Plasma Scattering Measurement Using a Frequency Tunable, Submillimeter Wave Gyrotron as a Power Source

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Objectives: Development of a high frequency gyrotron for high power source (-100 W) operating in the submillimeter wave range, conversion of the output into a high quality beam and its application to plasma scattering measurement.The results :1. Development of a submillimeter wave gyrotron as a power source for scattering measurement We stabilized the gyrotron oscillation at the second cyclotron harmonic (f=353 GHz, p=110 W) within 10% of output power during a pulse of 0.6 s. This oscillation is stable enough for scattering measurement. In order to get high frequency output, we have developed a new gyrotron using a 12 T superconducting magnet and succeeded in a single mode oscillation (f=595 GHz, p=220 W) at the second cyclotron harmonic. The maxim frequency attained up to now is 636 GHz.2. Production of a high quality beam We have made a quasi-optical antenna to converting a gyrotron output into a linearly-polarized, two dimensionally focusing beam. The antenna produces a well collimated beam to the extent of a diffraction effect. In order to improve the focusing and the mode purity of beam, we are making a new transmission line consisting of multi-mirrors, which converts a gyrotron output into a Gaussian beam and couples it to plasma.3. Plasma scattering measurement using a gyrotron as a power source We have constructed a scattering measurement system consisting of a gyrotron and a quasi-optical antenna and succeeded in observing drift waves in CHS (Compact Helical System in National Institute for Fusion Science) plasma. In the near future, we will improve a scattering measurement system by replacing the quasi-optical antenna with the new transmission line and will observe externally excited wave for plasma heating.