US2392155A - Electron beam tube - Google Patents

Description

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Y ELECTRON REAMTUBE x Filed June 27, 1940 3 Sheets-Sheet l l Pw `ian. 1946.

G. R. KiLGORE ELECTRON BEAM TUBE Filed June 27 1940 was 3 Shes-Shee't 2 ellllllbgg-j-T-w Jan. 1946. G. R. KILGORE ELECTRON REAM TUBE Filed June 27, 1940 3 Sheets-Sheet 3 INVENTOR GeoRef R. Knee/QE ATTORNEY acteristic of my invention are set forth with parascissa ELECTRGN BEAM TUBE George Ross lilgore, Verona, N. J., assigner to Radio Corporation o America, a corporation of Delaware Application June 27, 1940, Serial No. 342,625

15 Claims. (Si. 25u- 175) verse longitudinal section of one form of an electron discharge device made according to my invention, Figures 4 and 5 are enlarged sectional views of portions of the tube shown in Figure 3 and showing details of construction. Figure 6 is a transverse longitudinal section oi a modification or an electron discharge device'made ao- My invention relates to electron discharge devices, more particularly to such devices utilizing secondary emission electron multiplication.

Secondary emission multipliers of the voltage controlled type have oered a possibility of increasing the transconductance of amplifier tubes and more particularly the transconductance per unit capacity, which is recognized as the figure cordng to my invention and Figure 7 is a perof merit for Wide band amplication. However, spective o details of construction of the tube in tubes or this type where conventional space 1o shown in Figure S, and Figure 8 is a longitudinal charge control has been used in the input cersection of a still. further modification of my intain limiting factors are found. One is that the vention. ratio of transconductance to plate current In order to provide an electron discharge de- (gm /Ipl is comparatively low. lNow the currentvice or" the type described and having the charand transconductance increase together in the i5 acterstcs pointed out in the objects Of the nmultiplier and the maximum current is limited vention, I utilize, in accordance with my invenby space charge or sale dissipation of the secA tion narrow dense beams, that iS, beams 0f high ondary emitting surfaces so that the maximum current density combined with high deflection transconductance is limited by the h17m/Ip ratio. .sensitivity and secondary emission amplication. It is, therefore, desirable to obtain a control of go Whereas, in conventional beam tubes it is necthe electrons entering the multiplier which al essary to employ comparatively large currents lows a higher gm/Ip ratio. which make it diihcult to provide sharp beams It can also be shown that the signal to noise and high deflection sensitivity, by using a mulratio is a function of tiplier type of tube, smaller currents can be utig5 lized where the initial control is applied to the gl electron beam, resulting in greater deflection n u. n so that b increasm sensitivlty. rl'he smaller current in the beam r..-

jy g K duces space charge effects permitting a sharp if edge beam which with high deflection sensitivity so will result in a tube having a high transconductance. If high transconductance is obtained it is l. 4 possible to reduce the output capacity between ml'hel nmling ECGI'S in th@ COnvenional the screen and plate by reducing area of the control are the high input loading at ultra high screen and pmi-la The input apacity may be frequencies and the high capacities due to small 35 made Smaller than in conventional type tubes by electrode spacings. decreasing the deflecting plate capacity, which It 1s, therefore, an obJect of my invention to in deecton type beam tubes measures the inprovide an electron discharge device or" the mulput capacity' I accomplish this by having wider tipller type having improved .Operating Chala' spacing and smaller area for the deecting plates eristic More Spjecmcany it s a Objet 0f my 40 made possible by smaller initial current. This invention to provide such a tube having comin combination with'properly balanced voltages paratively high transconductance, low input and for reducing the transit time 0f the electrons output capacities and in which electron loading ast the deecting electrodes wm decrease the at the higher frequencies is small or substannput wading Gf the tum These various desir.. tilly Eliminatei i 45 able characteristics are obtained by the tubes de- A further Object 0f my invention iS 1X3 Provide scribed below and made according to my invensuch a tube having low noise characteristics. tion The novel features which I believe to be ohar- To better understand my invention the theory underlying my invention is discussed below. tlcularmf m the? appended Clams but the m' 50 Consider a sharply focussed rectangular beam Vention melf W1 best be understood by refer' deflected past a rectangular aperture in electrode ence to the following description taken in con- E at the and 0f a, pair of deecting plates p of nection with the accompanying drawings in length l and spacing d as shown in Figure -l. which Figures l and v2 are diagrams illustrating The transccnductance per cm. of length of such principles of my invention, Figure 3 is a. trans- 55 a device is equal to the product of the current in the proper manner a lower noise tube may also be produced.

