Coupled cavity traveling wave tube

Various embodiments of a coupled cavity traveling wave tube are disclosed herein. For example, some embodiments provide a coupled cavity traveling wave tube including a plurality of core segments arranged in spaced-apart fashion to form an electron beam tunnel, a first longitudinal member adjacent the plurality of core segments alternately extending toward and receding from successive core segments, and a second longitudinal member adjacent to the plurality of core segments alternately extending toward and receding from successive core segments. The first and second longitudinal members are offset to extend toward different core segments.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to PCT Patent Application No. PCT/US09/46305 entitled “Coupled Cavity Traveling Wave Tube”, filed on Jun. 4, 2009, and to U.S. Provisional Patent Application No. 61/059,182 entitled “Design of Ladder-based Coupled Cavity TWT System”, filed on Jun. 5, 2008. The aforementioned applications are assigned to an entity common hereto, and the entirety of the aforementioned applications are incorporated herein by reference for all purposes.

BACKGROUND

A traveling wave tube (TWT) is an amplifier that increases the gain, power or some other characteristic of a microwave or radio frequency (RF) signal, that is, electromagnetic waves typically within a range of around 0.3 GHz to above 300 GHZ. An RF signal to be amplified is passed through the device, where it interacts with and is amplified by an electron beam. The TWT is a vacuum device through which the electron beam travels, typically focused by a magnetic containment field to prevent the electron beam from directly touching the structure of the TWT.

The electron beam may be generated at the cathode of an electron gun, which is heated to typically about 1000 degrees Celsius. Electrons are emitted from the heated cathode by thermionic emission and are drawn through the TWT to a collector by a high voltage bias, focused by the magnetic field.

The TWT also contains a slow wave structure (SWS) such as a wire helix through which the RF signal passes. For example, in the case of the wire helix TWT, the electron beam passes through the central axis of the helix without significantly contacting or touching the inner walls of the helix. The slow wave structure is designed so that the RF signal travels the length of the TWT at about the same speed as the electron beam. As the RF signal passes through the slow wave structure, it creates an electromagnetic field that interacts with the electron beam, bunching or velocity-modulating the electrons in the beam. The velocity-modulated electron beam creates an electromagnetic field that transfers energy from the beam to the RF signal in the slow wave structure, inducing more current in the slow wave structure. The RF signal may be coupled to the slow wave structure and the amplified RF signal may be decoupled from the slow wave structure in a variety of ways, such as with directional waveguides that do not physically connect to the slow wave structure.

A number of different slow wave structures are known for use in traveling wave tubes, such as the wire helix TWT mentioned above, with corresponding advantages and disadvantages. For example, a wire helix TWT has a wide bandwidth, meaning that the RF signals that can be amplified in the wire helix TWT are less bandwidth-limited and may have a wider range of frequencies than in some other TWT designs. However, a wire helix TWT has some limitations when compared with other TWT designs. Another type of TWT is a coupled cavity TWT, in which the slow wave structure has a series of cavities coupled together. As the RF signal passes through the resonant cavities, inducing RF voltages in each cavity. When the velocity modulation of the electron beam passing adjacent the cavities is in phase, the RF voltages in each subsequent cavity increase in an additive fashion, amplifying the RF signal as it passes through the coupled cavity TWT. However, coupled cavity TWTs are often difficult to manufacture and assemble, including a large number of tiny components that must be precisely aligned and spaced. Although coupled cavity TWTs have relatively high gain, they also generally have narrower bandwidths than some other designs such as a wire helix TWT, leaving room for improvement in areas such as bandwidth and ease of construction.

SUMMARY

Various embodiments of a coupled cavity traveling wave tube are disclosed herein. For example, some embodiments provide a coupled cavity traveling wave tube including core segments arranged in spaced-apart fashion to form an electron beam tunnel, a first longitudinal member adjacent the core segments alternately extending toward and receding from successive core segments, and a second longitudinal member adjacent to the core segments alternately extending toward and receding from successive core segments. The first and second longitudinal members are offset to extend toward different core segments.

In an embodiment of the aforementioned coupled cavity traveling wave tube, the first and second longitudinal members are on opposite sides of the core segments

In an embodiment of the coupled cavity traveling wave tube, the core segments comprise rungs of a ladder.

In an embodiment of the coupled cavity traveling wave tube, the first and second longitudinal members each comprise a body and protrusions which extend from the bodies toward each corresponding core segment, wherein protrusions form a series of coupled cavities.

