Abstract
Localized measurements of internal MHD activity in PBX-M have been made using a heterodyne radiometer. The radiometer measures third harmonic optically gray ECE from the center to the outer edge of the plasma on 8 radially localized frequency channels from 112–127 GHz. The radiometer uses a focusing gaussian optics system which provides a vertical resolution of approximately 6 cm throughout the plasma. The radial resolution of the diagnostics is limited by the relativistic broadening to approximately 2–3 cm. Long wavelength (k<0.5 cm−1) temperature and density fluctuations can therefore be detected using this diagnostic. Correlation techniques have been used on the data to provide fluctuation sensitivity as low as 0.4%. An analytical theory has been developed which relates the measured amplitude of the coherent ECE fluctuations caused by a MHD mode to its mode characteristics. Using the analysis, the MHD mode displacement amplitude ξ(r) has for the first time been calculated for observed MHD activity in a tokamak plasma. ξ(r) and δP/P have been calculated for high n-number MHD modes in high-βp PBX-M plasmas. The results of these measurements will be presented.
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