2 essaies l' density at the aperture la by the deiiection sensitivity S. For the simple ease shown where V=average potential on the deiiecting plates with respect to the cathode potential. Now it can be shown that there is a theoretical limit to the current density which can be converged into a line focus from any electron source o! density Io and' this limit is given by the expression.'4

where e=electronc charge o=current density or source =angle of convergence of the beam T=temperature of the source V=energy of the electrons In the ycase of the deflection tube the angle e is small so that sin 0 is approximately d/z; thus the limiting current density becomes,

2Vs 5 d Ic=I Kr 2 (4) Combining (2) and (4) the following expression for the limiting transconductance is obtained,

Si mas-2%??? macs/cm. (5)

Three important facts vappear from this expression;

(l) The maximum om is independent of denecting plate spacing which means that the spacing can be made relatively large in order to i f semina/V The expression for am can then be written as aperfect ienssystemasharpedged image of ai can be' iormed at ai by adjusting the ratio of Il to Vg; and the current density at A: will depend on the current density at Af and the magnification ratio but not on the size oi' the aperture.

To obtain a sharp image the lens aberrations must be kept small. 'Eris can be done by keeping the total beam current iow to reduce space charge erlects, and by limiting the beam to a small fraction or the lens opening. The spreading oi the beam pawns through aperture A1 is due to three things; (l) space charge forces, (2) lens eect of aperture, (3) radial emission velocities. Now, except for extremely small apertures the most important diverging eect is that due to the lens action oi' tbe aperture. Assuming this to be the case and referring to Figure 2 the following approximate relation can be set up for the aperture lens eii'ect,

where K willdepend upon what fraction cf d is allowed for the spread of the beam. For practical cases, I believe, K can be taken as 2 in which case the beam width is roughly d/4 at the second aperture.

In my experimental tubes I have used apertures from .025 to .0025 cm. usually compromising on about .91 cm. I believe that with the smallest apertures I have used that the spreading due to the initial velocities is predominate so further reduction would not be o! much value. It is interesting that this size of aperture is also somewhere near the practical mechanical limit.

In Figure 3 the L-shaped exhausted envelope l0 is provided'with a cathode iI for supplying electrons and a beam forming and defiecting ar- D rangement comprising the tubular member. i2

From a consideration of the drop of deflection sensitivity with the transit angle through the deilecting plates it can be shown that the optimum transit time through the plates is roughly one half period.

It is dimcult to approach this theoretical limit of gm ior a number ci reasons including spreadtially cf an electron source X, a rectangular denins aperture A1 which is imaged on the nal rectangular aperture A: by the lens L. V1 and V: are the voltages applied to electrodes having apertures A1 and A: and delecting electrodes D; and Ds. The width of the aperture is W.

which shields the space in which the beam is formed and deflected and which supports the beam forming element i3 provided with limiting vaperture I3. A focusing element il forms with the deilecting electrodes i1 and i8 an electron lens for imaging the beam at the aperture i3' in the element i3, in the aperture i3 o! the intercepting electrode i8 which assists in the modulation oi the beam by electrodes i1 and I8 as will be described below. l

