US2541843A

US2541843A - Electronic tube of the traveling wave type

Info

Publication number: US2541843A
Application number: US761798A
Authority: US
Inventors: John W Tiley
Original assignee: Philco Ford Corp
Current assignee: Space Systems Loral LLC
Priority date: 1947-07-18
Filing date: 1947-07-18
Publication date: 1951-02-13
Anticipated expiration: 1968-02-13

Description

Feb. 13, 1951 J. w. TILEY ELECTRONIC TUBE OF THE TRAVELING WAVE TYPE Filed July 18, 1947 3 Sheets-Sheet i INVENTOR JOHN w. TILEY W a ZM ATTORNEY5 Feb. 13, 1951 J. w. TILEY 2,541,843

' ELECTRONIC was OF THE mvsunc WAVE TYPE Filed July 18, 1947 a Sheets-She et g v .3 g INVENTOR Q .9, -JOHN w. TILEY k BY ATTORNEYS ELECTRONIC TUBE OF THE TRAVELING WAVE TYPE ATTORNEYS Patented Feb. 13, 1951 ELECTRONIC TUBE OF THE TRAVEIJNG \WAVE TYPE John W. Tiley, Philadelphia, Pa., assignmto Philco Corporation, Philadelphia, Pa... a corporation of Pennsylvania Application July 18, 1947, Serial No. 761,798

17 Claims.

This invention relates in general to the electrontube art, and more particularly to a novel electromagnetic traveling wave tube having special application in high frequency and broad band electrical systems.. j

In electrical systems such as television, pulseposition modulation and radar, it has often been extremely difficult to obtain adequate and uniform amplification over the frequency spectrum encompassed by the signal. Ordinary triodes, even when i corporated in special circuits, fail to provide a usable gain in such applications. Recent designs, such as the lighthouse tube and the velocity modulation tube, as exemplified by the Klystron, can provide reasonable gain solely in narrow band width operation. An attempt to use a Klystron or like tube in the microwave region with a band width of fifty megacycles or greater will result generally in a gain of less than unity.

A recent electronic development, known as the beam traveling wave tube, overcomes the limitations of the more conventional tub types, and has been successfully tested as an efiicient amplifier of signals having a mean frequency of the order of thousands of megacycles with an overall band width of the order of eight hundred megacycles. Theoretically, even much wider bands may be amplified by the device without sacrifice of gain.

For a description and illustration of this development, reference is made to the publication, Bell Laboratories Record" of December 1946, and to the article therein entitled "The Beam Traveling Wave Tu by J. R. Pierce. In this reference, the traveling wave tube is described as constituted of an electron gun similar to those employed in cathode ray tubes. An electron beam generated by the gun is directed in a narrow beam along the axis of a long evacuated tube and impinged upon a collector anode. Within the tube, and surrounding the beam axis, is a closely wound wire helix which is excited at the electron gun end thereof by the weak signal to be amplified, and which providesat the collector end thereof the amplified output signal. The tube contains no signal grids in the conventional sense.

Broadly speaking, the applied signal travels along the wire helix as an electromagnetic wave at a speed approaching the speed of light. As is determined by the pitch of the helix, the wave travels axially of the tube at a fraction of the speed of light; and the electron gun and collector anode potentials are arranged so that the average 2 axial velocity of the electron beam through the helix is somewhat greater than the axial wave I velocity.

Interaction of the electron beam and electromagnetic field components extending from the.

helix' produces the signal amplification. The greater th electron cin'rent and the longer the helix, the greater is the gain. In transit through the helix, the average electron velocity is reduced,

and the energy drop represented by this dccreased velocity is imparted to the signal. The tube does not require a tuned circuit in the signal path, the wire helix being in effect an all pass transmission line. Hence the tube is capable of operation over an exceedingly wide frequency range. In practice, this range is limited somewhat by the impedance match of the helix to the external circuits.

In operation, the signal appearing on the helix acts on the electron stream and gradually produces fluctuations in velocity and density. The density modulated electron beam delivers energy to the wave and over the helix section nearest the collector there is a substantially uniform gain per unit length of travel. 7

The present invention contemplates and has as a primary object the provision of an electromagnetic traveling wave tube of novel and improved design having wide application as a broad band microwave amplifier, oscillator, frequency converter, or the like. The fundamental improvement of this invention is the utilization of wave guides for directing the input and amplified signals around the path of electron travel. These wave guides are formed to permit direct coupling of the electromagnetic fields therein and the tube electron beam while'precluding excessive energy if'ladiation and undesirable coupling with external elds.

In one form of the invention, signal amplification is obtained by causing an electron beam to traverse the axis of a helical wave guide section of a predetermined pitch and'number of turns energized by the weak incoming signal. In this manner the electron beam is bunched, or density modulated. The beam is then caused to traverse another helical wave guide section and to deliver energy thereto to provide the amplified output signal. As will be described in detail hereinbelow, maximum gain is obtained when certain conditions of beam velocity and helix design are properly satisfied. For oscillator operation, a fraction of the amplifier output is coupled to the input thereof in the correct phase. Coupling may be external of the tube and through frequency sta- 3 bilizing filter circuits or internal by suitable wave reflections.

It is therefore another object of the present invention to provide a beam traveling wave electron tube characterized by inherent stability and high gain.

A further object of the present invention is to provide a velocity modulation type electron tube utilizing wave guide sections for bunching and extracting energy from the electron beam.

Another object of this invention is to provide an electron beam traveling wave tube utilizing coiled half wave guide sections for establishing bunching electric fields and extracting energy from density modulated electron beams.

A still further ob ect of this invention isto provide a traveling wave electron tube of comparatively simple design and rugged construction.

These and other objects of the present invention will now become apparent from the following detailed specification when taken in connection with the accompanying drawings, in

which Figure l is a general side view, partially in section, of a helical wave guide traveling wave electron tube;

Figure 2 is a general side view, partially in section, of another embodiment of a traveling wave tube of the general type illustrated in Fi ure 1; and

Figure 3 is a side view, partially in section, of a still further embodiment of a traveling wave electron tube.

With reference now to the drawings. and more particinarly to Figurel, there is illustrated an electron tube incorporating the features of the present invention and comprising generally a centrally disposed, sealed and evacuated glass or similar dielectric cylinder 2|. Sealed into the left-end of the glass cylinder 2|, as viewed in Figure 1, are the electrodes of an electron gun 20, similar to those utilized in conventional cathode ray tube structures. As shown, the electron gun 2! comprises a heater 22 and its associated cathode 23 which serve, when suitably energized, as an electron source.

