Heavily-loaded wideband helix traveling wave tube

The experimental work for a wideband (6-18GHz) helix TWT with a heavy ceramic loading has been carried out at SNU. The well known issues of this kind of TWT are the maximum allowed 2^nd harmonic level at the lower frequency range and minimum allowed efficiency at the higher frequency range. The "negative" dispersion of a helical slow-wave structure (SWS) is a key concept to ensure a successful operation in wideband helix TWT. In this study, an unconventional "positive phase-velocity-tapering" with an unequal ceramic rod was employed to enhance the second harmonic level at the lower frequencies and efficiency at the higher frequencies. The dispersion and the interaction impedance of the structure were predicted by a 3-D HFSS code.