US2351757A - Electron discharge device - Google Patents

Description

June 20, 1944. F. GRAY ELECTRON DISCHARGE DEVICE Filed July 29, 1941 FIG. 2

INVENTOR E GRAY A T TORNEY Patented June 20, 1944 ELECTRON DISCHARGE DEVICE Frank Gray, East Orange, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N; Y., a corporation of New York Application July 29, 1941, Serial No. 404,467

6 Claims.

This invention relates to electron discharge devices and more particularly to such devices of the beam type and operable at ultra-high frequencies.

space charge tends to cause a spreading or divergence of the electron beam so that the current which can be transmitted through a slender tube is limited. Any electrons in the beam which, due to space charge efiects, are collected by the slender tube represent a loss and. this action results in a low operating efficiency and also in unstable operatingcharacteristics for the device.

One object of this invention is to improve the operating characteristics of electron discharge devices of the type wherein an electron beam is projected through a slender tube.

In the illustrative embodiment of this invention, an electron discharge device comprises a slender tube, a collector electrode or anode o-p- .posite or at one end of the tube, and an electron gun opposite the other end of the tube for projecting an electron beam thereinto.

In accordance with one feature of this invention, the electrode system constituting the electron gun, and the slender tube are so constructed and arrangedthat all of the electrons projected into the tube pass therethrough and are delivered to the anode or collector electrode.

The invention and the above-noted and other 7 features thereof will be understood more clearly and fully from the following detailed description with reference to the accompanying drawing in which:

Fig. 1 is in part a longitudinal view of an electron discharge device illustrative of one embodiment of this invention and in part a circuit diagram showing one manner in which the device may be operated; v Fig. 2 is a view similar to Fig. 1 illustrating another way in which the device may be operated in accordance with this invention; and

Fig. 3 is another view similar to Fig. 1 show- ,ing another illustrative embodiment of this invention. I

Referring now;to the drawing the electron discharge device shown in Fig. 1 comprises an evacuated enclosing vessel IU of vitreous material, having an elongated portion or tube H of relatively small diameter and at one end of which there is mounted a collector electrode or anode l2. Opposite the other end of the slender tube II is an electrode system, commonly referred to as an electron gun, including a cathode [3 having an electron emissive surface It, and a plurality of cylindrical electrodes l5, I6, I! and 18 mounted in coaxial relation with the cathode l3 and the tube l l. The cathode may be heated by a suitable heater element, such as a filament or coil l9. The electron gun, when the electrodes thereof are energized as described hereinafter, concentrates the electrons emanating from the cathode into a beam, circular in section, which is projected into thetube ll.

Externally of the tube I there may be mounted circuit elements for coacting with the electron beam to impart energy to the beam or conversely to extract energy from the beam in ways known in the art. For example, such elements may be one or more cavity resonators for producing a velocity variation of the beam or adapted to be excited by the beam to deliver power to aload circuit. One ofsuch cavity resonators is illustrated at 20 in Fig. land is toroidal in form and coaxial with the tube.

In order that an efiicient and effective coupling may be realized between the electron stream and the external circuit elements, such as cavity resonators, it is necessary that the tube H be of small diameter. However, inasmuch as, because of space charge, spreading or divergence of the electron stream occurs as it traverses the tube ll, if the tube diameter is made small, a number of the electrons in the stream may be collected by the tube so that low efiiciency operation and unstable operating characteristics would result.

It can be shown that the shape of a cylindrical electron beam traversing a space substantially free from external forces, for example a substantially field free space, such as the space in the tube II in the device shown in Fig. 1, that is the shape of the cylindrical sheath of electron paths that constitutes the boundary of the beam, is given by the equation:

at which the electrons are projected into the 1 space, Z is a coordinatetaken along the axis of the beam from the plane of its minimum cross section, 1' is a radial coordinate measured from the axis, To is the minimum radius of the beam in travers ing the space, and

e being the electroncharge. For a space, such as in the tube I I, having a diameter D and a'length L, if Z1 and n are taken as the coordinates on the periphery of the beam as it enters the spac Equation 1 may be written as ll 2 V3/4 Z1[7'1F(T0)] from which it will be seen that the maximum when has its largest permissible value and the quantity is a maximum. This quantity has a maximum value, it can be shown, when Inasmuch as the beam must not touch the wall of the tube I I and is symmetrical with respect to the plane of its minimum cross section, it follows that the largest permissible value of 1 occurs when the beam just fillsthe. end. efthe tube when it is projected into it and the minimum cross section of the beam. is located at the. midpoint of the tube. The maximum value of is, then, equal to Hence, the maximumcurrent'that can be-projected through a tube, such as the tube H, for any voltagevis given by the-equation! which equation is. obtained: by substituting. th

twomaximadeterminedabove for V and the function of in Equation 3.

