US2924739A - Traveling-wave electron discharge device - Google Patents

Description

R. H. GEIGER TRAVELING-WAVE ELECTRON DISCHARGE DEVICE Filed March 29, 1956 Feb. 9, 1960 2 Sheets-Sheet 1 INVENTOR R/CHA/eo MGE/GEA? AGENT WANN@ summit l.. l

R. H. GEIGER TRAVELING-WAVE ELECTRON DISCHARGE DEVICE Filed March 29, 1956 Feb. 9, 1960 2 Sheets-Sheet 2 INVENTR R/cHA/Po H. GE/QER AGENT United States PatentO'f TRAVELlNG-WAVE ELECTRON DISCHARGE DEVCE Richard H. Geiger, Emerson, NJ., assignor to International Telephone and Telegraph Corporation, Nutley, N J., a corporation of Maryland Application March 29, 1956, Serial No. 575,695

12 Claims. (Cl. 315-35) This invention relates to traveling-wave electron discharge devices and, more particularly, to gas discharge tubes for introducing a circuit loss along the path of the radio-frequency field of a traveling wave in such devices to prevent both undesired oscillations and modes of propagation therein, and for modulating the output of the traveling-wave device and for shifting the phase of the propagating wave.

The general structure and theory of operation of a traveling-wave type of electron discharge device or tube is well known, a comprehensive treatment being given in the book Traveling-Wave Tubes, by J. R. Pierce, published by D. Van Nostrand Company, Inc., New York, 1950.

It is recognized that the useful range of amplification of a traveling-wave tube is limited by a tendency to generate self-sustaining oscillations as the amplification is increased. This effect is usually due to mismatch between the output circuit of the tube and the load circuit over all or part of the wide range of frequencies to be amplified. Because of such mismatch, energy of at least certain frequencies is reected back toward the input end of the amplifying device. When the reflected wave is not attenuated in its travel along the helix, or other type of propagating structure, in a direction opposite to the motion of the electron stream, someenergy reaching the input end of the device is reflected therefrom causing the generation of self-sustaining oscillations. Thus, the energy reflected or transmitted back to the input must be attenuated if the tube is to operate in a stable manner.

Heretofore various means have been employed to overcome this tendency of generating self-sustaining oscillations. One such means employs a body of resistive material disposed on or adjacent to the propagating struct-ure, either in a confined location thereabout or coextensive therewith to absorb the reflected energy. Another such means employs wire-wound helices coaxial with the propagating structure and in coupled relation thereto for removal of reilected energy from the propagating structure for coupling to an appropriate termination thereof to overcome the tendency for self-sustaining oscillations. Another such means provides a coupled-helix circuit loss by coating adielectric sleeve coaxial with the propagating structure with a conductive film having` a predetermined conductivity. In still another such means, particularly useful where high-power amplification is required, a lossy fluid is circulated through a coupled helix to provide attenuation and remove heat generated.

These means for overcoming the tendency for selfsustaining oscillations have greatly improved the usefulness of the traveling-wave tube, at least for certain applications. However, in attempts to obtain high power and high gain, stability of the attenuation at high temperatures, broadband attenuation characteristics and reproducibility of electrical and mechanical properties of the attenuators, it has been found that the above-mentioned vtypes of attenuators fail to meet one or more of 2,924,739 Patented Feb. 9, 1960 ICC the above major physical and electrical properties. Thus the use of resistive or lossy material to attenuate the reflected waves, while advantageous in some respects, nevertheless possesses several drawbacks. Thus the distribution of the resistive material along the entire length of the helix, along a major portion thereof, in lump form, or as part of the helical conductor itself tends to limit the gain and power output of the device. Where the resistive material is spaced from the conductor, such as in the lumped form, it is less effective and requires a larger amount in order to absorb the reflected energy. Further, the utilization of an organic lossy material, such as carbon, or aquadag, makes the out-gassing of the traveling-wave device in some types of construction more diflicult inasmuch as the organic material entrapsv more gases than would normally be found had the organic material been omitted.

