US2985793A

US2985793A - Traveling-wave tube

Info

Publication number: US2985793A
Application number: US499586A
Authority: US
Inventors: Charles K Birdsall; George R Brewer
Original assignee: Hughes Aircraft Co
Current assignee: Raytheon Co
Priority date: 1955-04-06
Filing date: 1955-04-06
Publication date: 1961-05-23
Anticipated expiration: 1978-05-23

Description

May 23, 1961 c. K. BIRDsALL ErAL TRAVELING-WAVE TUBE Filed April 6. 1955,

-NSM-N.

2,985,793 Patented May 23, 1961 ICC TRAVELING-WAVE TUBE Charles K. Birdsall, Los Angeles, and George R. Brewer,

Palos Verdes Estates, Calif., assignors to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed Apr. 6, 195s, ser. No. 499,586

4 Claims. (c1. als-3.o

This invention relates to microwave tubes and more particularly to electron stream amplifiers of the travelingwave type.

In a traveling-wave tube a helical conductor is generally employed to propagate an electromagnetic wave at a velocity substantially less than the velocity of light. An electron stream is then projected lengthwise through the center of the'helix whereby a mutual interaction of the stream and the wave causes the wave to grow or to be amplified. According to small signal theory an electromagnetic wave, when launched along the helix, actually divides into three forward waves. These three forward waves comprise a growing wave, a decaying wave, and a wave of a relatively constant amplitude. As the electron stream enters the helix it begins to interact with the wave iields and becomes modulated. As the wave and stream progress the growing wave eventually dominates. The total voltage of this wave acts on the electron stream; this is the voltage representative of the toal A C. energy propagated by both the circuit or helix and the stream. This wave energy can also be divided into two portio'ns comprising that present in the circuit and that in the stream. These portions are represented by the two components of the total voltage, called the circuit voltage and the stream voltage. At the output end of he helix only he energy represented by the circuit voltage may be coupled to an external load. The power output and efficiency of a traveling-wave tube is therefore limited by the fact that the circuit voltage may be considerably less than the total voltage at the output of a helix. The elecron stream velocity is uniform along a single helix and is of course determined by the direct-current voltage of the helix. The stream velocity for maximum gain, however, is usually different than that required fo'r producing a maximum ratio of circuit voltage to total voltage.

It is therefore an object of this invention to provide means for increasing the 4power output of a travelingwave tube.

Brewer. The two helix segments, in the instant case, may thus be maintained at different direct-current potentials. The principal helix, which is positioned near the input end of the tube, is, in fact, maintained at a selected voltage to cause the electron stream to ow near its maximum gain velocity, i.e. that electron stream velocity which produces maximum gain. However, the smaller or output helix is maintained at a voltage to cause the stream to flow at a velocity which will produce a maximum transfer or maximum output coupling e'iciency of electromagnetic energy from the stream to the circuit.

The novel features lwhich are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawing in which several embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawing is for the purpose of illustration and description only, and is not intended as a definition of the limits of the invention.

Fig. 1 is a sectional view of a traveling-wave tube amplier illustrating one embodiment of the present inventio'n; and

Fig. 2 is a broken sectional view of an alternative embodiment of the invention.

Referring now to the drawing, there is shown in Fig. l an embodiment of the present invention which comprises a `traveling-wave tube 10 including input and output matching cavities 12 and 14 with input and output co- Y axial cables 16 and 18, respectively connected to the It is another object of the invention to provide means for increasing the efficiency of a traveling-wave tube.

It is a further object of the invention to provide means whereby `Wave energy may be more effectively extracted from the electron stream of a traveling-wave tube at the output end of the tube helix.

In accordance 'with the present invention the helix of a traveling-wave tube is severed near the output end and means are provided to couple the electromagnetic wave energy from one of the helical segments to the other. A similar structure is employed at the input end of the tube in a gain control system which is described and claimed in copending application Serial No. 470,205, entitled Traveling-Wave Tube Gain Control, filed November 22, 1954, by George R. Brewer, now U. S. Patent 2,869,-

022, issued January 13, 1959. A similar high gain strucseparate cavities. An envelope 20, which provides the evacuated chamber of traveling-wave tube 10, consists of a long cylindrical structure which has an enlarged portion at the left extremity, as viewed in Fig. l. Within the enlarged portion of the envelope there is located an electron gun 22 for producing a stream of electrons which is directed along a predetermined path that lies on the longitudinal axis of envelope 20.

