US2611879A - Electron discharge device - Google Patents

Description

Sept. 23, 1952 R. ADLER 4 2,611,879

' ELECTRON DISCHARGE DEVICE Filed. March 18, 1948 s Sheets-Sheet 1 FlG.l 1 FIG-2 so rikiffd \jzl 31A 3'3 I .1 1 1 A 15' egl ROBERT ADLER INVENTOR.

OUTPUT VOLTAGE Sept. 23, 1952 R, ADLER' 2,611,879

ELECTRON DISCHARGE DEVICE Filed March 18, 1948 3 Sheets-Sheet 2 FIG.6

R ADLER O BERTINVENTOR.

HIS AGE/VT ROBERT ADLEIR INVENTOR.

3 Shegts-Sheet 5 ms AGE/VT R ADLER ELECTRON DISCHARGE DEVICE Sept. 23, 1952 Filed March 1a, 1948 Patented Sept. 23, 1952 ELECTRON DISCHARGE DEVICE Robert Adler, Chicago, Ill., assignor to Zenith Radio Corporation, a corporation of Illinois Application March 18, 1948, Serial No. 15,713

16 Claims. 1

This invention relates to electron discharge devices, and more particularly, to such devices as those disclosed and claimed in my co-pending application, Serial No. 7864, filed February 12, 1948, for Electron Discharge Devices, now United States Patent No. 2,511,143, issued June 13, 1950, and. which is assigned to the same assignee as the present application.

In the aforementioned co-pending application, there is disclosed and claimed a novel type of electron tube which provides improved inherent limiting characteristics by way of improving the transconductance of a control grid. Improved transconductance of the control grid is effected by locating the control grid in a position along the electron stream following an accelerating electrode having an aperture, the Width of such aperture being less than the distance between such aperture and the control grid.

Also shown in the aforementioned co-pending application, as a preferred embodiment, is a novel electron tube which, together with its associated circuit elements, functions simultaneously as an amplitude limiter and a frequency demodulator. This preferred embodiment comprises, inessence, two control systems cascaded along a single electron stream, each of such systems including a high transconductance control grid, and is particularly well adapted for use as a combination amplitude limiter and frequency demodulator in a frequency modulation radio receiver.

In such an application, it is particularly desirable that substantially all amplitude modulation appearing in the input circuit be rejected. While the device set forth as the preferred embodiment in the aforementioned co-pending application rejects substantially all amplitude modulation of the input signal at low signal input levels, it has been found in practice that impaired amplitude modulation rejection is encountered in the case of large signal input levels, i. e., signal inputs greater than about 20 volts. Such impaired amplitude modulation rejection may be traced to changes, in response to changes in the input signal level, in the degree of convergence or divergence of the electron beam as it impinges upon the second or quadrature grid.

. It is a primary object of the present invention to provide an electron discharge device of the type described which is particularly suited for use as a combination amplitude limiter and frequency demodulator, the amplitude modulation rejection being substantially independent of the input signal level.

In accordance with the invention, a new and improved electron-discharge device comprises, in the order named, an electron gun for projecting an electron beam, an accelerating electrode having an aperture registering with the beam, a control grid, and an additional electrode responsive to an applied unidirectional operating potential for producing an accelerating electrostatic field adjacent the control grid on the side thereof remote from the electron gun. The, control grid is provided with a bulged central portion extending toward the aperture of the preceding accelerating electrode and spaced therefrom by a distance greater than at least one of the transverse dimensions of that aperture.

The term aperture, as employed throughout the specification and in the appended claims, is intended to define the entire electron-permeable portion or area of an otherwise electron-impermeable electrode.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may more readily be understood, however, by reference to the following description taken in connection with the accompanying drawings, in which like reference numerals indicate like elements, and in which:

Figure 1 is a schematic representation of the electrode arrangement in a preferred embodiment of an electron discharge device constructed in accordance with the invention.

Figure 2 is a graphical representation of operating characteristics of an electron discharge device constructed in accordance with the schematic representation of Figure 1.

Figure 3 is a graphical representation of certain voltage and current relationships in a device such as that shown schematically in Figure 1.

Figure 4 is a schematic representation of another embodiment of the invention.

