US3050657A

US3050657A - Slow wave structures

Info

Publication number: US3050657A
Application number: US481450A
Authority: US
Inventors: Jr Garland M Branch
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1955-01-12
Filing date: 1955-01-12
Publication date: 1962-08-21
Anticipated expiration: 1979-08-21

Description

Aug. 21, 1962 G. M. BRANCH, JR

SLOW WAVE STRUCTURES Filed Jan; 12, 1955 2 Sheets-Sheet 1.

l whiz NWQ 7 ll "In.

In ventor Gar/and M. Branch His Attorney Aug.21,1962

G. M. BRANCH, JR

SLOW WAVE STRUCTURES 2 Sheets-Sheet 2 Filed Jan. 12, 1955 Fig.9.

n &7 r n o e wi M n J a a w a N H 6 a UV 6 MJ United States Patent M 3,050,657 SLOW WAVE STRUCTURES Garland M. Branch, Jr., Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed Jan. 12, 1955, Ser. No. 481,450 11 Claims. (Cl. 315-3.6)

This invention relates to helical slow wave structures and, while this invention may be incorporated in a large number of diiferent types of apparatus, it is, by way of example, particularly described in connection with traveling wave interaction devices generally identified as traveling wave tubes.

In a traveling Wave tube utilizing a helix slow wave structure, an electromagnetic Wave is caused to follow the turns of the helix so as to result in a reduced velocity of electromagnetic wave propagation along the helix axis. An electron beam is caused to travel parallel to the axis of the helix and at a velocity so that interaction will take place between the electromagnetic Wave and the electrons in the electron beam. Customarily, where it is desired to amplify the electromagnetic wave energy, the electrons in the electron beam have an average velocity which is greater than the velocity of the electromagnetic wave along the helix axis so that energy is transferred from the electron beam to the electromagnetic wave.

The resulting transfer of energy from electrons in the electron beam to the electromagnetic wave results in a decrease in the electron beam energy and a consequent velocity and density modulation of the electron beam. In order to effect maximum energy transfer from the electron beam to the traveling wave, it would be desirable to be able to control the electron beam velocity at various points along the helix by applying different direct current potentials to discrete regions of the helix. In conventional helix traveling wave tubes this is not possible since the helix is highly conductive.

A solution to this problem which has been advanced, consists of breaking the helix up into a number of sections and coupling energy from one of the helices to a second section or helix by means of the velocity and density modulation of the electrons in the electron beam or alternatively to use two electron beams, one as an energy supplying beam and the other as an electromagnetic wave coupling beam.

Systems such as those above-described tend to be cum bersome and relatively inefiicient; however, slow wave structures of the type hereinafter described as part of this invention provided a convenient means of transferring electromagnetic wave energy between separate helices which can then be maintained at different potentials. Therefore, in accordance with this invention it is possible to maintain the beam potential at an optimum value and at the same time effectively and efiiciently transfer electromagnetic wave energy between the helix sections.

In addition to being able to maintain the electron beam at an optimum velocity it is possible in the practice of this invention to effect a substantially field free drift region between the helices so that electron bunches formed as a result of interaction of the electromagnetic wave with the electrons in the beam can drift and then recombine in proper phase with a second section of helix to further increase the energy transfer from the electron beam to the electromagnetic wave on the slow wave structure.

Another characteristic difficulty in the construction and operation of traveling wave tubes is that of stabilizing the tube by attenuating undesired frequency components and backward traveling waves at the operating frequency. In accordance with another aspect of this invention the attenuating structure is formed on an external portion 3,050,657, Patented Aug. 21, 1962 ICC of the vacuum enclosure of the traveling wave tube and therefore can be easily cooled and adjusted for optimum operation of the traveling wave tube.

It is therefore an important object of this invention to provide improved slow wave structures.

Another object of this invention is to provide improved slow wave structures for use in traveling wave interaction devices wherein the electron beam velocity can be easily and conveniently controlled throughout the interaction region to effect optimum operation of the device.

Another object of this invention is to provide an improved slow wave structure for use in a traveling wave interaction device wherein the combined efiects of electron bunching and electron interaction with a traveling electromagnetic wave are easily and conveniently effected.