Cathode 23 is followed by a disc-like, centrally perforated control grid 24, and by focusing and accelerating hollow

cylindrical electrodes

25 and 26, respectively. When the electrodes comprising the electron gun 20 are energized from a suitable power source (not shown), an axial beam of electrons of predetermined high velocity is generated and directed along the axis 2. of cylinder 2| toward a disc-shaped collector electrode 21 sealed into the right-hand end of the tube, as viewed in Figure 1. Structural means required for supporting the electron gun electrodes have been omitted for clarity.

As electron beam tubes of the type included within the Structured Figure 1 are suiliciently well known in the electronic art, further description thereof and of the energizing means therefor are considered unnecessary at this point. p s

In accordance with the broad principles of the present invention, the axial path of travel of the eectron beam through tube 2| is sub-' stantially enclosed within axially aligned helical wave guide structures.- As illustrated in Figure 1, the left-hand wave guide structure 30, designated herein as the input wave guide, is comprised of a thin, highly conductive, metallic strip 3|, wound edgewise about and in contacting re'- lationship with cylinder 2| at a predetermined uniform pitch.

The outer edge of the conductive strip 3| defines a hehx lying in electrical contacting relation with the inner cylindrical surface of an enclosing, hollow, metallicor otherwise conductive cylinder 33, co-axial with and extending substantially the length of tube 2|. The helical strip 3| and enclos ng metallic cylinder 33 there-- by define a helical passage 32 of uniform rectangular cross-section which progresses axially of tube 2|. In effect, therefore, helical passage 32 comprises a helical wave guide enclosed at any point by two adjacent turns of strip 3| and by the metallic surface of the enclosing cylinder 33. The innermost wall of the helical passage 32 lies in, the outer surface of dielectric cylinder 2|, and is accordingly open insofar as the radiation of electromagnetic fields is concerned.

A second wave guide structure 34, having the same general construction as the helical wave guide 3|, is formed within conductive enclosing cylinder 33 by helical conductive strip 36 and is herein designated as the output wave guide. As illustrated in Figure 1, however, the pitch of helical conductive strip 36 progressively diminishes as it approaches collector electrode 21 from a pitch equal to that of the input wave guide 3|. As in the case of helical wave guide 3|], conductive strip 36, enclosing cylinder 33 and glass tube 2| define a helical passage 34 of substantially rectangular cross-section which is electrically open for field radiation into tube 2|.

Coupling means are provided for the interchange of electromagnetic w ve energy with the helical wave guides 3| an 34. As illustrated, the outer cylindrical conductor ll of a co-axial line 39 extends through a perforation 4| in metal cylinder 33 and enters helical e 32. The end of inner conductor 42 of co-axial line 39 is curved as illustrated and electrically connected to the outer conductor 4. to form a small coupling loop 43. A similar co-axial line H, comprising an outer conductor 45 and an inner conductor ll, extends through a perforation 41 in cylinder 33 and is terminated by coupling loop 48 within the output helical wave guide 34. Itwillbenotedthatco-axialllnesfland enter wave guide es 32 and 35 at axially opposed ends thereof.

In order to maintain electrons traversing cylinder 2| within a sharply defined axial beam. means are provided for establishing a substantially uniform and unvarying axial magnetic ileld. As illustrated, this is accomplished by means of. a multi-layer, solenoid-type coil 5| co-axial with tube 2|, and wound upon an insulating cylindrical coil form 52, suitably secured over metallic cylinder 33 between co-axial connectors 33 and M. Coil II is preferably energized from a direct current source (not shown in the drawings). In accordance with well understood principles, an electron which tends to travel a path divergent from the axis 23 of tube 2|, and hence angularly of the field established by coil 8|, will be urged back to axis 23 as a consequence of its interaction with the axial magnetic field.

For successful operation of the electron tube structure of Figure 1, it is desirable to preclude possible interference from stray electromagnetic fields and, further, to preclude internal wave reflections. Insofar as external electric fields are concerned, the conductive cylinder 33 and the 7s helical strips II and 33 provide substantially comlight.

plete shielding. A magnetic shield may be employed if found necessar To minimize internal signal reflections, helical wave guides 30 and 34 have been suitably terminated and matched at the ends thereof. Thus, in the embodiment of Figure l, impedance matching is effected at all ends of the wave guides by means of four uniformly tapered, helical blocks of molded material 54. Blocks 54 are preferably constituted of a ceramic containing powdered graphite as the dissipating element; however, various other absorptive substances may be employed. I

By thus tapering the lossy material, a proper impedance match is readily obtained. This corresponds in effect with the utilization of tapered or wedge-shaped dissipative blocks as reflectionless terminations for conventional rectan l r and other wave guides. 1

To insure refiectionless termination, each of the four blocks 54 illustrated in Figure 1 is preferably made equal to at least-one wave length at the lowest frequency of operation as measured along the helix. Suitable terminations may also be effected by uniformly tapering the spacing of the end turns of each of wave guides 30 and 34 to zero and covering the tapering sections thereof with a power absorbing, semi-conductive substance. J

The operation of the traveling wave electron tube, illustrated in Figure 1, will now be described when efiective as a signal amplifier. The incoming weak signal to be amplified is applied to the tube through co-axial line 39 and the amplified output is extracted through co-axial line H. The input signal travels in a helical path through passage 32 at a velocity somewhat less than that of Standing waves are precluded by the impedance matching, dissipative blocks 53.

In accordance with the general principles of traveling wave electron tube operation, the velocity component of electromagnetic wave energy within wave guide 32 parallel to the axis 28 of tube 2|, is a fraction of the velocity of light, as determined by the pitch of the wave guide 32. As an example, if the length of the helical path through wave guide passage 32 is ten times as great as the axial distance encompassed thereby, then the velocity of wave propagation considered relative to the axis of tube 2| will be equal to one-tenth of the velocity of light. It is therefore apparent that if an electron in the beam generated byythe tube electron gun has an average axial velocity equal to one-tenth the velocity of .light, then the relative phase of the electromagnetic wave in helical passage 32 and this electron will remain unchanged.

It is preferable for amplifier operation that the average velocity of electrons in the beam traversing tube 2| be of the order of the relative axial velocity of the input electromagnetic wave traveling in helical wave guide 32. If the electron velocities are of the order of one-tenth the veloeity of light or less, then the following nonrelativistic equation may be utilized as a guide for determining the electron velocity:

v=5.93xl0" i, centimeters per second wherein E is equal to the voltage through which the beam electrons are accelerated.