Figure 5 is a schematic circuit diagram, incorporating the device schematically shown .in Figure 1, of a combined limiter-discriminator stage of a frequency modulation radio receiver.

Figure 6 is an idealized graphical representation of the demodulator characteristic of the circuit shown schematically in Figure 5.

Figure 7 is an exploded perspective view, partly in section, of a present preferred physical embodiment of the invention.

Figure 8 is a transverse section taken at 8-9 of Figure 7.

Figure 9 is a schematic representation of the electrod arrangement of an electron discharge am n substantially symmetrical relation with respect' to a reference plane, indicated by the letter R, such electrodes including, in'th'e order named, a cathode I0, a first accelerating electrode H, a

first control grid I2, a secondaccelerati'ng electrode l3, a second control grid I4; and an anode IS. A beam forming or directing plate I8, constructed of electrically conductive material, is arranged to surround the cathode ID in such away that electrons emitted by the cathode are formed into a convergent beam. The first accelcrating electrode H is provided with a centrally located apertureor slot'll, which registers with the electron'beam; a positive'potential appearing on electrode l l' results in an accelerating field between slot l7 andc'athode I0. outwardly extending portions l8" and [91 may be provided on eachsideiofslot' IT to adjust'the refractive power offlthe slot; it is preferred that portions l8 and I31 extendiparalle'l to the axisand in both directions fromslot IT, in order to insure that electronsie'mitted from the cathode are formed into a substantially parallel beam in the region between electrodes 1 land I 2.

Focu'sing means 20, such as a pair of'rods or a memberhaving a slot, preferably are inserted between the accelerating electrode II and the grid I 2. Focusing means insure that all the electrons emerging from slot I! are directed-to thegrid [2 in a substantially parallel beam.

, Asdisclosed in my above mentioned co-pendin'g application; it has been'found that whenever the IZis large compared to the width of the aperture I] itselfpit is possible to adjust the field con-- figurations; by suitabl'elocation of the various electrodes and suitable selection of the operating potentials applied thereto, in such a manner that .unsually high transconductance is obtained over a narrow range of voltages applied to control grid I2, while at more negative voltages the anode current remains zero and at more positive voltages it remains constant.

The second accelerating electrode I3 is provided with a slot or aperture 2| which registers with the electron beam projected through the first grid l2. Extending portions 22, extending on each side of slot 2| toward the first grid l2, are provided to produce the desired field configuration. The second control grid I4 is provided with a bulged or formed central portion 23-which extends toward the aperture 2| in the second acceleratin electrode [3. Anode I5 is provided witha bulged or formed central portion 24 substantially similar in contour to the central portion 23 of the second control grid M.

Second focusing means are inserted between the slot 2| in the second accelerating electrode l3 and the second grid l4, and means 25 also serve as an electrostatic shield for minimizin intergrid capacitance.

In order to insure that the electron beam pro- J'ected into aperture 2| of the second accelerating electrode l3 be substantially parallel upon 4 emergence from aperture 2 I, the first control grid l2, in the preferred embodiment, is made plane or unformed. In addition, the contour of first focusing means 20 is adapted in a preferred Way to insure substantially perpendicular incidence of the electron beam on the first grid [2, thereby to approach maximum transconductance of the input grid. m a

In i the device of Figure- 1,the-'input grid l2 has associated therewith a'n-anode' operating characteristic resembling a step function, with a region of constant anode current separated from the zero anode current region by a narrow region of high transcondu-ctance. By suitably spacing and arrangin the electrodes, the constant anode current region may be made to commence at a negative value of input grid voltage.

