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TWTs for space applications commonly have a helix pitch profile which incorporates a section with increased phase velocity followed by a negative phase velocity taper. A simple method is described for the initial design of a helix slow-wave structure of this kind to achieve high overall efficiency. It is shown that the use of a section with increased phase velocity increases the beam efficiency of a TWT while reducing the generation of second harmonic power. The technique is illustrated by its application to a 70 W Ku-band TWT and the performance is shown to be comparable with that of an existing TWT.