Fundamental and harmonic mode competition in gyrotron oscillator

Summary form only given. We present a theoretical study of the competition between fundamental and harmonic interactions in the gyrotoron oscillator. It is shown through the particle-in-cell simulation that, in the nonlinear stage, the fundamental harmonic interaction possesses a significant advantage over the harmonic interaction, which can become a deciding factor in the competition process. The beam-wave coupling coefficient is given by H sm =J 2 s-m (k mn r c )J' 2 S (k mn r L ), where s is the cyclotron harmonic number, J n (x) is a Bessel function of order n, x mn = k mn /r w , and x mn is the nth nonzero root of J s '(k mn r L ). H sm of a higher harmonic mode decreases much faster than a lower harmonic mode as the electron Larmor radius decreases. Hence, in saturated operation, H sm (averaged over all electrons) of a higher har-monic mode decreases rapidly along the interaction structure as the electron transverse energy is depleted, while H sm of a fundamental harmonic mode remains near its original value. This suggests an inherent advantage for fundamental harmonic mode.