Since the electron stream passes through the first grid in the form of a concentrated beam, it may be directed through a second system comprising thesecond slot 2| and the second grid M similar to the first system comprisin -the first slotmll and the first grid; I2; The secondgr-id [4 has a characteristic substantially similar-to that of the first-grid !2 but oflower transcon-t ductance, provided that the'yfirst gridis" operated Within the constant current region;- V 7 The device shown in Figure 1* is; particularly suited for use H in a frequency-modulation radio receiver, combining in a single electron stream the functions of amplitudelimiterand frequency emod o 1 In my above mentioned co-pending-application, thereis di-sclosed -and claimed a device similar to that shown inFigure l, the only difference be ing that no bulged orforrnedcentralportions 23" and 24 are provided onthe second gridyltand the anode l5. Inthat-application, there isshow-n an idealized representationof the anode currentinput grid voltage; characteristic of a-tube' of this nature. This-idealized representation is repro-- duced in Figure 2 of this application as the-curves 30, 3|, andltz. Curve 30 i the characteristic when'the voltage E02 on the-second-or quadra ture; grid 14 exceeds the level E2 requiredfor limiting, while curve 3| represents -the; characteristic when E02 equals zero and; curve 32 represems 'ule, idealized characteristic for v some arbie trary negativevalue of E02. "Ithas'been found, however, that in-practice, the operatingcharac teristics deviate somewhat from the idealized characteristics 31 and 32, such practically'realizablecharacteristics being shown as curves 31A- and 32A respectively. The result of these-practi cal deviations is impaired amplitude modulation" rejection, such impaired rejection being particu larly noticeable at: large'signal input levels El. The provisionof central-bulged portions'23 and" 24 on'the quadraturegrid l 4: and the anode f5" (Figure l), in accordance with" the present invention, results in operating characteristics as shown in Figure2 as curves 3l-B and 32B; suchcharacteristics very closely approaching the ideal ized characteristic's3l and 32.

- The operation of-the device shown in Figure l is as follows. The tube comprises a pair-barrowtrol grids l2 andMarranged in cascadei 'I'hse grids may be compare'dlwith a pair of interrupter switches, the passage of curren'tt'dtli 'an'ode 15 being dependent on 1 both switches being closed. An input signahappli'ed tothe first "lid [2 re-' sults in a substantially square wave spade current within the tube, the COn'dllCting and IiO llC O nduCt mg periods being equal in length. If a signal of the same frequency "is appnec to the second grid l4, space current flows only during those periods when the signals applied to both grids are simultaneously positive. If the signal applied to the second grid I4 is shifted 90 degrees with respect to the signal applied to the first grid l2, anode current flows during one quarter of the cycle, while no anode current flows during the remaining three quarters of the cycle. The average anode. current Ib, therefore, is one fourth the maximum instantaneous value of space current. Variation of the phase displacement between the two signals produces corresponding variations in length of the conductive periods, and therefore, in the average anode current. Such a characteristic is exactly what is desired for a combined limiter-discriminator stage in a frequency modulation radio receiver.

The switching action may be more fully understood by referring to Figure 3, in which certain relevant voltage and current relationships are graphically depicted. In this figure, conditions are illustrated for the case in which the voltage applied to the quadrature grid lags exactly 90 degrees behind the voltage applied to the input grid. Two conditions are illustrated, one based one a relatively large input signal voltage e r, and the other based on a much smaller input signal voltage egl. In order to simplify the explanation, sinusoidal waveforms, and a constant signal eg2 on the quadrature grid, are assumed.

That interval during which the signals applied to both grids are simultaneously positive, so enabling the space current to reach the anode, extends approximately from the instant denoted by the phase angle of 90 degrees to that denoted by 180 degrees. More accurately, anode current starts to flow at the time corresponding to the vertical line 40 when the voltage 632 on the quadrature grid becomes more positive than its cutoff potential Egz; the anode current reaches its full intensity at the time corresponding to vertical line 4| when the quadrature grid, its voltage still rising, attains the limiting level potential E2. During the period of rising anode current between times 40 and M, the potential of the first grid is greater than the potential E1 required for limiting; thus, operation in the region of constant current is assured, and the anode current during this period is not affected by the potential of the input grid. If an input signal e 1, which is large in comparison with E1, is applied to the input grid, the anode current it continues to flow with its full intensity until the time, marked by the vertical line 42, that the input grid, its voltage decreasing, becomes more negative than the limiting level potential E1. At this time, the anode current it begins to drop and reaches zero at the time, marked by line 43, that the input grid reaches cutoif Egl. It then remains zero until, approximately three quarters of a cycle later, the entire sequence is repeated.

If, new, the signal applied to the input grid is decreased to a smaller amplitude egi the constant anode current no longer flows until the time corresponding to line 42, but the anode current ib begins to drop at the earlier time corresponding to line 44. On the other hand, the decrease in anode current occurs much more slowly and zero anode current is not reached until the time corresponding to line 45, considerably later than it was reached in the case of larger input signal amplitude.

