Conversion of submillimeter wave gyrotron output into Gaussian beam and its application to plasma scattering measurement

Summary form only given. Plasma scattering measurement is effective technique to observe low frequency density fluctuations (f<1 MHz, k/spl sim/10 cm/sup -1/) 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 to offer small scattering volume and relatively large scattering angle 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 TE/sub mn/ mode structure. It is therefore necessary to convert the output radiation into a Gaussian beam (TEM/sub 00/ mode), which is suitable for an effective transmission and can be used as a well-collimated probe beam. A quasi-optical antenna is a suitable element for the conversion system under consideration since it is applicable to several TE/sub 0n/ and TE/sub 1n/ modes. It should be noted, however, that the far-field of the linearly-polarized beam produced by the antenna consists of side lobes and a main beam, which is similar to a bi-Gaussian beam. A Gaussian beam can be obtained by converting the mainbeam. A confocal mirror system with different focal lengths in different directions is used for the conversion. The stabilization of gyrotron output power and frequency are required to improve the performance of the measurement. They can be stabilized by decreasing the fluctuations of the cathode potential and the anode potential. These fluctuations are removed by using a smoothing circuits consisting of capacitor, coil and resistor.