' It will be noted from maximum current at a particular beamiv'oltage which 'canbe projected through a tube, such as the tube II 'in the device illustrated in Fig; ,1, without collection of any of the electrons by the tube, is a ,function of this" voltage an'dthe ratio of the tube diameter to the length of the tube? and that the attainment ofthis current Without'such collection of electrons by the tube necessitates a particular physical relation between the electron r'beam and the tube, namely that at the inlet end of the tube, the beam diameter must be substantially the same as the internal diameter of'the tube, and that the minimum cross section of the beam must occurat a regionmidway between the feature of this invention as described hereinafter.

' 3 A. current I is'a b The cathode I3 is, of course, constructed in accordancewith principles known in the art, so that the desired maximum current can be drawn f'romth'e emissive surface l4 thereof. As shown v in, Figil, thecylindrical electrodes l6 and I8 are connected together electrically by a conductor 2| and are maintained at a positive potential with respect to the cathode I3 by a suitable source, not shown, connected across the terminals22 whereby the desired potential of the projected .beam is oba the foregoing that'the tai'ned. The cylindrical electrode I5 is maintained at a negative potential with respect to the cathode I3 by a potentiometer including a resistance. 23 and a source, not shown, connected across the terminals 24. The cylindrical electrode I1 is-connected adjustably to'a resistance. 25 connected across the terminals 22. In. an illustrative case, the electrode l5xmay= be operated at a potential [Of the order of, BOO-volts negative with respect to the cathode I3 and the electrodes l6 and I8 may .be maintained at. a potential' of the. order of 2500 volts positive with respect to the cathode l3. The anode or collector electrode I2 is connected. directly to the electrode l8. v I

The electrode I5, itwillbeapparent, constitutes a; simple electronic lens, illustrated at L, in Fig. 1,

.and. the electrodesnIB, I1, and I8 constitute a compound; electronic lens,"i-llustrated at L2 in. Fig. 1." The focal points of these lenses'will bedependent upon the potentials of the several. electrodes constituting'the: lenses, as will the form of the electron beamz'projectedinto" the tube-I I. The form of the beam can be controlled by varyingthe potentials of the'electrodes l5 and I! to efiect adjustment. of the focal lengths of the lenses. a

: Now, .the maximum current 'obtainableis'calculable directly from Fig. 4, the volta'geV, diameter D and lengthL bein pf'known values. This current :is. then realizable in the actual device by adjusting the potentials of the electrodes I5 and 11 until the 'currentatthe collector electrode or anode I 12, as measured by a suitable instrument in circuit with theanode I2, corresponds to the .value determined from Equation' l.

-Thedetermination of the potentials of the electrodes'l5 and .I'I- requisite for 'the'attainment of the'maximum current maybe effected also with- "out recourse to; direct measurementof the current to the-anodes In devices ,whereinpotentials of the order mentioned *hereinabove are utilized, the fact that the beam may be of agreater cross I section: at the inlet endof the tube II than the internal diameter of this tube isdetermined by the phenomenon that insuch case the enclosing vessel of the device will fluoresceat the region adjacent the inlet end of the tube." Also the/tube 1 II will fluoresce' at regions at which electrons impinge. -Inlowervoltage devices or those in which thefvessel er a material which doesnot fluoresce at the'yolt'age's employed, the inlet end or thetube and thein'ner walls thereof may be coated withflu'orescent material. Hence, the requirement's'that the beam should just an theinlet end of the tube and should not impinge upon the tube may be met by varying the potentials of the electrodes I and I1 until no fluorescence of the tube and of the enclosing vessel adjacent the inlet end of the tube II is discernible and a small change in these values results in such fluorescence. A very small amount of inert gas may be left in the enclosing vessel of the device so that the form of the beam in traversing the tube II will be observable by the glow produced by the beam. The conditions for maximum current are obtained by varying the potentials of electrodes I5 and I! until, as determined by observation of the beam glow, the beam just fills the inlet end of the tube II and the minimum cross section thereof appears at a region midway between the end of the tube I I.

The device disclosed in Fig. 1, it will be noted, involves two .controls, namely of the potential of the electrode I5 and, that of the electrode IT. The invention may be embodied also in devices involving three controls, one of which devices is illustrated in Fig. 2 and is of generally the same construction as the device illustrated in Fig. 1. In the device shown in Fig. 2, however, the electrode I6 is provided with a separate leading-in conductor adjustably connected to the resistance 25 so that the potential of the electrode I6 may be varied. As in the device shown in Fig. 1, the potential of the electrode I8 is made such that the beam projected into the tube II is of the desired potential. The potential of the electrode I5 is made such that the required current is drawn from the emissive surface I4 of the cathode. The requisite form of the beam to give the maximum current to the anodes or collector electrode I2 as determined by Equation 4 is obtained by adjustment of the focal lengths of the electrostatic electron lenses L1 and L2 in the manner described heretofore in connection with Fig. 1.

The invention may be embodied also in devices utilizing a magnetic electron lens. One illustrative device is shown in Fig. 3 and is of generally the same construction as the devices shown in Figs. 1 and 2 except that it comprises three cylindrical electrodes I5, I6 and I1 and a magnetic coil 26 encompassing the vessel II] for producing a field coaxial with the longitudinal axis of the enclosing vessel III of the device. The coil 26 may be energized by a suitable source, such as a battery 21 connected thereto through a variable resistance 28. The electrode I5 is maintained negative with respect to the cathode as in the devices illustrated in Figs. 1 and 2 and the electrodes I6 and I! are maintained positive with respect to the cathode by suitable sources, not shown, connected across the terminals 29 and 36.

