Marginal stability design criterion for gyro-TWTs and comparison of fundamental with second harmonic operation

Stability properties of both the fundamental and second harmonic gyrotron travelling wave amplifier (gyro-TWT) are examined with multi-mode particle simulations. The second harmonic cyclotron interaction with an axis-encircling electron beam is found to be more stable to oscillations and can yield significantly greater power than the fundamental harmonic gyro-TWT. A multiple stage interaction structure based on a marginal stability criterion is proposed and illustrated with examples of a 128kW fundamental gyro-TWT and a 532 kW second harmonic gyro-TWT, Stable amplification at much higher power levels is in principle possible.