US3054016A - Traveling wave tubes - Google Patents

Description

Sept. 11, 1962 H. F. CHAPELL TRAVELING WAVE TUBES Filed Oct. 50, 1959 //VVE/VTOH HARRY E UHAPELL A TTOR/VE Y United States Patent Ofifice 3,d54,0lti Patented Sept. 11, 1962 3,954,016 TRAVELING WAVE TUBES Harry F. Chapell, Maynard, Mass, assignor to Raytheon Company, Waltham, Mass, a corporation of Delaware Filed Oct. 30, 1959, Ser. No. 851,190 Claims. ((11. 315-35) This invention relates to traveling wave electron discharge devices and more specifically to means for increasing the power rating of such devices by novel beam collection techniques.

Traveling wave electron discharge devices are widely used as amplifiers capable of operation over a large band Width or as oscillators capable of being tuned electronically over a considerable frequency range in the microwave region. Such devices utilize the interaction between an electron beam moving along paths adjacent a periodic non-resonant slow wave propagating structure and the electromagnetic field of the radio frequency wave propagating along said periodic structure. The electromagnetic field along such a periodic structure may be resolved into a number of superimposed traveling waves or space harmonics each having its own phase velocity. Some of the space harmonics of a phase velocity travel in the same direction as the wave energy or group velocity and are referred to as forward waves. Other space harmonies, on the other hand, have a phase velocity of opposite sense, that is, the phase velocity is in a direction opposite to the energy or group velocity. Such harmonics are referred to as backward waves. If the electron beam velocity is adjusted so that it is in substantial synchronism with the phase velocity of a given space harmonic, interaction between the electron beam and this component will occur, and energy will be transferred from the electron beam to the electromagnetic field. In a traveling wave amplifier, a radio frequency input signal is coupled to the periodic structure adjacent one end thereof, and, owing to the interaction between electron beams moving along a path adjacent the periodic structure and the electromagnetic field of the radio frequency wave propagating along said structure, amplification of the input signal may be obtained, under proper operating conditions. This amplified signal may then be extracted from the periodic structure adjacent the other (output) end thereof. In the forward wave amplifier the interaction is between the electron beam and a forward wave; the electrons thus are projected toward the output end of the periodic structure. In the backward wave amplifier, interaction occurs between the electron beam and a backward wave and the electrons then move toward the input end of the periodic structure. In a traveling wave oscillator, when the electron beam current exceeds a critical current at which oscillations can begin, and when the electron beam velocity is substantially equal to the velocity of one of the backward waves, oscillations may be generated within the device and the generated energy will propagate along the periodic structure and may be extracted at the end thereof away from which the electrons are moving.

In certain crossed field beam type systems particularly employed in traveling wave tubes of cylindrical construction, -a cathode structure is used which provides a relatively large surface and mass and power handling capacities in order to increase the power rating of the traveling wave tube.

In such cylindrical traveling wave tubes the electron gun may be positioned outside the interaction space between the periodic anode structure and the negative electrode or sole. In certain constructions the electron gun may be recessed into the sole. The electrons from the cathode of this electron gun may be accelerated by means of one or more beam forming electrodes of the electron gun in an initial direction which is parallel to the magnetic field into the interaction space, where, under the influence of the direct current electric and magnetic fields, 5 they will be diverted and caused to travel along the interaction space with a motion which is now perpendicular to said electric and magnetic fields and in a direction determined by the polarity of the magnetic field. In other gun constructions the electrons may be formed into a ribbon-type beam which may be projected substantially along only one equipotential level and perpendicular to the magnetic field.

Since the electron gun assembly is located outside those portions of the electrodes of the tube which bound the main interaction space, a cathode of relatively large area and mass may be used and may, in some designs, be spaced entirely outside such electrodes and extend the entire length of the tube. Consequently, such traveling wave tubes are capable of handling considerably more power than prior traveling wave tubes which contained cathodes of limited area.

A corollary to this is the requirement that such electron beam of increased power must be properly collected at the terminus of the periodic structure adjacent the other end.

