Oscillation characteristics of a high power 300 GHz band pulsed gyrotron for use in collective thomson scattering diagnostics

A high power sub-THz pulsed gyrotron with a frequency of 303 GHz has been developed for use as the power source of the collective Thomson scattering (CTS) diagnostics in the Large Helical Device (LHD) at the National Institute for Fusion Science. Based on careful design considerations, a whispering gallery mode TE22,2 was adopted to avoid mode competition. More than 320kW was attained with single-mode oscillation. The spectrum of the oscillation frequency is very narrow and stable during the entire pulse duration for pulse widths up to 100 μs. No parasitic mode is excited at the turn-on and turn-off phases nor during the steady state of the oscillation pulse. In addition to the adoption of the whispering gallery mode, a fast but finite rise time of the beam voltage is an important role for the realization of single-mode oscillation. Self-consistent calculations show that, for the finite voltage rise time corresponding to the present experiment, gradual evolution to the high power regime is attained from backward wave oscillation. This high power regime corresponds to the hard self-excitation region in the case of instantaneous voltage rise. © 2019 The Japan Society of Plasma.