A solenoid 24 is positioned coaxially about the elongated portion of envelope 20. An appropriate direct-current is maintained in solenoid 24 by means of a potential source, such as a battery 26, so as to produce a magnetic tield of the order of 1000 gauss, whereby the stream is confined over the complete length of the elongated portion of the tube 10.

lElectron gun Z2 comprises a filament 28, a cathode 30, a focusing electrode 32, and an accelerating ano'de 34. Filament 28, which is supplied with current by a potential source 36, is employed to heat cathode 30, the cathode being connected to the positive side of filament 28. Focusing electrode 32, sometimes called a Pierce electrode, has a -frusto-conical configuration with an internal surface of revolution which may be disposed 671/2 mechanical degrees from its axis of symmetry. Electrode 32 is maintained at a potential somewhat negative with respect to cathode '30 by a connection to the negative terminal of source 36. Ano'de 34 is maintained at ground by a suitable connection 35 thereto. `Cathode 30 is maintained at about 800 volts negative with respect to ground by a source 40, the negative terminal of which is connected to source 36. A contact arm 38 on source 40 is connected to ano'de 34 and hence to ground.

Disposed concentrically within envelope 20 about the electron stream path in the direction of ow are a matching ferrule 42 connected over a lead 44 to a principal helix 46. A secondary or output helix 48 is connected over a lead y49 to a matching ferrule 50.

Principal helix 46, by means of a connection to the positive terminal of source 40 through a ferrule 42, is maintained at a potential to produce maximum gain.

Output helix 48 is maintained at ground potential by a connection through ferrule 50. However, helix 48 may be maintained at any other potential which produces maximum eciency.

Disposed about envelope and contiguous thereto is coupling helix 68 which is disposed coextcnsive with the separation in space of principal and output helices 46 and 48. All of the

helices

46, 48 and 68 are constituted of a stiff metal such as tungsten or molybdenum since it is required that they retain their shape, especially with respect to their pitch -to diameter ratios. In this embodiment of the invention it is intended that the diameters and pitch angles of principal and output helices 46 and 4S be the same and that coupling helix 68 have a pitch angle which is the negative of that of principal and output helices 46 and 48. It is unnecessary to maintain coupling helix 68 at any specific reference potential.

The stream electrons are intercepted by a collector electrode 52 at the end of envelope 20 opposite electron gun 22. Collector 52 is sealed to envelope 28 so as to have a large surface external to the evacuated chamber for heat dissipation purposes and may include fins as illustrated to aid in conducting away the heat that is dissipated by the stream electrons when collected. Accordingly, collector 52 is preferably fabricated from a metal having good electrical and heat conducting properties such as, for example, copper or silver. A potential of the order of 200 volts positive with respect to that applied to the output helix 48 is applied to collector 52 in order to prevent secondary electrons from reaching the output helix 48 or ferrule S0. This potential is applied by means of a connection from collector 52 to the positive terminal of a source 54, the negative terminal of which is referenced to ground.

As previously mentioned principal helix 46 is connected to ferrule 42 by lead 44 and output helix 48 to ferrule 50 by lead 49. Leads 44 and 49 are located parallel to the electric fields excited within matching cavities 12 and 14. Matching cavity 12 has the configuration of a rectangular toroid with a concentric collar 56 disposed about matching ferrule 42 and spaced from envelope 20 and a slot opening adjacent to lead 44. An opening 58 in the end plate of cavity 12 facing the left end of principal helix 46 allows the full length of lead 44 to be energized and, in addition, decreases the tendency of the electric eld produced bythe potential in the cavity from disturbing the flow of the electrons in the streams. Cavity 14 is similarly shaped, having a corresponding concentric collar 60 arranged about matching ferrule y5t) and spaced from the envelope and an opening 62 facing the right end of output helix 48.

The two respective center conductors v6-4 and 66 of coaxial cables 16 and 18 extend through the apertures in the end plates of cavities 12 and 14 to be in electrical contact with collars 56 and 60, respectively, while the outer conductors of the cables 16 and 18 are bonded to the periphery of the apertures. Cavities 12 and 14 are fabricated with an inner surface composed of a highly conductive material and are broadly resonant so as not to limit the frequency of operation. The conguration shown for cavities 12 and 14 in the drawing may provide, Afor example, suitable matching from helix 46 to coaxial cables 16 and 18 over a 2:1 range of frequencies extending from 2000 to 4080 megacycles.