Various methods have been advanced for collection of such increased-power electron beams. One recently advanced practical method has been to spread the electron beam in the B field direction of the crossed or transversely directed direct current and magnetic fields, to thus provide an increase in collector area to dissipate the increased electron beam energy.

Early crossed field beam type tubes used collectors which were interposed directly in the path of the electron beam. With increased power tubes such collector means proved unable to dissipate the power.

Such prior art methods of collecting electron beams of increased power proved unsuccessful in that the beam could not be adequately controlled during collection and the electron beam, or more precisely the electrons thereof, escaped to the circular anode covers of such tubes of cylindrical construction in sufiicient numbers to melt these covers.

In accordance with this invention, an extension member, designated as a director, is provided in conjunction with a pro-collector electrode in order to direct the electron beam into a collection channel and area at the terminus of the periodic anode delay network so that such beam does not spread or escape to the circular anode covers.

It is thus an object of this invention to provide novel electron beam collection techniques.

A further object of this invention is to increase the electron beam collection area.

An additional and more specific object of this invention is to provide supplementary beam directing means which permit a controlled collection of high power electron beams.

A particular object of this invention is to provide means in a cylindrical traveling wave crossed field beam type tube to direct the spread electron beam onto a collection area which provides adequate surface and mass for power dissipation, while preventing the impingement of anysubstantial numbers of electrons onto areas of lesser powerhandling abilities, as for example the tube anode covers.

Other objects, advantages, and features of this invention will be understood more fully from the following detailed description of the invention with reference to the accompanying drawings wherein:

FIG. 1 is a transverse cross-sectional view of a traveling wave tube of cylindrical configuration accordingto the invention;

FIG. 2 is a detail view, to an enlarged scale, of a portion of the tube shown in FIG. 1;

FIG. 3 is a longitudinal view of the tube shown in FIG. 1;

FIG. 4 isa detail view showing a fluid-cooled precollector electrode which may be maintained at a potential difierent from that of the anode structure; and

FIG. 5 is a fragmentary view showing a modification of the supplementary beam directing assembly of FIGS. 1 to 3.

FIG. 1 shows a traveling wave tube 20 of cylindrical construction. Such a tube may include an electron gun and periodic anode structure which are in many respects similar to the construction shown in an application of Edward C. Dench, Serial No. 567,141, filed February 23, 1956, now US. Patent No. 2,890,372, dated June 9, 1959, and assigned to the same assignee. However, in accordance with this invention, the electrode 30, hereinafter referred to as a sole, is disposed in the electron beam collection area after the electrons have traversed the interaction space, and is of such dimensions in relation to the anode as to support a supplementary beam directing assembly extending into the anode structure but spaced therefrom, as will be described hereinafter in detail.

The tube 20 contains an anode assembly 21 including a periodic slow wave energy propagating structure 22, a sole electrode 30 maintained negative relative to the anode delay network 22, a lead-in assembly 40, an energy coupling means 50, an electron gun assembly 6t) including at least a cathode 61 and a heater 62, and a transverse magnetic field producing means, the pole pieces 80 and 81 of which are illustrated in FIG. 3.

The anode assembly 21 may comprise a circular interdigital delay line including a plurality of interdigital fingers 24 which extend from oppositely disposed annular members or crowns 25, as shown. Alternatively any suitable periodic slow wave propagating structure other than an interdigital line may be used, such as a helix, a vanetype structure, or the like. Members 25 are secured, as by brazing, to the shoulder portion of a cylindrical electrically conductive ring 27. The remainder of the anode assembly includes a pair of oppositely positioned cover plates 28 and 29 electrically sealed to ring 27. Reference numeral 100 indicates the outer Wall of a cylindrical water jacket which surrounds and cools the cylinder 27 and annular anode members 25. The water jacket is shown fragmentarily, since the details thereof are not a part of the present invention. i

The sole 30 consists essentially of a substantially cylindrical tubular member of electrically conductive material, such as copper, having a flange portion 32. Furthermore, the sole 30 may be either primarily or secondarily electron-emissive. A centrally located aperture is provided in the sole to permit connection of lead-in assembly 40 and to allow for passage of external connecting leads, in a manner to be shown subsequently. Alternatively, the sole may be a solidcylindrical member whose outer periphery coincides with that of the tubular member illustrated in FIGS. 1 to 3.