It is also an object of this invention to provide a slow wave structure having in corporated therein stabilization means having high heat dissipating characteristics and convenient reproducibility.

It is also an object of this invention to provide a slow wave structure for use in a traveling wave interaction device which has a substantially field free drift region the effective length of which can be easily and conveniently varied.

In accordance with an important aspect of this invention there is provided a slow wave structure comprising a plurality of helices. Between each of these helices there is provided at least one coupling helix which is oriented to transfer electromagnetic wave energy between each of said plurality of helices.

Other important objects and aspects of this invention will become apparent from the following specification and claims when taken in connection with the figures of the drawing wherein FIG. 1 illustrates an example of a traveling wave interaction device incorporating this invention; FIGS. 2 through 6 illustrate diagrams useful in explaining the theory of operation of this invention; and FIGS. 7 through 9 illustrate examples of other embodiments of this invention.

FIG. 1 illustrates a traveling wave interaction device, hereinafter referred to as a traveling wave tube which includes an electron gun 10 consisting of electron emitting cathode 11, accelerating anode 12, and heater 13 which is energized by power supply 14. The heater makes electrical connection to cathode 11 at junction 15 to provide the low potential connection through lead 16 to power supply 17. Electrons from cathode 11 follow the general beam path 18 and are collected by collector 19 which is connected by lead 20 to power supply 17, at a lower potential point than accelerating anode 12 in order to efiect deceleration of the electrons in beam 18. Solenoid 21 provides a magnetic field substantially parallel to the beam path to focus the beam along the desired path from the cathode 11 to the collector 19.

The traveling wave tube is provided with two helices within glass or ceramic vacuum enclosure 22. Helix 23 is provided with an input lead 24 through which electromagnetic wave energy can be easily coupled. Helix 23 is severed at point 25 to provide a gap 26 along the electron beam which extends between point 25 and point 27 on helix 28. Helix 28 is provided with. an output lead 29 from which amplified electromagnetic wave energy can be extracted. The accelerating anode and the helix 23 are maintained at the same potential through lead 30 which makes adjustable connection to power supply 17 while helix 28 can be maintained at the same potential, or, as herein illustrated at a slightly higher potential through separate lead 31.

As thus far described, this tube provides substantially no coupling between helix 23 and helix 28 for an electromagnetic wave, except through the relatively inetl'icient means of the electron beam 18. In order to transfer a maximum amount of electromagnetic wave energy from helix 23 to helix 28 there is provided coupling helix 32. Helix 32 is Wound in an opposite sense to

helices

23 and 28 and has substantially the same helix pitch angle. Helix 32 is further provided with an adjustable potential source 33. In order to attenuate undesired electromagnetic wave energy components which tend to render the traveling wave tube unstable there is provided an attenuating means 34 which may, for example, consist of an aquadag coating or a coating of other high loss material which is applied to the outer surface of the vacuum enclosure 22 and between a portion of the coupling helix 32 and the electron beam 18. Attenuators 34, which may for example consist of rods of high loss material are placed between the turns of coupling helix 32 to attenuate backward traveling wave energy, particularly at the operating frequency. It will be noted that this attenuator is isolated from the electron beam and therefore is not subject to electron beam saturation. In addition the attenuator 34' is easily cooled and may be varied to obtain optimum operation.

The traveling wave tube illustrated in FIG. '1 is operated by applying the necessary operating potentials so as to effect an electron beam flowing between cathode 11 and collector 19 wherein the average electron velocity is slightly greater than the velocity of an electromagnetic wave propagated along helix 13. The manner of computing these velocities and designing a helix conductor with the proper pitch to achieve maximum efiiciency and optimum electron beam coupling is Well known in the art.

An electromagnetic wave is applied to input lead 24 land is caused to be propagated along helix 23. The wave energy interacts with the electrons inthe electron beam to absorb energy therefrom. When the electromagnetic wave energy reaches the region where the coupling helix overlaps the input helix 23 electromagnetic wave energy is induced into the coupling helix. As will be hereinafter described if the overlap between coupling helix 32 and input 'helix 23 is one-quarter or an odd number of quarter space beat wave lengths of the coupled electromagnetic energy, there will be a complete transfer of the electromagnetic wave energy on helix 23 to the coupling helix 32 so that at point 25 there is substantially no electromagnetic wave energy on helix 23 since all of this energy has been transferred to coupling helix 32.

