US2782339A - Electron beam amplifier device - Google Patents

Description

Feb. 19, 1957 7|. 5; NERGAARD ELECTRON BEAM AMPLIFIER DEVICE Filed Jan.

3 Sheets-Sheet 2 YJIRL INVENTOR LEON -S. NERGAARD BY 7M ATT NEY.

Feb. 19, 1957 L s. NERGAARD. 2,782,339

, ELECTRON BEAM AMPLIFIER nsvzcs,

Filed Jan. 7, 1949 3 Sheets-Sheet 3 E lNVEINTOR LEON S NERGAARD ATTO EY United States Patent ELECTRON BEAM AMPLIFIER DEVICE Leon S. Nergaard, Princeton, N. 3., assignor to Radio Corporation of America, a corporation of Delaware Application January 7, 1949, Serial No. 69,634

8 Claims. (Cl. 315-4) This invention relates to electron discharge devices and systems, and more particularly, to traveling wave tubes useful as amplifiers in relay systems.

in a conventional traveling wave tube an electromag netic wave is coupled to one end of an elongated conducting helix of such diameter and pitch that the axial velocity of the wave along the helix is reduced to a fraction, say one-tenth, of the velocity of light, and an electron beam is projected along the helix, either inside or outside, at a velocity approximately equal to the axial wave velocity. Under such conditions, the electron beam interacts with the wave on the helix to cause the amplitude of the wave to grow exponentially, and hence, produce amplification.

At traveling wave tube has been proposed by C. W. Hansell, in a copending application, Serial No. 83,697, filed March 26, 1949, now Patent No. 2,684,453, dated July 20, 1954, assigned to the same assignee as the in stant application in which the conventional helix is replaced by a second electron beam. This proposal has several attractive features. One is that the tube is all electronic and need contain none of the circuitry. The second attractive feature is that, because of the forward motion of the two beams, it seemed unlikely that there would be a backward Wave. The absence of a backward wave would render unnecessary the use of circuit attenuation now used in conventional traveling wave tubes to keep regeneration within reasonable bounds. The attractiveness of these features is, however, contingent upon the gain per unit length of tube which can be achieved with practical beam currents and voltages.

In accordance with my invention, a signal is amplified by means of longitudinal or plasma oscillations in two electron beams of different velocities. When the relative plasma frequencies and relative electron velocities in the two beams are properly adjusted, the interaction between the two beams leads to a growing wave.

A simple example which suggests the possibility of obtaining a growing wave in this'way is the following:

Consider a stream of uniformly spaced electrons proceeding in the xdirection with uniform velocity 11,. If one of these electrons is displaced in the a c-direction from its equilibrium position, it will be acted upon by a restoring force arising from the fields of other electrons. This force is proportional to the electrons dis; P eme f m ilib um bsi sa at eas f s e l displacement-s. Hence, the electron will perform a s imple harmonic motion about .its equilibrium position, frequency of oscillation will ibedeterminedby the average spacing between electrons, i. e. by the average charge density in the beam.

Unless otherwise specified, all quantities will be expressed in electrostatic units. In these units, the specific relation between the angular frequency o, and the D. G. c r e ensity of e beam is Patented Feb. 19, 1957 where ag, is 21r times the natural frequency and e and m are the, charge and mass, respectively, of the electron. This frequency w, will be referred to as the plasma frequency, as named by Tonks and Langmuir, Phys. Rev. 3 3, (1928), during their work on oscillations in gas discharges. Suppose this beam of uniformly spaced charges passes through a very narrow gap at x= 0, and that each electron receives an impulse mAva- 1 in. crossing the gap, where Av is the change in velocity, and V is the voltage across the gap. Then each electron will execute a simple harmonic oscillation with an amplitu e e e m ned by e. mr e a f t i h displacement of an electron from its equilibrium position at a time t, the motion of the electron is given by Because the electron drifts a distance x=u,(t t in a time (1t,), in which t is the time at which the electron under consideration crosses x= '0, the space distribution ofdisplacements is i 6V a: e V 21m:

= m SID f mw u U1 mw u A1 where A, is the wavelength in the beam; This distribution of charge gives rise to an electric field 40V win 1 11.1