The interaction of an electromagnetic wave traveling within the helical wave guide 32 and an electron beam traversing the axis 28 of glass cylinder 2| is somewhat similar to the bunching action obtained in velocity modulation electron tubes. As will be demonstrated, the wave energy traveling within the guide 32 density modulates the axial electron beam in accordance with the input signal variations so that the electron beam, as it enters the axial portion of tube 2| between helical wave guides 30 and 36, is bunched. The density modulated beam is then utilized to excite a signal within wave guide 34 as it travels toward collector electrode 21.

The electromagnetic fields established by wave energy flowing within the input helical wave guide passage 32 may best be visualized if the region between the inner surface of conductive cylinder 33 and the axis of tube 2| is considered as half a rectangular wave guide, the broad walls of which are the metallic surfaces of adjacent turns of helical strip 3|. If the coupling loop 43, applying the input signal to be amplified to the helical wave guide 32, excites this wave guide in the TEm mode, then maximum electric field is produced between adjacent inner edges of the turns of helical strip 3|. The electric field is normal to the strip 3| and diminishes sinusoidally to zero at the inner surface of metallic cylinder Points A, B, C and D may be considered as defining the cross-section of half a rectangular wave guide operating in the aforementioned mode. As points C and D are metallically connected, there is no potential gradient therebetween. However, an electric field is established between points A and B by the exciting wave, which field cyclically varies in intensity as a function of time from a predetermined maximum in one direction through zero to a corresponding maximum in the opposite direction. The electric field between points A and B, of course, extends through the dielectric cylinder 2| andinto the region of electron flow therethrough. In view of the fact that the input electromagnetic wave constantly progresses from left to right, as viewed in Figure l, a given potential gradient may be considered as traveling around the inner edges of the helical strip 3| at a velocity approaching the velocity of light. Thus, if a voltage maximum in a particular direction appears between points A and B at one instant, it will appear at correspondingly later intervals of timebetween points E and F, B and G, F and H, and so on until the energy remaining therein is absorbed in the insulating block 54.

For purposes of illustration, let it be assumed that the dimensions of input wave guide 32 are such that electromagnetic wave energy, introduced by coupling loop 43, travels around one complete turn of the helical passage 32 in a time period corresponding to one-half cycle of the input wave. Under such conditions there is a sinusoidal voltage distribution between opposed inner edges of the helical strip 3|. If, under these conditions, and at a particular instant of time, the voltage between points A and B is a maximum in one direction, then the potential difierence between points E and F will be zero, and between goints B and G a maximum in the opposite direc- Within the glass cylinder 2|, the electric fields extending between opposed points on adjacent turns of the helical strip 3| may be resolved into components, one of which is parallel to the axis 28 of electron flow. The sense of these parallel field components is dependent upon the instantaneous polarity of the originating points. As wave energy passes from left to rightthrough the helical wave guide passage 32, these field components may be considered as progressing axially from left to right, while simultaneously rotating about the tube axis; As previously mentioned,

the axial velocity of propagation of the electromagnetic field components will be determined primarily by the pitch of the helical guide 32.

If the electron gun potentials are adjusted to provide an axial electron beam having a velocity substantially equal to the axial velocity of propagation of the input electromagnetic wave, a bunching action will take place as a res t of the interaction of the traveling electro beam and the axially moving electric field components. In a given electron beam, individual electron velocities extend over a spectrum from a velocity considerably below to one considerably above the average velocity. If at the time electrons enter that portion of the axis 28 of tube 2| encompassed by the input helical wave guide 32, point B is positive with respect to point A; the entering electrons will experience an acceleration toward point B, whereby instantaneously the average velocity of electrons so influenced is increased.

Electrons whose initial velocity is equal to or somewhat less than the axial velocity of the traveling wave will be ,continuously accelerated as they progress along with the electric field component in its travel between points A and B, E and F, B and G, and the like. On the other hand, electrons which enter tube M in the region of helical guide structure 30 with a velocity greater than the axial velocity of the traveling wave, will be retarded somewhat and will continue to decrease in velocity in transit through the tube.

Electrons which enter the region of the helical wave guide 30 when point A is positive with respect to point B, will be retarded. Of these retarded electrons, the ones having the-highest velocities may escape the retarding field and enter into a region of an accelerating field of the type previously described. The lowest velocity electrons may be sufllciently retarded to cause them to be overtaken by the following cycle of an accelerating field component.

Thus, as the electron beam traverses the section of tube 2| spanned by the helical wave guide 30, it is acted upon by the traveling wave field components so that electrons will become progressively bunched in the region of maximum positive potential points of the electric field. Y

The degree of bunching, or density modulation, depends of course upon the intensity of the input electromagnetic wave and the effective length of the helical waveguide passage 32. The electron beam, as it enters the region between helical wave guides 3| and 34, will therefore be density modulatedwith electron bunches spacedtherein in accordance with the pitch of helical metallic strip SI, and further, which have been reduced to a substantially uniform axial velocity equal to theaxial velocity of the traveling wave.

The density. modulated beam which is formed as hereinabcve described then enters the axial region of tube 1| encompassed by the output helical wave guide structure 34 and finally impinges upon collector plate 21. As the excitation of helical wave guide 34 is accomplished by the extraction of energy from the velocity modulated beam, the average electron velocity decreases as the beam approaches the collector plate 21. The uniform reduction of the pitch of strip 36 as hcreinabove described time permits maximum extraction of energy from the density modulated beam.

Since the electron beam is progressively reduced in velocity, as it travels through the region of output helical wave guide 34, it is preferable that the collector electrode 21 be connected to a source of potential (not shown) which is that required to produce such electron velocity, as determined by the above equation. It is evident that this potential is lower than that of the accelerating electrode 26 of the electrode 81m.

The signal induced in the output helical wave guide 34 is taken from the system by means of coupling loop 48 of co-axial cable 44. The intensity of the output signal is greater than the input signal at coupling loop 43 by virtue of the extraction of the kinetic energy of the electron beamand the conversion thereof to electromagnetic wave energy. The absorptive blocks 54 disposed'at the ends of output helical wave guide 34 preclude standing waves therein and the consequent reduction of energy signal output.

In the embodiment of the beam traveling wave illustrated in Figure 1, it is preferable, as mentioned above, that the average velocity of the electron beam entering the region of helical wave guide 30 be of the order of the axial velocity of the traveling wave. With this velocity relationship, a maximum bunching effect may be obtained with a minimum length of helical wave guide. If the velocity of the beam is widely different than that of the traveling wave the electrons do not remain under the influence of either an accelerating or retarding electric field for a sufilcient period of time to permit effective bunchmg.