In an embodiment of the coupled cavity traveling wave tube, the protrusions and the corresponding core segments comprise mating surfaces, wherein the mating surfaces of the protrusions are placed in contact with the mating surfaces of the corresponding core segments.

In an embodiment of the coupled cavity traveling wave tube, the mating surfaces are substantially flat.

An embodiment of the coupled cavity traveling wave tube includes a housing. The core segments and the first and second longitudinal members are substantially contained within the housing. The first and second longitudinal members extend from inner top and bottom walls of the housing

In an embodiment of the coupled cavity traveling wave tube, the core segments extend to inner side walls of the housing.

In an embodiment of the coupled cavity traveling wave tube, the core segments each comprise an inner surface defining a passage. Each of the core segments is aligned to form the electron beam tunnel.

In an embodiment of the coupled cavity traveling wave tube, the passages defined by the core segments have a circular cross-section.

In an embodiment of the coupled cavity traveling wave tube, the passages defined by the core segments have a hexagonal cross-section.

An embodiment of the coupled cavity traveling wave tube includes a coating on the core segments.

An embodiment of the coupled cavity traveling wave tube includes a radio frequency input waveguide at a first end of the coupled cavity traveling wave tube and a radio frequency output waveguide at a second end of the coupled cavity traveling wave tube.

Other embodiments provide methods of manufacturing a coupled cavity traveling wave tube. In one embodiment, the method includes forming slots in a ladder to form rungs,

forming a tunnel longitudinally through the ladder, and forming a first ridge having a group of protrusions forming a second ridge having a second group of protrusions. The method also includes aligning the first ridge adjacent a first side of the ladder so that the group of protrusions contacts an alternating sequence of the rungs. The method also includes aligning the second ridge adjacent a second side of the ladder so that the second ridge is offset from the first ridge, and the second group of protrusions contacts a second alternating sequence of the rungs

In an embodiment of the method, the first ridge is formed in a first portion of a housing and the second ridge is formed in a second portion of the housing. The alignment of the first and second ridges includes enclosing the ladder within the first and second portions of the housing.

An embodiment of the method also includes brazing the groups of protrusions to the rungs.

In an embodiment of the method, the slots are formed using photolithography.

An embodiment of the method also includes providing a coating on the ladder.

In an embodiment of the method, the thickness of the coating is graded.

Another embodiment of a coupled cavity traveling wave tube includes a ladder having a group of rungs. Each rung includes a core segment having an inner surface defining a passage with a circular cross-section. The core segments are arranged in a spaced-apart linear array, with the passages aligned to form an electron beam tunnel. A first ridge having a group of protrusions is positioned adjacent a first side of the ladder, so that the group of protrusions contacts an alternating sequence of the core segments. A second ridge having a second group of protrusions is positioned adjacent a second side of the ladder, so that the second ridge is offset from the first ridge, and the second group of protrusions contacts a second alternating sequence of the rungs.

This summary provides only a general outline of some particular embodiments. Many other objects, features, advantages and other embodiments will become more fully apparent from the following detailed description, the appended claims and the accompanying drawings.

DESCRIPTION

The drawings and description, in general, disclose a coupled cavity traveling wave tube (TWT). Various embodiments of the coupled cavity TWT provide benefits such as higher bandwidth and/or gain than other coupled cavity TWTs, as well as simple and precise manufacturing and assembly techniques. As illustrated inFIGS. 1-5, the coupled cavity TWT10has a central structure12with ridges14and16adjacent to the central structure12, all within a cavity or chamber20in a housing. The ridges14and16(also referred to herein as longitudinal members) are oriented along a longitudinal or Z axis22adjacent the central structure12. The central structure12and ridges14and16form a slow wave structure through which an RF signal passes.

The ridges14and16each have a number of protrusions (e.g.,24,26,30and32) extending toward alternating core segments (e.g.,34,36,40and42) in the central structure12. For example, the first ridge14extends toward the first core segment34with its first protrusion24, recedes from the second core segment36, and extends toward the third core segment40with its second protrusion26. The second ridge16is offset from the first ridge14, receding from the first core segment34, extending toward the second core segment36with its first protrusion30, receding from the third core segment40, and extending toward the fourth core segment42with its second protrusion32. The offset protrusions (e.g.,24,26,30and32) on the ridges14and16thus form a series of coupled cavities (e.g.,44,46,50and52). The cavities (e.g.,44,46,50and52) are coupled via the spaces or gaps (e.g.,54) between each successive core segment (e.g.,34and36), as well as via other open portions of the chamber20, if any, such as alongside the ridges14and16. In some embodiments, the protrusions (e.g.,24,26,30and32) may be referred to as supports, at least in part based on providing support to the core segments (e.g.,34,36,40and42) in the central structure12in these embodiments.

