The 2.5-dimensional (2.5-D) particle-in-cell (PIC) simulation code MAGIC and 3-D electromagnetic simulation code HFSS were used to investigate the helix traveling-wave tube (TWT) with positive-negative tapering for efficiency improvement. Some key parameters were analyzed, including high-frequency parameters, non-synchronous parameter, gain parameter, and electron phase. A positively-negatively tapered helix TWT were designed and fabricated. The measured efficiency of the tapered TWT reaches 29%, while that of the non-tapered TWT is 19%. The simulation method of this paper has the averagely relative error of about 5%.
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