Another serious disadvantage encountered with the utilization of relatively low electron velocities is that the average velocity of such beam will be increased as it passes through the input wave guide 3!. This velocity increase represents an overall rise in the kinetic energy of the beam, which energy obviously must be derived from the input signal source. This energy represents an efiective loading of the input signal source which, as in conventional amplifiers, measurably reduces the efiiciency and gain.

- For a wide range of input signal frequencies, the field configuration within the helical wave guides 30 and 34 and the velocity of propagation therethrough are substantially-independent of frequency. Accordingly, the electron tube amplifier of Figure 1 may be used to amplify high frequency signals over an extended frequency spectrum. These signals may be constituted of frequency components covering a comparatively wide. frequency band. Other than the normal frequency limitations of wave guides ll and 34, thereare no tuned circuits in the apparatus of Figure l.

It is preferable that the diameter of dielectric cylinder 2! be selected so that the substantially cylindrical wave guide defined thereby has a cut-oft frequency considerably above the operating frequency band of the amplifier. In this manner, direct wave propagation and wave reflection along the axis 28 of the tube, which might otherwise interfere with normal bunching, are precluded.

The amplifier electron tube illustrated in Figure 1 may be-utilizcd as a microwave sig al 9 feedback energy must, of course, be selected so that the feedback is regenerative to sustain oscillation. Since, as mentioned above, operation of the electron tube of Figure 1 is insensitive to frequency over wide bands, it is essential for stable oscillator operation to incorporate a frequency selective circuit in the feedback connection between the output wave guide 34 and the input waveguide 30. For this purpose a tunable resonant cavity (not shown) may be utilized so that the frequency of oscillation may be controlled.

The input wave guide 30 of the beam travelin wave tube, as previously mentioned, is formed by a helical metal strip 3| wound at substantially constant pitch. Accordingly, the electrical constants of wave guide 30, as for example its characteristic impedance and its velocity of propagation, are unvariable over its entire length.

On the other hand, the helical wave guide 34 has continuously variable electrical constants. Ordinarily the gradual variation of electrical constants will not seriously impair operation by signal reflection; however, under certain operating conditions it may be irable to modify helical wave guide 34 in suitable manner so that parameters thereof are constant over its entire length. This modification may comprise the insertion of a tapered dielectric material having the general form of the tapered blocks 54, or the outer diameter of the helix may bevaried uniformly ina predetermined relation.

Referring now to Figure 2, there is illustrated amicrowave amplifier or. oscillator having the general electrical features illustrated and described in connection with Figure 1, but differ ing somewhat in construction. The electron tube of Figure 2 comprises a. glass, or similar dielectric cylinder 6|, sealed and evacuated to permit the passage of an electron beam therethrough;

Sealed into the left-hand end of cylinder 6|, as viewed in Figure 2, is an electron gun comprising an indirectly heated cathode 62, a perforated disc-type grid 63, and focusing and accelerating cylindrical electrodes 64 and 65, respectively. As described in connection with Figure 1, when these electrodes are energized from a suitable power source (not shown), an electron-beam is generated having an average electron velocity determined as a function of the total accelerating potential in accordance with the equation set forth above.

The generated electron beam is directed along axis 66 toward a collector electrode 61, suitably sealed into the right-hand end of the cylinder 6|. For amplifier and oscillator operation, the potential of collector electrode 61 is'determined in a manner similar to that described hereinabove for collector electrode 21, shown in Figure 1.

In accordance with the principles of the present invention, an input helical wave guide 1| and an output helical wave guide 12 are co-axially disposed with respect to glass cylinder 6|' and positioned adjacent the electron gun end and the collector anode end, respectively. Helical wave guides 1| and 12 are preferably formed of coils of unitary U-shaped metallic channels, comprising side walls 13 and 14 rigidly spaced by integral outer metallic strip 15. As illustrated, the inner open face of each U-shaped channel is formed to lie in the outer surface of cylinder 6|.

Input and output helical wave guides 1| and 12, respectively, are of the same general construction; however, it is to be noted that the pitch of the input wave guide 1| is uniform, whereas the pitch of the output wave guide 12 progressively diminishes from a pitch equal to that of input wave guide 1|, in the direction of electron travel from left to right, as viewed in Fig. 2.

In order to prevent signal reflections and standing waves within the wave guides 1| and 12, helical dissipative blocks, such as 18, are secured within all four ends of the wave guide sections. As described in connection with the impedance matching blocks 54 of Figure 1, blocks 18 are preferably uniformly tapered and constituted of a material capable of efiectively dissipating microwave energy. The length of a block 18, when measured helically about the guide, is greater than a wave length at the lowest frequency of tube operation.

For reasons to be described in detail below.

the space between inp t and output helical wave guides 1| and 12 respectively, and surrounding cylinder 6|, may be filled as illustrated by a molded block of attenuating material 10, formed as dissipative blocks 18.

Electromagnetic wave energy is introduced into the input wave guide 1| through a co-axial line 16, the outer wall of which enters the wave guide section through a perforation 11 in the outer metallic strip 15. A loop 8| formed of the coaxial line inner conductor,-provides ample signal coupling. In a corresponding manner, a co-axial cable 82 is utilized to extract signal energy from the output helical wave guide 12. Thus, the outer conductor of co-axial cable 82 extends into the wave guide 12 through a perforation 83 therein, and a loop 84 in the inner conductor serves as the coupling means.

The outer helical surfaces of the wave guides 1| and-12 are substantially enclosed within a ba'selite or similar insulating cylinder 85, upon which a uniform, solenoid-type coil 86 is wound. When coil 86 is energized from a direct current source (not shown), a magnetic field is established having a large component parallel to the general, when used as an amplifier, the weak incoming signal energizes the input helical wave guide 1| by means of the coupling loop 8|. This input signal travels down the helical wave guide 1| in the direction of flow of electrons toward the collector anode 61. The actual axial velocity of the traveling wave is a fraction of the velocity through the wave guide 1|, due to the helical path traversed.

Dissipative blocks 18 ensure the absence of traveling waves in a direction other than the axial direction of electron flow. Between any two opposed points, such as M and N, of the open-sided helical wave guide 1|, an electric field is established which has a component within the tube 6| parallel to the axis 66 of electron flow. This electric field component may be visualized as spinning about the axis of the tube 6| at somewhat less than the velocity of light as accuses Figure 2.

The average velocity of electrons entering the space within tube GI encompassed by helical wave guide H is preferably of the order of the axial velocity of the wave. In this manner the elec tron beam is density modulated as it traverses the region of the helical wave guide 1i and emerges into the space between helical wave guides 1i and I! as a comparatively high energy bunched beam.