The instantaneous variation of the anode current with respect to time is plotted in the lower part of Figure 3 for both input signal amplitudes.

It is seen that the steep sl e of decreasing anode current is in the case of th large input signal amplitude egl is intersected symmetrically by the more gradual slope 111, corresponding to the smaller signal amplitude e'gl- The point of intersection of the two slopes corresponds exactly to the phase angle of degrees. I

In plotting the anode current in Figure 3, it has been assumed that the control characteristic of the input grid is approximately straight and symmetrical and that the control characteristics of the quadrature grid correspond to the idealized curves 3| and 32 in Figure 2. With these assumptions, the area of the trapezoid bounded by the anode current curve is not changed by the transition from the steep slope to the more gradual slope. Therefore, the average anode current It remains unchanged as the input signal amplitude is varied, provided only that the input signal amplitude is sufficiently greater than the limiting level potential E1 that the entire space current reaches the anode for at least a portion of the cycle.

In practice, however, as already set forth, the operating characteristics of a tube such as that disclosed and claimed in my above mentioned copending application deviate considerably from the idealized characteristics, such practical characteristics being shown as curves :HA and 32A (Figure 2). While these characteristics have no bearing on the behavior of the tube during that long period (between lines 4| and 42 in Figure 3) when it operates in the constant anode current region, they influence the short period at the beginning of the conductive cycle, between lines ii) and M, when the input grid is at its positive peak and the quadrature grid is rising from cutofi' Egz to the limiting level E2. During that period, amplitude modulation of the input signal e 1 may produce corresponding audio frequency components in the anode current. This effect is particularly pronounced at large input signal levels, since the characteristics diverge at a greater rate at such levels.

While it would seem possible to improve the amplitude modulation rejection by increasing the transconductance of the quadrature grid, thereby shortening the length of the transition period, it has been found in practice that increasing the transconductance of the quadrature grid results in further deviation of the operating characteristics MA and 32A from the ideal conditions 3| and 32. Thus, a compromise is required between a desired maximum transconductance on the one hand and parallel straight operating characteristics on the other.

The deviation of the practical operating characteristics MA and 32A from the idealized curves 3! and 32 may be explained in terms of a change in the focus of the electron beam as it leaves the slot 2| in the second accelerating electrode [3 (Figure i). The input grid [2, focusing means 20, and the second accelerating electrode l-3 together function as an electron lens, the focal length of which lens varies as the potential on the input grid is made more positive; the width of the beam in the plane of the input grid also decreases with an increase in the positive potential on the input grid. The combined result of these lens effects is a gradual transition from divergence to convergence of the electron beam emerging from slot 2! in the second accelerating electrode [3 as the potential of the input grid I2 is made more positive.

Now, the second system consisting of the sectrodes II and I3 are interconnected, either internally or externally, and connected to the positive terminal of a suitable source of positive unidirectional operating potential, here shown as a battery 54, the negative terminal of which source is connected to terminal I. An oscillatory circuit 55, comprising an inductance 56 and a capacitance 51, is connected between the second grid I4 and terminal 5|. It is to be understood that capacitance 5'! may comprise the distributed capacitance of thecircuit, in which case no separate unitary circuit element 51 appears. Audio frequency output voltage is derived from an anode load resistance 58, connected between the anode I5 and the positive terminal of source 55, by.

means of a pair of output terminals 59 and 00. Output terminals 59 and 60 are bypassed for the intermediate frequency by a capacitance BI.

In operation, the parallel resonant circuit 55 is tuned to resonate at the intermediate frequency. As is taught by Zakarias in his U. S. Patent No. 2,208,091, a unilateral space charge coupling exists from the input grid I2 to the quadrature grid I4. That is to say, a voltage is induced on the second grid l4 by the space current passed from the first grid l2; however, the potential of the first grid I2 is unaffected by the potential variation of the second grid I5. This space charge coupling betweengrids is equivalent phasewise to a one way negative capacitance; consequently, the voltage induced on the'second grid I4 lags the voltage of the firstgrid l2 by exactly 90 degrees whenever the input voltage varies at exactly the frequency to which the oscillatory circuit .55 is tuned. Frequency deviation of the input voltage applied to grid [2 results in .a change in the phase displacement between the voltages appearing on the first and second grids I2 and I4, and thus in a change in the average anode current. As a result, the output voltage appearing across output terminals 59 and 60 varies in amplitude in accordance with the frequency variation of the input signal applied between input terminals 50 and 5|.