Positioned between the apertured elements I3 and il maybe placed a secondary emission screening electrode i! which may be used for preventing the returnof secondary electrons from the focusing electrode il to the aperture i3 in the apertured electrode i3 and thus setting up undesirable interference with the beam. The details of construction of the beam forming and deiiecting electrode, which is the control element, and the lens action is shown in more detail in Figure 4.- It will be seen that aperture i3' of the element Il forms and directs a beam of electrons through the aperture il' in the ele,-

ment i4. which with the deiiecting electrodes I1" and I8 provides an electron lens for focusing the beam on the aperture I8' of the element I8. The electrode I5 filters out electrons which have ianned out between the elements i3 and Il, thus maintaining a sharply dened beam to the electron lens by electrode il and deilecting plates i1 and I8. By the action of this lens the electron beam is focused into asharp dense beam at the aperture il'.

The deilection sensitivity at low frequency for ,ver

assenso idVi where I is the length of the plates with the aperture at the end oi the defiecting plates as shown in Figure l and d the spacing between. Hence oy making i longer the deilection sensitivity is increased. However, this increases the capacity but by increasing the distance d between the plates the increased capacity can be oiset by an increase in spacing. Thus, while the capacity is maintained at a small value the deection sensitivity is increased.

By means of my invention I obtain a dense beam which is narrow and sharp and in which the deflection sensitivity is high so that the resulting tranconductance is also high. The beam. after passing through the aperture i' is directed through the shielding and accelerating electrode i9 provided with aperture i9', the beam being directed to the first secondary emitting surface 20. and in succession to the secondary emitting surfaces 2i, 22, 23, and 24 and ilnaily being received by the output electrode e, these electrodes being maintained at successively higher potentials by the voltage sources 30 and 3i so that the electrons are continuously accelerated toward the output electrode 25.

.a voltage source 21 maintains the defiecting electrodes i'! and i8 at a higher positive potential than the cathode while the voltage source 2B maintains the tubular member i2 and elements i3, i4, l5, and i6 at a higher positive potential than the cathode. The input voltage is applied to the deecting electrodes by means of secondary of input transformer 2S, the output being applied to the output circuit 32. By-passing accomplished by condensers 33. 34, 35, 35, and

3l, the nrst three of which are preferably mounted inside the bulb for very high frequency operation.

In operation an electron beam is directed through the beam forming lens system and deflecting arrangement to the secondary emitting surfaces and the output received at the output electrode 25. As pointed out above, due to the increase of sensitivity very small currents can be initially utilized thereby making it possible to more easily control the beam and focus it into a sharp small beam, resulting in a tube having a higher transconductance and a higher ratio of transconductance to current which is a desirable feature. The input capacities are decreased by proper design and spacing of the delecting electrodes, for example, in a, tube made according to my invention 40 mil spacing is used instead of the usual 5 to 10 mil spacing of conventional grid controlled tubes. As pointed out above vbecause smaller output currents are used due to the high gm/Ip in the tube, a smaller output electrode and screen can be used which thus results in smaller output capacities. Input loading is reduced by reducing the transit time of the electron beam .between the deecting plates by applying proper voltages to these plates and by using plates which are not'too long.

In one form of tube constructed according to l my invention employing a live-stage electron multiplier arrangement, I obtained a gsi equal to 50,000 ambos with only l milliampere plate current which is considerably better than conventional tubes. The total input and output capacity was only 7 :wf-

Another improved i'orm of my invention is shown in Figures 6 and 7 in which a simplified improved gun structure and improved output system is shown. In this arrangement theenvelope encloses a cathode il and the beam forming and deiiecting system comprises a tubular member d2, the limiting apertured electrode i3, the lens element 4d, deiiecting electrodes E and il insuiatingly supported by glass beads i8 and apertured element i5 for modulation. An

element for assisting in the beam formation and electrically connected to the cathode is shown at 49. Shielding electrode 50 is mounted adjacent the end of the beam forming and deflecting arrangement and has an aperture registering with the aperture in the element e5. This element and the electrode 5| electrically connected to it are maintained at cathode potential so that electrons passing through the aperture in the electrode 50 ar deflected to the first multiplier electrode 52, the electrons moving to 53 and ed and output anode 55 as indicated by the dotted lines, output anode $6 being screened by screen electrode 5l lying along an equipotential of the multiplier system. As in connection with the tube described above. the apertured electrode 4 together with the deecting plates 46 and :31 form a. converging lens which is used to image the rst aperture 33' in element 3 on the final aperture 45 in the element 45. The

size and spacing othe apertures is so proportioned that a sharp image is formed in the iinal aperture. The length of the deiiecting plates may be chosen to obtain the desired maximum sensitivity at the predetermined frequency..

