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The results of an experimental program with period tapering of the slow-wave structure of a traveling-wave tube which yielded a significant improvement in the efficiency of high-power TWTs are discussed. A method is outlined whereby these results may be applied to other tubes to improve efficiency. The study utilized a novel traveling-wave tube with a helical interaction structure independent of, and outside of, the vacuum envelope. This allowed the use of many different slow-wave structures with the same vacuum envelope; it resulted in fewer variables and greatly increased the number of experiments that could be performed.