The circuit described is substantially unresponsive to amplitude variations of the input signal because of the relations explained in connection with Figure 3. The response to amplitude variations of the input signal is further reduced by the substantially constant amplitude of the quadrature signal, which develops on the second grid I4 as a consequence of unilateral space charge couplingfrom the input grid I2 for all input signal .voltages exceeding a certain minimum. This stability of the quadrature voltage results from the] fact that the space charge which develops in the vicinity of the quadrature grid [4 is a function of the space current, and that in a tube constructedin accordance with this invention, thespace current varies between zero and a constant intensity.

The demodulator characterictic for the circuit shown and described in Figure 5 is shown in Figure 6 as curve 70. The demodulator characteristic for the'well-known combination of an amplitude limiter and a conventional double-diode phase sensitive discriminator is shown as the dotted curve II, the coordinate axes of curve I! having been displaced for the purpose of facilitating comparison between the two systems. The output of the demodulator of Figure 5 is similar to that of the double-diode discriminator within the substantially linear region, and the characteristic peaks of curve II are absent in the case of curve It. It is to be noted that the circuit shown in Figure 5 provides a wider linear region of operation than the conventional circuit, thus permitting improved tuning and greater output.

There is shown in Figure 7 an exploded perspective view, partly in section, of a combination limiter-discriminator tube constructed in accordance with the arrangement shown and described in connection with Figure 1. The electrodes'are supported between a pair of mica tube mounts 8t and 8| within an evacuated envelope 82, the customary getter 83 being provided for absorbing any residual ga after evacuation. The electrodes are brought out through a sealed. base 84 to respective terminals or pins 85.

Figure 8 is a transverse section taken at8-B of the view of Figure 7, and is included in order to show more completely the structure of such a tube.

Merely by way of illustration, and in no sense by way of limitation, it has been found that the following dimensions may be used in producing a tube of the type shown and described in connection with Figures 1, 7 and 8.

. Inch Width of cathode l0 .050 Width of slot II .030 Distance from cathode I0 to slot IL"--- .112 Length of extending portions I8 .047 Length of extending portions I9 .025 Width of input grid I2 .106 Distance from slot I1 togrid l2 -s .101 Width of slot 2! .050 Width of quadrature grid I4 .178 Distance from slot 2| to grid I2 .072 Distance from slot 2| to central portion 25 of grid l4 .093 Distnce from slot 2| to main portion of grid I4 .123

All electrodes have a longitudinal dimension of of an inch. All of the sheet metal electrodes are constructed of 4-mil stock. The inputgrid I 2 is formed by mounting 88 turns per inch of 1.5- mil Wire on a pair of supporting rods, and the quadrature grid I4 is constructed in a similar manner, using turns per inch of 2-mil wire. In addition, the accelerating electrodes II and I3 are internally interconnected, as are the cathode I0, the beam forming plate I6, the focusing electrode 20, and the shield 25, in order to minimize the number of external connections.

While the present invention has been shown and described in connection with a present preferred embodiment specifically suitable for use as a combined amplitude limiter and frequency demodulator in a frequency modulation radio receiver, and while the objects and advantages of the invention have been pointed out in connection with improved amplitude modulation rejection, one other very important advantage accrues from the use of the present invention. The

warped configuration of the quadrature grid results in a space charge saturation between the accelerating slot precedin the grid and the grid itself which is substantially independent of small changes in the degree of convergence or divergence of the electron beam impinging on the grid. In practice, and particularly in mass production, it has been found that small changes in the focusing of the electron beam result from varia,--

tions in the dimensions of and the spacing between the various electrodes within the tolerances practically attainable with mass production tech niques. .Such changes in the focus of the elecsuch as disclosed and claimed in my aforementioned co-pendingapplication. The present-in- .-verition,.however,.by automatically compensating forsuchsmall changes in:fcus,.resu1ts.in. a substantial-uniformity of operating characteristics between. individual tubes whilemaintainin practical manufacturing tolerances.

fIfhus, in accordance with the present 1 invention, and of, .coursewith. some sacrificeof trans- ..conductance, substantial. uniformity ofoperating ;characteristics...may.be obtainedin electrondischarge devices -comprisin a single;v control -sys-' .temsuch as; shownand-described in Figure .2 of

my aforementioned .coepending application.

