Efficient excitation of high axial modes in simulations of low-voltage gyrotron

The gyrotron operation on the modes with high axial indices is an attractive method to provide the electronic frequency tuning which is desirable for DNP/NMR-aimed sub-THz devices. However, gyrotron efficiency typically drops drastically with increase of axial index of the operating mode. In numerical simulations we reveal that low-voltage operation at fundamental cyclotron resonance dramatically increases gyrotron efficiency at higher axial modes. Moreover, for magnetic fields corresponding to the gyro-TWT regime the gyrotron output power at the second and third axial modes can be even higher than at the first axial mode. Further improve of frequency tuning takes place in gyrotron with reversed direction of power output.