In a like manner the electromagnetic wave energy is propagated along coupling helix 32 and is subsequently transferred to output helix 28 where it further interacts with the electrons in the electron beam so that an enhanced output is obtained from output lead 29.

It is apparent, then, that there is provided a means for transferring electromagnetic wave energy from a first helix to a second helix without forming any direct current connection therebetween and in such a manner that the severed section of the helices do not have to be specially terminated and can be operated at the correct direct current potential to obtain optimum operation of the traveling wave tube.

Before describing the additional features of this invention, it is considered desirable in order to obtain a corm plete understanding thereof to discuss the phenomena which occurs when energy is transferred between the helices. FIG. 2 illustrates two conducting lines which for purposes of this discussion may be considered representative of one of the helices within the vacuum enclosure and of the coupling helix, respectively. For example, line 1 can be considered to represent helix 23 and line 2 to represent helix 32. Curve 35 illustrates the manner in which electromagnetic wave energy or power is transferred between the helices when they are properly oriented and so spaced as to effect a power transfer therebetween.

It will be noted that substantially all of the energy,

4 under idealized conditions is transferred from one line to the other line every quarter of a space heat wave length Thus it may be seen that a space beat wave length, which will be further described in subsequent paragraphs, amounts to two complete cycles of power transfer between

lines

1 and 2.

The manner of power transfer will become more apparent from a consideration of FIG. 3. It is well known that if coupling exists between two transmission lines, such that electromagnetic wave energy traveling in one of them induces an electromagnetic wave in the other line that travels in the same direction, the power originally fed to one of the lines will gradually transfer to the other. Then the reverse process starts, i.e. the power tends to transfer back to the original line as has been described in connection with FIG. 2. The two requirements are that the individual transmission lines have substantially the same velocities of propagation and that the coupling provides a forward traveling wave.

FIG. 3 illustrates the normal coupling between two conductors of different transmission lines wherein there is shown the electric and magnetic vectors E and H and the resulting Poynting vector S which determines the direction of wave energy propagation along the transmission line. The differential induced electric and magnetic fields dB and dH over the distance dZ are shown. Here the resulting wave travels in the opposite direction as shown by dS so that the coupling illustrated in FIG. 3 does not result in what is generally termed spacial beating and from which the term beat wave length is derived.

Thus, if two helices are wound in the same direction there is relatively loose coupling therebetween and substantially no energy is transferred therebetween; however, if a pair of concentric helices are wound in opposite senses as illustrated in FIG. 4, spacial beating does occur and as a result of the relatively strong coupling therebetween there is a highly eflicient energy transfer.

In FIG. 4 it will be noted that a wave impressed on the outer helix, line 1, travels down and to the right. Where the pitch angle is relatively small, a wave is induced on the inner helix, line 2, that travels up but again progressing toward the right. This backward coupling over an incremental distance, together with forward coupling in the overall structure results in spacial beating and strong coupling between the oppositely wound helices. Thus, it may be seen that it is possible to exchange power between the two helices and that the inner helix may be the traveling wave tube helix in the vacuum enclosure while the second or coupling helix may readily be oriented outside of the vacuum enclosure.

FIG. 5 illustrates the instantaneous amplitudes of the electromagnetic waves on a pair of coupled helices. The coupled wave is always degrees out of phase with the induced wave, as is well known, so that the secondary effect of the induced wave coupled back to the first helix gives a degree phase shift to subtract power from the original wave.

FIG. 5 illustrates the beat wave envelopes 36 and the instantaneous waves 37 and 38. From these illustrations it is apparent that under idealized conditions there is a transfer of electromagnetic energy from one helix to the other helix every one-quarter of a space beat wavelength so that, referring back to the illustration of FIG. 1, if there is an overlap of one-quarter or an odd number of quarter wavelengths between coupling helix 32 and input helix 23 there will be, under idealized conditions, no power on helix 23 at point 25 so that no special termination is necessary and there will be maximum power on coupling helix 32. In a like manner, no electromagnetic energy at terminal 39 of helix 32 and maximum energy on output helix 28.