Hence, the apparent angular frequency w of this force is given by The resonant or plasma frequency of an electron in the second beam is 01,. Suppose this frequency is adjusted to equal the apparent frequency to so that w d g u u Then an electron in the second beam will behave like a resonant system driven at its resonant frequency and the amplitude of oscillation will build up linearly with time. Of course, as the electrons in the second beam start to oscillate, they produce an electric field which reacts back on the first beam and gives rise to a rather complex be; havior. However, the initial linear rise in amplitude suggests strongly that some more complex manifestation of this mechanism may lead to a growing Wave when the voltage V across the gap is made periodic in time. It is interesting to note that the relation & 7 i-uz which gives the initial linear rise in amplitude in this simple example is one of the conditions for maximum gain in the more elaborate analysis below. The other condition is current densities and velocities.

where w is the angular frequency of the signal wave to be amplified by the tube.

The principal object of my invention is, therefore, to provide a novel traveling wave tube having two electron beams which interact with each other by means of longitudinal or plasma resonance of the electrons forming the two beams.

Another object is to provide a system incorporating such a tube.

A further object is to provide a novel method of operating a two beam traveling wave tube and associated circuit.

These and other objects and advantages of the invention will be apparent from the following detailed description in connection with the annexed drawings, in which:

Figs. 1, 2 and 3 are graphs referred to in the description of the invention;

Fig. 4 is a plan view in axial section of one embodiment of an electron discharge device made according to my invention; Figs. 5 and 6 are transverse section views taken on the lines 55 and 6-6, respectively, of Fig. 4;

Fig. 7 is a detail view taken on the line 77 of Fig. 4,

showing the two grids;

Fig. 8 is a schematic diagram of a circuit for the de- 2 from the mechanism contemplated above, a simple model of a two-beam tube was analyzed. The model consists of two admixed beams, infinite in extent, with arbitrary The velocity vectors are assumed to be in the same direction. All D. C. charge efiects are neglected. Let the beams be designated as beams 1 and 2, and let the symbols pertaining to beams 1 and 2 carry the subscripts 1 and 2, respectively. Further, let

p=D. C. space-charge density of the electrons o'=A. C. space-charge density of the electrons u=D. C. velocity of the electrons v=A. C. velocity of the electrons E=A. C. electric field w='\/i=plasma frequency Let the velocities of both beams be directed in the x-direotion. Then the equations of motions of the electrons in the two beams are The equations of continuity in the two beams are Finally, the divergence theorem is It should be noted that all second order terms have been I dropped in the above equations."

Now assume that all A. C. quantities are periodic in time with the signal frequency w/21r, and also assume that the space distribution of A. C. quantities may be represented by e, where I is the propagation constant. Then the above equations become This equation gives the values of I for which solutions of this above set of equations exist. Because the secular equation is a quartic, there will be four possible values of 1. The possible values of the I"s depend on two parameters which are conveniently taken to be In practical terms and units, these may be written 05 v wit w .12 V1 V2 1 h y/2 W. V. 1 V1 1/4 V2 l/4 et a) a) j1=current density in beam 1 in amperes/cm. jz=current density in beam 2 in amperes/cm. V1=D. C. velocity in beam 1 in electron-volts V2=D. C. velocity in beam 2 in electron-volts where where Another relation of interest is in the same terms and units.

An analysis of the behavior of the four possible Ps as functions of the parameters on and 8 shows that for certain ranges of these parameters, two of the possible I"s are complex. When any 1 is complex, and in particular has .the form I=I1-iIt, the corresponding exponential factor e will take the form H E-I'm: rm: H31: 0 ==e e The factor e is a function which increases exponentially with the distanct x. Hence, when 0: and p are such that one of the I"s is complex, all the A. C. quantities pertaining to the two beams increase exponentially with x and a growing wave exists.

The complete analysis isypresented in my paper'entitled Analysis of a simple model of a two-beam gr Wave tube, RCA Review, December 1948, vol. IX, No. 4', pages 585-601. This analysis shows that the power gain per unit length of the beams is given by G=8.686I'i (db per cm.)

It is convenient to express the gain in terms of a dimensionless quantity X which is defined by the relation The gain function X is plotted as a function of a with 8 as a parameter in Fig. 1 and as a function of 1/ c and withp as a parameter in Fig. 2. It' will be noted that the gain function has a maximum value of /2 for a-l.l and 5:1. The exact values of a and B for maximum X, i.- e., maximum gain, as determined by the mathematical solution of the secular equation given above, are oz=2/ 3 and p=1. Substituting these values in the above definitions of or and ,8 gives the following conditions for maximum gain.