Lead-in assembly 40 includes an electrically conductive cylindrical sleeve 42 inserted in an aperture in cover plate 28 and securely attached thereto. A section of glass tubing 43 serves to interconnect metal sleeve 42 and a second metal sleeve 44. The other end of sleeve 44 is' provided with a glass seal 45 for sealing tube 20 after evacuation. Sleeves 42 and 44 preferably are constructed of a material having an expansion coefiicient closely approximating that of tubing 43. The assembly 40 further includes an elongated electrically conductive hollow supporting cylinder 46 which serves as the main support for sole 30. One end of cylinder 46 is fastened to the pe riphery'of aperture 31 in the sole. The other end of cylinder 46 contains an outwardly flared portion 47 which isconnected to the inner surface of sleeve 44. V The neces sary leads for the electron gun are fed through support- 4 ing cylinder 46 and are insulatedly supported therefrom and from one another by one or more glass beads 49'.

The coaxial coupling means 50 is sealed in an opening in cylindrical wall 27 of the anode assembly and is impedance matched to the anode delay network 22. The inner conductor 52 of the coaxial coupling means 50 is connected to a finger 24' at or adjacent one end of the periodic anode delay network. Although the device of FIG. 1 has been shown as an oscillator having but a single energy coupling means, this invention also is applicable to an amplifying device having input and output coupling means at opposite ends of the delay network 22. As more particularly shown in FIGS. 2 and 3, an extension member 34 is attached to the sole 30, as by screws 39, and, in conjunction with a pro-collector member 23, directs the electron beam into a controlled collection area at the end of anode delay network.

' The sole extension member 34 will be designated as a director, and such member is attached to the outer periphery of sole 30 in the electron beam collection area which begins at the termination of the delay network. The last reentrant finger of the delay network, 24", is indicated at the top of FIG. 1.

As shown more clearly in FIG. 3, the director is L-shaped in cross section, and the outer edge thereof extends into the arcuate collection space in which the beam is permitted to spread perpendicular to the electric field along the magnetic field lines. The configuration of the director is such as to direct the electron beam, 36, into the channel formed by the cylinder wall 27 and the two anode annular members or crowns 25 in the arcuate portions thereof which are devoid of the fingers 24. As shown in FIG. 1, this is the arcuate portion between the last anode finger 24 and the first anode finger 24' to which the coaxial coupling means 50 may be connected.

Thus, when the electron beam spreads vertically, it is intercepted by the arcuate anode crowns 25 and the inner periphery 27' of cylinder wall 27, both of which are well cooled by means of the water jacket 100. The effective electron beam collection area is thus occupying the same space as in that of earlier tubes, while at the same time the anode covers 28 and 29 are shielded from the electron beam. Equally important, a more uniform distribution of electrons over the entire collector area may be obtained in a relatively simple manner by varying the spacing between the director 34 and the cylinder 27. Inspection holes 38 may be mounted in the lower anode member 25 at various intervals along the collection region to permit a visual check on the director periphery and location. Similar holes may be provided in the upper anode member 25 if desired, and the provision of such holes will not cause any excessive loss of beam electrons to the covers 28 and 29. A supplementary benefit of such inspection'holes may be that of providing an increased effective area of the collection region.

A pro-collector 23 is attached to the inner wall 27' of cylinder 27. Element 23 in conjunction with the director 34, controls the electron beam optics at the entrance to the collection region. As indicated in FIG. 2, the beamof electrons 36 as shown by the arrows having traversed the interaction area 35, is directed into the collection area including channel 37.

