Extended RF Behavior of a 77/154 GHz, 0.5 MW Continuous Wave Gyrotron

An extended RF-behavior study of a dual regime gyrotron for large helical devices (LHDs) is presented in this paper. The frequencies of taken operation are 77 and 154 GHz as used in the LHD experimental campaign. The mode-pair selected, after careful consideration of practical tradeoff parameters, for successful realization of the design is the TE
_15,4
and TE
_28,8
modes as the mode-pair for operation at 77 and 154 GHz, respectively. Rigorous mode competition and cold-cavity analysis have been done to ensure the feasibility of this mode-pair. For RF behavior, an extended cavity, which is comprised of a down-taper and mid-section conventional cavity followed by a nonlinear uptaper section, is considered. Self-consistent single-mode and time-dependent multimode computations are also carried out for both frequencies. These calculations were performed with a nonuniform magnetic field over the extended cavity section before and after space-charge neutralization using realistic beam parameters obtained from the magnetron injection gun calculations. From these rigorous calculations 0.5 MW, continuous wave power is obtained for chosen mode-pair with efficiency more than 38%.