US2695973A - Reflex traveling wave amplifier - Google Patents

Description

NOV. 30, 1954 GINZTQN 2,695,973

REFLEX TRAVELING WAVE AMPLIFIER Filed 001:. 27. 1949 2 Sheets-Sheet 1 GAIN FREQUENCY INVENT R ATTORNEY 1954 E. L. GINZTON REFLEX TRAVELING WAVE AMPLIFIER 2 Sheets-Sheet 2 Filed Oct. 2'7, 1949 INVENTOR Bow/m L G/NzTo/V ATTORNEY United States Patent Ofiice 2,695,973 Patented Nov. 30, 1954 REFLEX TRAVELING WAVE AMPLIFIER Edward L. Ginzton, Menlo Park, Calif., assignor to The Board of Trustees of the Leland Stanford Junior University, Stanford University, Calif., a legal entity having corporate powers of California Application October 27, 1949, Serial No. 123,820

9 Claims. (Cl. 315-3) This invention relates to the art of amplification of high frequency wave energy, and more particularly to improvements in travelling wave amplifiers, wherein an electron stream reacts with a travelling or running wave to increase the amplitude thereof.

Prior travelling wave amplifiers in general operate by projecting a stream of electrons in the direction of travel of the wave, but at a somewhat greater velocity. Under proper conditions, the electrons give up kinetic energy to the wave, which thus grows in amplitude as it travels with the stream.

According to the present invention, electrons are projected in a stream transversely of the direction of travel of the wave and the electron stream is bunched or density modulated in such manner as to reinforce the wave. Reference is made here to copending application Serial No. 86,018, filed April 7, 1949, by Edward L. Ginzton and entitled Electron Discharge Devices.

Said copending application describes a travelling wave amplifier including two spaced parallel wave guides or conduits, with means for projecting a sheet of electrons first through one conduit and. then through the other. Input wave energy applied to the first conduit produces velocity modulation and consequent bunching of the electron stream, which induces an amplified travelling wave in the second conduit. The process is continuous throughout the active length of the conduits, causing the induced wave to grow in amplitude as it travels.

The present invention deals with amplifiers which are similar in some respects to those of said copending application, but involve a single wave conduit, with input energy applied to one end and output energy taken from the other. The electron stream passes twice across the conduit, being velocity modulated by the first interaction with the wave, and delivering energy to the wave by the second interaction.

It is one of the principal objects of this invention to provide travelling wave amplifiers which can be designed to give substantial voltage gain at microwave frequencies, at any desired frequency throughout a relatively wide band.

Another object is to provide travelling wave amplifiers utilizing the principle of reflection bunching, whereby a single wave guide or conduit of reasonably short length may be used to achieve a desired degree of amplification.

A further object is to provide a type of travelling wave amplifier which is simple to design and requires a minimum of precision mechanical construction.

One of the more specific objects of the invention is to provide methods and means for causing ubstantially unilateral amplification in a device of the described type.

The invention will be described with reference to the accompanying drawings, wherein:

Fig. 1 is a longitudinal section of a travelling wave amplifier embodying the invention,

Fig. 2 is a cross section of Fig. 1 in the plane 2--2,

Fig. 3 is a graph showing typical gain vs. frequency characteristics of a device like that of Fig. i,

Fig. 4 is a perspective view in section of a modification of the structure of Fig. 1, and

Fig. 5 is a transverse cross section of another modification of the device of F ig. 1.

Referring first to Figs. 1 and '2, a wave guide 3 is provided with a longitudinally extending opening or slot 7 along the center of its upper wall. The opening 7 contains an electron permeable conductive grid 9. The lower wall of the guide 3 is vformed with reentrant portions 11 which define an Opening 13. parallel to the opening 7. The. upper end of the opening 13 is provided with a grid 15 adjacent the grid 9, and the lower end includes a similar grid structure 17.

A strip-like cathode 19 is disposed below and parallel to the grid 17, and is provided with a heater 21 which is connected between a pair of terminal posts 23 and 23. A focussing electrode 25 extends along both sides of the cathode 19 and is connected thereto. A reflector or repeller electrode 27 is supported on a pair of terminal posts 29 above and parallel to the upper grid 9. This assembly of elements constitutes an elongated electron gun for projecting a sheet beam of electrons transversely through wave guide 3.

The terminal posts 29, and also the terminal posts 23 and 23 are supported in insulating seals 31 in the walls of a vacuum-tight envelope 33. The envelope 33 surrounds all of the above described structure and is provided with end walls 35, one of which includes an exhaust stem 37 through which the envelope is evacuated. The envelope 33 may be made of metal, as shown, or of other suitable material such as glass. Insulating seals 39 are provided in the wave guide 3 in the vicinity of the end walls 35.

