Design consideration and oscillation characteristics of high-power 300 GHz gyrotron

A 300 GHz pulse gyrotron has been developed for collective Thomson scattering diagnostics in the Large Helical Device with a careful design consideration. Single-mode oscillation of more than 320 kW was attained. The frequency spectrum was very narrow and stable across the pulse width up to 80 μs. No parasitic mode was observed during the whole pulse width including the turn-on and turn-off phases. No-parasitic-mode oscillation is likely due to a very fast rise time of the beam voltage as well as due to adoption of a whispering gallery mode.