Upon entering the region of tube 8| spanned by the helical wave guide I2, the bunched electron beam is retarded and delivers energy to the wave guide 12 by excitation of a traveling wave therein which progresses at a speed approaching the speed of light toward output coupling loop 84 of co-axial cable 02. The pitch of the output helical wave guide I! is progressively diminished" as has been previously described, to accommodate for this reduction in beam velocity as it moves toward the collector electrode 81. In this manner, maximum energy is extracted from the bunched electron beam and delivered over co-axial cable 82 to a suitable load circuit (not shown).

By utilizing a constant width U-shaped channel for the structure of helical wave guides 'II and 12, the electrical constants, such as the characteristic impedance of these wave guides, are constant over their entire length and are independent of the variable pitch construction inwithin the cylinder ii, of comparatively uni-.

guide inter-turn space I flowing from the output guide 12, and vice versa. are substantially eliminated. As will be discussed below in connection with the embodiment of' Figure 3. the inter-tum spaces ll and .1 may themselves be filled with dissipative material (not shown in Figure 2) to preclude the flow of energy therein.

Referring now to Figure 3, there is illustrated a beam traveling wave tube, combining the novel features of the structures illustrated in Figures 1 and 2, within an enclosure which permits of simpower source (not herein illustrated), this electron gun provides an electron beam directed along the axis 98 of the tube toward a collector electrode Ill, suitably sealed within the end 53 of the tube.

. Input and output helical wave guides I03 and III are disposed within the cylindrical envelope 9i. These .wave guides are in all respects similar to the helical wave guides II and II, described in formly weak electric field, that is to say, that the electricfleld is at all times concentrated between opposed broad walls II and 14 of the wave guide. In efiect, therefore, regions within the cylinder Ii, such as that between points N and connection with the tube of Figure 2, with the V exception that they do not enclose an evacuated within region ill to the axis ll, an insulating The electron tube of Figure 2 may be connected for oscillator operation by connecting a portion of the signal output obtained in co-axial cable diameter of cylinder 8| be chosen such that when the volume enclosed thereby is considered as a cylindrical wave guide, it has a cut-oi! frequency above'the range of operation of the tube, whereby direct wave propagation and wave reflections at the signal frequency tending to distcrt the normal electric fields are precluded.

The cylindrical block ll of dissipative material functions to minimize energy transfer from output to'input circuits of the tube whether or not the attenuating and matching inserts 18 are utilized. In addition, block ll serves to attenuate wave energy flowing into the inter-turn spaces II and I! of the input and output wave guide recvlinder III is positioned over the central portion of envelope 9! and is wound with a uniform solenoid-type coil I. when energized from a direct current source, coil I" establishes a magnetic field parallel to the path 9| oi' electron. beam travel. Electrons which tend to diverge from the axis 98 are urged back into the axial beam by virtue of their interaction with the magnetic field.

The outer conductor of\ an input co-axial cable ill extends through a seal l l I in the envelope 9! and through a perforation III in the outer wall of wave guide ill. Loop Ill in the inner conductor provides a coupling means between the cable and the wave guide Ill; An insulating seal III is utilized within the co-axial cable ill to preclude air leakage into the evacuated env l pe ll.

An output coupling is provided by means of a co-axial cable iil, the outer conductor of which extends through a seal I22 in the cylinder II and through a corresponding perforation in the outer wall of helical wave guide ill. An output coupling loop Ill is formed on the end of the inner conductor of co-axial cable III. As in co-axial 10' cable I, a seal I is disposed between inner spectively. Thus, undesirable interaction with the electron beam of wave energy in the input II dissipative blocks III areinsertedintheendsof and, outer conductors of cable ill to preclude leakage of air into the electron tube.

As described in connection with the helical wave guide structures of Figure 2, four tapered wave guides I 03 and I04 to insure refiectionless transmission of energy therethrough.

For the purpose of preventing the fiow of energy within the inter-turnspaces I08"and I of input and output wave guides I03 and I04, respectively, helical dissipative blocks I08 and I09 may be inserted therein. These dissipative elements I08 and I0! are similar in form to the wave guides I03 and I04 and do not enter into or impede the flow of electrons through cylindrical space I00. Fbr reducing the inter-turn space-energy to a minimum, the helical members I08 and I09 are preferably made to extend from end to end of their respective wave guides. In a manner similar to that described in connection with block" in Figure 2, the region I01 between wave guides I03 and I04 may be filled with attenuating, power absorptive material, with the exception, of course, of an axial cylindrical opening for the passage of the electron beam.

In operation as an amplifier, the incoming signal is utilized to energize input helical wave guide I03 over co-axial cable III. The interaction of the fields established between the open inner edges of the helical wave guide extending into region I00 and the electron beam passing. therethrough, velocity modulates the electron beam.

The passage of the velocity modulated beam through the helical wave guide I04 delivers energy thereto, which is delivered to a load circuit by co-axial cable I2I. As in the examples of the preceding figures, input helical guide I03 is of substantially uniform pitch, whereas output wave guide I04 is of progressively diminishing pitch to accommodate the retardation of the electron beam as it passes axially therethrough.

The fundamentals of amplifier and oscillator operation of the device illustrated in Figure -3 are essentially no different from those already treated in connection with the operation of the device shown in Figure 2, and accordingly further details are believed unnecessary.

As a result of the novel construction illustrated in Figure 3, the electric field established by the wave guides in region I00 does not extend through a glass cylinder, such as members 2| and 6| of Figures 1 and 2, respectively. As a consequence, the dielectric losses of the electron tube shown in Figure 3 are negligible, permitting operation at maximum efiiciency. Frequency and band width considerations are no different from those of the preceding tube structures, with the exception that lower dielectric losses somewhat extend the upper frequency limit.

From the foregoing description of the present traveling wave electron tubes, it is apparent that these oifer design possibilities unattainable with other known means. -Although described above as functioning as signal amplifiers, these tubes may be employed as microwave modulators and detectors, and in other circuits where conventional tube types fail to give the desired performance.

Numerous modifications of the apparatus herein disclosed are possible without departing from the spirit of the present invention. As described, a helical path is employed for signal propagation to reduce the axial velocity of wave propagation to the average electron velocity. A large pitch, necessitating fewer turns for the input and output wave guides, may be used if the wave guides are filled with a solid, low loss dielectric substance. This is possible since the velocity of wave propagation in a solid dielectric is substantially less than the velocity in free space.

In order'to increase of the tubes illustrated, the helical wave guides may be operated with a uniform increasing po tential gradient. Thus, the guides may be formed of a resistive material rather than of highly conductive substances as hereinabove described, and a direct voltage maintained between the ends thereof, the positive end being furthest along in the direction of electron travel. This positive gradient will accelerate and add to the kinetic energy of the electrons, and permit the extraction of increased power from the bunched beam.