The ridges thus comprise protrusions (e.g.,24,26,30and32) or supports and, in some embodiments, a longitudinal backbone portion or body (e.g.,56) running parallel with the Z axis22. The ridge backbones (e.g.,56) may have any suitable height58. The ridge backbones (e.g.,56), if included, enhance the mechanical, structural and thermal properties of the design. However, the height58of the ridge backbones (e.g.,56) may be adjusted to tune the bandwidth of the TWT10, including to a zero thickness.

The chamber20is formed in a housing to be described below, with any suitable cross-section shape to the inner and outer walls. For example, as illustrated inFIG. 3, the chamber20may have an inner wall having a cross-section that is substantially square or rectangular. In other embodiments, the chamber20may have a rectangular cross-section with rounded corners, or a round, elliptical or oval cross-section, or any other suitable shape to provide the desired performance characteristics and to provide ease of manufacturing. A substantially square or rectangular cross-section in the chamber20is particularly simple to produce using a number of fabrication techniques ranging from conventional machining techniques such as using a rotating cutting bit to mill the chamber20with its ridges (e.g.,14and16) and protrusions (e.g.,24and26) from a solid block of material to microfabrication techniques and various hybrid manufacturing techniques. In other embodiments, the ridges (e.g.,14and16) may be independent elements that are separately formed and mounted within the housing. An electron beam tunnel60is formed along the Z axis22through the core segments (e.g.,34,36,40and42in the central structure12. The shape of the cross-section of the tunnel60may be adapted to give the desired operating characteristics and based on manufacturing constraints. For example, the inner wall of the beam tunnel may have a cross-section with a circular, square, rectangular, hexagonal, oval, elliptical or any other desired shape based on factors such as ease of manufacturing and coupling requirements between the electron beam and the slow wave structure. The hexagonal tunnel60illustrated inFIGS. 1-3can be manufactured by bending and joining two ladder halves without drilling as will be described in more detail below. The circular tunnel62illustrated inFIG. 6can be manufactured by drilled along the Z axis22which may require more precision in the machining process but which generally provides greater coupling between an electron beam passing through the tunnel62and the RF signal traveling through the central structure12and ridges14and16making up the slow wave structure.

In one embodiment, the ridges14and16are positioned on opposite sides of the central structure12, extending from inner top and bottom walls64and66, respectively, along an X axis70. (SeeFIG. 3) In this embodiment, the protrusions (e.g.,24and26) extend from the ridges14and16along the X axis70. The width of the ridges14and16and protrusions (e.g.,24and26) along a Y axis72can be varied as desired.

For example, the14and16and protrusions (e.g.,24and26) may be about as wide as the core segments (e.g.,34) as illustrated in the drawings, or may fully extend between the inner side walls74and76to fill the chamber20from side to side if desired, although the operating characteristics of the TWT10will vary with these changes. It is important to note that the terms top, bottom and side are used herein merely to distinguish various surfaces inside the TWT10and do not imply any particular rotational orientation about the Z axis22. It is also important to note that the variations of the above embodiments are meant as examples of the present invention and are in no way limiting of all of the potential embodiments of the present invention especially in terms of size, shape, overlap, extending of, number and placement of, etc. the protrusions, ridges, and other geometrical shapes, positions, types, etc.

A single unit cell is illustrated in shown inFIGS. 2-4, which may be repeated as desired along the Z axis22to provide a particular amplification or gain to an RF signal.

Referring now toFIG. 7, an example of a cylindrical housing80is shown, being formed in two halves82and84with the central structure12sandwiched inside the housing80between the two halves82and84. As with previous embodiments, the inner cross-section of the chamber20is substantially rectangular, with rounded corners (e.g.,86) which may minimize edge effects in the RF signal, although numerous other shapes and styles can be used for the present invention. The housing80may serve as a vacuum envelope in some embodiments, or a vacuum may be alternatively provided for as desired and as needed.

