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 investigates the linear behavior of the TM modes and reveals for the first time that certain TM modes might be suitable for gyrotrons --- ECM based devices. In addition, non-linear but non-self-consistent model shows that for a fixed field profile the efficiency could be as high as the transverse electric (TE) modes. Such interesting findings deserve more theoretical and experimental studies, and might facilitate some applications.