P roduction of an intense, high quality submillimeter wave probe-beam and its application to plasma scattering measurements

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The design of the quasi-optical transmission line was based on gyrotron output at 354 GHz in order to carry out plasma scattering. This system consists of a quasi-optical antenna to convert the output into an linearly-polarized beam and an elliptical mirror to collimate the beam. Although the beam shape of the beam produced is different from that of calculated result, the size of the beam produced (58mm*55mm) is in broad agreement with that of the calculated result (42mm*67mm).We have used this intense beam to study low frequency density fluctuations by plasma scattering in Compact Helical System (CHS) plasmas at the National Institute for Fusion Science. A peak is found around 150 kHz in the upper spectrum. Although the frequency of the electron drift waves deduced from the plasma parameters changed from 100 kHz to 300 kHz, the observed peak frequency did not change. This suggests that the peak is not merely due to the electron drift waves.A high frequency gyrotron delivers moderate cw outputs (P=20W) at submillimeter wavelengths (f=302GHz). Time resolved frequency measurements show that the frequency fluctuation levels of the cw outputs are within 2MHz. The frequency of the cw output is modulated by the potential modulation of the body. A peak-to-peak value of 120V can produce the frequency modulation of 29MHz.