Obtaining higher efficiency during the development of space Traveling Wave Tubes (TWTs) is always one of the most important goals for scientists. In this paper, a scheme of obtaining the maximum theoretical overall efficiency is explored by optimizing the helix pitch profile of a TWT based on the collectability of spent beam. The collectability of the spent beam was evaluated by the maximum collector efficiency, and this maximum collector efficiency was employed to calculate the maximum theoretical overall efficiency. The energy distribution of the spent beam and the output power of TWTs were calculated by the 3-D large signal Beam-Wave Interaction Simulator (BWIS) of MTSS. The detailed design of a Ku-band helix TWT is described according to three optimization goals (theoretical overall efficiency, theoretical collector efficiency and electronic efficiency). The simulation results indicate that the optimization for high interaction circuit efficiency or collector efficiency by itself is not adequate to obtain maximum overall efficiency. The maximum theoretical overall efficiency of 77% was achieved via the optimization of slow wave structure for theoretical overall efficiency.
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