for region 1) for the case where 01 is large.

F0129, large we have Y which is identical with the result of other workers obtained Vby different methods in the investigation of noise in a long electron beam. Y

As a measure of 'the improvement to be gained by the present invention, let us consider the values of I for an electrode arrangement as in Fig.r3 `of the accompanying drawings for lthe two cases Where the separation between cathode 10 and grid 12 is small, in accordance with the invention, and where it is large, as obtaining in prior electron gun'arrangements incorporating a control grid. For convenience we shall assume that the grid 20 is at the same potential as grid 12 and also that the interception factors are zero. Typical results are Case 1 Case 2 f 01:3.0; 62:12.() radians I"62=19, Io2=0.90Y u Y In order to evaluate the spacings,V and voltages required in a practical embodiment of the invention, it is important that We should not neglect the'separation between Y Vcathode and potential minimum as is done in many transit time approximations; this would lead to results giving us smaller clearances than are, in fact, required.V We should also take. into account the initial velocity'of the electrons at the space `charge minimum. For any given transit angle having a corresponding transit time t between space charge minimum and grid Y12 the distance d2 between space charge minimum and grid is given by aff/ m Y d2- GEOIU-l-uat Y when e, is the permittivity of free space Y Vl -9 aefrX 10 fared/metre and where la is the mean velocity of the electrons leaving the plane ofthe potential minimum.

5 If V be the required equivalent voltage of the grid 12,

It should be mentioned that, in order't obtain agreementV between the values of V and d2 given by Equations 17 Vand 18 with the corresponding values given by Langmuirs but must be taken as a Yfunction'of VV-lV-Vm, having a mean value for voltages between 0.1 and 1 volt ofv In order to evaluate zu,L more exactly and to obtain Vm andVi AWe use Langmuirs (E, 11) tables and let Y qi i uzzIGT where A is given, for low anode voltages, to a sucient approximation by the relation For a givenV construction: we may'A obtain the average transit angle between space charge minimum and grid for any given current density In, and, cathode temperature T by determining Langmuirs E; in terms of` 11, from the tables, and hence :3, and 11,. .using the above expression for A and substitutingrin the expression;

1.5X103 5 103 104 amps/metre?. 1.325X105- 6.83)(105.. 4.48X10-l metre.

42 0.30 0.23 volt.

1.0 4.50 volts. 3.52X10-5-. 5.57 l0 metre. 4.20)(10-5-. 6.05X105 metre.

Assuming the same value of cathode emission current and a fixed spacing of 1.6)(10*5 metre, the frequency fo at which this separation gives a transit angle of l radian between space charge minimum and grid varies for different values of beam current Io as follows:

Io fo 104 amps. lmetre?.

1.5Xl03-. 3X103 42 3,105 megacycles/sec.

An embodiment of the invention as applicable to a helix type of travelling Wave tube is shown in Fig. 4. In this embodiment the envelope is divided into two portions, 23 and 24, sealed to either side of a centrally apertured disc electrode 25 which forms the nal anode of the electron gun. A screen grid type of electron gun is indicated diagrammaticaliy, the cathode heating arrangements 11 and the grid 12 being connected as described With reference to Fig. 3, the capacitance 16 between grid 12 and cathode 10 being shown for convenience externally of the tube. Cathode and grid assembly are housed within the screening electrode 26. The helix 27 of the travelling Wave tube is connected by a straight section of rod 28 to the disc electrode V25 at one end and to a collector electrode 29 at the other. The ends of the helix project into wave guides 30 and 31 respectively, as in known travelling wave ampliers and a magnetic focussing solenoid 32 is provided around the helix. The disc 25 clips into an annular member 33 within the wave guide 30 so that disc 25 and member 33 together constitute a door-knob probe pick up for the helix. This arrangement is adopted, apart from its other advantages, so as to bring the electron gun as close as possible to the interaction space and so reduce the transit angle between grid 12 and anode 25. Grid 12 is polarised with respect to cathode 10, as in Fig. 3, by connection through the decoupling resistance 18 to the bias source 19, the screen grid 26 is polarised by source 3d which is in series with source 35, and the collector electrode 29 is shown connected to a tapping point on the source 35. The screen and nal anode potentials of the electron gun, together with the potential of grid 12, must be chosen so that the region between the cathode and grid 12 is fully space charge limited, but after passing the grid 12, the beam may be accelerated or decelerated as required. A

