A design process for a second harmonic operation of a low ohm lossy TE03 mode 0.25 THz gyrotron has been presented.
Mode competition and mode selection are carefully studied through the linear theory of CRM. The cavity are designed
and optimized by using a time domain open cavity calculation code, and validated by using the famous FEM code HFSS.
Interaction numerical investigations are carried out by using a self-consistent nonlinear theory cod. The influences of the
magnetic field, current, voltage and the velocity ratio of the electron beam under the interaction between the electron
beam and RF field are analyzed. The 14-kW 0.25 THz gyrotron with a predicted device efficiency of 39% is driven by a
25-kV 1.5-A (<i>v</i>⊥/<i>v</i><sub>//</sub> = 1.5, ▵<i>v</i><sub>z</sub> / <i>v</i><sub>z</sub> = 6%) electron beam from a magnetron injection gun. A tapered magnetic field is
adopted in the large signal simulation to prompt the electron efficiency.