5 .Therelisshown 'in- -Ei 'gure. 9 .a device..of this type incorporating the present invention. .The structureshown schematically in. Figure 9..is.-sub- .stantially .the same as the..first portionof .the

structure schematically showninFigure .1, the second control system of Figure .1: comprisingthe second accelerating electrode l3, secondfocusing means 25, second control grid l4, and anode [5 being replaced by a single anode 80. The configuration of focusing-meansZO issimplified to comprise a relatively large-aperture, -and =extending portions IBo'neither side-obslot l of-' the first -acclerating -electrode ll have been eliminated. r

In accordance with the present I invention, the

control grid.-l-2' is provided with a formed "or bulged central portion -8| extending toward the slot H inthe-accelerating electrode l-l. In-order to -maintain a-- substantiallyuniform accelerating field in'th'e region-between control; grid IZ -and anode--80, the anode is provided with a central bulged portion '82, substantially similar incontour tothe central-portion-tl -of-controlgrid [2.

While 5 there 1 have been shown and .described certain present rprefered embodiments of the-invention,; i s to .to be understood thatfnumerous variations-and -modifications may ,be made, .and

. it ris contemplated in .the appended 7 claims,- to

cover :all such modifications :as {fall within the true-spirit-and scopeoftheinvention. aI-claim: l I =l..--An electron discharge. device comprising .in the :order named an electron, gun. for projecting an: electronabeam; an accelerating eleCtrodehaving an aperture-registering withsaidbeamy-a control grid having a; bulged: central portion; extending toward said aperture-and spaced therefromlby adistance greater thanatleast oneof the; transversedimensionsbf said erture; and an additional electrode responsive .-to...an: applied unidirectional. operating potential for producing an. accelerating electrostatic field: adjacent .said grid on. the side thereof.remoterfromsaid electron gun. 1

.2. An electron discharge. device comprising. in V the order, named: .anelectron gun .forprojecting an electron beam; an accelerating. electrodeineluded. int-said electron gun. and. having.an...aperturefregistering with said beam; a control grid includinga .plurality .of substantially parallel conducting elementsdisposed .across the path. of said beam and .having abulged centrallportion extendingtoward said aperture andspaced therefromby a. distance greater than at 7 least one of the; transversedimensions of said aperture; and

an additional! electrode responsive .to. an. applied unidirectional operating potential Jifor producing an accelerating electrostatic field adjacent said .a -sheetelikef electron .beam of substantially rectangular ,-cross=section; an..accelerating electrode .having...a.,slot registeringwith said'beam; :a control. grid having .a. bulged. central. portion. extending-towardsaid slot-and spaced therefrom by a distance greater than -.the ,smallest transverse dimension .of (said slot and an. additional electrode responsive rtoiuan applied unidirectional operating otential. for producing an accelerating electrostatic .fi'eldadjacent said grid on the. side thereof, remote from said. electron gun.

..;5. .An. electron discharge device comprising in the orderrnained: an; electrongun for projecting a .sheetelike electron beam .of substantially rectangular.crossesectiomsan accelerating electrode included. insaid {electron gun .and' having a. slot registering Withsaid ,beam; ..a.. controLgrid in cluding a. plurality-pf. substantially parallel conducting elements disposed. acrosslthe path. of. said beam .and having...a .biilge'd central portion extending .toward said...slot.-. and spaced therefrom by a .distance reatenthan the.smallest transverse dimensionbf. said slot;..and an additional electroide resnonsivetoanapplied unidirectional operating potential for. producing an. accelerating electrostaticfield adjacent said grid on theside thereoiremote. fromsaid electron gun.

..6. Anf .eleotron discharge .device comprising in the, ordernamed -anLelectron. gun for projecting a sheet-like electron beam .ofsubstantially rectangularz cross-section; an accelerating electrode includedL-insaid.electron gun and having a slot registering.withsaidbeam; acontrol grid having a; bulged ..c'entr.a1 portion extending .toward said slotand spaced "therefromby a distance greater than .thesmallest ,transverse dimension of said slot; .Landananodehaving. a .contour. substantially similar. to that..of..said. grid.