In the foregoing discussion of FIGS. 2 to 5 the description has been qualified with the statement that these relations apply under idealized conditions. It should be clearly understood that the effective transfer of electromagnetic wave energy over broad bands is possible in any properly designed system and that the losses introduced, for example, by helix resistance, space charge effects and the effects peculiar to the material and form. of the vacuum envelope, merely change the well known design parameters and do not alter the fundamental concept of being able to transfer electromagnetic energy be tween helices which are otherwise insulated for direct currents and voltages.

FIG. 6 further illustrates the particular phenomena accompanying this form of broad band coupling and, specifically, it is a plot of the amplitude of the axial component of the electric field associated with a traveling electromagnetic wave. FIG. 6a illustrates a plot of the two energy modes which are characteristic of two coaxial helices wound in the opposite sense wherein there is strong cross coupling therebetween. The inner and outer helices are diagrammatically indicated by the circles located along the Zero axis line and curve 40 illustrates one mode of energy propagation and curve 41 illustrates a second mode of energy propagation.

Modes

40 and 41 are propagated along the helices in a direction substantially perpendicular to the surface of the drawing and at different velocities. Mode 40 is characterized by in-phase currents of approximately equal amplitudes in the two helices and mode 41 by substantially 180 degree out-of-phase currents. Thus, for example, if one helix, say the inner helix 28, is terminated by an open circuit such as open circuit at point 27, so that no current can flow in helix 28 at point 27, an approximately equal mixture of the two modes is excited when an electromagnetic wave is propagated in the outer helix, for example, coupling helix 32.

It is interesting to note that the conditions prevalent at point 27 and throughout the gap 26 in FIG. 1 are generally illustrated in FIG. 6b by curve 42 which illustrates the distribution of electromagnetic field across the tube when substantially all of the electromagnetic wave energy is on the outer helix. It will be seen that under these conditions there is a very strong electromagnetic field between the helices and having a peak centered over the outer coupling helix 32 and that there is substan tially no electromagnetic field in the region 43 between the wires of the inner helices. This intermediate region 43 is the region through which the electron beam passes and the distribution of the electromagnetic fields immediately suggests that there is established a substantially field free drift region in gap 26 which is further enhanced by the shielding eflect of the high loss coating 34. Electrons in the beam can drift through this field free region and bunch in accordance with the velocities imparted thereto by the interaction of these electrons with the electromagnetic wave on helix 23.

Again considering FIG. 6, it is noted that the two modes are then propagated down the coaxial structure until an elapsed phase angle of one of the modes is 180 degrees greater than that of the other mode at which point the two modes now interfere to effectively cancel the currents on the outer helix as illustrated by curve 44 in FIG. 6b. Thus, if the outer helix is discontinued at this point the wave propagates along the inner helix alone and the radial frequency or electromagnetic energy has been efiectively transferred from the outer helix to the inner helix.

A consideration of the energy distribution in curve 44 of FIG. 6b immediately suggests that substantially all of the energy on the helices is within the vacuum enclosure so that there is strong interaction between the electron beam and the electromagnetic wave energy propagated along the inner helix.

In view of the foregoing it is readily apparent that the above mentioned teachings can be applied to the traveling wave tube structure illustrated in FIG. 1 to effect the objects of this invention. It is noted, and it can easily be shown that coaxial helix couplers introduce very small reflections over extremely wide bandwidths such as, for example, frequency ranges in the order of 5 to l, Where such wide band acceptance is desired. For example, a traveling wave tube of the type illustrated in FIG. 1 can readily be designed to operate efliciently and effectively over a frequency range from approximately 300 to 1500 megacycles.

