TE-and TM-Mode Competition in Subterahertz Gyrotron Using Axis-Encircling Electron Beam

For a long time, most gyrotron oscillators have avoided using TM modes because of the concerns about strong bunching competition and relatively weak beam-wave coupling. However, this work demonstrates that an axis-encircling electron beam with high mode selectivity is adapted to preclude most parasitic modes and makes the TM12-mode oscillation in an open-cavity-type gyrotron system feasible. Considering the modes excited at the fundamental cyclotron harmonic, the TE12 mode remains to be the only competitor to the targeted TM12 mode; however, it will be effectively suppressed by the axial velocity spread. Operating with 70-kV beam voltage and 1-A beam current, the output power of the TM12 mode may reach the several-kilowatt level, verified by both nonlinear frequency-domain and time-domain simulations. Nonetheless, as the modes at high cyclotron harmonics are included, the second-harmonic TE24 mode and the third harmonic TE36 mode would potentially hazard the proposed TM12-mode operation. Even so, the particle-in-cell (PIC) Computer Simulation Technology (CST) simulation results still show a tunable window of the TM12 mode, where all parasitic TE-mode oscillations are fully suppressed. This work manifests the importance of considering the competition from TM modes in the designs of gyrotron devices, especially for the cases using axis-encircling electron beams.