At millimeter-wave and terahertz frequencies, vacuum electronics amplifiers have promising potential for high-data-rate secure communications, surveillance, and remote-sensing applications. However, the power and efficiency of vacuum electronic slow-wave circuits at these high frequencies is limited by the small size of the slow-wave circuit. Relatively small dimensional variations resulting from conventional micromachining techniques that are adequate for lower frequency operation can be large enough to cause serious degradation and variation of performance at higher frequencies. To alleviate this problem, a new design procedure for significantly improving the power, efficiency, and robustness of high-frequency vacuum electronic amplifiers was created and developed in an in-house research effort at the NASA Glenn Research Center.
Four periods, consisting of vacuum between copper walls, of a folded waveguide slow-wave circuit are shown. The entire copper circuit consists of several hundred periods with varying lengths. Amplification is obtained as the millimeter wave or terahertz wave travels through the serpentine path of the folded waveguide while interacting with an electron beam passing through the circular apertures. The length of a single period is approximately 500 μm for the 94-GHz circuit and 150 μm for the 400-GHz circuit.
Several vacuum electronics slow-wave circuit geometries were investigated with a commercial three-dimensional electromagnetics simulation code (ref. 1) at frequencies of 94 and 400 GHz, and the folded waveguide geometry shown in the preceding illustration was found to be the most robust with respect to currently available manufacturing tolerances. A number of micromachining techniques were investigated and compared with respect to applicability, expected tolerances, and cost. Wire electrodischarge machining appears to be the best currently available technique for manufacturing folded waveguide circuits at these frequencies.
A robust optimization algorithm based on simulated annealing (refs. 2and 3) was created to design slow-wave circuits consisting of a series ofseveral hundred folded waveguide periods with varying lengths. This algorithm differs from previous slow-wave circuit design techniques in that the dimensional tolerances are taken into account during the optimization. Designs optimized for robustness were developed for folded waveguide circuits at 94 and 400 GHz.
Simulated efficiency values of a 94-GHz circuit design obtained with the new robust optimization algorithm compared with those for a baseline design obtained with a standard simulated annealing algorithm. For each design, 200 runs with pseudorandom dimensional variations were performed.
Long description of figure 2.
The graph shows the simulated statistical performance of a 94-GHz circuit design obtained with the new robust optimization algorithm compared with that of a baseline design obtained with a standard simulated annealing optimization algorithm (ref. 2). For each design, 200 runs with pseudorandom dimensional variations were performed. The results show that the efficiency distribution is significantly improved with the new algorithm; similar results were obtained with the 400-GHz design. These results also indicate that this design procedure can significantly alleviate the performance degradation caused by manufacturing tolerance variations in high-frequency vacuum electronics amplifiers.
Find out more about the research of Glennís Communications Division: http://ctd.grc.nasa.gov
Dr. Jeffrey D. Wilson, 216-433-3513, Jeffrey.D.Wilson@nasa.gov
Analex Corporation contacts: Christine T. Chevalier, 216-433-6082, Christine.T.Chevalier@nasa.gov; and Dr. Carol L. Kory, 216-433-3512, Carol.L.Kory@nasa.gov
Authors: Dr. Jeffrey D. Wilson, Christine T. Chevalier, and Dr. Carol L. Kory
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Programs/Projects: Space Operations Mission Directorate
Last updated: October 16, 2006
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