In the operation of the tube 10 an input signal is applied to the center conductor 64 of coaxial cable 16. An electric eld is thereby excited within resonant cavity 12 which energizes lead 44 to launch a traveling-wave along principal helix 46. The electron stream supplied by gun 2.?. is then directed through principal helix 46 to interact with the wave. Principal helix 46 is maintained at or near the direct-current potential necessary for producing maximum gain. Near the output end of the tube it) the wave energy is coupled from principal helix 46 to output helix '48 by coupling helix 68, output helix 48 being maintained at or near a direct-current potential for producing a maximum circuit voltage at the output end of output helix 48. Output helix 48 thus efciently demodulates the modulation of the stream electrons, the electrons being subsequently intercepted by collector 52. The circuit voltage on output helix 48 being a maximum, a maximum transfer of wave energy is effected as the wave flowing along lead 49 excites an alternating electric field within cavity 14. The energy is then withdrawn from cavity 14 when the output signal is launched on the center conductor 66 of output coaxial cable 18.

An additional explanation of the physical mechanism of operation of the present invention may help to make clear its novel features. As mentioned before, the interaction between the electron stream and the waves propagating on the circuit results in electron bunching so that A.C. energy can be associated with the fields of the space charge and with the elds on the circuit. Only the wave on the circuit is coupled out to a useful load. It is there fore desirable that as much as possible of the space charge energy be converted to circuit energy prior to the output coupling for maximum power output and efciency. A single-circuit tube is ordinarily adjusted for maximum Ipower output so that this voltage must be a compromise between that required for maximum gain and that for maximum conversion of space charge-to-circuit energy. This compromise will not in general result in optimum performance. The present invention allows adjustment of the electron velocity to achieve each `of these effects separately; namely, adjustment of the principal helix potential for maximum gain, and adjustment of the output helix potential for maximum power output. By the use of the structure disclosed in the present invention, more nearly optimum performance of the traveling-wave can be obtained.

Another method for producing a direct-current potential separation and an alternating-current coupling for the principal and output helices employed in the present invention is shown in Fig. 2. This broken section of the output end of a traveling-wave tube exhibits an evacuated envelope enclosing a principal helix 122 with an output helix 124. In order that an intermediate helix be eliminated, output helix 124 has been contra-wound about a portion of principal helix 122, i.e. output helix 124 has a negative pitch angle, i.e. an angle equal in magnitude but opposite in sign to the pitch angle of principal helix 122. Furthermore, output helix 124 has a larger diameter than that of principal helix 122.

A helix is not, of course, the only type of slow-wave structure with which the present invention may be employed. Numerous other types, such as a disc-loaded waveguide, rare illustrated in Traveling-Wave Tubes by J. R. Pierce, D. Van Nostrand and Co., Inc., New York, 1950. Another type-is also shown in copending application Serial No. 450,987, entitled High Power Microwave Tube, tiled August 18, 1954, by C. K. Birdsall, now U.S. Patent 2,957,103, issued October 18, 1960. In other slow-wave structures a principal and an output section would thus be electrically insulated. The present invention would also include means for maintaining the sections at appropriate direct-current potentials and means for coupling electromagnetic energy from one section to the other.

The pitch and diameter of principal helix 46 and output helix 48 are determined by the operating frequency band desired and the power requirements of the tube 10. As stated previously, principal helix 46 is maintained at a potential to produce maximum gain of the growing wave. All that is left in the design of the tube 10 is then the respective optimum lengths of principal helix 46 and of output helix 48 and the determination of the potential of output helix 48 at which maximum eiciency will occur. By employing well-known wave propagation lequations in a traveling-wave tube, thegain of a travelingwave tube may be plotted versus the direct-current pctential of output helix 48 for different lengths. A curve of maximum gain then may be plotted versus tube length. Where maximum gain itself reaches a maximum value, an optimum length for output helix 48 may be found. By plotting gain versus the direct-current voltage of output helix 48 `for the optimum length of output helix 48, an optimum voltage for output helix 48 may be determined. An overall tube length will generally be assumed or will be determined by the tube power requirements. Thus the length of principal helix 46 may depend only on the optimum length of output helix 48.

In the design of the tube of the present invention, curve calculations and plotting is suggested as a preferred method of determining an optimum design because of the complexity of traveling-wave tube wave propagation equations. A direct analytical approach is feasible, but generally impractical.