In the device illustrated in Fig. 3, the electrode I1 is maintained at a fixed potential to provide the desired voltage on the electron beam projected into the tube II. The electrode I5 functions primarily to control the beam current and the potential thereof is made such thatthe requisite beam current is drawn from the cathode I3. Incidentally, this electrode acts as a weak electrostatic electron lens L0 to cause convergence to a small extent only of the electron stream. The electrode I6, the potential of which is adjustable, defines a strong electrostatic lens L1, and, as noted above, the coil 26 acts as a second lens L2. As in the devices illustrated in Figs. 1 and 2, the two lenses L1 and L2 in the device shown in Fig. 3 have an appreciable separation and the focal lengths thereof are adjustable by varying the po- 76 tential of the electrode I6 and the current through the coil 26. By proper adjustment of these focal lengths, as in the manner described heretofore in connection with Fig. l, the requisite form of the projected beam and its relation to the tube II for the transmission of the maximum current through the tube II are obtained.

It will be appreciated that this invention enables the construction of electron beam discharge devices which will transmit a desired current at a desired voltage and wherein the tube II will be of the minimum diameter permissible with transmission of the maximum current therethrough so that not only is a close and efiicient coupling obtained between the beam and external circuit elements, such as thecavity resonator'2fl.

but also maximum efliciency and stable operating characteristics are realized.

Although the invention has been described with reference to particular embodiments wherein the tube constitutes a portion of the enclosing vessel, it is obvious that it may be embodied in other constructions, for example, constructions wherein the tube is mounted within the enclosing vessel of an electron discharge device. It will be understood also that various modifications may be made in the devices shown and described without departing from the scope and spirit of this invention as defined in the appended claims.

What is claimed is:

1. An electron discharge device comprising an elongated tube, an electron receiving electrode opposite one end of said tube, and means opposite the other end of said tube for projecting an electron stream thereinto and coaxial therewith, said means including a cathode and a pair of electron lenses in cooperative relation with said cathode and having such focal lengths that at said other end of said tube the electron stream has substantially the same cross section as said tube and the current transmitted through said tube is substantially equal to where D and L are the diameter and length, respectively of said tube and V is the voltage at which said stream is projected into said tube.

2. An electron discharge device comprising an elongated tube, an electron receiving electrode opposite one end of said tube, a cathode opposite the other end of said tube, and means for concentrating electrons emanating from said cathode into a stream coaxial with said tube and having a minimum cross section substantially midway between the ends of said tube and a cross section :at said other end of said tube substantially equal to the cross section of said tube, said concentrating means including means defining a pair of coaxial, axially spaced electron lenses between ;said cathode and said other end of said tube and where D and L are the diameter and length, respectively, of said tube and V is the potential at "which the beam is projected into said tube, said :means comprising a cathode, means including a cylindrical electrode adjacent said cathode defininga first electron lens coaxial with said tube and means, defining a second electron lens between said first lens and said, tube, said lenses being coaxial with said tube and axially spaced. y H 4. An electron discharge-device comprising an elongated tube; an electron receiving electrode oppositeone end of said tube, a cathode opposite the other end of said tube and coaxial therewith; and an electrode system for; concentrating elec: trons emanating from said cathode into a stream projected into said tube and having, at said other end of said tube, a cross section substantiallyequalto the cross section of said tube, and having i a minimum cross section substantially midway between the ends'of said tube, said system comjecting an electron stream into; the otherendof csaid'tubeand coaxial with said tube, saidmeans comprising an electrode system coaxial-with said tube, and opposite the other end thereof and ineluding a cathode and a cylindrical electrode defining a first electron lens adjacent said cathode, said means comprising also a magnetic coil between said cylindrical electrode and saidother stream is concentrated within a boundary having,

" at said other end offlsaid tube, a cross section enclosing vessel having elongated, vitreous cylindrical portion, a collector electrode opposite end of said tube and defining-a second electron lens, and said first and second electron lenses having such focal lengths that said electron substantially equal to that of said tube and the current transmitted through said tube is substantially equalto where -D and L are the diameter and length respectively of said tube and Vlis the voltage at whicl'i'said stream is projectedinto said tube. 1 a

6 An e1ectrondi'scharge device comprising an one end of said cylindrical portion, 'andan electrode system opposite theotlier'end of saidpo'r tion and coaxial therewith for iprojecting an electron stream into said portion, said system comprising a cathode; a positive electrode for drawing froni'saidcat'hode a current of substana yno a v aswa z @3 10- where D and L are the diameter and length respectively oi said portion and V is the potential at which said stream is projected into said portion and means including a plurality of electrodes coaxial with said portion defining a pair. of axially spaced electron lenses having. such focal lengths that at its planeof injection into said portion said, stream is of substantially the same cross' section as said portion and all of said current is transmitted through said portion.

GRAY.

Images