In utilizing the electron tubes of Figures 1 to 3, inclusive, for signal generation, it is possible to induce oscillation without an external feedback loop. Thus, by proper selection of the physical dimensions of the tube, the waves may be refiected through the cylindrical region carrying the electron beam. If such waves are of proper phase, self-oscillation is initiated.

In view of the many possible structural and electrical design modifications possible, it is preferred that the present invention be defined solely by the appended claims.

I claim:

1. A beam traveling wave electron tube comprising in combination an evacuated dielectric cylinder, an electron gun disposed at an end of said cylinder for generating an electron beam of predetermined average electron velocity, a collector electrode positioned within the opposed end of said dielectric cylinder, means for directing said generated electron beam axially through said dielectric cylinder and impinging said beam upon said collector electrode means disposed adjacent said electron gun for velocity modulating said electron beam comprising a first helical wave guide of substantially uniform pitch, said helical wave guide being a channel shaped conductor of substantially rectangular cross-section and being co-axial with said dielectric cylinder, the open wall of said channel lying in the cylindrical outer surface of said dielectric cylinder, means for in troducing microwave energy to said first helical wave guide for establishing a traveling electromagnetic wave therein, the axial component of the velocity of said travelling wave along said guide, as determined by said pitch, being substantially equal to the average velocity of said electron beam, means disposed adjacent said collector electrode adapted to extract energy from a velocity modulated electron beam comprising a second helical wave guide formed as said first helical wave guide and co-axial with said dielectric cylinder, said second helical wave guide having a progressively diminishing pitch in the direction of travel of said electron beam toward said collector electrode, the maximum pitch of said second helical wave guide being equal to the pitch of said first helical wave guide, said second helical wave guide being thereby adapted to extract maximum energy from an electron beam of uniformly diminishing average velocity.

2. A beam traveling wave electron tube comprising in combination an evacuated dielectric cylinder, an electron gun disposed at an end of said cylinder for generating an electron beam of predetermined average electron velocity, a collector electrode positioned within the opposed end of said dielectric cylinder, means for directing said generated electron beam axially through said dielectric cylinder and impinging said beam upon said collector electrode, means disposed adjacent said electron gun for velocity modulating said electron beam comprising a helical wave guide of further the energy output in the outer cylindrical surface oi'said dielectric cylinder, means for introducing microwave energy to said helical wave guide for establishing a traveling wave therein, said helical wave guide permitting the interaction of microwave energy flowing therein and said electron beam, and a wave energy dissipative member disposed in the region between turns of said helical wave guide.

3. A beam traveling wave electron tube comprising in combination, within an evacuated substantially cylindrical envelope, an electron gun at one end of said envelope for generating an electron beam of predetermined average electron velocity, an axially opposd collector electrode at the opposite end of said envelope, said electron beam being directed axially toward said collector electrode, means disposed adjacent said electron-gun for velocity modulating said electron beam comprising a first helical wave guide of substantially uniform pitch, said helical wave guide comprising a channel shaped conductor of rectan ular crosssection and being co-axial with said envelope, the

open wall of said helical wave guide channel defining a cylindrical surface enclosing said electron beam, means for introducing microwave energy to said first helical wave guide for establishing a traveling electromagnetic wave therein, the

axial component of the velocity of said travelling wave along said guide, as determined by said pitch, being substantially equal to the average velocity of said electron beam, means disposed adjacent said collector electrode adapted to extract energy from an electron beam velocity modulated by said first helical wave guide comprising a second helical wave guide formed as said first helical wave guide and co-axial with said envelope, and

being formed of channel shaped conductor of rectangular cross section with an open-wall defining said cylindrical surface, said second helical wave guide having a progressively diminishing pitch in the direction of travel of said electron beam toward said collector electrode, the maximum pitch of said second helical wave guide being equal to the uniform pitch of said first helical wave guide, said second helical wave guide being thereby, adapted to extract the maximum amount of energy from a velocity modulated electron beam of uniformly diminishing velocity, coupling means including said opening in said second wave guide for taking energy from said second helical wave guide, andwave energy dissipative helical members substantialy filling the inter-turn spaces of said first and second wave guides. whereby energy flow insaid spaces is substantially eliminated.

4. A beam traveling wave electron tube comprising in combination, within an evacuated envelope, an electron gun at one end of said envelope for generating an electron beam, means for producing a predetermined electron velocity of said electron beam, an axially opposed collector electrode at the opposite end of said envelope, a centrally disposed dielectric cylinder for containing said electron generating means and said collector electrode, the diameter 01' said cylinder being such that the substantially cylindrical wave guide formed thereby has a cut-oi! frequency considerably above the operating frequenelectromagnetic wave therein, the axial component of the velocity of said travelling wave alon said guide, as determined by said Ditch, being substantially equal to the average velocity of said electron beam, a second helical wave guide surrounding said dielectric cylinder and axially spaced from said first wave guide, said second helical wave guide having a progressively diminishing pitch in the direction of travel of said electron beam toward said collector electrode, the maximum pitch of said helical guide being equal to the uniform pitch of said first helical wave guide, means for maintaining the parameters of said second helical wave guide constant over its entire length comprising tapered dielectric material, coupling means at the output end or said second wave guide for absorbing energy from said second helical wave guide, wave energy dissipative helical members substantially filling the interturn spaces of said first and second wave guides for eliminating energy flow in said spaces, and wave energy dissipative elements at the ends of said wave guide for preventing standing waves.

5. A beam traveling wave electron tube comprising in combination, within 'an evacuated envelope, an electron gun at one end of said envelope for generating an electron beam. means for producing a predetermined electron velocity of said electron beam, an axially opposed collector electrode at'the opposite end oi said envelope, a centrally disposed dielectric cylinder for containing said electron generating means and said collector electrode, means for directing said electronsaid guide, as determined by said pitch, being substantially equal to the average velocity of said electron beam, a second helical wave guide surrounding said dielectric cylinder and axially spaced from said first wave guide, said second helical wave guide having a progressively diminishing pitch in the direction of travel of said electron beam toward said collector electrode, the maximum pitch of said helical guide being equal to the uniform pitch ot said first helical wave guide, coupling means at the output end of said second wave guide for absorbing energy from said second helical wave guide, and tapered blocks at the ends of said wave guide for providing impedance match. r

6. A beam travelling wave electron tube comprising in combination, within an evacuated envelope, an electron gun at one end of said envelope for generating an electron beam, means for producing a predetermined electron velocity of said electron beam, an axially opposed collector electrode at the opposite end of said envelope, a centrally disposed dielectric cylinder for containing said electron generating means and 17 along said guide, as determined by said pitch, being substantially equal to the average velocity of said electron beam, a second helical wave guide surrounding said dielectric cylinder and axially spaced from said first wave guide, said second helical wave guide having a progressively diminishing pitch in the direction of travel of said electron beam toward said collector electrode, the maximum pitch of said helical guide being-equal 'to the uniform pitch of said first helical wave guide, coupling means at the output end of said second wave guide for absorbing energy from said second helical wave guide, and tapered blocks at the ends of said guide for providing impedance match, each block being made equal to at least one wave length at the lowest frequency of operation as measured along the helix.