One end of the wave guide 3, for example the left hand end, is adapted to be coupled to a source (not shown) of high frequency waves which are to be amplified. The other end of the guide 3 is adapted to be coupled to a load for utilization of the amplified waves. A low voltage source, such as a battery 41, is connected between the heater terminal posts 23 and 23'. The wave guide 3 is grounded, together with the envelope 33 and any other metallic parts connected to it.

A relatively high voltage source 43 is arranged to maintain the cathode assembly, which includes the cathode 19 and the focussing electrode 25, at a negative potential with respect to the wave guide 3 and the grid 17. A further source 45 is shunted by an adjustable voltage divider 47, Whose movable tap 49 is connected to the repeller electrode 27 through one of the terminal posts 29. The source 45 and the voltage divider 47 are arranged to maintain the repeller 27 at a negative potential of adjustable magnitude.

In the operation of the system of Figs. 1 and 2, the cathode assembly produces a sheet-like stream of elec' trons. The grid 17, being positive with respect to the cathode, accelerates the electrons to a substantial velocity. The electric field between the grid 17 and the focussing electrode 25 is such as to direct the electrons in a relatively thin sheet through the grids 17, 15 and 9 toward the repeller electrode 27.

Input wave energy applied to the guide 3 sets up a travelling wave therein, so that the instantaneous voltage between two juxtaposed points on the grids 9 and 15, such as the points 51 and 53, varies cyclically at the input frequency. Most of the electrons emerging from the accelerator grid 17 have substantially the same velocity, and they enter the grid 15 at that velocity. However, those which cross the gap between the points 51 and 53 at a time when the point 51 is positive with respect to the point 53 will be further accelerated; those which cross the gap a fraction of a cycle later, when there is efiectively no voltage between the points 51 and 53, will emerge from the grid 9 at their original velocity, and those which cross while the point 51 is negative with respect to the point 53 will be decelerated.

Thus the element of the electron stream which goes through the points 53 and 51 is velocity modulated, substantially in the same manner as the beam in a klystron tube is modulated by the field in its buncher resonator. The electrons emerging from the upper grid 9 approach the repeller 27, but its potential is adjusted to a value such that their kinetic energy is overcome and they are forced to travel back toward the grid 9. The more rapidly moving electrons approach the repeller more closely before their flight is reversed, just as a ball thrown vertically up with a high velocity will go higher than one thrown with a low velocity. Similarly, the high velocity electrons will take longer to return to the grid 9, just as a fast ball will take longer to return to the ground than a slow one.

As a result of the difference between the times of flight of the fast and slow electrons, a slow electron will reach a certain point in the return path at the same time as a fast electron which emerged earlier from the grid 9. Thus the velocity modulated stream becomes bunched or density modulated, since each electron which crosses the gap between points 53 and 51 without change in velocity tends to become the center of a cloud of electrons which have emerged earlier at higher velocity and later at lower velocity.

The potential of the repeller 27 is adjusted to make the bunching occur in the vicinity of the points 51 and 53. [t is also necessary to make the relationship between the repeller and accelerating voltages such that each bunch will cross the gap at a time when it will reinforce the wave travelling along the guide. In order to do this, it is found that the bunch must arrive slightly in advance of a voltage maximum between the points 51 and 53, when the gradient is in such direction as to absorb energy from the electrons, i. e. when the point 51 is positive with respect to the point 53.

In making the return trip from the point 51 to the point 53, the electrons give up part of their kinetic energy and emerge from the lower grid 53 at a relatively low velocity. They are collected by the reentrant walls 11 and other conductive parts of the structure. Like the element 1 of the stream which goes through the points 53 and 51, each other element is modulated, bunched, and delivers energy to the wave, the wave being operated upon in succession by the successive stream elements as it travels along the guide.

Viewed in one aspect, the bunched stream acts like a uniform admittance sheet across the guide, in which the conductance component is negative. The wave amplitude increases exponentially, since the greater its amplitude as it reaches any given part of the guide, the greater is the bunching and the more is its amplitude increased. However, the actual voltage amplitude that can be attained is limited by the accelerating voltage, because when the amplitude of the voltage between the grids 53 and 51 becomes high enough, the returning bunched electrons will not have suflicient kinetic energy to re-cross the gap, and thus cannot add any energy to the wave.

An important point to be noted in the operation of the system of Fig. l is that since the structure is electrically symmetrical, it will amplify waves travelling in one direction as well as waves travelling in the other. While this characteristic might be advantageous under some circumstances, it makes it necessahy to provide a good impedance match between the ends of the guide 3 and the respective source and load elements which are coupled thereto. The reason for this is that any substantial reflections will set up a standing wave in the guide, and the standing wave will be amplified in both directions, producing sustained oscillation of the system.

