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Fast electromagnetic wave and undulating electron beam interaction structure

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US3129356A
US3129356A US81654059A US3129356A US 3129356 A US3129356 A US 3129356A US 81654059 A US81654059 A US 81654059A US 3129356 A US3129356 A US 3129356A
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stream
wave
electron
waveguide
electrons
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Robert M Phillips
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General Electric Co
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    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01JELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J25/00Transit-time tubes, e.g. klystrons, travelling-wave tubes, magnetrons
    • H01J25/34Travelling-wave tubes; Tubes in which a travelling wave is simulated at spaced gaps
    • HELECTRICITY
    • H05ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05HPLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
    • H05H9/00Linear accelerators

Description

Aprll 14, 1964 R. M. PHILLIPS 3,129,356

FAST ELECTROMAGNETIC WAVE AND UNDULATING ELECTRON BEAM INTERACTION STRUCTURE Filed March 28, 1959 5 Sheets-Sheet 1 II.E l N P951427 PMAA/P! INVENTOR. BY%

Aprll 4, 1964 R. M. PHILLIPS 2 FAST ELECTROMAGNETIC WAVE AND UNDULATING ELECTRON V BEAM INTERACTION STRUCTURE Filed March 28, 1959 5 Sheets-Sheet 2 i IE II IE- INVENTOR.

Apnl 14, 1964 R. M. PHILLIPS 3,129,356

FAST ELECTROMAGNETIC WAVE AND UNDULATING ELECTRON BEAM INTERACTION STRUCTURE Filed March 28, 1959 5 SheetS-Sheet 3 K E] E4 K; EwAP VMPMZZ/Q J INVENTOR.

Apnl 14, 1964 R. M. PHILLIPS 3,129,356

FAST ELECTROMAGNETIC WAVE AND UNDULATING ELECTRON BEAM INTERACTION STRUCTURE Filed March 28, 1959 5 Sheets-Sheet 4 iii R 11M I M W? i m H 41 W e I Q U IMHII x; H n l k L,

III z... :LII]

INVENTOR. BY Z- Apnl 14, 1964 R. M. PHILLIPS 3,129,356

FAST ELECTROMAGNETIC wAvE AND UNDULATING ELECTRON BEAM INTERACTION STRUCTURE Filed March 28, 1959 5 Sheets-Sheet 5 20552; A/a4 A06 *3 INVENTOR.

United States Patent 3,129,356 FAST ELECTRQMAGNETIC WAVE AND UNDU- LATING ELECTRON BEAM INTERACTION STRUCTURE Robert M. Phillips, Redwood City, Calif., assignor to General Electric Company, a corporation of New York Filed May 28, 1959, Ser. No. 816,540 25 Claims. (Cl. 31539.3)

This invention relates to high frequency energy interchange devices of the type wherein an interchange of energy takes place between a stream of electrons and an electromagnetic wave in a waveguide. The invention includes devices wherein the electromagnetic wave abstracts energy from the electron stream as in amplifiers, oscillators or the like as well as devices wherein the electromagnetic wave imparts energy to the electrons in the stream as in the case of an electron accelerator.

In particular, the invention relates to the class of beamtype interaction devices in which it is not necessary either to employ a slow-wave circuit or to hide the stream from the wave over a portion of its travel. The interaction used in these devices will be referred to as a fast-wave interaction to distinguish it from the interaction used in conventional high frequency energy interchange devices.

In the conventional high frequency amplifier or oscillator of the traveling-Wave variety, of which the travelingwave tube, backward-wave amplifier and backward-wave oscillator are examples, the unidirectional energy of the electron stream is converted to radio frequency energy through a continuous and cumulative interaction between the beam and a component of the electric field of the radio frequency wave. The continuous and cumulative interaction is accomplished by slowing the velocity at which the wave progresses axially along the length of the guide (termed the phase velocity v -for the particular wave under consideration in the particular guide) to approximately the velocity of the stream and providing a component of electric field in the direction of stream travel. The field component then causes the electrons in the stream to form in bunches. The bunches form in a favorable phase of the wave and are slowed by the electric field. A portion of the beam kinetic energy is thus imparted to the radio frequency wave. The slowing of the wave phase velocity is accomplished in a number of ways. The most often used device for slowing the Wave is the helix, which may be thought of as one Wire of a two-wire transmission line. The distance traveled by a wave on the conductor coiled into the form of a helix is greater than the distance traveled down the axis of the helix, the path followed by the beam. Hence, although the wave velocity along the coiled conductor isapproximately the velocity of light, the axial component of this velocity can be much less than that of light. Another circuit which is used to slow the phase velocity of the wave is the loaded waveguide. The introduction into a smooth waveguide of periodic loading obstacles causes the wave to be broken up into a number of components,

able interaction and bunching. Serpentine waveguides are examples of circuits of this type. The action of these devices may be shown mathematically to be equivalent "ice to synchronizing with a spatial harmonic in a loaded waveguide, hence are considered here to be in the same class.

In the conventional high frequency amplifier or oscillator of the standing-wave variety, of which the klystron is the prime example, the unidirectional energy of the electron stream is converted to radio frequency energy through an interaction between a stream and an electric field component .of a standing wave at two or 'more discrete gaps. The standing wave is excited in a resonant cavity which surrounds the interaction gap, and consists of two traveling-waves of equal amplitude traveling in opposite directions. The stream is not exposed to (is hidden from) the standing wave in'the region between cavities (drift region). In a two-cavity klystron amplifier, it is the purpose of the first cavity to impart a velocity modulation to the stream. This is converted to current modulation in the drift region. In other words, electron bunches are formed in the drift region. The bunches of electrons are decelerated by the standing wave in the second cavity, thus giving up a portion of their kinetic energy to the radio frequency energy in that cavity. The process of exposing the bunched beam to the electric field in a discrete gap can be shown to be equivalent to synchronizing the beam with that spatial harmonic of the standing wave which travels in the direction of and with the velocity of the electron stream. Working from this picture, the short gap klystron can be seen to be a special case of the extended interaction klystron which consists of two or more resonant slow-wave circuits of any of the types previously discussed, supporting standing rather than traveling Waves. The action of this device can be thought of as a cross between the conventional traveling-wave tube and the discrete gap klystron. The action of the first resonant slow-wave section is .to bunch the beam in a continuous fashion much as in the traveling-wave tube. The action of the second resonant section is to extract energy from the bunches. The action of the electron accelerator may be said to be the reverse of the traveling-wave amplifier. The electron bunches are maintained in such a phase in the wave that the Wave imparts energy to the beam.

