Summary form only given. Our second-harmonic TE/sub 21/ gyro-TWT with an 80 kV, 20 A, v/sub /spl perp//v/sub /spl par//=1.0 MIG electron beam has yielded 210 kW at 16 GHz with 13% efficiency, 16 dB saturated gain, 2% saturated bandwidth and zero-drive stability. It remained stable because the dominant gyro-BWO modes, the second-harmonic TE/sub 11/ and third-harmonic TE/sub 31/, had been suppressed by slicing the cylindrical waveguide with two cuts separated in azimuth by 90/spl deg/. Harmonic gyro-TWTs not only require a substantially weaker magnetic field and offer nearly the same high efficiency as fundamental gyro-TWTs, but also have the potential to stably generate significantly higher levels of power because the threshold beam current level for oscillation is raised dramatically due to the relatively weaker harmonic interaction. The design for a 1 MW third-harmonic TE/sub 31/ gyro-TWT utilizing a MIG will also be presented.