10.3: Experimental measurement of picosecond pulse amplification in a 140 GHz Gyro-TWT

We report the first experimental studies of picosecond pulse amplification and broadening in a gyrotron amplifier. These studies allow investigation of the amplification process in a new physical regime, where wave dispersion and finite gain bandwidth play a major role. The experiments were performed on a 140 GHz gyro-traveling wave tube (Gyro-TWT) operating in a higher order mode of a confocal waveguide. We observed significant pulse broadening for pulses shorter than 1 ns when the operating point of the amplifier was close to the waveguide cut-off. For example, a 580 ps pulse at 137.30 GHz, a frequency near cutoff, was measured to be broadened by 100 %, while pulses at higher frequency, further from cutoff, showed much smaller broadening. The results of these experimental studies show that for the picosecond pulse amplification necessary in applications such as Electron Paramagnetic Resonance (EPR), the operating point of the gyro-TWT should be chosen to be at least a few percent above the cut-off to avoid pulse distortion.