All of the conventionalde-vices discussed above have in common a circuit or waveguide for slowing the phase 'velocity of the electromagnetic wave and providing a component of electric field along the axisof the electron stream. The structure of the wave propagating guide is relatively complex and'difficult to fabricate. This is particularly true when the frequency band of interest becomes higher as for millimeter and submillimeter wavelengths where the slow-wave circuit becomes vanishingly small. Inhigh power tubes, the slowwave circuit is difficult to cool regardless of the frequency. Thus, for high power and millimeter wavelengths, fabrication and cooling of the slow-wave transmission line present particularly acute problems.

The class of device proposed eliminates the problems associated with the slow-wave transmission line by providing'a structure in which interaction takes place between an electron stream and a fast Wave, i .e'., having a phase velocity greater than the velocity of light. in

such devices, the electron stream rather than the circuit is made periodic. The class of devices includes those in which the periodicity necessary to'the interaction'is in beam position or beam velocity (speed and/ or direction). One difliculty with prior fast-wave interaction schemes is that they do not exhibit the first order axial bunching of the beam which is characteristic of conventional devices and hence are not capable of the efficiency exhibited by the conventional devices. Bunching in such devices,

of the type wherein the energy interchange takes place between an electron stream and an electromagnetic wave which may be a fast wave and wherein the axial energy of the electron stream is available.

Another object of the present invention is to provide an energy interchange device of the electron stream type wherein the interaction between the electromagnetic wave and the electron stream results in a first order axial bunching of electrons in the electron stream.

Another difliculty encountered in known fast-wave devices is that of maintaining the electron stream in a condition for obtaining an interchange of energy between the electron stream and the fast wave. One difiiculty is that of maintaining the periodicity in the beam required for interaction. The other is in focusing the beam. Part of the difficulty involved in maintaining periodicity and in focusing the stream results from the existence of a componet of radio frequency electric field perpendicular to the natural focused path of travel of electrons in the stream. This component of force tends to deform and defocus the beam. Another cause of difficulty in maintaining beam periodicity is that periodicity is intimately related to the strength of the focusing force, thus, requiring that the strength of the focusing force be exact.

Accordingly, a further object of the present invention is to provide a high frequency energy interchange device of the type under consideration wherein individual electrons in the stream are not necessarily subjected to components of force perpendicular to their focused path of travel.

Another object of the invention is to provide such a device wherein the focusing forces for the electron stream are not critical.

In carrying out the present invention, a radio frequency propagating structure which may be a fast-wave waveguide provides an interaction region and an electron stream is projected along the length of the region in such a manner that electrons in the stream have a transverse velocity, the direction of which varies periodically along the guide. A radio frequency electric field is introduced in the waveguide having a phase velocity related to the stream velocity and a mode such that a transverse component of the radio frequency electric field produces a modulation in the transverse velocity of electrons in the stream which modulation is converted into modulation in the axial velocity by a non-time varying focusing force such as a non-time varying periodic magnetic field. Such a field converts changes in the transverse momentum of the electrons in the stream into changes in the axial momentum While leaving the stream energy essentially unchanged. The momentum conversion thus changes transverse velocity modulation in the stream into axial electron bunching, that is, bunching of electrons along .the axial length of the stream. The transverse component of electric field in the radio frequency wave then abstracts energy from the transverse momentum of the beam. Thus, the ultimate source of energy causing the radio frequency wave to grow is the axial velocity of the stream. Stated in another way, it may be said that the interaction mechanism depends upon an intermediate momentum conversion, that is, conversion of the axial momentum of the beam into transverse momentum with the subsequent interchange of energy between the transverse components of electric field in the radio frequency wave and transverse momentum of electrons in the stream.

The novel features which are believed to be characterist-ic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings in which:

FIGURE 1 is a perspective view of a model utilized in describing and illustrating the high frequency energy interchange mechanism of the present invention;

FIGURE 2 is a side elevation of a portion of the device of FIGURE 1;

FIGURE 3 is a plan view of a portion of the device of FIGURE 1;

FIGURE 4 is an end view of a coaxial waveguide utilized in describing the interaction mechanism as applied to a guide of such configuration;

FIGURE 5 is a partially broken away perspective view of the coaxial device of FIGURE 4 showing electron trajectories therein;

FIGURE 6 is a central vertical longitudinal section through the coaxial guide of FIGURES 4 and 5 additionally illustrating means for focusing the electron stream in the guide;

FIGURE 7 is an end view of a circular waveguide utilized in describing the interaction mechanism of the present invention as applied with a guide of this configuration;

FIGURE 8 is a perspective view of the circular guide of FIGURE 7 illustrating electron trajectories necessary for the interaction;

FIGURE 9 is a central vertical longitudinal section through a portion of the circular guide of FIGURE 8 illustrating the electron stream focusing arrangement;

FIGURE 10 is a partially broken away side elevation of a preferred embodiment of a high frequency energy interchange device utilizing the present invention;

FIGURE 11 is a plan View showing a portion of the waveguide employed in the device of FIGURE 10 and its focusing system as seen along section lines 11--11;

FIGURE 12 is a transverse section through the rectangular body portion of the device of FIGURE 10 taken along section lines .1212 of FIGURE 11;

FIGURE 13 is a side elevation partially broken away and partially in section of a high frequency energy interchange structure which employs the principles of the invention to obtain a fast wave interaction in an extended interact-ion klystron; and

FIGURE 14 is a transverse section through a coupling section taken along line 14-14 of FIGURE 13.

