A V-Band Overmoded Coaxial Millimeter-Wave Oscillator Based on a New Method of Asymmetric Modes Suppression

In the design of the coaxial millimeter-wave Cerenkov devices due to the low starting current and high temporal growth, asymmetric quasi-TE
_v1
modes are always preferentially excited. This article proposes a new method of hybrid modes coupling to suppress the asymmetric modes. The results show that when the quasi-TEM (N - 1)π/N mode couples with the TM
₀₁
(N - 3)π/N mode, the asymmetric modes can be suppressed effectively. The external Q-factor, electric field distribution, and the beam conductance of the hybrid modes are analyzed theoretically to explain the mechanism of the asymmetric modes suppression. According to this method, a coaxial V-band millimeter-wave oscillator is proposed. The 3-D particle-in-cell simulation results show that the symmetric modes are generated and all the asymmetric modes are suppressed successfully. When the diode voltage is 355 kV and beam current is 3 kA, the dominant frequency is 62.6 GHz and the output power is 322 MW, corresponding to a beam-wave conversion efficiency of 30.2%.