When the first of these relations is expressed in terms of the beam current densities and voltages, it becomes 1'1 J'z V'13/2.' V23/2 Hence if '1 and ]'2 are obtained from planar diodes of the same spacing or, more generally, from diodes of the same perveance per unit area, then the relation 1 is satisfied whatever the voltages V1 and V2 are. The perveance of a diode is the ratio of the diode current to the 3/2 power of the diode voltage.

An examination of Fig. l or Fig. 2 shows that the possible gain diminishes as ,8 departs from the optimum value of unity. It will also be noted that for each value of [3 there is an optimum value of oz for maximum gain.

The parameter a was defined as where Xrnax is the maximum value of X for a given ,8 and a is the corresponding 0:. By writing in which Aw b andwidth X 211' grams the following formula for the fractional bandwidth may be deduced V IDHX where Gmax is the total power gain of the tube when a= -a This formula shows that the fractional bandwidth varies inversely as the square-root of the total power gain, and with 13, which determines the value of j1=0.l00 ampere per square cm. V1=5OO volts 7 0): l .88 X radians per second L=30 cm.=length of tube Then the theory yields jz=0.072 ampere per square cm. V2=403 volts G=4.0 decibels per centimeter Gmax= decibels=total power gain Aw/21'r=860 mc. =bandwidth From the analysis which has been outlined above, it is clear that (1) by the method of operation utilizing the interaction of two streams of electrons does lead to growing waves, (2) the conditions which lead to growing waves can be realized with practical .current densities and voltages, (3) the rate of growth of the waves with. practical structures can be made adequate to give useful power gains, and (4) the bandwidths obtainable with these power gains is more than adequate for most communication systems.

Some tube structures which utilize the method of operation described above are shown in Figs. 4 through 12. Figs. 4-6 show one form of tube according to the invention which produces a growing wave from an initial. modulation produced by space charge control grids in two electron beams from two cathodes disposed on opposite sides of the tube axis. The two cathodes and two grids lie in concentric spherical surfaces and produce two beams which merge within an elongated hollow conductor which encloses the interaction space.

As shown in Fig. 4, the tube envelope comprises a cup-shaped insulating member 2 which contains the two electron gun structures 3 and and is connected by a metallic sealing ring 5 to an elongated cylindrical metallic drift tube 6. The opposite end of the tube 6 is connected in sealed relation to a coaxial cup-shaped metallic anode or collector 8 by means of metallic sealing rings 9 and 10 and an insulating ring 11. Theanode 8 is spaced from the tube 6 to form an output gap 12.

The two gun structures 3 and 4 comprise two cathodes 13 and 14 in the form of spaced segments of a spherical surface having its center on the longitudinal axis of the tube 6, two cathode heaters 15, and two grids 16 and 17 consisting of two sets of curved grid wires 18 mounted on two semi-circular mounting members 19 and 20 as shown in Fig. 7. The grid wires 18 lie in a spherical surface concentric with that of the two cathodes to, provide equal spacing between the cathodes and grids of the two diodes. The cathodes and grids are supported on a button stem 22 forming the base of cupshaped member 2 and their spacing is accurately maintained by the stem leads 24 and suitable supporting rods 25 and insulating beads 26. The stem leads 24 are so arranged. that the input circuit may comprise a distinct circuit for each diode.

