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.