The coupled cavity TWT10is not limited to any particular central structure12. In one embodiment illustrated inFIGS. 8 and 9, the central structure12comprises a ladder90having a number of rungs (e.g.,92and94). The ladder90can be manufactured in as few as one or two pieces using techniques such as lithography and machining, and can be assembled quickly and easily with high precision. A series of slots (e.g.,96and100) may be cut or otherwise formed in the ladder90to separate and define each segment of the central structure12. The width of the slots (e.g.,96and100) may be adapted as desired to provide the required operating characteristics. Parameters and properties such as the length, spacing, thickness, periodicity, etc. can be varied along the length dimension of the structure in linear, power-law, exponential, and any other way imaginable, realizable, etc. to provide desired performance behavior (i.e., gain, linearity, efficiency, power, etc.) and enhancements. A circular tunnel62may be formed, for example, by drilling longitudinally through the ladder90using any technique, including but not limited to conventional drilling, end milling, EDM, laser milling, laser ablation, micromachining, etching, plasma processing, etc. In another embodiment, the ladder90may be formed of two halves which are mated and connected to form the tunnel, or as a single piece with two halves formed side by that is folded over. For example, a hexagonal tunnel60may be formed by bending each half to form a three-sided half-hexagonal core segment and mating the two halves to form a hexagonal tunnel60. A circular tunnel62may be formed by milling, micromaching, or otherwise creating a semicircular trough along the Z axis22of each half and mating the two halves to form the circular tunnel62. The two halves may be aligned using traditional techniques such as registration marks or pins, or by self-alignment techniques, microfabrication, micromaching, MEMS, etc. and mated or connected by brazing, bonding, electrically conductive adhesives, or any other suitable technique.

By ending the slots (e.g.,96and100) in the ladder90short of the edges102and104, the ladder90remains in a single integral piece that maintains the desired gap between each segment. The slots (e.g.,96and100) may be formed to fully extend between the side walls74and76as illustrated inFIG. 7, or may stop short of the side walls74and76if desired although the coupling between cavities (e.g.,44and46) will be reduced. The segments of the ladder90comprise core segments (e.g.,34) through which the tunnel62passes with wings106and110extending from the core segments (e.g.,34). The wings106and110may be thinner along the X axis70as illustrated in the drawings or may be as thick as or thicker than the core segments (e.g.,34) if desired. The wings106and110extend at least to the side walls74and76for ease in manufacturing and to provide support to the core segments (e.g.,34) beyond that provided by the ridge protrusions (e.g.,44and46), as well as to provide a thermal connection between the housing80and the ladder90to dissipate heat.

The core segments (e.g.,34) of the ladder90have mating surfaces (e.g.,112) that are substantially matched to corresponding mating surfaces on the ridge protrusions (e.g.,24) to form a connection between the core segments (e.g.,34) and the protrusions (e.g.,24). These mating surfaces (e.g.,112) provide an electrical, mechanical and thermal connection between the ladder90and the ridges14and16to conduct electricity, provide support to and conduct heat from the ladder90, and substantially separate adjacent but non-coupled cavities. The ladder90and the ridges14and16may merely be held in contact physically or may be brazed, connected by adhesives or attached in any other suitable manner. Although the ladder90and the ridges14and16are shorted together from a DC standpoint, the slow wave structure including the ladder90and the ridges14and16are adapted to provide the desired impedance from an AC standpoint at the RF operating frequencies of the TWT10.

The core segments (e.g.,34) of one embodiment have a cross-section with an outer hexagonal shape112, although the TWT central structure12is not limited to this configuration. Other embodiments may have any shape suitable to achieve the desired operating characteristics and ease of manufacturing, such as a square, circular, elliptical or oval, rectangular or any other desired cross-section.

A ladder-based central structure12has been described above as one particular embodiment. However, the central structure12is not limited to this configuration. The central structure12may comprise other structures that combine with the offset ridges14and16to form coupled cavities. For example the central structure12may comprise a helix, double helix, ring bar structure, etc.

Referring now toFIG. 10, an example of a cylindrical housing80formed in two halves (e.g.,84) is illustrated. A cylindrical housing80is convenient for mounting external electron beam containment magnets to form a pencil beam through the tunnel62, although the housing80is not limited to this configuration. As discussed above, the ridges (e.g.,14) and protrusions (e.g.,24) may be machined, micromachined, milled or otherwise formed directly in the body of the housing80, or may be separately formed and attached to inner surfaces in the housing80. Note that the housing80is not limited to two halves, but may be formed in other manners. As illustrated inFIG. 11, the ladder90may be enclosed in the TWT10between the portions82and84of the housing80so that the protrusions (e.g.,24) are aligned with the core segments (e.g.,34). The housing80may be assembled in any suitable manner, such as with mechanical connection elements, brazing, bonding, adhesives, etc.