The input circuit comprises a cylindrical conducting shell 28 divided along a longitudinal axial plane into 7 two compartments 30 and 31 by a central conducting bafile 32. A tubular conductor 33 is mounted within each of the compartments in a plane normal to the baffle 32 and is capacity coupled to one of the grids 16 and 17 by an insulating sleeve 34, a connector 35 and stem lead 35?, as shown. Each compartment is closed at the end remote from the evacuated enclosure by a disc 36 which is apertured to receive and support the tubular conductors 33. The two cavity resonators formed by the shell 28, the central baflle 32 and the two tubular conductors 33 may be tuned by separate apertured shortcircuiting pistons 37. The input signal may be coupled into one or both of these resonators by means of a coaxial line 37' as shown in Fig. 4. A disc or header 38 is positioned within the shell 28 adjacent to the button stem 22, the header being apertured at 39 to clear the grid, cathode and heater leads. The cathode and heater leads 24 are by-passed to the header 38 by condensers consisting of metallic strips 40 attached to the header by screws extending through an intervening mica half ring 41 disposed on each side of the baffle 32, with insulating bushings 42 to prevent short circuit of the strips to the screws and header. The strips 40 are brought out through apertures 44 in the shell 28 to permit separate D. C. connections to be made to the cathodes and heaters. The D. C. connections are made to the grids by means of leads extending through the conductors 33 and connected to the connectors 35. The structure described gives the four degrees of freedom necessary to adjust the two D. C. beam velocities and two plasma frequencies to the optimum values for the desired method of operation, and makes it possible to apply the input signal to be amplified to either or both of the beams, and to adjust the phase of the signal impressed on one beam with respect tothat of the other beam so that maximum gain results.

The drift tube 6 enclosing the interaction space is coupled to the cathodes 13 and 14 by means of a transmission line or cavity resonator comprising the tube 6, an extension 28' of the shell 28, the header 38, the by-pass condensers 40, 42 and the leads 24. This line may be tuned to an electrical length of M2, wherein the tube 6 is effectively short-circuited to the cathodes, by a shortcircuiting ring slider 45. It may be desirable to tune this line to produce regeneration to enhance the gain of the tube. The tube may be made to act as an electroncoupled oscillator by adjusting this line to produce selfsustained oscillation in the input circuit and taking the amplified output from the output circuit. To produce a more complete admixture of the beams, together with a longitudinal focusing to produce an axial beam, it may be desirable to use a short magnetic lens in the plane A-A of Fig. 4.

The output tank circuit may consist of an annular cavity resonator surrounding the gap 12 and through which the electrons pass. The resonator shown comprises a hollow cylindrical shell 46 surrounding the rings 9, l and 11 and connected to the tube 6 and anode 8 by flanged rings 47 and 48. The shell 46 may be split as shown to facilitate assembly. Output energy may be obtained from the output resonator by suitable means, such as the coaxial line 50, 51 and coupling loop 52 shown. In some cases it may be desirable to increase the collector efiiciency by operating the collector 8 at a potential lower than that of the drift tube 6. This may be accomplished by interposing a coaxial capacitor being the collector 8 and the ring 48, for example.

The circuit diagram of Fig. 8 shows a convenient arrangement of the voltage supplies for the tube shown in Figs. 4-7. The voltages E01 and E82 between the cathode and grids control the D. C. space charge densities p1 and p respectively, and hence, the plasma frequencies m and m of the two beams. The voltage EB between the cathode 13 and the drift tube controls the D. C. velocity in of one beam, and the voltage AEB controls the potential difference between the two cathodes and (with En) controls the D. C. velocity as of the other beam. 'It should be noted that the entire exterior of the circuit shown'in Figs. 4 8 is at ground potential, so that none of the line couplings or tuning controls require blocking capacitors or other insulating means.

In the embodiment of the invention shown in Figs. 9-11 the signal to be amplified is impressed on the two beams by velocity modulation. In addition, this tube employs an electron gun structure, comprising two concentric annular cathodes and a single grid, which automatically maintains the condition for maximum gain:

- liq 1L2 1 and employs a novel output system which providesa means for exploiting the large electronic bandwith of the tube.

Referring to Fig. 9, a cup-shaped insulating member containing the electron gun structure is coaxially connected by a metallic sealing ring 61 to a relatively short cylindrical metallic tube 62 which, in turn, is connected to an elongated cylindrical metallic drift tube 64 by metallic sealing rings 65 and 66 and an insulating ring 67. The ring 65 may be an extension of the ring 61, as shown. The outer end of the drift tube 64 is connected to a cupshaped metallic anode or collector 68 by means of two metallic sealing rings 69 and an elongated insulating tube 70 which forms part of a novel output system to be described later.