In order to obtain an understanding of the principle of operation of the present invention, reference should be made to FIGURES l, 2 and 3 of the drawings which schematically illustrate a waveguide 16 consisting of a pair of parallel conductive planes 11 and 12 which are considered to be infinite in extent for purposes of discussion. Let us assume that the guide 10 is excited by a radio frequency electromagnetic Wave of the transverse electric mode known as TE This mode may be considered the fundamental mode of an ordinary rectangular waveguide which has its narrow dimension extended to infinity. By definition, the TE mode has lines of electric force or electric flux which are all parallel to the two parallel plates 11 and 12 and perpendicular to the axial dimension of the guide 10 as indicated by the arrows E, and the lines of magnetic flux are perpendicular to the lines of electric flux as indicated by the arrow H. Further, the electromagnetic wave is propagated down the length of the guide perpendicular to the lines of electric force E. There can not be any tangential electric field at the surface of the planes i1 and 12 because they are conducting planes. Therefore, the electric field intensity, 211- though everywhere horizontal in direction, diminishes to zero at the parallel surfaces 11 of the guide It). That is to say, that the electric field intensity is zero at the surface of the conducting planes 11 and 12 and increases to a maximum at the center as indicated by the flux density diagram marked E in FIGURE 1.

A sheet electron stream 13 which is to interact with the electromagnetic Wave propagated down the Waveguide is directed down the length of the region between the two conducting plates 11 and 12. The phase velocity (v of the electromagnetic wave propagated down the guide is greater than the velocity of light (0) whereas electrons in the stream 13 must be less than the velocity of light. Utilizing the structure described thus far, there is no net interaction between the electromagnetic wave and the electron stream 13. The interaction mechanism utilized makes use of an interchange of energy between electrons in the stream and transverse components of the radio frequency electric field E in the electromagnetic wave rather than, as is the usual case, with the axial components of the electromagnetic wave. This mechanism may best be understood by a consideration of the plan view of the guide 1% as seen in FIGURE 3. In considering FIGURE 3, it should be noted that the representation of the radio frequency electric field is an instantaneous one and therefore the electromagnetic wave appears to be stopped in its travel along the length of the guidelt), whereas, in practice, the regions of electric field having vectors in a given direction move down the length of the guide at the velocity which we have called the phase velocity. Electrons in the sheet stream 13 are made to move from side to side in the waveguide 19 as they move down the axial length in order to obtain the desired interchange of energy. The transverseundulating motion of four such individual electrons 14, 15, 15 and 17 is illustrated in FIGURE 3 to facilitate description of the principle.

In order to cause the electrons in the stream 13 to have the transverse undulating motion illustrated, they may be subjected to a series of non-time varying spatially periodic magnetic focusing fields which have lines of force perpendicular to the conducting planes 11 and 12 of the waveguide Iltl. Thus, the magnetic focusing fields of interest are perpendicular to the direction of propagation of the electromagnetic wave, the direction of flow of the electron stream 13 and also perpendicular to the plane in which it is desired to cause electrons to undulate. As illustrated, the periodic magnetic focusing field is provided by a series of magnets 18 which have poles located on opposite sides of the guide 19 adjacent the conducting planes in such a manner that a nort magnetic pole on one side of the guide is directly opposite a south magnetic pole on the opposite side of the guide It) and progressing down the guide in the direction of propagation of the electromagnetic wave, the polarity of themagnetic poles alternate. That is, for example, progressing down the top plane 11 of the guide 10 in the direction of propagation first a north pole is encountered, then a south pole, next, another north pole and then another south pole, and so on. Thus, the magnetic field always has a component perpendicular to the conducting planes 11 and 12. but alternates in the directional sense as indicated by the lines marked B in FIGURE 2 of the drawings.

Due to the fact that the force exerted by a magnetic field on a charged particle is always perpendicular to the direction of motion of the particle, the electrons are not displaced toward or away from the parallel plates 11 and 12 of the guide as they move axially down the guide 19, but have a side-to side serpentine motion in the plane of entry.

For example, as seen in FIGURE 3, the charged particles 14, 15, 16 and 17 move into a magnetic field which is said to be down, i.e., from the top to the bottom of the guide 10. Thus, the charged particlesstart to'move ina curved or circular path in the'clockwise direction. However,'before they curve all the way around, they move into a magnetic field which is in the opposite sense and are therefore caused to move in a curved path with the opposite rotation; that is, they move in a path which rotates in a counterclockwise sense (the terms clockwise and counterclockwise are used here to describe rotation as seen looking down from the top of the guide 10 as illustrated in FIGURE 3). Thus, the individual charged particles move down the length of the guide undulating from side to side in the direction of the transverse electric field lines E while maintaining their original plane of entry.

As has already been pointed out, a fixed relationship (a synchronism) must be maintained between the electrons in the electron stream and the interaction component of the electromagnetic wave as they move down the waveguide in order for a net interchange of energy, either from the stream to the wave, or vice versa, to take place.

Since the interaction in question involves electromagnet ic waves which have a phase velocity (v greater than the velocity of light (0) and an electron stream in which electrons have a velocity less than that of light, the condition responsibe for interaction deserves some special comment. In order to assist in the discussion, a coordinate system has been superimposed on FIGURES 1, 2 and 3. As may be seen in FIGURE 1, the planar surfaces 11 and 12 are parallel to the YZ plane with the electron stream 13 directed in the Z direction and the Y axis directed out of the paper and parallel to the electric field lines E. The X axis then is perpendicular to the conductive planes 11 and 12. Thus, in the side elevation of FIGURE 2, the paper represents the XZ plane while in the plan view of FIGURE 3, the paper represents the YZ plane. The terms period and periodic length are designated by the symbol :P and used to denote the interval or distance in the axial (2) direction between corresponding points on any closest two like pole pieces. For example, in FIGURE 2, P is designated as the distance in the Z or axial direction between center points of two north poles on the upper Wall 11 of the guide It) which are closest together.

In the fast-Wave interaction of the apparatus of FIG- URES 1, 2 and 3, the :condition which is called synchronous is obtained between the electron stream and the electromagnetic wave by forcing the velocity direction of the stream 13 to vary periodically at such a rate that an individual electron in the stream travels one periodic length P in the time that the wave travels one periodic length P plus one Wavelength of the electromagnetic wave.