In my copending application, Serial No. 465,513, tiled October 29, 1954, now Patent No. 2,788,464, I have` solved some of the foregoing problems associated with the use of traveling-wave electron discharge devices by providing an absorptive gas discharge plasma along the path of the radio-frequency field of a traveling wave, thereby introducing a desired attenuative circuit loss along this path. However, loss of useful information because of phase shift, for example, has not been eliminated by this method. Thus, as is well known, the amount of useful information that can be transmitted by an electromagnetic wave is a function of three parameters ofthe wave: frequency, amplitude and phase. tortion of any of these three parameters can result in undesirable output characteristics.

It is an object of the present invention, therefore, to provide an improved means for introducing a circuit loss in the path of the radio-frequency wave of a traveling- Wave device -so as to prevent self-sustaining oscillations, obtain high gain and maximum output power, and at the same time provide phase compensation or phase shifting ofthe output Wave.

It is a further object to provide the above means for introducing a circuit loss in combination with means for modulating the output of a traveling-wave tube.

One of the features of `this invention is the utilization of a gas discharge device having an electron density upon i ionization such that the collision frequency of the elecp trons therein is approximately equal to the angular frequency of the propagating ,microwave energy to intercept part of the high-frequency field of the propagated energy 1 propagating structure, one of vsaid devices acting principally as an attenuator, and another of said devices acting as a modulator or phase shifter of the propagated energy.

Another feature of this invention is the use of a capillary gas discharge tube in single or multiilar counterwound coniiguration with respect to the main propagating helix for attainment of maximum radio-frequency coupling.

The above-mentioned and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawings, in which:

Fig. l is a diagrammatic representation of a travelingwave electron discharge device incorporating a unitary with this invention;

Fig. 2 is an elevational view partly in section showing Dist a traveling-wave electron discharge device incorporating a plurality of gas discharge devices; and

Fig. 3 is a sectional view partly in elevation showing a capillary gas discharge tube in single iilar counterwound configuration with respect to the main'helix.

Referring to Fig. l, therev isshownl an illustrative embodirnent of a traveling-wave tube adapted'to be used as an amplifier at ultra-high frequencies. The arrangement shown comprises an electron-beam tube including an evacuated envelope 1. The envelope 1 may be constituted of a low-loss insulating material, such as glass or quartz, or a non-magnetic type of metal. The shape of envelope 1 may have a uniform diameter as illustratedin Fig. l or may include an elongated portion coexte'nsive with the .interaction section 2 and in supporting' relationshipl with the propagating structurey 3 VincludedE therein.' The envelope 1 is provided at one end with means, such as a' known type of electron gun-4 for producing an Aelectron beam or stream'. The electron stream emerges from the electron gun 4 and travels along a rectilinear" path through an input coupler 5 and axially down the evacuatedl envelope 1. The electron stream is further' concentrated and guided along ythe substantially axial path by a'l long-itudinal magnetic field produced by magnet 6 which may be either an electromagnet or a permanent magnet. Electrode 7 serves to collect the electrons arrivingat the end of envelope 1.

The propagating structure 3, which serves as a path along which the radio-frequency -Wave may be propagated, is illustrated as including a helix 8. Helix 8 is Wound with several turns per wavelength along its axis, which may preferably be of a plurality-of wavelengths of the frequency being amplied. The helix is illustrated as being supported by a series of non-conductive rods 9 equally spaced about the circumference thereof and supported in an axial relationship with respect to envelope 1 by disks 10. As is known in this art, helix S may be supportedby means of a ceramic tubing or by so shaping envelope 1 to provide an elongated portion in contact with the helix'S for support thereof.

lThe helix 8 is joined at the input coupler 5 by an input impedance matching section V11 and the output coupler 12 by the output impedance matching section 135. These matching sections are simply extensions of the helix in which the spacing between turns is increased along the circumference of the helix. Thereby, these sections act as tapered transmission lines to provide a wave transmission path of uniformly changing impedance from the relatively low impedance at the end of the couplers 5 and 12 to the relatively high impedance of the central portion'of helix 8, with a minimum reflection of energy back to `the signalsource.

