Plasma scattering measurement using a submillimeter wave gyrotron as a radiation source

Plasma scattering measurement is effective technique to observe low frequency density fluctuations excited in plasma. The spatial and wave number resolutions and the S/N ratio of measurement depend on the wavelength range, the size and the intensity of a probe beam. A well-collimated, submillimeter wave beam is suitable for improving the spatial and wave number resolutions. Application of high frequency gyrotron is effective in improving the S/N ratio of the measurement because of its capacity to deliver high power. Unlike the molecular vapor lasers, the gyrotrons generate diverging beam of radiation with TEmn mode structure. It is therefore necessary to convert the output radiation into a Gaussian beam. A quasi-optical antenna is a suitable element for the conversion system under consideration since it is applicable to several TE0n and TE1n modes. In order to apply the gyrotron to plasma scattering measurement, we have stabilized the output (P=110W, f=354GHz) of gyrotron up to the level (DP/P<1 %, Df<10 kHz). The gyrotron output can be stabilized by decreasing the fluctuation of the cathode potential.