Plasma and Fusion Research

Volume 14, 1406104 (2019)

Regular Articles

Oscillation Characteristics of a High Power 300 GHz Band Pulsed Gyrotron for Use in Collective Thomson Scattering Diagnostics
Teruo SAITO, Shunsuke TANAKA, Ryuji SHINBAYASHI, Yoshinori TATEMATSU, Yuusuke YAMAGUCHI, Masafumi FUKUNARI, Shin KUBO1), Takashi SHIMOZUMA1), Kenji TANAKA1) and Masaki NISHIURA1)
Research Center for Development of Far-Infrared Region, University of Fukui, Fukui 910-8507, Japan
1)
National Institute for Fusion Science, National Institute of Natural Sciences, Toki 509-5292, Japan
(Received 26 March 2019 / Accepted 24 April 2019 / Published 19 June 2019)

Abstract

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 320 kW 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.

Keywords

Sub-THz gyrotron, whispering gallery mode, single-mode oscillation, collective Thomson scattering, LHD

DOI: 10.1585/pfr.14.1406104

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