In order to utilize the device in an operable system, there is provided an incoming wave path represented by thedotted input waveguide 14 into which there Ais introduced the input wave signal to be amplified. An output wavepath, shown as the output lwaveguide 15, serves -to transfer the amplified output wave to a load circuit. As the electron beam and the radio-frequency wave-travel axially of the helix at substantially the same linear velocity, an interaction takes place whereby energy is transferred from the beam to the wave, thereby greatly amplifying the Wave. As the amplified wave reachesy the output end of the helix 8, it is transferred to the output waveguide 1S by'means of output coupler-12. As the amplilied wave reaches the impedance matching section 13, even with an extremely favorable termination, at'a'given band of frequencies, there will still exist reflected waves at frequencies both inside and outside the given band at the output end of the-helix. This'reiiected wave is very little affected by the electron streamand hence will propagatebacltralon'g the helxfS toward the-input end with attenuation. The reiiected Wave will reach the-input end ofthe helix 8 with attenuation equal to the circuitattenuation and will in turn be reflected back 'toward' the outputA end ofthe helix. Itis obvious that where there is not enough circuit attenuation to dampen the reiiected energy, this energy will result in self-sustaining oscillations. It will thus be seen that the deliberate introduction of an artificial loss within the electromagnetic field of the helix so as to provide a dissipation or removal of the reflected wave will serve to greatly increase the range of the useful amplification which canbe achieved witha device of this type. It is of course an essential feature of travelingwave tube construction that this attenuation must be provided. In addition to providing this attenuation, it is highly important for many applicationsin this field to be able to selectively and controllably modulate the electromagnetic Wave output, correct any phase distortion thereof, and alter the phase thereof as desired.

In accordancewith the principles of this invention, au artificial loss is introduced along a portion of the helix S which will increase the range of useful amplification, but whichr will not reduce the gain. and powenoutput of the traveling-wave device; The articial loss of' this invention introduced in juxtaposition to helix y8 isprovided by gas discharge device 16. In the. embodiment illustrated, this device comprises anelongated envelopef17 disposed to be in contact with .the support rods or tubing 9 of the helical propagating structureV 8. The. inner surface. of envelope 17 is brought as close as possible. tothe electromagnetic field present abouty the helix 8'. Theenvelope 17 is iilled with a suitable rarer gas, such as helium, neon, argon, krypton and xenon, and is causedto ion-ize to function as an attenuator for thepropagating structure of a traveling-wave electron. discharge device'.

As is known, a gas discharge plasma may act as a propagating medium forA microwave energy, and. if the electron density ofthe discharge plasma is adjusted so that the collision frequency of the electrons of the gas is approximately equaLto the angular'frequency of thepropagating microwave energy, maximalabsorption of the microwave energy propogating through the gas plasma occurs. The degree ofrabsorption is furtherdetermined by the magnitude and distribution of the. electron density in the radiofrequency fields. Thus, it'has been discovered that if a gas dischargeplasma islocated close to the turns of a helical structure, .such as helix 8, wherethe electromagnetic field distribution isat a maximum betweenthe turns of a helix, effective absorptionor attenuation will occur.

Discharge device l16 is provided with electrodes 1S and n 19 which in turn are placed across k apotential source 20,

as illustrated in Fig. l, which, when switch 21-is closed, Will establish a potential difference between electrodes 18 and19 of sutiicient value to ionize the gas within envelope 17 to produce a discharge plasma of suiicient electron density whose collision frequency is approximately equal to the angularfrequency of the propagated'energy such that an absorption or attenuation` of the energy will take place. Envelope 17 is provided with ends 2v2 which are tapered lradially with respect to the helix to reduce reflections from the glass envelope'17 andthe discharge plasma.

