The present invention relates to the microwave generation, amplification, and processing arts. It particularly relates to traveling wave tubes for microwave amplifiers and microwave oscillators, and will be described with particular reference thereto. However, the invention will also find application in other devices that operate at microwave frequencies, and in other devices that employ slow wave circuits.
Traveling wave tubes typically include a slow wave circuit defined by a generally hollow vacuum-tight barrel with optional additional microwave circuitry disposed inside the barrel. An electron source and suitable steering magnets or electric fields are arranged around the slow wave circuit to pass an electron beam through the generally hollow beam tunnel. The electrons interact with the slow wave circuit, and energy of the electron beam is transferred into microwaves that are guided by the slow wave circuit. Such traveling wave tubes provide microwave generation and microwave amplification.
Heretofore, commercially produced traveling wave tubes have been limited to about 65 GHz. However, future applications call for traveling wave tubes that operate at frequencies of 100 GHz or higher. For space-based applications these high frequency devices will probably be driven at operating voltages of 20 kV or less in accord with presently available space-based electrical power sources.
Construction of high frequency traveling wave tubes is difficult using existing traveling wave tube manufacturing techniques. Designs for high frequencies call for microwave circuitry with very small features (for example, repetition periods of less than 0.2 mm), and greatly reduced quantities of dielectric insulation material in the tube to reduce dielectric loading. Moreover, adequate heat sinking becomes an increasingly significant issue as the operating frequency increases.
In one known method for manufacturing traveling wave tubes, a fragile three-dimensional microwave circuit shell, such as a metallic helix, is compressively secured in a generally hollow cylindrical barrel. Dielectric rods arranged parallel to the helical axis of the microwave circuit act as standoff insulators that align and secure the compressed microwave circuit shell inside the barrel.
To ensure good thermal contact between the components, the compressive forces are substantial. The fragile microwave circuit shell and dielectric rods are compressively secured in the barrel by briefly heating the barrel during insertion to induce temporary thermal expansion of the barrel. The microwave circuit shell/dielectric rods combination has close tolerances with respect to the barrel, and so when the barrel contracts upon cooling the interior components are compressively secured in the barrel. However, the heating and compression can damage the slow wave circuit, and mass production by this method is difficult. Moreover, this technique is not well suited for the small structures used in devices that are preferred for 100 GHz or higher operation.
To achieve features on the delicate scale called for in high frequency operation, lithographic techniques are regularly used in the electronics industry. However, these techniques are generally applied to planar wafer substrates of silicon or other semiconductor materials. Lithographic techniques are not readily adapted to produce the types of finely detailed three-dimensional structures called for in traveling wave tubes designed for high frequency operation.
To reduce dielectric loading, the dielectric rods can be replaced by thin dielectric standoff chips of natural diamond. In one constructed device described in A Millimeter-Wave TunneLadder TWT (D. Wilson, NASA Contract Report 182183, 1988), diamond chips with heights of 250 microns each were used in a traveling wave tube that operated at 28 GHz. However, this device has not been replicated to date due to the cost of natural diamond and the assembly difficulties, especially relating to positioning of the diamond chips. Moreover, scaling such a device up to 100 GHz or higher frequency would call for a large number of diamond chips (e.g., about 80-150 diamond chips for a 2-3 cm long traveling wave tube designed for 100 GHz operation) each having a height of about 75 microns or thinner. These reduced dimensions and increased numbers of diamond chips would further exacerbate an already difficult manufacturing process.
The present invention provides an improved apparatus and method.
According to one embodiment, a slow wave circuit of a traveling wave tube is provided, including a three-dimensional conductive structure. A dielectric film coats selected portions of the three-dimensional conductive structure. An outer housing surrounds the three-dimensional conductive structure. The outer housing includes interior surfaces that contact the dielectric film.
According to another embodiment, a method is provided for generating or amplifying microwave energy. A generally hollow three-dimensional electrically conductive structure is formed. The three-dimensional conductive structure is laser micromachined to define a selected generally periodic pattern on the conductive structure. The conductive structure is arranged inside a generally hollow barrel. An electron beam is passed through the generally hollow three-dimensional conductive structure. The electron beam interacts with the conductive structure and the hollow barrel to generate or amplify the microwave energy.
According to yet another embodiment, a slow wave circuit of a traveling wave tube is provided. The slow wave circuit includes a three-dimensional electrically conductive shell having at least one laser micromachined gap defining a pattern selected to interact with microwaves in the traveling wave tube. The conductive structure is disposed inside a generally hollow barrel. The barrel includes interior vanes. Dielectric standoff insulators are arranged between the interior vanes of the barrel and the electrically conductive shell. The dielectric standoff insulators include laser micromachined gaps corresponding to the at least one laser micromachined gap of the three-dimensional shell.
Numerous advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading the following detailed description of the preferred embodiments.
The invention may take form in various components and arrangements of components, and in various process operations and arrangements of process operations. The drawings are only for the purpose of illustrating exemplary embodiments and are not to be construed as limiting the invention.
With continuing reference to
In one exemplary embodiment designed for operation at about 100 GHz, the slow wave circuit 10 has a length of about 2 cm to 3 cm, and the helical conductive structure 30 has a helical pitch (axial distance between adjacent helical turns) of around 0.20 mm to 0.22 mm. In this design, the metal helical turns are each around 0.06 mm to 0.08 mm wide, and the helical gap is around 0.14 mm wide. For a length of 2 cm to 3 cm, the helical conductive structure 30 includes around 90 to 150 helix turns. Thus, it will be appreciated that the FIGURES show substantially enlarged views of small exemplary portions of the total length of the slow wave circuit 10.