The gunstructure comprises two cathodes 72 and 73 in the form of spaced concentric annular segments of a spherical surface having its center along the tube axis, cathode heaters (not shown) and a single grid structure. The grid structure is made up of .a mounting ring 75 to which are attached uniformly spaced grid wires 76 which lie in 'a spherical surface concentric with that of the two cathodes as in the embodiment of Figs. 4-8. The two cathodes and the single grid are maintained in spaced relation and are mounted on the button stem by stem leads 78, supporting rods 79 and insulating beads 80. The stem leads 78 provide means for making separate D. C. connections to the cathods, heaters and the grid. Since the two cathodes have the same spacing to the grid, they form with the grid a pair of diodes having the same perveance per unit area. Hence, the relation Ti 1L2 is automatically satisfied at the surface of the grid, whatever the diode voltages. As the beams pass through the grid and merge within the drift tube enclosing the interaction space they depress the potential within the drift tube. This effect has been minimized by using annular cathodes so that the admixed beams lie close to the drift tube. To compensate for the remaining unavoidable depression of potential, the drift tube is insulated from the grid so that its potential may be raised above that of the grid until the beams travel with the velocities they had at the grid. Thus the relation fl-fli u u is maintained throughout the interaction space.

The signal is impressed on the beams by means of an annular cavity resonator structure 82 surrounding the gap 83 between the tubes 62 and 64. This resonator structure is formed by the adjacent ends of the tubes 62 and 64, a pair of flanged rings 84 and 85, a cylindrical shell 86 and an annular tuning slider 87. A coaxial line coupling loop 89 is provided to introduce the signal to the resonator.

The novel output system shown in Fig. 9 consists of two coaxial metallic helices 90 and 91 positioned inside and outside of the insulating tube 70. The inner helix diameter throughout its length. The outer helix 91, which is connected to the inner ring 69 and to the output line 92, is tapered along its length in such manner that the ratio of the diameters b and a of the two helices varies as E22 E: 60 6 L in which We is the ratio of the phase velocity along the line to the phase velocity in free space, Z is the surge impedance of the tapered line at x=L, and L is the length of the tapered line. Both helices are proportioned so that the velocity of propagation along the line is approximately equal to the average velocity of the electrons in the beams. In theory, this coupling network (1) has a phase velocity equal to the velocity of the electrons in the beams, so that energy may be extracted continuously from the beams over a long distance, (2) displays a constant impedance to the beams, of a character appropriate to the extraction of energy therefrom, and (3) presents a resistance to the output line at its output terminals, so that if the surge impedance of the line is made equal to this resistance no reflection will occur. The result is an output system which, in theory, has a bandwidth limited only by the electronic bandwidth of the tube. Because it is impossible to build a tube and circuit which conforms exactly to theoretical conditions, or to formulate an exact theory for a practical system, the practical performance will be somewhat short of the theoretical performance. However, the bandwidth obtainable with this coupling system is expected to far exceed that of currently used coupling systems. It should be noted that this output system is not limited to the particular use shown herein, but instead, may be applied to conventional traveling wave tubes employing a single beam by suitable modifications. Moreover, a similar coupling system may be used as an input system in place of the input systems of Figs. 4-8 or 9-11, or in conventional traveling wave tubes.

As shown in the circuit diagram of Fig. 11, the total voltage Be and EB between outer cathode 72 and the drift tube 64 controls the D. C. velocity of the outer beam, and voltage AE controls the potential difference between the two cathodes and (with E0 and EB) controls the velocity uz of the inner beam. The voltage EB is a small voltage which corrects for space charge depression.

Fig. 12 shows another embodiment of the invention which employs an electron gun structure which automatically makes u u and has the new and improved coupling system at both the input and output ends of the tube. The electron gun structure is the same as that of Fig. 9. The sealed enclosure is formed by the cup-shaped insulating member 60, sealing ring 61, a relatively short flanged metal tube 101, an insulating tube 102, a metal drift tube 104 flanged at both ends, a second insulating tube 105 and a flanged metal anode or collector 106. The input system comprises a uniform metal helix 103 disposed inside the tube 102 and having a pitch and diameter such that the phase velocity of the wave along the helix is equal to the average velocity of the electron beams, and an exponential metal cone 109 coaxially disposed outside the tube 102. The cone is tapered to conform to the relation given above in connection with Fig. 9. The smaller end of the cone 109 is clamped to one flange of the drift tube 104 by means of rings 110 and 111 and the other 10 end is grounded through a metal ring 112 to the tube 101. The helix 108 is connected at its output end to the drift tube 104 and has its input end brought out through the tube 102 to an input coaxial line 114. The output coupling system is similar to the input system except that the output terminal of the helix 115 is connected to the anode 106, and the anode and the exponential cone 116 are connected to the inner and outer conductors 117 and 118, respectively, of a coaxial output line extending along the axis of the tube.

