Abstract
The transverse magnetic (TM) modes have long been considered as the unsuitable waveguide modes for the operation of the electron cyclotron maser (ECM). This study found that the interaction efficiency for some TM modes is as high as the transverse electric (TE) modes. Detailed theoretical analysis reveals that the bunching mechanism of TM modes is good for backward-wave operation. Unlike the TE modes, the TM modes has z-component of the electric field. This causes the bunching behavior to be more complicate than TE modes. The authors also found that for the backward-wave interaction the azimuthal bunching and axial bunching will have the same bunching point, which will be beneficial to TM mode operation. However, we found out that the forward wave perform well in the simulation of TM11 mode gyrotron oscillator system. The result of electron phase analysis indicated that azimuthal bunching is more important than axial bunching. Focusing on azimuthal bunching, the transverse electric field and longitudinal electric field present a competitive relation for backward wave.