In the exemplary 100 GHz design, the barrel defined by the assembled outer shell pieces 12, 14 has an inner diameter of about 0.7 mm, while the helical slow wave circuit 10 has an outer diameter of about 0.3 mm. In design simulations, a ratio between a thickness of the dielectric standoff insulator film 36 and an inwardly extending length of the vanes 20, 22, 24 of about 1:4 has been found to provide especially good microwave characteristics. In the exemplary 100 GHz design, this corresponds to a thickness of the dielectric standoff insulator film 36 of about 40 microns, and an inwardly extending length of the vanes 20, 22, 24 of about 160 microns.
These values are for an exemplary 100 GHz design. Those skilled in the art can readily compute specific dimensions of the slow wave circuit that provide selected electrical and microwave characteristics. Generally, reducing the thickness of the standoff insulator film reduces dielectric loading and enhances performance. However, at some point continued thinning of the dielectric produces excessive electrical conductance or other deleterious effects. Preferably, the diamond film is less than about 90 microns thick.
In operation, the slow wave circuit 10 is inserted into an evacuated portion of a traveling wave tube, and an electron beam is passed through the hollow interior of the conductive structure 30, that is, along the helical axis. Interaction with the slow wave circuit 10 causes energy transfer from the electron beam to microwaves guided in the slow wave circuit 10.
With reference to
As shown diagrammatically in
Diamond insulating stripe coatings deposited by chemical vapor deposition are preferred. Such stripe coatings are readily manufactured and provide high thermal conduction that is uniform along the stripes 50, 52, 54. However, other dielectric materials and methods can be used. For example, thin dielectric strips of insulating material can be brazed onto the metal shell 40.
With reference to
For example,
The slow wave circuit 110 also includes a three-dimensional electrically and thermally conductive structure 130 formed by two symmetric, mating ladder structure pieces 132 which in one suitable embodiment are shaped metal sheets. The metal sheets are shaped so that the two mating structure pieces 132 define a generally cylindrical central hollow region coinciding with the central portion of the hollow waveguide. The ladder structure pieces 132 are brazed or otherwise bonded together.
With continuing reference to
In one suitable embodiment for 100 GHz operation, the slow wave circuit 110 has dimensions of order 2-3 cm in length, cylindrical diameter of order 1 mm, and a ladder period of less than 0.5 mm, and preferably about 0.1-0.3 mm. Hence, it will be appreciated that the slow wave circuit 110 includes of order 50-300 or more gaps 33. The FIGURES show an exemplary enlarged axial portion of the slow wave circuit 110 and components thereof. These values are for an exemplary 100 GHz design. Those skilled in the art can readily compute specific dimensions of the slow wave circuit that provide selected electrical and microwave characteristics.
With reference to
The stripe 152 is preferably a diamond coating deposited by chemical vapor deposition. Such a coating is readily manufactured and provides intimate contact between the stripe 152 and the formed metal sheet 150 which promotes high and uniform thermal conduction therebetween. However, other dielectric materials and methods can be used. For example, a thin dielectric strip of insulating material can be brazed onto the formed metal sheet 150.
The metal sheet 150 with the diamond stripe coating 152 defines an intermediate structure 160. The intermediate structure 160 is suitably processed by laser micromachining to cut the slots or gaps 133 in the metal sheet 150 and the diamond stripe 152 to produce the final ladder structure piece 132. Although laser micromachining is a preferred method for forming the slots or gaps 133, other methods can be used, such as lithographic methods.
The described slow wave circuits 10, 110 are exemplary only. Other types of high frequency microwave circuitry can be similarly constructed. Laser micromachining enables substantially any type of cut, slot, or other opening to be formed into a metal or dielectric structure. Chemical vapor deposition combined with laser micromachining and/or lithography can be used to place diamond standoff insulators essentially anywhere on the microwave circuitry. Other dielectric materials besides diamond can also be used, such as diamondlike carbon (DLC), born nitride or other boron-based films, or the like.
Additionally, the dielectric standoff insulator films can be deposited on interior surfaces of the exterior housing rather than on the three-dimensional conductive structure. Although generally cylindrical barrel-shaped exterior housings are illustrated, rectangular or otherwise-shaped housings can also be used. Waveguides other than ridged waveguides can be used. Moreover, the methods described herein are also applicable to fabricating three-dimensional microwave circuitry for other applications besides traveling wave tubes.
Those skilled in the art will recognize substantial performance benefits in the disclosed traveling wave tubes and their equivalents. The combination of laser micromachining and chemical vapor deposition enable reliable and readily manufacturable formation of smaller features (e.g., periodic ladder structures, helices, standoff insulator heights and lateral dimensions, and the like) compared with previously employed methods. Smaller features enable higher frequency operation and reduced dielectric loading.
Static forces between the barrel and the conductive structure disposed therewithin are generally noncompressive. Rather than a compression fit typically used heretofore in traveling wave tube construction, the exemplary traveling wave tubes here described employ brazed joining of component parts of the traveling wave tube. This substantially reduces compressive forces on the fragile conductive structure 30, 130 disposed in the barrel.
These aspects enable substantial improvement in the gain-bandwidth product. Low dispersion traveling wave tubes in which the phase velocity is substantially independent of frequency are readily achieved. Thermal aspects are also improved due to the intimate physical and thermal contact between the deposited diamond layer and the three-dimensional conductive structure.
The invention has been described with reference to the exemplary embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.
This application claims the benefit of U.S. Provisional Application No. 60/356,524, filed Feb. 13, 2002, inventor James A. Dayton, Jr.
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Number | Date | Country | |
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20030151366 A1 | Aug 2003 | US |
Number | Date | Country | |
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60356524 | Feb 2002 | US |