Although the wave guide 3 may be matched at both ends, and therefore operate as a non-resonant, i. e. periodic device, it will be found that any particular adjustment of the repeller and accelerator voltages will provide amplifica' tion only at certain frequencies, rather than continuously throughout the wide pass band of the wave guide. This is because the average travel time of the electrons in the space between the grid 9 and the repeller must bear a definite relationship to the frequency, i. e. the electron bunches must be made to arrive in the proper phase relationship with the wave. Since the travel time may be approximately one, two, three, or any other small number of cycles, one set of adjustments will provide amplification throughout several discrete frequency bands, as shown in Fig. 3.

As a result of the wide distribution of the gain vs. frequency characteristic, the wave guide in Fig. 1 must be matched not only at the particular frequency where amplification is desired, but also at all the other frequencies where substantial amplification can occur, if uncontrolled oscillation is to be prevented. In view of this rather stringent requirement, it may be preferable to employ a modification of the structure of Fig. 1, including means to make the device unilateral, providing amplification of waves travelling one way and attenuation of waves travelling the other way.

The desired unilateralamplification characteristic may be achieved by giving the electrons a velocity component which is in the direction of wave propagation in the wave guide. Then, with the accelerator and repeller voltages adjusted to make the electron bunches arrive at the guide in the proper phase to reinforce a wave travelling in one direction, a wave travelling in the opposite direction will be attenuated. a

Referring to Fig. 4 the structure of Fig. 1 is modified by making the electron gun assembly 19, 25 and the grid 17 diverge from the wave guide 3 in the direction of propagation in which amplification is to occur. The axes of the elements remain in a common plane, as in Fig. l. The arrangement of Fig. 4 causes the electrons to follow paths crossing the gap between the grids 9 and 15 at one point anddrecrossing at another point which is further along the gut e.

The electron trajectories may be controlled in the required manner for unilateral amplification by means of a tranverse magnetic field, as shown in Fig. 5. Here a pair of magnetic pole pieces 57 and 59 extend lengthwise of the structure, facing each other between the cathode 19 and the lower side of the wave guide 3. The poles 57 and 59 may be energized by a suitable winding, not shown. Although the accelerating grid may be arranged in the same way as in Fig. 1, it may be preferable to extend the walls 11 below the guide as at 11', placing the accelerating grid 17 below the pole pieces 57 and 59.

In the operation of the system of Fig. 5, the electrons are first accelerated upwardly, i. e. directly toward the grid 17, and they emerge therefrom at substantially uniform velocity. The transverse field between the poles 57 and 59 deflects the electrons in the direction of wave propagation along the guide, so that bunches formed by the field in one part of the guide will return to and reinforce the field in another part further along the guide.

It will be evident without further illustration that other arrangements, for example combinations of the systems shown in Figs. 4 and 5, may be used to impart a longitudinal velocity component to the electron stream for obtaining unilateral amplification.

Since many changes could be made in the above construction and many apparently widely different embodiments of the invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

I claim:

1. A reflex travelling wave amplifier, including an aperiodic wave conduit having input and output openings at its respective ends and having two opposed substantially parallel electron permeable longitudinal walls, the distance between said walls being very short compared to their length to define a gap that is narrow laterally of said conduit but wide longitudinally of said conduit, the electric vector of energy propagated by said conduit extending between said longitudinal walls, means outside said conduit and adjacent one of said walls and substantially longitudinally coextensive with said gap for producing and projecting a flat sheet stream of electrons across said gap, and means outside said conduit and adjacent the other of said walls for changing the direction of flow of said electrons after they cross said conduit to make them recross said gap in substantially the opposite direction.

2. In a unilateral wave guide amplifier, a wave guide having an input opening at one of its ends and an output opening at the other of its ends, the length of said guide being substantially greater than its major cross sectional dimension, an electron gun outside and adjacent said wave guide and substantially longitudinally coextensive therewith for producing a flat sheet stream of electrons and projecting said electrons across said wave guide, said gun being inclined with respect to the axis of said guide to impart said electrons with a component of velocity parallel to the axis of said wave guide, and a reflector electrode outside said wave guide and on the opposite side thereof from said electron gun for reflecting said electrons to recross said wave guide.

3. In a reflex travelling wave amplifier, an aperiodic hollow wave conductor provided with an input opening at one of its ends and an output opening at the other of its ends, said hollow conductor having two opposed electron permeable longitudinal walls defining a gap that is short transversely of said conductor relative to its length longitudinally of said conductor, means for projecting an electron beam across said gap with a component of velocity parallel to the longitudinal axis of said conductor, and means for reflecting said electron beam for a return transit across said gap with a substantially equal component of velocity parallel to said axis.