7. A beam traveling wave electron tube comprising in combination, with an evacuated envelope, an electron gun at one end of said envelope for generating an electron beam, means for producing a predetermined electron velocity of said electron beam, an axially opposed collector helical wave guide having a progressively diminishing pitch in the direction of travel of said electron beam toward said collector electrode, the

maximum pitch of said helical guide being equal to the uniform pitch/oi said first helical wave guide, coupling means at the output end of said second wave guide for absorbing energy from said second helical wave guide, dissipative members in the spaces between said first and second wave guides for minimizing energy flow between said first and second wave guide.

9. A beam traveling wave electron tube comprising in combination, within an evacuated envelope, an electron gun at one end of said envelope for generating an electron beam, means for producing a predetermined electron velocity of said electron beam, an axially opposed collector electrode at the opposite end of said envelope, a centrally disposed dielectric cylinder for containing said electron. generating means and said collector electrode, means for directing said electron beam axially toward said collector electrode,

- a first helical waveguide of substantially unielectrode at the opposite end of said envelope, a

centrally disposed dielectric cylinder for containing said electron generating means and said collector electrode, means for directing said electron beam axially toward said collector electrode, a first helical wave guide of substantially uniform pitch surrounding said dielectric cylinder, means for introducing micro-wave energy to said first helical wave guide for establishing a traveling electromagnetic wave therein, the axial component of the velocity. of said travelling wave along said guide, as determined by said pitch, being substantially equal to the average velocity of said electron beam, a second helical wave guide surrounding said dielectric cylinder and axially spaced from said first wave guide, said second helical wave guide having a. progressively diminishing pitch in the direction of travel of said electron beam toward said collector electrode, the maximum pitch of said helical guide being equal to the uniform pitch of said first helical wave guide, coupling means atthe output end of said second wave guide for absorbing energy from said second helical wave guide, and tapered blocks at the ends of said wave guide for providing impedance match, the length of said blocks when measured helically about said wave guide being greater than a wave length at the lowest frequency of said tube operation.

8. A beam traveling wave electron tube comprising in combination, within an evacuated envelope, an electron gun at one end of said envelope for generating an electron beam, means .-for producing a predetermined electron velocity of said electron beam, an axially opposed collector electrode at the opposite end of said envelope, a centrally disposed dielectric cylinder for containing said electron generating means and said collector electrode, means for directing said electron beam axially toward said collector electrode, a first helical wave guide of substantially uniform pitch surrounding said dielectric cylinder, means form pitch surrounding said dielectric cylinder, mean for introducing micro-wave energy to said first helical wave guide for establishing a traveling electromagnetic wave therein, the axial component of the velocity of said travelling wave along said guide, as determined by said pitch, being substantially equal to the average velocity of said electron beam, a second helical wave guide surrounding said dielectric cylinder and axially spaced from said first wave guide, saidsecond helical wave guide having a progressively diminishing pitch in the direction of travel of said electron beam toward said collector electrode, the maximum pitch of said helical guide being equal to the uniform pitch of said first helical wave guide, coupling means at the output end of said second wave guide for absorbing energy from said second helical wave guide, wave energy dissipative members in the spaces between said first and second wave guides for minimizing energy fiow between said first and second wave guide, and wave energy dissipative elements at the ends of said wave guide for preventing standing waves.

10. A beam traveling wave electron tube comprising in combination, within an evacuated envelope, an electron gun at one end of said envelope for generating an electron beam, means for producing a predetermined electron velocity of said electron beam, an axially opposed collector electrode at the opposite end of said envelope, means for directing said electron beam axially toward said collector electrode, a, first helical wave guide of U-shaped conductive channel and of substantially uniform pitch, means for introduclng micro-wave energy to said first helical wave guide for establishing a traveling electromagnetic wave therein, the axial component of the velocity of said travelling wave along said guide, as determined by said pitch, being substantially equal to the average velocity of said electron beam, a second helical wave guide of U-shaped conductive channel and axially spaced from said first wave guide, said second helical wave guide having a progressively diminishing pitch in the direction of travel of said electron beam toward said collector electrode, the maximum pitch of said helical guide being equal to the uniform pitch of said first helical wave guide, the inner edges of said U-shaped conductive channels of said helical wave guides defining a cylinder containing said electron beam generating means and collector electrode, the cylinder being of sufiicient diameter t permit unimpeded passage thercthrough of the electron beam, coupling means at the output end of said second wave guide for absorbing energy from said second helical wave guide, dissipative helical membersisubstantially filling the inter-turn spaces of said first and second wave guides for eliminating energy fiow in said spaces, and wave energy dissipative elements at the ends of said wave guide for preventing standing waves.

11. A beam traveling wave electron tube comprising in combination, within an evacuated envelope, an electron gun at one end of said envelope for generating an electron beam, means for producing a predetermined electron velocity of said electron beam, an axially opposed collector electrode at the opposite end of said envelope, means for directing said electron beam axially toward said collector electrode, a first helical wave guide of U-shaped conductive channel and of substantially uniform pitch, meansfor introducing micro-wave energy to said first helical wave-guide for establishing a traveling electromagnetic wave therein, the axial component of the velocity of said travelling wave along said guide, as determined by said pitch, being substantially equal to the average velocity of said electron beam, a second helical wave guide of U-shaped conductive channel and axially spaced from said first wave guide, said second helical wave guide having a progressively diminishing pitch in the direction of travel of said electron beam toward said collector electrode, the maximum pitch of said helical guide being equal to the uniform pitch of said first helical wave guide, the inner edges of said U-shaped conductive channels of said helical wave guides defining a cylinder'containing said electron beam generating means and collector electrode, coupling means at the output end of said second wave guide for absorbing energy from said second helical wave guide, dissipative helical members substantially filling the inter-turn spaces of said first and second wave guides for eliminating energy fiow in said spaces, and wave energy dissipative elements at the ends of said wave guide for preventing standing waves.

