Plasma scattering measurement using a high frequency gyrotron as a radiation source

The scattering method with electromagnetic wave makes it possible to observe frequency and wavenumber of the fluctuations directly with a high spatial resolution. There is an optimum wavelength of the probe beam in consideration of the spatial and wavenumber resolutions. The range of the available wavelengths is in submillimeter region, which allows refractive effects to be sufficiently small while still permitting large scattering angle. Application of high frequency gyrotron is effective in improving the S/N ratio of the measurement because of its capacity to deliver high powers. In order to meet the high requirements needed for measurement, the output radiation of the Gyrotron FU Ⅳ has been stabilized by introducing a smoothing circuit in the high-voltage power supply. In addition, conversion of the output radiation into a Gaussian beam (TEM00 mode) is necessary for an effective transmission and irradiation of the plasma by a well-collimated probe beam. A final goal is to produce an optimal probe beam (High quality, intense submillimeter wave beam) and to apply it to plasma scattering measurements with the aim of optimal performance of the measurem