What is claimed is:

1. A microwave tube comprising electron gun means for projecting a stream of electrons along the axis of said tube, a principal slow-wave structure helix for propagating traveling waves to modulate said electron stream, an output slow-wave structure helix aligned coaxially with and disposed adjacently to but direct-current insulated from said principal slow-wave structure helix and being exposed to the modulated electron stream for propagating electromagnetic waves in energy exchange relationship therewith, and helical means disposed intermediate and coaxially with said helices for coupling said electromagnetic energy from said principal slow-wave structure to said output slow-wave structure helix Without appreciable phase delay therebetween, said principal slow-wave structure helix being of a predetermined length and being maintained at a direct-current potential to produce maximum amplification of said electromagnetic energy, and said output slow-wave structure helix being of a substantially shorter predetermined length and being maintained at a direct-current potential to produce a maximum ratio of circuit voltage to total voltage associated with said electromagnetic energy.

2. A traveling wave type amplifier tube comprising means for projecting an electron stream along the axis of said tube, a principal conductive helix disposed concentrically about said electron stream for propagating an electromagnetic Wave at a predetermined velocity and for modulating said electron stream and being maintained at a direct-current potential to produce maximum amplification of said electromagnetic wave, an output helix disposed in electromagnetic communication and energy exchange relationship with said stream and aligned coaxially with and having a length substantially less than that of said principal helix and disposed adjacently thereto but direct-current insulated therefrom, helix means disposed intermediate and coaxially with said helices for coupling electromagnetic wave energy substantially without phase delay from said principal helix to said output helix so that said output helix subsequently propagates said electromagnetic wave, and means for maintainingr said output helix at a direct-current potential to produce maximum output coupling eciency from said stream to said output helix.

3. A traveling Wave amplifier tube comprising an evacuated envelope and an electron gun disposed at one end of said envelope for producing an electron stream, means for directing said stream along a predetermined path including the longitudinal axis of said envelope, a collector electrode disposed at the opposite end of said envelope for intercepting said stream electrons, a principal helix disposed about said path adjacent said electron gun for propagating an electromagnetic wave at a .predetermined velocity n energy exchange relationship with said electron stream for modulating said stream, said predetermined velocity being small in comparison to the velocity of light, an output helix disposed sequentially and coaxially along said path and exposed to the modulated stream for propagating said wave at a velocity greater than said predetermined velocity, said output helix being substantially shorter than said principal helix, helical means coaxial with said helices for coupling said electromagnetic wave without phase delay from said principal helix to said output helix, means for maintaining said principal helix at a direct-current potential to produce maximum amplification of said wave and means for maintaining said output helix at a direct-current potential to produce a maximum ratio of circuit voltage to total voltage.

4. A traveling wave amplier tube comprising an evacuated enevlope, an electron gun disposed at one end of said enevlope for producing an electron stream, means for directing said stream along a predetermined path including the longitudinal axis of said envelope, a principal helix of -predetermined length, pitch and diameter for modulating said stream to provide maximum amplification of traveling waves propagating therealong at a first predetermined velocity and being disposed concentrically about said axis adjacent said electron gun, an output helix exposed to the modulated electron stream and having a predetermined length, pitch and diameter to provide maximum coupling from said stream and being disposed sequentially along said path adjacently to said principal helix but direct-current insulated therefrom for propagating said wave at a velocity greater than said iirst velocity, said output helix being substantially shorter than said principal helix, external contrawound helix means for coupling said electromagnetic wave from` said principal helix to said output helix without appreciable phase delay, means for maintaining said principal helix at a direct-current potential to produce maximum amplication of said Wave and means for maintaining said output helix at a direct-current potential to produce a maximum ratio of circuit voltage to total voltage to thereby maximize the output coupling efficiency of said tube.

References Cited in the tile of this patent UNITED STATES PATENTS 2,541,843 Tiley Feb. 13, 1951 2,588,831 Hansell Mar. 11, 1952 2,588,832 Hansell Mar. l1, 1952 2,623,193 Bruck Dec. 23, 1952 2,660,689 Touraton et al. Nov. 24, 1953 2,733,305 Diemer Ian. 31, 1956 2,802,136 Lindenblad Aug. 6, 1957 2,804,511 Kornpfner Aug. 27, 1957 2,849,651 Robertson Aug. 26, 1958 2,869,022 Brewer Ian. 13, 1959 2,885,593 Cook May 5, 1959 2,894,168 Wing et al. July 7, 1959 2,905,858 Cutler Sept. 22, 1959 2,928,979 Kompfner Mar. 15, 1960 FOREIGN PATENTS 969,886 France May 3l, 1950 1,071,367 France Mar. 3, 1954 OTHER REFERENCES Article by P. D. Lacy, pages 132-135, Electronics for November 1954.