The attenuator 34 illustrated in FIG. 1 as has been previously mentioned, may consist of any lossy material such as an aquadag coating which can be easily and conveniently sprayed on the outer surface of the vacuum enclosure 22. Thus, by placing theattenuator external to the vacuum enclosure, the amount of attenuation can be easily controlled, is more easily cooled, is relatively free from saturation effects due to the action of the electron beam, and furthermore is in a strong electromagnetic field as a result of substantially all of the power being transferred to coupling helix 32. Aquadag coating 34 extends a small distance over the regions where the inner helices are terminated; however, it does not extend beyond the outer ends of the coupling helix since to do so would result in ineflicient transfer of power to the coupling helix and from the coupling helix back to the inner helix. Thus, attenuator 34 stabilizes the tube against regeneration and oscillation by damping spurious oscillations and the electromagnetic waves associated with these oscillations which are reflected from the output termination or load and propagated back along the slow wave structure of the tube input.

Very often, the reflected electromagnetic waves at the operating frequency are at such a high power level that it is necessary to provide additional attenuation such as lossy rods or members 34 which are herein shown as being placed between the turns of the coupling helix 32. Substantially all of the electromagnetic wave energy at the operating frequency passes through the coupling helix and therefore any reflected wave energy, which would tend to render the interaction device unstable, can be absorbed by this easily cooled and controlled attenuating means.

As has been previously mentioned the quarter space beat wave length overlapping section of coaxial crosswound double helices provides a very wide-band reflectionless transition between the inner helix 23 and the coupling helix 32 in a relatively short physical distance. The inner helix, which is effectively severed, can have its input and output sections, 23 and 28, respectively, operated at different voltages to provide optimum gain and/ or efliciency without the customary necessity of introducing long tapered attenuators near the severed ends. Such attenuators interfere with the interaction between the slow wave on the inner helices and the electron beam and reduce the efiiciency as well as the gain of the tube.

It is apparent, from FIGURE 6, that by increasing the diameter of the outer or coupling helix the field strength in the proximity of the electron beam due to the electromagnetic wave energy on the coupling helix will decrease. If it is desired to design a relatively narrow band pass coupling helix it is only necessary to utilize a coupling helix having a relatively large diameter compared to the diameter of the inner helices.

As has been previously mentioned, in connection with the discussion of the curves in FIG. 6 of the drawing, it is theoretically possible to transfer all of the electromagnetic wave energy from the inner helix to the coupling helix such that there is substantially no electromagnetic field in the vicinity of the electron beam and consequently the electromagnetic wave is completely decoupled from the electron beam. Under ideal conditions an outer helix which has a diameter approximately one and one-half times the diameter of the inner helix is suflicient to decouple completely the traveling wave from the beam.

Thus, the attenuator inside the coupling helix and outside of the inner helix gap 26 as well as the greater diameter and lower axial impedance of coupling helix 32, reduces the direct coupling between the beam and the outer helix so that a substantially field free drift region 26 is provided between the two

helices

23 and 28. Thus electrons which have interacted with the electromagnetic wave on helix 23 are permitted to drift through region 26 and bunch or group in accordance with the velocities imparted thereto by the electromagnetic Wave. That is, those electrons moving slower than the actual velocity of the electromagnetic wave are accelerated and those moving faster than the electromagnetic wave are decelerated so that there is a grouping or bunching in the gap or drift space 26.

If the drift region 26 is made sufi'lciently long and helix 28 is of the proper length the electron bunches will have completely formed and will interact in proper phase with the electromagnetic wave which is transferred from the coupling helix 32 back to the helix 28 so as to result in a considerably enhanced electromagnetic wave output from lead 29 as a result of this combined effect. The length of the drift region 26 can be initially adjusted so as to effect the proper spacing between the

helices

23 and 28 and, in addition, power supply 33 is provided so that the potential of the coupling helix can be varied and the effective length of the drift region controlled for optimum bunching and phasing of these bunches with the electromagnetic wave. What has just been described then, amounts to recombining the density modulated beam at the end of the drift space 26 in proper phase with the electromagnetic wave energy transmitted through the coupling helix so as to result in enhancement of the efficiency and gain of the tube.