This may best be understood :by consideration of the electron trajectories illustrated in FIGURE 3. Since the :individual electrons undulate from side to side as they move down the waveguide, their velocity in the Y direction .(side-to-side velocity) must 'be zero at points where the electrons are changing directions. Half way between these points, the velocity in the Y direction is maximum. If the electrons in tthe stream are so phased that their velocityin" the Y direction'is a maximum when the decelerating force due to the radio frequency electric-field E is a maximum,.and, if :the electrons reverse direction at the same instantthat the radio frequencyelectric field E seen by the electrons reverses direction, the electron will continue to experience a periodicdecelerating force in the Y direction throughout itstravelas long as the proper phase relationship is maintained. The electric field will-reverse each-time the electrons velocity reverses if theelectron travels oneperiod (the distance P) in the time that the radio frequency wave travels one periodP plus one radio frequency wave length.

Under the conditions described, the forces exeprienced by the electrons '14, 15, 16 and 17in the electron stream are such as to cause themto impart energy'to the electromagnetic wave. The electrons :14, 15, 16 and17, considered, are selected in the most favorable phase. A considerationof the principles described revealsthat, for the synchronous condition described, other electrons, i.e., electrons in the most unfavorable phase, receive energy from the wave in similar fashion and electrons intermediate to two extreme phases impart or receive energy in varying degrees. Like the conventional traveling-wave tube,

the net energy interchange between the electron stream and electromagnetic wave is zero for exact synchronism. Also, as in the case of the conventional traveling-wave tube, there is a net transfer of energy from the stream to the wave when the electron stream has a velocity Which is slightly greater than the exact synchronous velocity. Thus, the condition for providing amplification is available.

An extremely important property of the interaction described is that the electrons in the stream are formed into bunches along the axial length of the stream much as the bunches of electrons which occur in conventional traveling-wave tubes. It is felt that the present discussion is not the place for a mathematical proof of this effect. However, in simple terms, it may be stated that this bunching of electrons is caused by the periodic magnetic field through which the electrons must pass. The changes in the transverse beam momentum caused by the transverse electric fields are converted into changes in axial momentum by the periodic magnetic field. In a sense, the magnetic field borrows energy from the axial velocity and turns it over to the radio frequency electric field in the form of transverse velocity. Hence, the energy available to the radio frequency wave is not simply the relatively small energy in the transverse motion but the much larger amount of energy introduced to accelerate the electrons in the axial direction.

In order to verify these results, the equations of motion for an electron in the undulating beam device illustrated were solved on an analogue computer. Solutions were obtained for eight electrons equally spaced in the radio frequency electromagnetic wave. The results of the study showed good first order axial bunching of the electrons in the stream and good efiiciency of conversion of beam energy to radio frequency energy. Further, one of the preferred embodiments of the high frequency energy interchange device (described subsequently) experimentally verifies the results obtained analytically.

The apparatus of FIGURES 1, 2 and 3 utilized thus far in describing the interaction utilizes a waveguide defined between two conducting plates 11 and 12 which are considered to be of infinite extent. Obviously, such a device can not be built. However, the electric field configuration in the guide as illustrated and the undulating electron stream can be reproduced in various ways. In other words, the relationship between electron trajectories and electric fields described may be obtained using other configurations. The model illustrated in FIGURES 4, 5 and 6 represents one such configuration. The model illustrated in FIGURES 4, 5 and 6 represents one such configuration. The device of FIGURES 4, 5 and 6 may be considered as a development of the device of FIGURES l, 2 and 3 which is obtained by wrapping the planes 11 and 12 around an axis centrally located beneath the lower conducting plane 12 and extending parallel thereto in the axial (Z) direction. This development produces a waveguide 20 which consists of concentric conductive right circular cylinders or pipes 21 and 22. The Waveguiding portion comprises the area between the two concentric waveguiding pipes 21 and 22.

If the device of FIGURES 4, 5 and 6 is considered as a wrapped up development of the device of FIGURES l, 2 and 3, it is seen that the required radio frequency electric field lines must extend circumferentially around the interior of the guide 20 and the electric field must be of maximum intensity approximately midway between the individual cylindrical conductors 21 and 22 as illustrated by the field lines E, and further, the electron stream must be a hollow cylindrical sheet stream in which individual electrons must be made to undulate from side to side.

The electromagnetic wave necessary to produce the electric field which has circumferential components as illustrated by the lines marked E in FIGURE 4 is produced by a coaxial waveguide mode of the TE type. A conventional hollow stream electron gun may be used to produce the stream of electrons within the waveguide, and periodic magnetic fields which are radial may be used to cause electrons in the stream to undulate as the stream passes down the length of the waveguide.

The periodic magnetic field of the required configuration may be furnished in a number of Ways. One way, for example, is to provide spaced apart disc-shaped radially magnetized magnets 23 inside the inside of the inner conductive pipe 22, and also provide disc-shaped magnets 24 surrounding outer conductor 21 which are coaxial with respect to the inner magnetic discs 23, occupy the same plane, and are of opposite polarity. Both the inner and the outer magnets 24 alternate in polarity down the length of the guide 20 as illustrated in FIGURE 6. Thus, the periodic magnetic field is provided down the length of the waveguide. The particular hollow stream electron gun utilized is not illustrated since any conventional hollow stream gun which does not cause the stream to spin may be used. For example, a hollow stream gun which might be used is illustrated and described in a paper entitled A C-W UI-IF 'IVVT Power Amplifier of Extended Bandwidth, by Ward A. Harman, published in pages 36 through 40 of the 1957 Proceedings of the National Conference on Aeronautical Electronics, sponsored by the Institute of Radio Engineers on May 13, 14 and 15, 1957, in Dayton, Ohio. It should be noted that the periodicity of the magnetic field in the guide should bear the same relationship to the electron stream velocity and electromagnetic wave as described in connection with the apparatus of FIG- URES 1, 2 and 3 to obtain the fast-Wave type of interaction described. For the coxial type waveguide 20 illustrated, it has been found that an amplitude of undulations of electrons in the stream which is satisfactory to produce good interaction may be on the order of one fourth of the radial distance between the inner and outer conductors 21 and 22 of the waveguide.