A cross-sectional view of the coupled cavity TWT10is illustrated inFIG. 12. An electron gun120is connected to one end of the TWT10and a collector122is connected to the other end. An ion pump124or other vacuum forming device is also connected to the TWT10to evacuate the TWT10. (Details of the electron gun120, collector122and ion pump124are not shown in the cross-sectional view ofFIG. 12, as the TWT10is not limited to use with any particular type of electron beam and vacuum equipment.) An RF input130and output132are connected at couplers134and136at the ends of the TWT10. For example, hollow waveguides having with RF-transparent windows140and142to maintain a vacuum in the TWT10may be used. As shown inFIG. 13, devices to form a magnetic field, such as periodic permanent magnets (e.g.,144and146) are placed around or adjacent the TWT10to steer the electron beam through the tunnel62between the electron gun120and collector122. Note that the TWT10ofFIGS. 12 and 13has a different number of core segments34than other drawings. As discussed above, the TWT10may be extended, modified, augmented, enhanced, increased, etc. based on the desired amplification.

During operation, the ion pump124produces a vacuum within the TWT10, the electron gun120is heated and a large bias voltage is applied across the electron gun120and collector122. This generates an electron beam between the cathode of the electron gun120and the collector122. The electron beam is focused or contained in the tunnel through the central structure12by a magnetic field generated by, for example, the periodic permanent magnets (e.g.,144and146). An RF signal is applied at the RF input130and is coupled to the slow wave structure including the central structure12(e.g., the ladder90) and the ridges14and16connected in alternating, offset fashion to the central structure12by the protrusions (e.g.,24). The TWT10is adapted to cause the RF signal to travel along the length of the TWT10at about the same speed as the electron beam, maximizing the coupling between the electron beam and the RF signal. Energy from the electron beam is coupled to the RF signal, amplifying the RF signal, and the amplified RF signal is decoupled from the slow wave structure to the RF output132before the electron beam reaches the collector122.

Dimensions of one non-limiting example of a Ku band coupled cavity TWT10are provided in Table 1 below. Dimensions will vary based on the RF frequency, desired bandwidth, and design variations as discussed above. Dimensions are identified inFIGS. 4,6and8.

The coupled cavity TWT10, including the housing80, ladder90and ridges14and16, may comprise any electrically conductive material selected based on the required operating characteristics, such as copper, a copper alloy, molybdenum, tantalum, tungsten, etc, providing a suitably high melting point and conductivity. One or more severs may be provided at various locations along the TWT10to control the gain by absorbing energy in order. This prevents reflections from the output end of the TWT10to the input end which would cause oscillations in the TWT10. In addition to or in place of the severs, a coating or film may be applied to the ladder90and/or the ridges14and16to control the gain, using any suitable material having the desired conductivity and patterned in any way or form including, but not limited to, two and three dimensional patterns and tapers. Any method of coating (i.e., thin film, thick film, sputtering, physical vapor deposition, chemical vapor deposition, pyrolysis, thermal cracking, thermal evaporation, plasma and plasma enhanced deposition techniques, plating, electro-deposition, electrolytic, etc. may be used to achieve the desired results. Because the ladder90may be formed as an integral unit, the thickness and placement of a coating may be controlled relatively easily and applied by a number of suitable techniques such as sputtering, vapor deposition, etc. as discussed above. The thickness or conductivity of the coating may be varied along the length of the TWT10if desired to control the conductivity as needed.

Referring now toFIG. 14, a method for manufacturing a coupled cavity traveling wave tube includes creating slots in a ladder to form rungs (block200) and forming a tunnel longitudinally through the ladder. (Block202) The method also includes forming a first ridge having protrusions (block204) and forming a second ridge having protrusions. (Block206) The first ridge is aligned or positioned adjacent a first side of the ladder with the protrusions contacting an alternating group of the rungs. (Block210) The second ridge is aligned adjacent a second side of the ladder with the second ridge offset from the first ridge so that the first ridge protrusions and second ridge protrusions contact different rungs. (Block212)

While illustrative embodiments have been described in detail herein, it is to be understood that the concepts disclosed herein may be otherwise variously embodied and employed.