Effects of beam velocity spread on two-stage tapered gyro-TWT amplifier

The recent successful operation of a two-stage K_a-band gyro-TWT amplifier at the Naval Research Laboratory has demonstrated its viability as a source of broadband high-power millimeter waves. However, it has been observed experimentally that sharp gain reductions occur at certain frequencies. In this paper, an analytical theory is presented which describes the effect of electron beam quality on the device´s instantaneous bandwidth. It is shown that these observed gain dips are due to electron phase-mixing, resulting from finite beam velocity spreads, in the frequency-dependent sever. In addition, the theory demonstrates that the velocity-spread induced dips can be shifted or reduced by adjusting the magnetic field profile, circuit length, and beam axial velocity, consistent with experimental observations