The configuration of the guide 20 has the advantage that the peak electric interacting field as illustrated in FIGURE 4 for this device is removed as far from each of the walls of the waveguide as is possible, i.e., a maximum distance away. Therefore, the electron stream may also be a maximum distance away from the walls of the guide. Further, the arrangement allows for easy adjustment of the periodicity of the magnets to improve efficiency since the magnets are entirely external to the waveguide which must ultimately be evacuated. These advantages are common to all of the structures which employ the present invention.

Another structure which may be used to obtain the correct field configurations for the interaction described, is illustrated in FIGURES 7, 8 and 9 of the drawings. All of the elements of the structure illustrated in these figures are common to the device illustrated in FIGURES 4, 5 and 6; therefore, common components of the two devices are given like reference numerals in order to simplify the description and drawings. The principal structural difference between the two devices is that the device of FIGURES 7, 8 and 9 does not have the central conductor 22 and its associated magnetic discs 23 as found in the previously described structure. In other Words, the main waveguide 20 is a simple hollow conductor of circular cross section. The magnetic field required is furnished by the external disc-shaped magnets 24. That is, the magnetic field thus produced will cause electrons in an electron stream directed down the waveguide to undulate as they pass down the guide rotating alternately in a clockwise and anticlockwise direction.

The interaction may be obtained using any one of the family of TE circular waveguide modes although the most advantageous mode is the TE mode. The electric field for the TE mode is illustrated in the end view of the circular guide of FIGURE 7. From an inspection of the electron trajectories and the configuration of the electric field E in the guide 20, it is seen that the condition for interaction described with respect to the planar guides of FIGURES 1, 2 and 3 may exist. That is to say that, if the velocity direction of the electron stream is made to vary periodically at such a rate that an individual electron in the stream travels one periodic length P in the time that the electromagnetic Wave travels one periodic length P plus one wavelength of the electromagnetic wavelength, the synchronism required for interchange of energy between the stream and wave exists. The structure has the advantages attributed to the two previously described fast-wave devices.

A preferred embodiment of the invention (the one previously referred to) is illustrated by FIGURES 10, 11 and 12 of the drawings. This device may be considered to be a vertical longitudinal segment of the apparatus described and illustrated in FIGURES 1, 2 and 3. Thus, the central waveguide portion 30 of the device constitutes a waveguide of rectangular cross section.

In addition to the central waveguide portion 39, the traveling-wave tube includes an electron gun 33, which is encapsulated in one end for the purpose of producing and directing a stream of electrons along the axis of the waveguide Si! and a cooled collector 134 located in an enlarged enclosure 4 at the opposite end of the tube for the purpose of collecting electrons from the gun 33.

In order to launch the electromagnetic wave in the waveguide portion 35), an input waveguide section 27 of rectangular cross section is positioned at the gun end of the guide 30 with its longitudinal axis perpendicular to the longitudinal axis of the main guide 3%. A window (not shown) is positioned within the input guide section 27 in order to provide a vacuum tight seal. A similar guide 23 is positioned in a like manner at the collector end of the device to provide an output transmission path for the amplified electromagnetic waves. The interacting electromagnetic wave launched within the guide 36 is the fundamental transverse electric mode for the "guide, i.e., the TE mode.

The gun 33 consists of a cathode 3S and a cathode heater 36 wmch is connected to a suitable energizing source (not shown) and which causes the cathode to emit electrons when heated. A centrally apertured electron stream focusing electrode 37 and a correspondingly centrally apertured electron beam accelerating anode 38 are provided for causing the electrons emitted by the cathode 35 to be projected outwardly along the axis of the waveguide structure 33 in a stream as depicted by broken lines 39. In order to simplify the description and drawings, the energizing voltage supply for the electron gun electrode is not shown. The electron stream produced by the gun 33 is of circular cross section but electrons in the stream may be made to undulate as described in connection with the discussion of the apparatus of FIGURES l, 2 and 3. I

As described in connection with the planar version of FIGURES l, 2 and 3, the desired periodic magnetic field is provided by a series of magnets 31 located on opposite sides of the guide. The magnets 31 are positioned ad jacent the narrow walls in such a manner that a magnetic north pole on one side of the guide is directly opposite a magnetic south pole on the opposite side of the guide and progressing down the guide in the direction of propagation of the electromagnetic wave, the polarity of the magnetic poles alternate. For example, progressing down one narrow wall of the guide 30 in the direction of propagation of electromagnetic waves, first a nort magnetic pole is encountered, then a south.p.ole, nex another north pole and then another south pole, and so on. In this manner, the magnetic field always has a component perpendicular to the narrow walls of the guide 31) but alternates in the directional sense.

Thus, the configuration of radio frequency electric fields and electron trajectories in the guide are similar to those described in connection with FIGURES l, 2 and 3. Consequently, the same interaction is obtained. A modiv 10 fication of the device of FIGURE 10 which can be made with a resultant reduction instream velocity is to bring the focusing magnets 31 in closer to the axis of the waveguide 30. This provides a higher concentration magnetic field between opposing pole pieces which increases the focusing force and allows the magnetic pitch P to be reduced.