For example, tubes designed for maximum sensitivity at 500 megacycles have a static transconductance (gm) of 200,000 umhos with l5 ma. plate current and a gain of more than 20 to l at a frequency at 450 megacycles with a l0 megacycle band width, which is considerably better than present tubes.

A further improvement resides in the output electrode system in which the screen 5l is positioned on an equipotentiai surface of the multiplier system. A number ci leads and supports 58, 59 and S0 are brought out from the screen and the plate through theend of the tube. The construction shown gives good screening and low inductance and allows the leads to be extended, ii desired, into a concentric line circuit although in the gures shown the concentric line circuit is not used.

The beam forming and deiiecting members of the tube are maintained at a positive potential with respect to cathode 4i by means of voltage source 53 and the deiiecting electrodes 48 and i1 had a positive potential different from the rest of the beam forming and defiecting arrangement by voltage source S2. The input voltage is applied to deiiecting plates $6 and 4l by inductance 5l which may form the secondary of an input transformer. The secondary emitting elements are maintained at successively higher potentials by means of voltage source 66. Bypassing is provided by-means of condensers S6, 61, E8, 69, l0. and ll, which though not so shown are preferably mounted inside the bulb for ultra high frequency operation. The output circuit 65 is connected to the output anode 66 as shown.

In one specific example of my invention; the

cathode was provided .with a coated area of .5 x 8v mm. and separated from the first apertured electrode, for example, 43 in Figure 6 a distance of 3 mm., apertured electrodes il and 45 being spaced and 25 mm., respectively, in succession from each other. The aperture i3 in electrode 43 was '.l mm. x 5 mm., the aperture in ii l mm. x 10 mm. and aperture d5 .g5 mm. x 5 mm. The deilecting plates 46 and 31 were lo x lo mm. respectively and separated by a spacing of 1 mm. The cylindrical shield d2 and disc members had applied a voltage of 300 volts positive with respect to the cathode andthe deiiecting electrodes had applied a voltage of about 140 volts. The total current between the plates was approximately 300 milliamperes and through the final aperture 150 ma. giving a maximum gm of 1Go micromhos for the gun structure shown. This tube can be operated successfully up to a 4frequency of SUO megacycles.

In one form ci device made according to my invention and in which the transit time of the electrons is reduced. is shown in Figure 5. In this case the end of the beam forming and deecting arrangement, which may 'ce referred to as an electron gun, is placed on anecuipotential of the multiplier structure and with the proper potential so that electrons are introduced with the velocity they would have if they came from a preceding stage.l The advantage oi this input system is that the electrons are not slowed downI to zero and thus the transit time is small. In the form shown here the envelope lo contains cathode l, shield and focusing element '33, the

beam forming and deecting. structure comprising tubular member l2 and apertured electrodes 13, l-I, and l5, dedecting electrodes "i8 and l1 being provided intermediate elements 14 and 75. The element `l5 is positioned to lie along an equipotential and the secondary emitting electrode 78 and Bil being positioned to receive the beam, the anode Si being enclosed and shielded by the screen S2, both being supported by support and lead wires 83, 34, and 8S. The voltage sources or'pro'perly biasing the various electrodes are indicated at B1, 88, and 39, the input being provided through the secondary of thel transformer N and output circuit being indicated at di).

Tubes made according to my invention have shown remarkably improved characteristics, the transeonductance being comparatively high and ratio of transconductance to output current being high resulting in the numerous improvements noted above. Because o the improved ratio of transconductance to current. the noise factor is very much improved. The tube results generally in a more emcient tube for use at the ultra high frequency.