4. An amplifier comprising means including a strip cathode and conformal focusing and accelerating electrodes for producing a substantially planar sheet electron beam, means including opposed magnetic pole pieces on opposite sides of the plane of said beam and extending along said plane substantially parallel to said cathode for providing a magnetic field directed at right angles to the plane of said beam, means including an electron permeable wave guide in the path of said beam with its longitudinal axis in said plane and substantially parallel to said cathode, said guide having an input opening at one of its ends adapted to be coupled to an external source of high fre quency energy for providing an alternating electromagnetic field with its electric vector parallel to a component of the velocity of electrons in said beam, and means including an electrode adjacent the side of said guide opposite said cathode and substantially coextensive with said cathode for providing an electrostatic field for reflecting said ieilectrons to retraverse said alternating electromagnetic old.

5. In an amplifier, an aperiodic wave conduit having electron permeable walls substantially longer than the major cross sectional dimension of said conduit, said conduit having an input opening at one of its ends and an output opening at the other of its ends, said input opening being adapted to be connected to a source of high frequency energy for exciting a travelling electromagnetic wave in said conduit, means including an electron gun adjacent one of said walls and substantially longitudinally coextensive therewith for producing a sheet stream of electrons and projecting said stream through said walls, each lineal element of said sheet crossing the longitudinal axis of said conduit at a respective first point, and means including a reflector electrode adjacent said conduit on the side remote from said electron gun, for directing said electron stream to make each said lineal element recross the longitudinal axis of said conduit at a second respective oint. p 6. An electron discharge device including an aperiodic wave conduit having opposed longitudinally extending electron permeable wall portions substantially longer than the major cross sectional dimension of said conduit, means including an input aperture at one end of said conduit for applying wave energy to be amplified thereto, means including an output aperture for leading away energy from the other end of said conduit, a strip cathode and a focusing electrode extending along said conduit adjacent and substantially longitudinally coextensive with one of said walls for producing a sheet-like stream of electrons and projecting said stream through said electron permeable wall portions in succession in a direction transverse to the direction of wave propagation in said wave conduit,

and an electron repeller electrode spaced from and generally parallel to said conduit and on the opposite side thereof from said cathode, for reversing the flow of said electrons to make said stream re-traverse said conduit.

7. An electron discharge device including an aperiodic wave conduit having opposed longitudinally extending electron permeable wall portions substantially longer than the major cross sectional dimension of said conduit, an input aperture at one end of said conduit for applying wave energy to be amplified thereto, means including an output aperture for leading away energy from the other end of said conduit, a strip cathode and a focusing electrode adjacent one of said wall portions and substantially longitudinally coextensive therewith for producing a sheet-like stream of electrons and projecting said stream through said electron permeable wall portions in succession in a direction transverse to the direction of wave propagation in said wave conduit, an electron reflector electrode spaced from and generally parallel to said conduit and on the opposite side thereof from said cathode, for reversing the flow of said electrons to make said stream re-traverse said conduit, and means between said cathode and said conduit for deflecting said electrons in a direction parallel to that of wave propagation in said conduit, whereby an electron crossing said conduit initially at one point will recross it at another point.

8. The invention set forth in claim 7, wherein said last mentioned means includes opposed magnetic pole pieces on opposite sides of the plane of said beam and extending along said plane substantially parallel to said cathode.

9. A reflex travelling wave amplifier, including an aperiodic wave guide with two opposed substantially parallel longitudinally extending electron permeable walls, said wave guide having an opening at one of its ends adapted to be coupled to a source of high frequency Waves to be amplified, and an opening at its other end adapted to be coupled to utilization means, the electric field of said waves extending between said walls, a strip cathode substantially longer than the major cross sectional dimension of said wave guide and adjacent one of said electron permeable Walls and longitudinally coextensive therewith substantially throughout the length of said cathode, and a reflector electrode approximately the same length as said cathode, adjacent the other of said electron permeable walls and longitudinally coextensive with the wave guide throughout the length of said reflector electrode.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,122,538 Potter July 5, 1938 2,153,728 Southworth Apr. 11, 1939 2,241,976 Blewett et al. May 13, 1941 2,367,295 Llewellyl Jan. 16, 1945 2,450,026 Tomlin Sept. 28, 1948 2,452,075 Smith Oct. 26, 1948 2,462,085 Fremlin Feb. 22, 1949 2,468,152 Woodyard Apr. 26, 1949 2,472,038 Yando May 31, 1949 2,509,374 Sunstein May 30, 1950 2,579,654 Derby Dec. 25, 1951

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