The pro-collector 23 is tapered so that its radial dimension is decreased in the direction of movement of the electron beam. The degree of taper will depend upon a particular tube design, including such factors as power handling'requirements, tube operating voltages, and the like. The pro-collector electrode 23, being in contact with the large thermally conductive cylinder 27, is able to dissipate heat quite readily, particularly when the cylinder is surrounded by fluid-cooling means 100. Some of the electron beam 36 is collected via the precollector 23 and the amount of collection is dependent, at least partially, upon the degree of taper. If the taper is too small, that is, if the periphery of the pro-collector 23 approaches the back wall 27 too slowly, the pre-collector will be unduly bombarded with electrons near the leading edge and destruction of the pre-collector may result. If, on the other hand, the taper is too large, there may be little or no collection accomplished by the precollector; this results, of course, in a waste of collector space in the tube. Furthermore, the electric field at the approach to the collection region of the tube, in the case of a pre-collector of large taper, will be such that the electron beam will not be directed properly in the region of the director and the electrons may be directed along irregular paths, thereby striking either the end plates or bombarding the post-collector so heavily as to burn out the latter electrode.

The pre-collector electrode 23 need not be connected directly to the anode structure, but may be electrically insulated from the cylinder 27. The pre-collector 23 may be fluid-cooled by means of fluid conduits passing through the tube envelope and into one or more hollow passages in the pre-collector, as shown in FIG. 4. The support assembly 95 of FIG. 4 includes a tube 96 sealed to cylinder 27. A tubular member 97 is attached to tube 96 by way of a glass seal 98. An inlet water pipe 101 and outlet water pipe 102 are brought up through member 97 and are sealed to member 97. The pre-collector electrode 23 of FIG. 4 contains a curved conduit or passage, not shown, into the ends of which the inlet and outlet pipes 101 and 102 are inserted. The pipes 101 and 102 may be brazed to the openings in the passage or may be provided with threaded fittings engaging threaded ends of the passage. These pipes may be sufficiently rigid to provide mechanical support for the pre-collector 23. An appropriate bias voltage between the pre-collector and the anode may be maintained by a battery 15 or other source of unidirectional voltage connected, for example, between the tube 96 and member 97.

The pre-collector electrode 23, in addition to aiding in the collection of the electron beam, serves to maintain the width of the collection space 37 in the region occupied by the director of the same order of magnitude as the width of the interaction space 35 between the sole 30 and the delay line elements 24 in the interaction region. In the absence of a pro-collector electrode, there would be a sudden decrease in electric field as the electron beam leaves the interaction region 35 and enters the collection region 37, since the spacing between the positive back wall 27' and the negative director 34 would be greater than the separation of the peripheral portion 30' of the sole 30 and the delay line elements 24 in the interaction region. Because of the resulting decreased radial component of the electric field and increased axial components of the electric field which are so directed as to cause the beam to spread outward toward the end plates and crowns, the end plates and crowns may be bombarded by an excessive number of electrons if the pre-collector 23 were to be omitted. The portion of the periphery of the director 34 facing the pre-collector electrode 23 is substantially parallel to the juxtaposed pre-collector surface. The director 34 includes a portion extending beyond the pro-collector 23 in applications in which additional heat dissipative area is required over and above that obtainable in the region occupied by the pre-collector 23; this additional portion of the director is generally more or less concentric with the back wall 27 A post-collector 33 is provided in order to remove any electrons which manage to tarverse the entire collectordirector channel.

Nominally the director 34 may be made of a material of the same type as that used for the sole 30. It is known, however, to provide lossy material in order to damp out rapid mode excitation problems in crossed field tubes. To provide such an effect, the entire director 34 may be fabricated from a lossy material such as carbon. Alternatively, the director could be made from a magnetic material, if it is desirable to increase the loss. The director dimensions are relatively uncritical, and the placement of the lossy material in the director element can be readily made without destroying the electron optics in the interaction region along the delay network.

