We report experimental and theoretical results from research into high
power X-band traveling wave tube amplifiers designed to eliminate sid
ebands caused by reflections from the output of such structures. These
amplifiers have a low energy velocity, such that the time it takes a
wave to be reflected from the output to the input-is of the order of,
or greater than, the electron beam pulse duration. The elimination of
sidebands and the effects of reflections is achieved by this transit t
ime isolation. The bandwidth of the output spectrum is limited by the
low pass-band of the periodic structures. Such amplifiers have been op
erated at power levels of up to 160 MW at 9 GHz for 50 ns pulse durati
ons.