In each of the devices described, a fast-wave interaction is provided which eliminates the need for a slow-wave circuit. Further, there an extremely large interaction area in each of the devices when compared with a conventional high frequency energy interchange device. The use of such a guide substantially eliminates matchingand reflecting problems between the main waveguide and input and output sections. The fact that the electrons in the streams undulate along the lines of electric fiuxof the transverse electric field rather than across them eliminates a whole class of stream focusing problems commonly encountered in other known devices of the fast wave interaction type. Each of the structures also allows external variation of the position of focusing magnets which means that the periodicity of undulations in the internal electron stream may be adjustedto improve efliciency. An additional advantage of the structureis that the peak intel-acting electric field is far from the wall. This minimizes stream interception problems and allows one to place the beam at the point of maximum field. H

One structure in which the fast-wave interaction described is used to particular advantage is the extended interaction klystron 4d illustrated in FIGURES 13 and 14. As previously indicated, the conventional klystron is dependent upon the exposureo-f an electron stream to the standing wave excited a resonant cavity to convert the unidirectional energy of the electron stream to radio frequency energy. The exposure of the stream to the wave must be accomplished in a narrow discrete. gap. Also, 'as was previously pointed out, the action of the klystron is improved in many respects by providing resonant slow-wave circuits, -i.e., slow-wave circuits in resonant cavities, so that the electron stream maybe exposed to the electric field of the resonant cavity throughout its travel in the cavity rather than being exposed to electric fields in very narrow discrete regions. Since the interaction mechanism of the present invention allows interaction between an electron stream and .fast electromagnetic waves, the interaction maybe obtained as described with respect'toFIGURES 1, 2 and 3 between an electron stream having undulating electrons therein and electric fields in a cavity which does not have narrow discrete gaps and which does not contain a slow-wave circuit.

The structure may best be seen by reference to FIG- URES l3 and 14- wherein the extended interaction klystron is illustrated without an electron gun or a collector. The'gun and collector are broken away due to the fact that they are exactly the same as those components illustrated inFIGURES 10, 11 and 12. Corresponding reference numerals are used in FIGURES 10 and "13 to identify the position of these elements.

Thus, the tube includes an electron gun not 'shown) which is encapsulated in one end for the purpose of producing and directing a stream of electrons along the axis of the structure 40. A cooled collector (not shown) is located in an enlarged enclosure at the opposite end of the tube for the purpose of collecting electrons from the gun. The interaction circuitry is located between the electron gun and the collector. The interacting circuitry includes an input cavity 41 which has the configuration of a right circular cylinder with an opening in its opposite end walls to provide a passage for an electron stream 42. The interaction circuitry also includes a hollow cylindrical pipe-like conductive portion 43 and an output cavity 44.

The input cavity may be considered a first resonant Waveguide much as the cylindrical waveguide illustrated and described in connection with FIGURES 7, 8 and 9.

Like the cylindrical Waveguide described in connection with those figures, one of the most advantageous modes for interaction is the TE circular waveguide mode. This is accomplished by the waveguide coupling section 45 Which will be described in more detail subsequently. Since the electric field associated with the TE circular waveguide mode has been previously described, the description will not be reiterated at this point. However, it should be noted that the input waveguide cavity 4-1 is made an integral number of half Wavelengths long in order to provide the resonant condition desired. For example, the cavity 41 illustrated is three halves wavelength long. Similarly the output cavity 44 may be considered a second resonant Wave guide which may be substantially identical to the above-described input cavity or first reso nant waveguide 41.

In order to give electrons in the stream 42 the proper trajectories for interaction, discs of radially magnetized material 46 are positioned around the input cavity in such a manner that they produce the spatially periodic magnetic fields necessary to cause electrons in the stream to undulate as described with respect to the circular device of FIGURES 7, 8 and 9. For example, the first disc 46, do, the disc which produces the magnetic field first encountered by the electron stream 42, is illustrated as producing a magnetic north pole, the second disc, a magnetic south pole, and the third disc, a magnetic north pole. Once again, the velocity of the electron stream is selected so that electrons in the stream travel one period P as previously defined while the forward traveling component of the standing electromagnetic wave travels one period plus one radio frequency wavelength. Thus the conditions previously described for interaction are met and the standing wave in the input cavity pro duces a velocity modulation on electrons in the electron stream.

The hollow conductive portion 4-3 which is located intermediate the input and output cavities 41 and 44, respectively, constitutes an electron drift channel where the electron stream is not exposed to standing waves in either cavity. Consequently, the velocity modulation imparted to the electron stream is converted to current modulation in this guide. The length of the channel 43 is selected to provide maximum conversion from velocity modulation of the stream to current modulation. For the apparatus illustrated, this distance is on the order of one to ten times the cavity length. The current modulated electron stream 42 then passes into the output cavity 44.

As previously pointed out, the output cavity or second resonant waveguide 44 may be of substantially identical structure to the input cavity or first resonant Waveguide 41. As a consequence, the current modulated electron stream excites the output cavity 44 and gives up a portion of its kinetic energy to the radio frequency energy in the output cavity 44. Although it is possible by utilizing special techniques to extract energy from the output cavity without causing the electrons from the stream to experience several undulations within that cavity, the best energy conversion is obtained by causing the electrons to undulate and thereby converting axialmomentum of the electron stream into transverse momentum with the subsequent interchange of energy between the transverse components of the electric field and transverse momentum I of electrons in the electron stream. Therefore, the output to a circular guide and vice versa. For the best understanding of the configuration of the input and output couplers 45 and 48, respectively, reference should be had to FIGURE 14. The coupler is made up of a straight section of rectangular waveguide 5t; which is brought into another waveguide or rectangular cross section Sll which has the form of a torus. The straight section 5t) contains a vacuum tight window 56 which provides a seal. The narrow Walls of the wrapped up portion iii of the coupler 45 form inner and outer Walls 53 and 54, respectively, which define the inner and outer diameter of the torus 51. The inner Wall defines a centrally located aperture through the coupling structure to accommodate an electron stream. The one flat broad wall of the circular portion 51 of the input coupler 45 which is adjacent the input cavity 4-1 is provided with four coupling slots 52. These slots or apertures are spaced equidistant about the broad wall and extend radially outward. The coupling slots 52 are open to the interior of the input cavity 41 for the purpose of coupling energy into the input cavity 41.

Thus it is seen that the objects and advantages of the present invention are obtained in high frequency energy interchange devices of the type which rely on an interchange of energy between an electron stream and radio frequency fields. While particular embodiments of the invention have been shown, it Will, of course, be understood that the invention is not limited thereto, since many modifications, both in the circuit arrangements and in the instrumentalities employed, may be made. It is contemplated by the appended claims to cover any such modifications as fall within the true spirit and scope of the invention.

