Development of high-frequency, highly stable gyrotrons as Millimeter to submillimeter wave radiation sources

High-frequency, frequency-tunable, medium-power gyrotrons (Gyrotron FU series) are being developed in Fukui University as millimeter to submillimeter wave sources. The gyrotron series has achieved frequency tuneability in a wide range from 38 to 889 GHz and medium output power from 0.1 kW to several kilowatts in pulse mode (the pulse width is 1 ms and repetition rate 1 Hz) and several tens of watts in CW mode. Stabilizations of both amplitude and frequency have been achieved, for the application to many fields. High purity mode operations by installation of carefully designed cavity were also achieved for highly efficient conversion to Gaussian beam. In this paper, the advances in recent few years are summarized.