Development of input and output structures for high power X-band TWT amplifiers

Our recent research into multi-stage X-Band TWT amplifiers producing output powers of 100-200 MW has shown that it is essential to minimize the reflections in each stage of the amplifier in order to avoid sideband development. These reflections also cause fluctuations in the RF output power envelope. One solution to this problem is to isolate the two stages of the amplifier. Following extensive PIC code simulations and analytical work we have designed the first amplifier stage to provide beam modulation over a range (~200 MHz) of input frequencies. The second stage is a quasi-periodic structure designed to minimize reflections, and allow the radial or longitudinal RF power extraction to be distributed over an extended region. A second approach uses tapers that adiabatically reduce the loading in the output sections of the amplifier to provide a smooth, broad-band transition from the slow-wave structure to cylindrical waveguide. We are also developing mode converters that will allow extraction in TEM and subsequently in a TE mode of rectangular waveguide