Under certain conditions it may be necessary or desirable to have a relatively small gap between the ends of the

helices

23 and 28. If this is the case, the necessary attenuation can be obtained by applying aqua-dag coating to other regions of the outer surface of vacuum enclosure 22 and using a short section of coupling helix to transfer some of the wave energy to the outer surface so as to provide a strong electromagnetic field in the region of the attenuating means. Alternatively a coupling helix of lossy material can be used which couples some of the energy to the outer helix from one or more of the inner helices but in which there is no physical inner helix gap. It is also readily apparent that Where it is desirable a relatively long traveling wave tube having a number of such drift regions can be provided so that the tube will in effect consist of a plurality of drift regions 26 with associated coupling helices 32.

FIG. 7 illustrates an alternative embodiment of this invention wherein electromagnetic wave energy is introduced to inner helix 46 through coupling helix 45 and is extracted from inner helix 47 by means of coupling helix 48. Coupling between helices 46 and 47 is elfected by means of outer coupling helix 49. This embodiment is provided with stabilizing aquadag coating 50 and an attenuator 51. Since the coupling helix 49 is somewhat remote from drift region generally indicated by 52, under some conditions it may be desirable to provide a drift tube in the gap between helices 46 and 47. This drift tube 53 consists of a conducting tubular member through which the electron beam can pass so that it will be in a completely field free region. In addition there is provided a tap '54 so that the drift tube 53' can be maintained at the desired potential and so that the effective length of the drift tube can be conveniently varied. It is apparent that this drift tube can be used in combination with any or all of the embodiments previously shown and described in connection with the illustration of FIG. 1 and that the accompanying electron beam forming means, magnetic field and potential supplies are not shown in FIG. 7 merely as an aid in reducing the complexity of this description.

Thus, it is apparent that the field free drift region and therefore the drift time can be varied by varying the length of the gap, the length of drift tube 53 and/or the potentials applied to the coupling helix or the drift tube or both. Energy is transferred to the helix 46 and from the helix 47 by the same mechanism as hereinbefore described in connection with the transfer of energy from an inner helix to an outer helix. The advantages of this method of coupling are readily apparent in that they reduce the number of glass-to-metal seals that must be formed in order to extract energy from the inner helix and require less complicated terminating structures at the ends of the helices within the vacuum enclosure.

FIG. 8 illustrates an alternative construction that can be utilized to effect elficient attenuation of undesired modes and to obtain some degree of electron bunching. In FIG. 8 there is shown input helix section 55 and output helix section 56 separated by lossy helix section 57 so that there is an effective gap for radio frequency currents in the intermediate region defined by the lossy section of helix 57. The lossy section 57 can be formed, for example, by applying a high loss material to a wire helix. Effective radio frequency electromagnetic wave energy coupling between

helices

55 and 56 is effected by coupling helix 58 and some stabilization is provided by the lossy section. An attenuator for the operating frequency reflected electromagnetic wave energy can be placed in juxtaposition to the helix 58 and therefore be in a strong electromagnetic wave energy region and yet be easily cooled and varied for optimum tube operation. It is apparent that the structure herein illustrated is not suited for operating conditions where it is desired to operate the input helix section and the output helix section at different potentials since lossy helix section 57 provides a relatively good direct current path between

sections

55 and 56.

FIG. 9 illustrates a plurality of helix sections 59, 60 and 61 which are coupled by coupling helix sections 62 and 63. Potential leads 64, 65 and 66 are coupled to helices 5'9, 60 and 61, respectively, and provide a means for maintaining these helix sections at the proper potential for optimum operation of a traveling wave tube. It is apparent that this multiple helix structure can be provided with any one or all of the other features hereintofore described in order to provide a multigap structure having substantially field free drift regions between the respective inner helices. Thus, there is provided an effectively periodic traveling wave tube structure wherein there are provided several beam voltage steps to maximize the tube efficiency.

In view of the foregoing, it is readily apparent that among the adjustable parameters, whose optimum values can be determined by theory or easy experimentation, include the length and pitch of the input helix, length and pitch and relative diameter of the coupling helix including the length of the drift section and the length and pitch of the output helix. Thus, it is possible to design an efficient and effective traveling wave tube incorporating this invention which can be operated over a wide frequency band or a relatively limited frequency band depending on the desired service and the band previously mentioned may be placed anywhere within a large portion of the radio frequency spectrum. In addition, it is apparent that this invention can be applied to interaction devices using low density annular electron beams and to any of the many other applications of slow wave structures.