What I claim is new and desire to secure by Letters Patent of the United States is:

1. A high frequency energy interchange device including in combination a Waveguide structure adapted to propagate high frequency electromagnetic waves having a transverse electric ON type mode, means for causing propagation of electromagnetic waves along said waveguide structure in said transverse electric ON type mode, electron gun means positioned for directing a stream of electrons down the length of said waveguide, and periodic electron stream focusing means for causing the electrons to have a periodic undulatory motion along the electric flux lines of transverse electric field of the said electromagnetic waves, said electron gun means directing the electron stream down said guide at such a velocity that the electron stream progresses down said waveguide one period while said electromagnetic wave is propagated substantially one period plus one wave length.

2. In combination in a high frequency energy interchange device a waveguide structure adapted to propagate electromagnetic waves having a transverse electric mode of the ON type, means for causing an electromagnetic Wave to be supported in said Waveguide structure in said transverse electric ON type mode, electron gun means positioned adjacent one end of said Waveguide for directing a stream of electrons down the length of said guide, and periodic magnetic focusing means positioned adjacent said waveguide whereby electrons in said stream have a periodic undulatory motion along lines of electric flux of said transverse electric field produced by the said electromagnetic Wave, said electron gun means directing said electron stream down said guide at a velocity such that the stream progresses one period while said electromagnetic wave propagates substantially one period plus one wavelength.

3. In a high frequency energy interchange device, the combination of a rectangular Waveguide structure adapted to support propagation of an electromagnetic wave of a transverse electric ON type mode, means for causing an electromagnetic Wave to be supported in said waveguide structure in said transverse electric ON type mode, an electron gun means positioned at one end of said guide for directing a stream of electrons down waveguide, a

13 collector means positioned at the opposite end of said guide for dissipating residual energy in said stream and periodic electron stream focusing means positioned along the length of said waveguide for imparting a substantially periodic undulatory motion to electrons in said stream which motion is along the lines of electric fiux of the transverse electric field of said electromagnetic wave, said electron gun means imparting such a velocity to electrons in the stream that they travel down said waveguide one period while said electromagnetic wave is propagated substantially a like distance plus one wavelength.

4. A-high frequency energy interchange device for providing interaction between a fast electromagnetic wave and a stream of electrons including a waveguide structure of rectangular cross section adapted to support an electromagnetic wave having a transverse electric mode of the ON type, means for causing an electromagnetic wave to be supported in said waveguide structure in said transverse electric mode, electron gun means and electron collector means positioned at opposite ends of said waveguide for directing electrons down the length of said guide and receiving electrons, respectively, and a spatially periodic magnetic focusing means positioned near opposite sides of said guide for producing a periodic magnetic field down the length of said guide which has components perpendicular to the direction of flow of electrons in said guide and in such a direction as to cause said electrons to have a periodic undulatory motion along introducing an electromagnetic wave in said waveguide structure for propagation therealong in said mode, means to direct a hollow stream of electrons coaxially down the length of said-guidebetween inner and outer conductors and spatially periodic-electron stream focusing means down the length 'of said guide for rotating electrons in said hollow stream in opposite senses along electric field lines as they move down the guide length whereby they have a periodic undulatory motion, said electron stream directing means imparting such a velocity to said electron stream that electrons travel the distance of one period down said waveguide while said electromagnetic wave travels a distance which is greater by substantially one wavelength.

6. A device of the type defined in claim 5 wherein the said spatially periodic electron stream focusing means is magnetic.

7. In combination in a high frequency energy interchange device of the type which depends upon an energy interchange between an electron stream and an electromagnetic wave, a waveguide structure of circular cross section adapted to propagate an electromagnetic wave having a circular transverse electric mode of the ON type, means for causing propagation of an electromagnetic wave along said waveguide structure in said circular transverse electric mode, means to direct a stream of electrons down the length of said Waveguide, and spatially periodic focusing means positioned down the length of said guide to cause electrons in said stream periodically to undulate back and forth along lines of electric flux of the electromagnetic wave, said means for directing the electron stream imparting such a velocity thereto that electrons in said stream progress one period while said electromagnetic wave propagates substantially one period plus one wavelength.

8. A high frequency energy interchange device as de- 'fined in claim 7 whereinsaid spatially periodic focusing means includes a spatially periodic magnetic structure.

9. A high frequency energy interchange device includ- -ing.in combination first and second resonant waveguide structures of like cross section adapted to be excited in a transverse electric mode of ON type; a drift channel having a cross section similar to said first and second sections but with dimensions as to preclude propagation of the excited electromagnetic waves from said guide, said drift channel being coaxially positioned between said first and second guides and defining a continuous open path therethrough; means for directing a stream of electrons down the path through said first waveguide, said drift channel and said second waveguide, respectively; means for exciting electromagnetic waves in said transverse electric mode in said first waveguide for modulating said electron stream, said second waveguide being adapted for excitation-of electromagnetic waves therein in response to the modulated electron stream; and periodic electron stream focusing means adjacent said first and second waveguide structures for causing electrons passing therevthrough to have a periodic undulatory motion along lines .of electric flux of the transverse electric field of the electromagnetic waves, said means for directing the stream of electrons imparting a velocity to the stream such that electrons in said first and second waveguide structures progress substantially one undulatory period while the forward component of the electromagnetic waves in those structures progress one period plus one wavelength.

10. A high frequency energy interchange device of the type defined in claim 9 wherein the said periodic electron stream focusing means includes periodic magnetic means;

11. In a high frequency energy interchange device of the class which depends upon interaction between elecltronsiin a stream and electromagnetic waves the combina- 'a stream of electrons down the path through said first waveguide, said drift channel and said second waveguide, respectively; means for exciting electromagnetic waves in said transverse electric mode in said first waveguide for modulating said electron stream, said second waveguide being adapted'for excitation of electromagnetic waves therein in response to the modulated electron stream; and periodic electron stream focusing means adjacent said first-and second waveguide structures for causing electrons passing therethrough to have a periodic undulatory motion along lines of electric flux of the transverse electric field of the electromagnetic waves; said means for directing the stream of electrons imparting velocity to the stream such that electrons in said first and second wave-guide structures progress substantially one undulatory period while the forward component of the electromagnetic waves in those structures progress one period plus one wavelength.