.7. .;In an electron discharge .device,..a control systemcomprisingan apertured accelerating electrode followed .bya control grid having a. bulged central ,portion..-extendi-ng-.toward the aperture of. said accelerating electrode and spaced therefrom by-a;distance greater than at least one of thetransversadimensions. of said aperture.

8.. .In. .an electron .-,discharge device, a control system comprising aslotted accelerating electrode followedby a control gridhaving abulged central portion .extendingatoward the slot of said actoward the aperture of the preceding accelerating electrode.

10. In an electron discharge device, a pair of control systems arranged in cascade along the path of a single electron beam, each control system comprising a slotted accelerating electrode followed by a control grid at a distance greater than the smallest transverse dimension of the slot of the accelerating electrode, the control grid of the second system having a bulged central portion extending toward the slot of the preceding accelerating electrode.

11. An electron discharge device comprising in the order named: an electron gun for projecting an electron beam; a first accelerating electrode included in said electron gun and having an aperture registering with said beam; a first control grid spaced from said first accelerating electrode by a distance greater than at least one of the transverse dimensions of said aperture; a second accelerating electrode having an aperture registering with said beam; a second control grid having a bulged central portion extending toward the aperture of said second accelerating electrode and spaced therefrom by a distance greater than at least one of the transverse dimensions of the aperture of said second accelerating electrode; and an anode.

12. An electron discharge device comprising in the order named: an electron gun for projecting an electron beam; a first accelerating electrode included in said electron gun and having an aperture registering with said beam; a first control grid spaced from said first accelerating electrode by a distance greater than at least one of the transverse dimensions of said aperture; a second accelerating electrode having an aperture registering with said beam; a second control grid having a bulged central portion extending toward the aperture of said second accelerating electrode and spaced therefrom by a distance greater than at least one of the transverse dimensions of the aperture of said second accelerating electrode; and an anode of contour substantially similar to that of said second grid.

13. An electron discharge device comprising in the order named: an electron gun for projecting a sheet-like electron beam of substantially rectangular cross-section; a first accelerating electrode included in said electron gun and having a slot registering with said beam; a first control grid spaced from said first accelerating electrode by a distance greater than the smallest transverse dimension of said slot; a second accelerating electrode having slot registering with said beam; a second control grid having a bulged central portion extending toward the slot of said second accelerating electrode and spaced therefrom by a distance greater than the smallest transverse dimension of the slot of said second accelerating electrode; and an anode.

14. An electron discharge device comprising in the order named: an electron gun for projecting a sheet-like electron beam of substantially rectangular cross-section; a first accelerating electrode included in said electron gun and having a slot registering with said beam; a first control grid spaced from said first accelerating electrode by a distance greater than the smallest transverse dimension of said slot; a second accelerating electrode having a, slot registering with said beam; 3, second control grid having a bulged central portion extending toward the slot of said second accelerating electrode and spaced therefrom by a distance greater than the smallest transverse dimension of the slot of said second accelerating electrode; and an anode of contour substantially similar to that of said second grid.

15. An electron discharge device comprising in the order named: an electron gun for projecting an electron beam; an accelerating electrode having an aperture registering with said beam; a control grid having a bulged central portion extending toward said aperture and spaced therefrom by a distance greater than at least one of the transverse dimensions of said aperture; and an anode having a bulged central portion substantially similar to that of said grid and having marginal portions spaced. from said grid by a distance less than the spacing between said central portions.

16; An electron discharge device comprising in the order named: an electron gun for projecting a sheet-like electron beam of substantially rectangular cross-section; an accelerating electrode having a slot registering with said beam; a control grid having a bulged central portion extending toward said slot and spaced therefrom by a distance greater than the smallest transverse dimension of said slot; and an anode having a bulged central portion substantially similar to that of said grid and having marginal portions spaced from said grid by a distance less than the spacing between said central portions.

ROBERT ADLER.

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

UNITED STATES PATENTS Number Name Date 2,400,753 Haeff May 21, 1946 2,416,302 Goodall Feb. 25, 1947 2,433,634. Stone Dec. 30, 194'?

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