  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.