While I have indicated the preferred embodiments of my invention of which l am now aware and have also indicated only one speciilc' application for which my invention may be employed. it will be apparent that my invention is by no means limited to the exact forms illustrated or the use indicated, but that many variations may be made in 'the particular structure used and the purpose for which it is employed without departing from the scope of my invention as set forth in the appended claims. 4

1. An electron discharge device having a source of primary electrons, secondary emitting means for receiving said primary electrons and an electrode for receiving secondary electrons, means positioned between said primary electron source and said secondary emitting means for focusing and controlling the number o1' primary electrons impacting on the secondary emitting means, said focusing and controlling means for primary elecassenso trode for receiving secondary electrons. means positioned between said primary electron source and said secondary emitting means for focusing and controlling the number of primary electrons impacting on the secondary emitting means, said .focusing and controlling means for primary electrcns comprising a tubular member supporting a pair ci transverse aperturedelements with the apertures in alignment, de'ecting plates positioned between said apertured elements and closely adjacent one oi said apertured elements forming with said one apertured element an electron lens for providing a sharply imaged beam at the aperture of said apertured element nearest the secondary emitting means.

3. An electron discharge device having a source of'primary electrons, secondary emitting means for receiving said prunary electrons and an electrode for receiving secondary electrons, means positioned between said source oi' vprimary electrons i and said secondary emitting means and comprising a tubular member having a plurality of apertured electrodes positioned successively along its length and having the apertures aligned between the cathode and the secondary emitting means, a pair of defiecting electrodes positioned between two o! said apertured electrodes adjacent the secondary emitting means and being closer to the apertured element farthest removed vfrom said secondary emitting means for providing an electron lens for forming a sharply focused electron beam passing through the apertured electrodes to said secondary emitting means.

4. An electron discharge device having a source of primary electrons, secondary emitting means for receiving said primary electrons and an electrode for receiving secondary electrons, means positioned between said source of primary electrons and said secondary emitting means and comprising a tubular member having a plurality o apertured elements positioned successively along its length and having the apertures aligned between the source of primary electrons and the secondary emitting means, a pair of -deiiecting electrodes positioned between two of said apertured elements adjacent the secondary emitting means and .being closer to the apertured element farthest removed from said secondary emitting means for providing an electron lens for forming a sharply focused' electron beam passing through the apertured elements. lto said secondary emitting means. and a shielding and accelerating element positioned between the secondary emitting means and said last apertured electrode.

5. An electron discharge device having a source of yprimary electrons, secondary emitting means for receiving said primary electrons, means positioned between said primary velectron source and said secondary emitting means ior focusing and controlling the number of primary electrons impacting on the secondary emitting means, said focusing and controlling means for primary electrous comprising a tubular member supporting a pair of transverse apertured elements with the assente l apertures in alignment, deecting plates pcsitioned between said apertured elements and closely adjacent one ci said apertured elements and forming with one of said apertured elements an electron lens ior providing a sharply imaged beam at the aperture of said one of said apertures elements closest said secondary emitting means, an output anode positioned to receive the secondary electrons and having a surface inclined to the path of the electrons and a screened electrode positioned adjacent the anode and lying along an equipotential of said anode.

6. An electron discharge device having a source of primary electrons, secondary emitting means for receiving said primary electrons, means positioned between said primary electron source and said secondary emitting means for focusing and controlling the number of primary electrons impacting on the secondary emitting means, said focusing and controlling means ior primary electrons comprising a tubular member supporting a pair of transverse apertured elements with the apertures in alignment, deiiecting plates positioned between said apertured elements and closely adjacent one of said apertured elements and forming with said apertured element an electron lens for providing a sharply imaged beam at the aperture of the other apertured element. an output anode positioned to receive the secondary electrons and having a surface inclined to the path of the electrons and a. screened electrode positioned adjacent the anode, and lead and support wires extending through one -end oi' the envelopeand supporting said screen and anode.