In view of the foregoing, it is readily apparent that this invention is subject to a large number of modifications and variations and that the examples herein described are considered to be representative only. Therefore, it is intended to include in the appended claims all such modifications and variations as come within the true spirit and scope of this invention.

What I intend to claim and protect by Letters Patent of the United States is:

1. In a traveling wave interaction device including means for producing a beam of electrons within a vacuum enclosure, a slow wave structure comprising first and second helical sections oriented within said enclosure and in energy transferring relationship with the beam of electrons, the adjacent ends of said helical sections being in spaced, longitudinal relation with respect to the propagation of high frequency waves, a coupling helix placed outside the vacuum enclosure and oriented in overlapping relation with the adjacent ends of said helical sections to transfer electromagnetic wave energy between said helical sections, said coupling helix having a pitch substantially equal to the pitch of said helical sections but opposite in direction, a first attenuator oriented between the coupling helix and the interior of the vacuum enclosure and a second attenuator coupled to the coupling helix to stabilize the interaction device, whereby heat energy in the attenuators is easily dissipated and the amount of attenuation is easily controlled to effect idealized operation of the interaction device.

2. In a traveling wave interaction device including means for producing a beam of electrons, a slow wave structure comprising first and second helical sections oriented in energy transferring relationship with the electron beam to define a gap between the helices along the electron beam, a coupling helix overlapping the adjacent ends of the first and second helical sections to transfer electromagnetic wave energy across the gap and between the helical sections, said coupling helix having a pitch substantially equal to the pitch of said first and second helical sections but opposite in direction whereby the electrons crossing said gap are in a substantially field free region and can bunch in accordance with velocities imparted to the electrons as a result of electron interaction and with electromagnetic wave energy on said first helical section, said gap being of proper length so that said bunches interact in phase with electromagnetic wave energy transferred to said second helical section by the coupling helix whereby the gain and efficiency of said interaction device are enhanced.

3. In a traveling wave interaction device including means for producing a beam of electrons, a slow wave structure of the type defined by claim 2 wherein a conductive member is oriented in said gap and in proximity to said electron beam to effect a more completely field free drift region within the gap in the interaction device.

4. In a traveling wave interaction device including means for producing a beam of electrons, a slow wave structure of the type defined by claim 3 wherein a potential is applied to the conductive member to control the effective electrical length of the field free drift region.

5. In a traveling wave interaction device including means for producing a beam of electrons, at slow wave structure comprising a first helix and a second helix oriented in energy transferring relationship with the electron beam to define a gap between the helices and along the electron beam, a coupling helix overlapping the adjacent ends of said first and second helices to transfer electromagnetic wave energy across the gap and between the helices, said coupling helix having a pitch substantially equal in magnitude to the pitch of said first and second helices but opposite in direction, a lossy material surround ing said gap and oriented between said coupling helix and the electron beam whereby beam electrons crossing said gap are in a substantially field free drift region and can bunch in accordance with the velocities imparted to the electrons as a result of interaction of the electrons with the electromagnetic wave energy on said first helix, said gap being of proper length so that said electron bunches interact in proper phase with the electromagnetic wave energy transferred to said second helix by the coupling helix whereby the gain and efficiency of said interaction device are enhanced.

6. In a traveling wave interaction device including means for producing a beam of electrons, a slow wave structure comprising a first helix and a second helix oriented in energy transferring relationship with the electron beam to define a gap between the helices and along the electron beam, means applying electromagnetic wave energy to said first helix and means for extracting electromagnetic wave energy from said second helix, a coupling helix overlapping at least a portion of the adjacent ends of said first and second helices to transfer electromagnetic wave energy across the gap and between the helices, said coupling helix having a pitch substantially equal in magnitude to the pitch of said first and second helices but opposite in direction, a lossy material surrounding said gap and oriented between said coupling helix and said electron beam whereby electrons crossing said gap are in a substantially field free region and can bunch in accordance with the velocities imparted to the electrons as a result of interaction of the electrons with the electromagnetic wave energy on said first helix, means for applying a potential to said coupling helix to control the effective length of the field free drift region so that said bunches interact in phase with the electromagnetic wave energy transferred to said second helix by the coupling helix to result in enhanced gain and efliciency of said interaction device.

