Excitation of backward waves in Gyro-TWTs and Gyrotwystrons

Summary form only given. Gyroamplifiers are promising candidates for high power microwave generation in the mm wavelength regime. Their operation at cyclotron harmonics is also attractive since it allows one to reduce the external magnetic field by a factor equal to the harmonic number. For optimizing the operation of travelling wave devices (gyro-TWT´s and gyrotwystrons) the excitation of parasitic backward waves is a critical issue. We have developed the formalism describing the self-excitation of backward waves in the presence of forward waves within the gyrotwystron and gyro-TWT in which both forward and backward waves can be excited at arbitrary cyclotron harmonics. The effect of the input signal power on the starting current of the backward waves in gyro-TWT´s and gyrotwystrons is analyzed.