12. A beam traveling wave electron tube comprising in combination, within an evacuated envelope, an electron gun at one end of said envelope for generating an electron beam, means for producing a predetermined electron velocity of said electron beam, an axially opposed collector electrode at the opposite end of said envelope, means for directing said electron beam axially toward said collector electrode, a first helical wave guide of U -shaped conductive channel and of substantially uniform pitch, means for introducing micro-wave energy to said first helical wave guide for establishing a traveling electromagnetic wave therein, the axial component of the velocity of said travelling wave along said guide, as determined by said pitch, being substantially equal to the average velocity of said electron beam, a second helical wave guide of U-shaped conductive channel and axially spaced from said first wave guide, said second helical wave guide havin a progressively diminishing pitch in the direction of travel of said electron beam toward said collector electrode, the maximum pitch of said helical guide being equal to the uniform pitch of said first helical wave guide, the inner edges of said U-shaped conductive channels of said helical wave guides defining a cylinder containing said electron beam generating means and collector electrode, coupling means at the output end of said second wave guide for absorbing energy from said second helical wave guide, and wave energy dissipative elements at the ends of said wave guide for preventing standing waves.

13. A beam traveling wave electron tube comprising in combination, within an evacuated envelope, an electron gun at one end of said envelope for generating an electron beam, means for producing a predetermined electron velocity of said electron beam, an axially opposed collector electrode at the opposite end of said envelope, means for directing said electron beam axially toward said collector electrode, a first helical wave guide of U-shaped conductive channel and of substantially uniform pitch, means for introducing micro-wave energy to said first helical wave guide for establishing a traveling electromagnetic wave therein, the axial component of the velocity of said travelling wave along said guide, as determined by said pitch, being substantially equal to the average velocity of said electron beam, a second helical wave guide of U-shaped conductive channel and axially spaced from said first wave guide, said second helical wave guide having a progressively diminishing pitch in the direction of travel of said electron beam toward said collector electrode, the maximum pitch of said helical guide being equal to the uniform pitch of said first helical wave guide, the inner edges of said U-shaped conductive channels of said helica wave guides defining a cylinder containing said electron beam generatin means and collector electrode, coupling means at the output end of said second wave guide for absorbing energy from said second helical wave guide, and wave energy dissipative elements for preventing standing waves.

14. A beam traveling wave electron tube comprising in combination, within an evacuated envelope, an electron gun at one end of said envelope for generating an electron beam, means for producing a predetermined electron velocity of said electron beam, an axially opposed collector electrode at the opposite end of said envelope, means for directing said electron beam axially toward said collector electrode, a first helical wave guide of substantially uniform pitch, mean for introducing micro-wave energy to said first helical wave guide for establishing a traveling electromagnetic wave therein, the axial component of the velocity of said travelling wave along said guide, as determined by said pitch, being substantially equal to the average velocity of said electron beam, a second helical wave guide surrounding said dielectric cylinder and axially spaced from said first wave guide, said second helical wave guide having a progressively diminishing pitch in the direction of travel of said electron beam toward said collector electrode, the maximum pitch of said helical guide being equal to the uniform pitch of said first helical wave guide, means for maintaining the parameters of said second helical waveguide constant over its entire length comprising tapered dielectric material, coupling means at the output end of said second wave guide for absorbing energy from said second helical wave guide, wave energy dissipative helical members substantially filling the inter-turn spaces of said first and second wave guides for eliminating energy flow in said spaces, and wave energy dissipative elements at the ends of said wave guide for preventing standing waves.

' velope, an electron gun at one end of said envelope for generating anelectron beam, means for producing a predetermined electron velocity of said electron beam, an axially opposed collector electrode at the opposite end of said envelope, means for directing said electron beam axially toward said collector electrode, a first helical wave guide of substantially uniform pitch, means for introducing micro-wave energy to said first helical wave guide for establishing a traveling electromagnetic wave therein, the axial component of the velocity of said travelling wave along said guide, as determined by said pitch, being substantially equal to the average velocity of said electron beam, a second helical wave guide surrounding said dielectric cylinder and axially spaced from said first wave guide, said second helical wave guide having a progressively diminishing pitch in the direction of travel of said electron beam toward said collector electrode, the ma um pitch or said helical guide being equal to the uniform pitch of said first helical wave guide, means formaintaining the parameters of said second helical wave guide constant over its entire length comprising tapered dielectric material, coupling means at the output end of said second wave guide for absorbing energy from said second helical wave guide, wave energy dissipative helical members between said first and second wave guides for eliminating energy flow in said spaces, and wave energy dissipative elements for preventing standing waves in said wave guides.

16. A beam traveling wave electron tube comprising in combination, within an evacuated envelope, an electron gun at one end of said envelope for generating an electron beam, means for producing a predetermined electron velocity of said electron beam, an axially opposed collector electrode at the opposite end of said envelope, means for directing said electron beam axially toward said collector electrode, a first helical wave guide oi substantially uniform pitch, means for introducing micro-wave energy to said first helical wave guide for establishing a traveling electromagnetic wave therein, the axial component of the velocity of said travelling wave along said guide, as determined by said pitch, being substantially equal to the average velocity 01' said electron beam, a second helical wave guide surrounding said dielectric cylinder and axially spaced from said first wave guide, said second helical .wave

guide having a progressively diminishing pitch in the direction of travel of said electron beam toward said collector electrode, the maximum pitch or said helical guide bein equal to th uniform pitch oi said first helical wave guide, means for maintaining the parameters of said second helical wave guide constant over its entire length comprising tapered. dielectric material, coupling means at the output end of said second wave guide for absorbing energy from said second helicalwave guide, wave energy dissipative helical members between said first and second wave guides for eliminating energy flow in said spaces, and wave energy dissipative elements at the ends oi said wave guide for preventing standing waves.

17. A beam traveling wave electron tube comprising in combination, within an evacuated envelope, an electron gun atone end of said envelope for generating an electron beam, means for producing a predetermined electron velocity of said electron beam, an axially opposed collector electrode at the opposite end of said envelope, means for directing said electron beam axially toward said first wave guide, said second helical wave guide having a progressively diminishing pitch in the direction of travel of said electron beam toward said collector electrode, the maximum pitch oi said helical guide being equal to the uniform pitch of said first helical wave guide, means for maintaining the parameters of said second helical waveguide constant over its entire length comprising tapered dielectric material, coupling means at the output end of said second wave guide for absorbing energy from said second helical wave guide, and wave energy dissipative elements at the ends of said wave guide for preventing standing waves.

' JOHN W. TILEY.

REFERENCES CITED The following references areof record in the file of this patent:

. UNITED STATES PATENTS Number