It is an essential feature of this invention that modulating means 23 are provided which can effectively vary the density of the gas discharge plasma and thereby provide modulation of the output Wave. In the'ernbodiment of modulating means illustrated in Fig. l, one of many such means is 4illustrated schematically, utilizing a triode 24 connected across electrodes 18 and 19 of the gas discharge tube 1,6. The plate 25 of the triode is biased by means of 'a voltage source 26 and variable resistive means 27. The iiow of conventional direct current from the plate 2'5v to the cathode 28 of the triode is controlled by the ybias on the grid 29, as is, of course, well-known in this art. Any desired signal of` a selected waveshape or other characteristics may be applied to the grid, as shown by arrow 29a, vto vary-the flow of currenty in the triode circuitin any desired periodic or aperiodic manner. Ef-

- fectivelythen, this iiow of current serves to modulate the potentiallsource appliedacrossfthe electrodes of-the gas ganarse' discharge device 16, thereby effectively altering the density of the gas plasma and consequently varying the degree of attenuation of the gas discharge. The net result, then, is modulation of the output wave of the traveling-wave amplier. Generally, it is preferredto arrange the relative voltages of the potential source and of the grid circuit so that there -is always a gas discharge maintained independently of the action `of the modulating means 23. What is effectively varied lin a desired `modulating manner is the degree of attenuation obtained in the device by varying the density of the Vgas plasma.

The manner in which the gas discharge plasma interacts with the electromagnetic wave of the traveling-wave tube has been described in my copending application` herein` before referred to, Serial No. 465,513, filed October 29, 1954, now Patent No. 2,788,464. As mentioned therein, the characteristic of the loss medium is such that most of the electromagnetic energy in the propagating structure is absorbed, and the signal is transmitted through the loss section by the modulation signal energy in the electron stream. At or near the output end of the loss section, the electromagnetic wave is re-excited in the lossless section of the propagating structure by the modulated electron beam. The location of discharge device 16 is a predetermined distance from the input end of structure 8 such that the equation CNB is satisfied, where C is the gain parameter relating the degree of interaction between the electron beam and electromagnetic wave propagated along the propagating structure and N is the number of wavelengths from the input end. Provision of an input portion of helix having a length corresponding to the aforesaid equation insures that the electron stream becomes suiciently excited according to the `radio-frequency signal at the input such that an electromagnetic wave is re-excited in the output portion of the helix after encounter-ing the attenuator. This re-excited wave in turn interacts with the modulated beam in a continuous manner as the wave and the beam progress down the tube at practically the same velocity in such a manner that the electromagnetic wave gains in amplitude. The length of the output or lossless section of the helix should satisfy the equation G=A-|BCN-aL, where G equals the gain of the tube desired, A is a coupling loss gure relating the voltage associated with the increasing wave to the total applied voltage, B is a` parameter associated with the increasing wave, C is the gain parameter, Ny is the length in wavelengths of the propagating structure, a is the fraction of the cold loss which should be subtracted from the gain of the increasing component of the propagated wave and L is the loss in decibels per wavelength. Providing a length of input section greater than that corresponding to CN 03 would result in little or no added gain if the attenuation of the electromagnetic wave by the attenuation medium is high enough to remove most ofthe electromagnetic energy. The length of the output section may be adjusted within reason to provide the desired gain for the wave coupled from the output of the propagating structure.

In certain instances and applications a separate external source of potential and the electrodes within the discharge device may be omitted. In these instances. the gas within the discharge device would be ionized by pro- Viding sufficient power in the radio-frequency tields` of` the waves propagated on the helix, such as occurs in TR devices. This eiect can be greatly enhanced by utilizing the longitudinal magnetic iield generated by magnet 6 if the magnetic field is the gyroresonant eld for the electrons of the gasat the microwave frequency of the waves on the helix. The influence of gas plasma on microwave energy and the gyroresonant effect therein is described in greater detail in the copending application of L. Goldstein et al., Serial No. 232,148, filed June 18, 1951, now Patent No. 2,773,245.

While one embodiment of modulating means has beenV illustrated in Fig. l, it is, of course, understood that many such means are known, used and available; and this invention is `directed to the combination of modulating and/or phase-shifting means in combination with the attenuation provided by a gas dischargeplasma. It is, therefore, considered well within the skill of those versed in this art to select the particular modulating means and signal source suited to the specific requirements on hand. Many such modulating means, for example, are described4 by Dr. Britton Chance in chapter 1l of Waveforms, vol. 11 of the M.I.T. Radiation Laboratories Series, published by McGraw-Hill Book Company, .1949; and re-` course may be had thereto for a choice of desired modulating means. 4