It will be noted that in each of the embodiments shown the two electron beams are directed in the same general direction throughout the major portion of the length of the drift tube.

Although several specific embodiments of the invention have been described for purposes of illustration, it will be apparent that many variations may be madein the particular structures employed Without departing from the scope of the invention as defined in the appended claims.

What I claim as new is:

1. An electron discharge device comprising an envelope containing means for producing two convergent electron beams, said means including a pair of spaced adjacent cathodes, a pair of spaced grids respectively one adjacent each of said cathodes and separate leads connected to each of said cathodes and grids and extending through said envelope to permit the application of different potentials to said cathodes and grids for imparting difierent velocities and space charge densities to said beams, separate tunable cavity resonator means coupled between the leads connected to each cathode and the grid adjacent thereto, means coupled to each of said cavity resonator means for applying a signal thereto, an elongated drift tube adjacent to said grids and through which said convergent beams are directed during operation of said device, and a collector disposed in the path of said beams beyond said drift tube.

2. An electron gun structure for producing two convergent electron beams of different velocities along intersecting paths comprising cathode means and grid means located adjacent said cathode means and uniformly spaced therefrom, whereby the perveance per unit area is uniform over the surfaces of said means, said cathode means being divided into two separate adjacent parts to permit the application of different potentials thereto for imparting different velocities to said beams, the emitting surfaces of said cathode parts being so oriented relative to each other that said two beams will merge during operation of said gun structure.

3. An electron gun structure for providing two electron beams of different velocities comprising means including a first pair of uniformly-spaced cathode and grid electrodes for producing and directing a first beam of electrons along a first predetermined path and means including a second pair of uniformly-spaced cathode and grid electrodes adjacent said first pair of electrodes for producing and directing a second beam of electrons along a second path intersecting said first path, the spacing between the cathode and grid electrodes being substantially the same for both of said pairs of electrodes, whereby the perveance per unit area of each of said pairs of electrodes is substantially the same.

4. An electron gun structure in accordance with claim 2, wherein said cathode means and grid means conform to and lie in two concentric spherical surfaces.

5. An electron gun structure in accordance with claim 3, wherein said cathode electrodes and said grid electrodes conform to and lie in two concentric spherical surfaces.

6. An electron discharge device comprising a collector electrode, an electron gun structure spaced from said collector electrode and an elongated drift tube extending between and aligned with said gun structure and said collector electrode from a point closely adjacent said gun structure, said gun structure comprising two spaced adjacent cathodes and grid means adjacent to said cathodes and uniformly spaced therefrom, the emitting surfaces of said two cathodes being inclined relative to each other and said drift tube and collector electrode so that the two electron streams from said cathodes merge within said drift tube and extend as an intermixed stream to said collector electrode.

7. An electron discharge device comprising an envelope containing means for producing two electron beams in the same general direction and in space charge coupling relation with each other, said means including a pair of spaced adjacent cathodes, a pair of spaced grids respectively adjacent to each of said cathodes, and separate leads connected to each of said cathodes and grids and extending through said envelope to permit the application of different potentials to said cathodes and grids for imparting different velocities and space charge densities to said beams, separate tunable cavity resonator means coupled between the leads connected to each cathode and the grid adjacent thereto, means coupled to each of said resonator means for applying a signal thereto, an elongated drift tube adjacent to said grids and through which said beams are directed during operation of said device, and a collector disposed in the path of said beams beyond said drift tube.

8. A method of operating a beam type amplifier tube including electron gun means for producing and directing two electron beams having different average velocities in and uz and different plasma frequencies where p and p2 are the average space charge densities of the two beams, and e and m are the charge and mass of an electron, respectively, in the same general direction and -in space charge coupling relation with each other, and

beam modulator means positioned to apply a signal to at least one of said beams; said method comprising the steps of applying a signal of angular frequency w to said beam modulator means, and applying such potentials to said electron gun means that the following relations are substantially satisfied:

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