In some cases, as shown in FIG. 5, the pre-collector electrode 23 may contain more than one tapered surface. The pre-collector 23 of FIG. 5 is doubly tapered. This arrangement is particularly useful in very high power tubes of relatively small diameter wherein a pre-collector of single taper may be so short in length, in order to avoid excessive electron bombardment at the tip, as to prevent adequate area for heat dissipation. By using a multiple tapered pre-collector, the heat dissipative area of the pre-collector may be increased so as to avoid burnout, While simultaneously providing the proper taper to prevent burnout at the end of the pre-collector. The director 34, in such cases, also is doubly tapered in the region facing the pre-collector.

The post-collector electrode 33 which is used to intercept stray electrons which are not directed to the collection area, may similarly be formed of a lossy material if required. However, in its nominal form the post-collector takes the configuration of a projection from the back Wall 27 of the anode cylinder 27, and is formed integrally and of the same material as that of the cylinder 27.

The electron gun assembly 60 for the tube includes a cathode 6-1, a heater 62, and a beam forming plate or electrode 65. Electrode 65, in elfect, is an accelerating anode serving to aid in the attainment of the desired electron beam trajectory. In accordance with known construction, the cathode 6-1 may be positioned within a recess or slot in the wall or sole 30. Further, a grid and durnmy anode may be provided, if desired, in order to obtain optimum electron beam configuration.

A uniform magnetic field transverse to the direction of propagation of the electron beam and the electrical field between the anode delay network and sole is provided by a permanent magnet or an electromagnet having cylindrical pole pieces and 81 positioned on or adjacent the tube. Pole piece 80 is apertured to receive the lead-in assembly 40 and pole piece 81 is apertured to maintain symmetry of the magnetic field. The flux lines should be concentrated in the interaction space 35 between sole 30 and anode delay network 22. By proper adjustment of the magnitude and polarity of the transverse magnetic and electric fields, the electron beam may be made to follow a circular path about interaction space 35 under the combined influence of these fields.

This invention is not limited to the particular details of construction, materials and processes described, as many equivalents will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

1. A traveling wave electron discharge device comprising a periodic slow wave propagating structure at one potential, an elongated electrode at another potential spaced from and coextensive with said structure, said structure and said electrode defining an interaction space and bounding an electric field in said interaction space, an electron source at one end of said interaction space, means producing a transverse magnetic field in said space, said electric and magnetic fields combining to compel electrons to move as a beam through the center of said interaction space in energy exchanging relationship with said waves, and means for collecting electrons which traverse said interaction space including means located outside said interaction space for spreading at least a portion of said electron beam by compelling electrons which emerge from said interaction space to move along paths substantially parallel to the direction of said magnetic field.

2. A traveling wave electron discharge device comprising a curved slow wave propagating structure at one potential, an arcuate electrode at another potential spaced from and coaxial with said structure defining an interaction space therebetween and bounding an electric field in said space, an electron source at one end of said space, means producing a transverse magnetic field in said space, said electric and magnetic fields combining to compel electrons to move through the center of said interaction space in energy exchanging relationship with said waves, and means for collecting the electrons after traversing said interaction space including an electron collecting space extending from said interaction space substantially parallel to said coextensive electrode and turning abruptly so that at least a portion of said electrons moving through said collecting space are collected at said turn.

3. In a traveling wave electron discharge device of the crossed electric and magnetic field type including a cathode emitting a beam of electrons, a slow wave propagating structure at one potential and an elongated electrode at another potential spaced from and coextensive with said structure defining an interaction space and bounding said electric field therebetween, means for collecting said beam of electrons after traverse of said 8 interaction space including an electron collecting space extending from said interaction space, and means producing an electric field in saidcollecting space whereby said beam is spread and collected.

4. A device as in claim 3 wherein said electron collecting space is defined by second and third electrodes connected to said structure and said elongated electrode, respectively.

5. A device as in claim 3 wherein said second elec trode includes conductive surfaces defining three sides of said electron collecting space and said third electrode forms the fourth side of said electron collecting space so that said electric field in said collecting space is shaped, to compel beam electrons to spread for collection by said second electrode surfaces.

References (Iited in the file of this patent UNITED STATES PATENTS Bryant et a1 Nov. 20, 1956

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