For `certain applications, it is considered desirable to separate the device providing the attenuating function from that providing the modulating or phase-shifting means. Such an arrangement allows for considerably more flexibility. Illustrated in Fig. 2 is a traveling-wave tube utilizing a plurality of gas discharge devices. The rst such gas discharge device30 functions as an attenuator and is provided with potential source 31 for controlling the discharge. Gas discharge device 32 is used for the modulation or phase-shifting function. Modulating4 means 33, which may include a source of potential, serves to providey a controllable, variable-density gas discharge plasma. During the period that gas discharge device 32 is inactivated, only the attenuation function is effective. During the on period of gas discharge 32, any desired combination Iof attenuation and modulation or phase shift may be obtained by independent control of gas discharge devices 30 and 32.

In Fig. 3 is illustrated an adidtional embodiment of gas discharge device 34. As is known, the maximum electromagnetic eld on a helical structure occurs between the turns of the helix. Therefore, if a gas tube or gas plasm-a could be positioned between turns of helix -8 or in suitable coupled relation thereto, maximum. absorption by the gas plasma would be achieved. This may be accomplished by employing a capillary glass tube 35 in which the gas plasma is positioned to be coupled with turns of the helix 8. This lamentary gas tube 35 is wound in counterfashion to helix 8. This counterwound arrangement is considered particularly effective for providing a maximum degree of interaction between the iield of the gas discharge plasma and that of-the propagating wave along the helix 8. Modulator 36, which preferably includes a potential source, is applied to gas tube 3S. The helix 8 may be supported for axial alignment with the electron beam by means of non-conductive rodsv or tubing,

as illustrated in Figs. 1 and 2.

As an important feature of this invention, by varying the density of the gas discharge plasma, modulation as well as phase shifting of the output of the electromagnetic wave of the traveling-wave tube may be obtained. Suchl phase shifting may be particularly important Where phase compensation of a distortion of a propagating. wave is required or where it is desired for certain applications to shift the phase of the propagating wave. In general, atmicrowave frequencies, a relatively high gas pressure of the order of 10 to 20 millimeters favors absorption of the electromagnetic wave by the gas discharge plasma. The determining relationship is basically that the collision frequency of the electrons of the gas be approximately equal to the angular frequency of the propagating microwave energy. Such absorption is utilized in obtaining the desired modulation. Where itis desired to have the phenomenon of phase shifting predominate, the use of low gas pressures is considered desirable, i.e., those pressures below l millimeter. The governing relationship here is that where fs is the signal frequency of the propagated electro- Y magnetic wave and fp is the electron plasmafrequency.i

I7 where N is the density electrons per cubic centimeter. Phase shifts up to an order of l21T may be readily obtained by appropriate adjustment of the gas pressure. Thus, means for both modulating of the output wave of a traveling-wave tube and shifting its phase may be readily obtained in this manner.

While I have described above the principles of my invention in connection with specific apparatus, itis to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof andv in the accompanying claims.

1. In a traveling-wave electron discharge device having a slow-wave propagating structure for transmission of radio-frequency energy therealong and means to project a beam of electrons parallel to the axis of said propagating structure for interaction with the electromagnetic field of the radio-frequency energy tnansmitted by said propagating structure; an attenuator `disposed adjacent a given portion of said propogating structure for absorption of radio-frequency energy propagated therealong, said attenuator comprising a gas discharge device disposed in juxtaposition to the electromagnetic eld of said propagating structure, means to ionize the gas of said gas discharge device produces kabsorption of and 4hence attenuation of the energy propagated along said propagating structure, a lmodulator disposed adjacent another given portion of said propagating structure for modulation'and phase shifting of radio-frequency energy propagated therealong, said modulator comprising another gas discharge device disposed in juxtaposition to the electromagnetic iield of said propagating structure, means to ionizel whereby ionization yof the gas of said another gas discharge device produces modulation and phase shifting of the energy propagated alo-ng said propagating structure, and means for controllably varying the electron density of each of said ionized gases. p

2. A device according to claim l, wherein the gas of said gas discharge device is selected from the group consisting of helium, neon, argon, krypton and xenon.

3. A device according to claim l, wherein the pressure of the gas of said gas discharge attenuating device is maintained between'lO and 20 millimeters of mercury.