7. In a traveling wave interaction device including means for producing a beam of electrons, the combination comprising first and second helical sections of conducting material positioned in energy exchanging relation with said beam of electrons and having the adjacent ends thereof positioned in spaced longitudinal relation with respect to the path of the beam and the propagation of high frequency waves and a coupling helix extending between said helical sections and overlapping the adjacent ends thereof, said coupling helix having substantially the same pitch as said first and second helical sections but being wound in the opposite direction to provide for the transfer of energy between said first and second helical sections through said coupling helix.

8. In a traveling wave interaction device including means for producing a beam of electrons, the combination comprising first and second helical sections of conducting material positioned in energy exchanging relation with said beam of electrons and having the adjacent ends thereof positioned in spaced longitudinal relation with respect to the path of the beam and the propagation of high frequency waves and a coupling helix extending between said helical sections and overlapping the adjacent ends thereof by an amount corresponding substantially to an odd number of one-quarter wave lengths at the space heat wave lengths of the electromagnetic energy to be coupled, said coupling helix having substantially the same pitch as said first and second helical sections but being Wound in the opposite direction to provide for the transfer of energy between said first and second helical sections through said coupling helix.

9. In a traveling wave interaction device including means for producing a beam of electrons, the combination comprising first and second helical sections of conducting material positioned in energy exchanging rela tion with said beam of electrons and having the adjacent ends thereof positioned in spaced longitudinal relation with respect to the path of the beam and the propagation of high frequency waves, a coupling helix extending between said helical sections and overlapping the adjacent ends thereof, said coupling helix having substantially the same pitch as said first and second helical sections but being wound in the opposite direction to provide for the transfer of energy between said first and second helical sections through said coupling helix and means maintaining said coupling helix at a direct current potential different than the direct current potential of said first and second helical sections.

10. In a traveling wave interaction device including means for producing a beam of electrons, at slow wave structure comprising first and second helical sections, said first and second helical sections being wound in a first direction and oriented in energy transferring relationship with the beam of electrons and a coupling helix wound in a sense opposite to that of said first and second helical sections and oriented to transfer electromagnetic wave energy between said first and second helical sections, said coupling helix having substantially the same pitch as said first and second helical sections and having a position coaxial With and overlapping with respect to the adjacent ends of said helical sections.

11. A traveling-Wave tube amplifier comprising an electron gun including a cathode maintained at a predetermined reference potential for producing an electron stream, means for directing said stream along a predetermined path, a collector electrode disposed opposite said electron gun to intercept the stream electrons, an input helix maintained at a first predetermined potential with respect to said reference potential and disposed about said path adjacent said electron gun for propagating an electromagnetic Wave at a predetermined velocity, said predetermined velocity being small in comparison to the velocity of light, a principal helix maintained at a second predetermined potential with respect to said reference potential and electromagnetically coupled to said input helix and disposed between said input helix and said collector electrode for propagating said wave, said first potential being of such a value as to maintain said input helix at a direct-current potential to minimize the coupling loss between both of said helices with regard to the growing wave portion of said electromagnetic Wave.

References Cited in the file of this patent UNITED STATES PATENTS 2,584,308 Tiley Feb. 5, 1952 2,588,832 Hansell Mar. 11, 1952 2,616,990 Knol et al. Nov. 4, 1952 2,623,193 Bruck Dec. 23, 1952 2,636,948 Pierce Apr. 28, 1953 2,660,689 Touraton et a1 Nov. 24, 1953 2,733,305 Diemer Jan. 31, 1956 2,782,339 Nergaard Feb. 19, 1957 2,793,315 Haeif et al. May 21, 1957 2,804,511 Kompfner Aug. 27, 1957 2,806,177 Haeif Sept. 10, 1957 2,811,673 Kompfner Oct. 29, 1957 2,814,779 Mendel Nov. 26, 1957 FOREIGN PATENTS 668,168 Great Britain Mar. 12, 1952