12. A high frequency energy interchange device as defined in claim 11 wherein the said periodic electron stream focusing means includes periodic magnetic means.

13. In combination in a high frequency energy interchange device of the type which depends upon an energy interchange between an electron stream and an electromagnetic wave, a resonant waveguide structure of circular cross section adapted to be excited by an electromagnetic wave having a circular transverse electric mode of the ON type, means for exciting an electromagnetic wave in said transverse electric mode for propagation in said waveguide structure, means to direct a stream of electrons down the length of said waveguide, and spatially periodic focusing means positioned down the length of said guide to cause electrons in said stream periodically to undulate back and forth along lines of electric flux of the electromagnetic wave, said means for directing the electron stream imparting such a velocity thereto that electrons in said stream progress one period while the forward component of said electromagnetic wave propagates substantially one period plus one wavelength.

14. A high frequency energy interchange device as defined in claim 13 wherein said spatially periodic focusing means includes a spatially periodic magnet structure.

15. In combination: a waveguide for propagating electromagnetic waves along an axis thereof; an electron gun disposed opposite one end of said waveguide for projecting a stream of electrons along said axis; a magnetic structure disposed along the length of said waveguide for providing a magnetic field having a component thereof directed perpendicularly to said axis, the direction of said magnetic field component reversing at intervals along said length for causing said stream of electrons to undulate in a direction transverse to said axis; and launching means for launching an electromagnetic Wave along said waveguide wherein an electric field component of said wave is directed perpendicularly to both said axis and said magnetic field component for providing interaction between said stream of electrons and said electric field component in a direction substantially perpendicular to said axis.

16. The combination of claim 15, wherein said launching means is disposed near one end of said waveguide and further including output transmission path means disposed near the other end of said waveguide.

17. The combination of claim 16, further including electron collector means disposed opposite the other end of said Waveguide.

18. The combination of claim 17, wherein said waveguide comprises a hollow conductive member and wherein said member is evacuated.

19. The combination of claim 15 wherein said intervals are uniform.

20. The combination of claim 15 wherein said waveguide is rectangular in cross-section.

21. The combination of claim 15 wherein said wave guide comprises a coaxial waveguide structure.

22. The combination of claim 15 wherein said waveguide is circular in cross section.

23. In combination: an elongated hollow member for supporting electromagnetic energy therein; means for projecting a stream of electrons along the longitudinal axis of said member; a magnetic structure disposed along the length of said member for providing a magnetic field having a component thereof directed perpendicularly to said axis, the direction of said magnetic field component reversing at intervals along said length for causing a component of velocity of said stream of electrons transverse of said axis; and means for causing electromagnetic energy to be supported in said member wherein an electric field component of said energy is directed perpendicularly to both said axis and said magnetic field component for causing a modulation by said electric field component of said transverse component of velocity of said stream of electrons.

24. In combination: an electron gun for projecting a stream of charged particles along an axis; means disposed along said axis for providing a steady magnetic field component oriented perpendicularly to said axis, the direction of said magnetic field component alternating as a function of distance along said axis for forcing said stream to undulate in a direction transverse to said axis; and means for propagating an electromagnetic wave along said axis, said wave having an electric field component oriented perpendicularly to both said axis and said magnetic field component for providing interaction between said stream and said electric field component in a direction substantially perpendicular to said axis.

25. The combination of claim 24, wherein the axial velocity of the electron stream projected by said gun is adjusted for a synchronous relationship between said stream and said wave, said synchronous relationship occurring when in the interval required for an electron in said stream to travel along said axis between two adjacent regions of like magnetic field component said wave travels along said axis a distance required to provide like electric field components at each of said regions.

References Cited in the file of this patent UNITED STATES PATENTS 2,241,976 Blewett et al. May 13, 1941 2,249,494 Ramo July 15, 1941 2,296,355 Levin Sept. 22, 1942 2,409,991 Strobel Oct. 22, 1946 2,414,121 Pierce Jan. 14, 1947 2,591,350 Gorn Apr. 1, 1952 2,650,956 Heising Sept. 1, 1953 2,794,936 Huber June 4, 1957 2,808,510 Norton Oct. 1, 1957 2,860,278 Cook et al Nov. 11, 1958 2,925,520 Cutler et al Feb. 16, 1960 FOREIGN PATENTS 995,137 France Aug. 14, 1951 699,173 Great Britain Nov. 4, 1953

Claims (1)

1. A HIGH FREQUENCY ENERGY INTERCHANGE DEVICE INCLUDING IN COMBINATION OF WAVEGUIDE STRUCTURE ADAPTED TO PROPOGATE HIGH FREQUENCY ELECTROMAGNETIC WAVES HAVING A TRANSVERSE ELECTRIC ON TYPE MODE, MEANS FOR CAUSING PROPAGATION OF ELECTROMAGNETIC WAVES ALONG SAID WAVEGUIDE STRUCTURE IN SAID TRANSVERSE ELECTRIC ON TYPE MODE, ELECTRON GUN MEANS POSITIONED FOR DIRECTING A STREAM OF ELECTRONS DOWN THE LENGTH OF SAID WAVEGUIDE, AND PERIODIC ELECTRON STREAM FOCUSING MEANS FOR CAUSING THE ELECTRONS TO HAVE A PERIODIC UNDULATORY MOTION ALONG THE ELECTRIC FLUX LINES OF TRANVERSE ELECTRIC FIELD OF THE SAID ELECTROMAGNETIC WAVES, SAID ELECTRON GUN MEANS DIRECTING THE ELECTRON STREAM DOWN SAID GUIDE AT SUCH A VELOCITY THAT THE ELECTRON STREAM PROGRESS DOWN SAIDE WAVEGUIDE ONE PERIOD WHILE SAID ELECTROMAGNETIC WAVE IS PROPAGATED SUBSTANTIALLY ONE PERIOD PLUS ONE WAVE LENGTH.
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