4. A device according to claim 1, wherein the pressure of the gas of said gas discharge modulating and phase shifting device is maintained below one millimeter of mercury. n

5. A device according to claim 1, wherein said propagating structure includes a helical transmission line and said modulating gas discharge device include s'a capillary tube interwoven with said helical transmission line in counterwound conguration, thereby placing the ionized gas of said gas discharge device in the maximum eld region of the electromagnetic field of said helical transmission line.

6. A device according to claim l, wherein said gas discharge device is disposed longitudinally within said propagating structure and the outer surface of said gas discharge device is tapered radially with respect to said propagating structure at the extremities thereof.

7. In a traveling-wave electron discharge device having a slow-wave` propagating structure for transmission of radio-frequency energy therealong and means to project a beam of electrons parallel to the axis of said propagating structure for interaction with the electromagnetic eld of the radio-frequency energy transmitted by said propagating structure; a plurality of gas discharge devices each disposed adjacent given spaced portions of said propagating lstructure for absorption of radio-frequency energy propagated therealong, saidgas discharge devices being disposed in juxtaposition to the electromagnetic eld of said propagating structure, -means to ionize the gas of said gas discharge devices to attenuate,

modulate and phase shift the v`energy propagated along said propagating structure, and means for controllably varying the electron density of the ionized gas of at least one of said gas 'discharge'devices 8. A device according to claim 7, wherein the pressure of the gas in said `controllably varied gas discharge device is maintained .between l() and 2O millimeters of mercury. l

9. A device according to claim 7, wherein the pressure of the kgas in said controllably varied gas discharge device is maintained below one millimeter of mercury.

10. In a traveling-wave electron discharge device having a conductor in the form of a helix for propagation of radio-frequency energy therealong, input means to couple radio-frequency energy to the input of said helix, output means to couple radio-frequency energy from the output of said helix, dielectric material disposed about said helix extending from theinput end and the output end of said helix to a given central portion of said helix to cooperate in providing a rectilinear propagating structure, a magnet concentric to and substantially coextensive with said propagating structure to provide a longitudinal magnetic eld to maintain the electrons of said beam parallel to the axis of said propagating structure for substantially the entire length thereof, and means to project a beam of electrons axially of said helix for interaction with the electromagnetic iield of the radiofrequency energy propagated along said helix; an attenuator disposed in said given central portion for absorption of radio-frequency energy propagated therealong, said attenuator comprising an elongated glass envelope disposed to be coextensive with said given central portion and in a coupling relation with the electromagnetic iield of said helix, .said envelope having the ends thereof tapered radially with respect to said helix, ahigh electron density gas enclosed within said envelope, the energy of the radio-frequency wave and the longitudinal magnetic eld of said magnet cooperating to ionize said gas for attenuation of the energy propagated along said helix, a modulator and phase shifter disposed in axial alignment with said attenuator and between said attenuator and the output section of the traveling-wave device, said modulator and phase shifter comprising a second elongated glass envelope disposed to be coextensive with said given central portion and in a coupling relation with the electromagnetic iield of said helix, said second envelope having the ends thereof tapered radially with respect to said helix, a high electron density gas enclosed within said second envelope whereby the energy of the radio-frequency wave and the longitudinal magnetic field of said magnet cooperate to ionize said gas for attenuation of the energy propagated along said helix, electrodes disposed within said envelope, and a signal source coupled to said electrodes for controllably varyingthe electron density of said gas.

11. A device according to claim 10, wherein the pressure of the gas in said controllably varied gas discharge device is maintained between 10 and 20 millimeters of mercury, thereby providing attenuation modulation of the output of said traveling-wave electron discharge dcn vice. l

l2. A device according to claim l0, wherein the pressure of the gas in said controllably varied gas discharge device is maintained below one millimeter of mercury, thereby providing a shift in phase of the output of said traveling-wave electron discharge device.

References Cited in the le of thispatent UNITED STATES PATENTS 2,721,953 Rothstein Oct. 25, 1955 2,788,464 Geiger ..-r Apr. 9, 1957 2,806,974 Haeff Sept. 17, 1957

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