7. An electron discharge device having a source of primary electrons, secondary emitting surfaces for receiving said electrons for releasing secondary electrons when impacted by said primary electrons, and an anode for receiving the secondinous screen in the path of the secondary elecrons and completely enclosing said anode, the apertured element in said beam forming amidenecting means lying along an equipotential surface adjacent the secondary emitting surface whereby the electron beam passing therethrough impinges on the secondary emitting surface without deceleration.

9. An electron discharge device having a cathode ior providing electrons, an electrode for receiving said electrons,y means positioned between the cathode and said electrode for receiving the electrons, said means for focusing and controlling the number of electrons impacting on the receiving electrode, said focusing and controlling means comprising a tubularmember supporting a pair of transverse apertured elements with the apertures in alignment. defiecting plates positioned beary electrons, a beam forming and dedectingrw mount positioned between the cathode and the secondary emitting surfaces and comprising an elongated tubular electrode surrounding the path of the electrons from the cathode to the secondary emitting surfaces and provided with a plurality of transverse aperto-.red electrodes, a pair oi' beam deilecting plates adjacent one of said apertured electrodes and providing therewith an electron lens, shield electrodes positioned at opposite ends of said beam forming and denecting mount to be maintained at a potential not greater than that of the cathode potential and a deecting electrode electrically connected to the shielding electrode adjacent the secondary emitting electrode whereby the beam of electrons emerging from the beam forming and deiiecting mount will be deflected toward the surface of the secondary emitting elements, said anode being of triangular cross section and a screen positioned between it and the secondary emitting means and lying along an equipotential of the anode established during operation of the tube. i

3. An electron discharge device having a source of primary electrons, secondary emitting means for receiving said primary electrons, means positioned between said primary electron source and said secondary emitting means for focusing the electrons into a beam and controlling the number of primary electrons impacting on the secondary emitting means, and including an electron lens and an apertured element. and means between the lens and the apertured element for deilecting the focused beam across the aperture in the apertured element, and an anode for receiving the secondary electrons, and a shield having a ioramtween said apertured elements and adjacent one of said apertured elements to form with said apertured element an electron lens for forming a sharply iocused electron beam at the aperture nearest the electron receiving electrode.

10. An electron discharge device having a cathode for providing electrons, an electrode for receiving said electrons, means positioned between the cathode and said electrode for receiving the electrons, said means for focusing and controlling the number of electrons impacting on thereceivi ing electrode, said focusing and controlling means comprising a hollow conductive member supporting a pair of transverse apertured elements with` the apertures in alignment with each other, deiiecting plates positioned between said apertured elements and adjacent one of said apertured elements to' form with said apertured element an electron lens for forming a sharply focussed electron beam at the aperture nearest the electron receiving electrode, the length of the deecting plates being substantially equal to the distance traveled by an electron in one half the period of the voltage applied to said deecting plates during operation oi the electron discharge device.

11. An electron discharge device having a cathode for providing electrons and means for receiving said electrons, means positioned between the cathode and the electron receiving means and comprising a tubular member having a plurality oi epertured elements positioned successively along its length and having the apertures aligned between the cathode and the electron receiving means, a pair or' denecting electrodes positioned between two apertured elements. adjacent the electron receiving means and being closer to the apertured element farthest removed from 'the electron receiving means for providing an electron lens for forming a sharply focussed electron beam passing through the apertured element nearer the electron receiving means.

12. .an electron discharge device having a cathode' for providing electrons and means for receiv. ing said electrons, means positioned betweenl the cathode and the electron receiving means and comprising a tubular member having a plurality of apertured elements positioned successively along its length and having the apertures aligned with each other between the cathode and the electron receiving means, a pair of deiiectlng electrodes positioned between two apertured elements adjacent the electron receiving means and being closer to the apertured element farthest removed from the electron receiving means for providing an electron lens for forming a sharply focussed electron beam passing through the apertilted element nearer the electron receiving

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