New helix tapers boost space TWT efficiency to 55%, broadband

The double-tapered helix was introduced fifteen years ago by Thomson-CSF in satellite TWTs. It brought a substantial increase of beam efficiency without degradation of phase linearity. However no real design methodology was then existing in the world. A basic study recently performed by the space tubes R&D group of the Electron Tubes Division of Thomson-CSF has led to a new double taper design, using an original method for pitch profile synthesis and optimization. The basic principle of this method is to significantly increase the RF beam current in the large signal region. The result is a large improvement of beam efficiency, without sacrifying phase linearity and intermodulation. This helix design method has been applied to tubes operating at 7 and 12 GHz with output power levels ranging between 40 and 130 W. Beam efficiencies in the range of 22 to 25%, with phase shifts as low as 20 to 40°, have been achieved in 20% bandwidths, resulting into overall efficiencies reaching 55%.