In the present invention shot noise reduction in an electron beam is obtained by-using a grid adjacent to the cathode to maintain the region bounded byl these electrodes fully space charge` limited, the transit angle between space charg'einduced potential minimum and grid being small. As has been shownbyfthe examples quoted, at microwave frequencies this criterion leads to very close inter-electrode spacings. From Equations 14 and 16, as an alternative to the present invention-it may be suggested that the transit angles in the regions I and 2 of Fig. 3, or in the equivalent regions in Fig.,4, should be madesuch that cancellation occurs. This can be done by adjusting the magnitudes of" 01 and 02 so that F02 is minimised. It must be remembered however, that when cancellation is obtained by adjustment o f the electron trajectories in twoY separate regions itis'not sui'licient for the average electron trajectory to obey the cancellation condition. Each individual electron must travel along a path obeying the condition. It is found in practice that such systems of cancellation only produce minor improvement in noise factor because the above mentioned considerations involve a degree of electron optical homogeneity which is not obtainable. For instance, in any real non-infinite plane parallel beam radial space charge forces exist and cause axial electrons to travel more slowly than peripheral electrons, so that it is impossible to satisfy the required condition over the whole beam.

In the invention described use is merely made of space charge debunching eiects in a single region which can be made to approximate the ideal plane parallel system rather closely. Beyond the space-charge forming grid it is not material if a variation in transit angle exists.

While the principles of the invention have been described above in connection with specific embodiments, and particular modiiications thereof, it is to be clearly understood that this description is made only by Way of example and not as a limitation on the scope of the invention.

What I claim is:

l. An electron velocity modulation apparatus comprising means deining a space having an electromagnetic field therein, electron gun means for projecting an electron beam through said space to produce interaction of the beam with the electromagnetic iield and velocity modulation of said beam, said gun having cathode and grid positioned outside said space, means for maintaining said cathode and grid at the same high frequency potential, means for producing fully space charge limited electron beam current throughout a region defined by the space between the cathode and grid, said grid being mounted at a distance from the potential minimum of the space charge produced in said region in greater than one radian of the electron transit angle at the highest operating fre- Y quency of said field.

2. An electron velocity modulation tube for the ampliiication of electromagnetic waves comprising means deiining an interaction space for said electro-magnetic waves and an electron beam, an electron gun having a cathode and a grid closely adjacent said cathode dening therewith a region shielded from external electromagnetic tields of the frequency of the wave to be amplilied, said cathode and grid being positioned :outside said space means for producing throughout said region fully space charge limited electron current, said grid being mounted at a distance from the potential minimum of the space charge produced in said region in greater than one radian of the electron transit angle at the highest operating Jfrequency of said lield.

3. Apparatus according to claim 2 including means for guiding the electromagnetic waves to be amplified along the path of the beam to interact continuously therewith.

' priemgmeans providing for sudn'giheWavess'helixsurrqunding theapathbf the beam. a'rwave guideprobe terminating-Said' helix at. .input 4sind, Said-e1etr0ngun indudingal anode frqmwhich Saidrprobeextends.;.-; Y .r `r5- Ah electron 'Yvelcity modllation, apparatus, ,00u11 Y a vspew@ Yhvig an alternating elecfrO-magilstic eld ,thereinl anY electron :gun ,for Ypro-Y hiding-.al1 eleqrolrbeam'throngh said spacefsor imeraftism, e

with saidleldgiisaid Agun Vhaying cathode 'and aigrid @Quilted Qutsd Said'spacernd means provdnga .beam

accelerating potential -1 whereby VYa Aspiace charge induced potential minimum is produced between saidcathQle and said Yaccelerating-potential prpduingvmeans, saidlathode and grid being Aclosely Yspaced with4 respect ne another;

to provide a region in which the electron beam current is substantially f ully space charge limited, said grid being greater than one radian of the electron transit anglet the highest operating frequency of said field and means for intercupling'said-cathode and grid'to maintain themv at' substantially the same high frequency potential.,`

, j *.leflfenceslite'd in the le of this patent f :UNITED STATES PATENTS t 2,280,824 Hansen et a1 Apr. 2s, 1942 l2,484,643 Y vPeterson f Oct. 11, 1949 2,519,420 Varian ,.c Aug-20, 195Q 2,584,597 Landauer'; Feb.5, 1952 OTHER REFERENCES p Travelingwave Tubes, by LR, Pierce, Ypublismiwby D; Van Nostrand C'o.,'Inc.', N. Y., 1950.4 Y'

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