Plasma density turbulence has been measured with the beam emission spectroscopy (BES) diagnostic system, using a low-power neutral beam with He0 and H0 as beam species. In general, He0 (588 nm) provided the best signal-to-noise ratio due to its lower edge plasma background interference. Simultaneous measurements of edge density fluctuations have been made with BES and Langmuir probes; the spectra are seen to be essentially identical, and the fluctuation amplitudes from both diagnostics are in close agreement. A poloidal coherence length of about 2–4 cm was observed. Radial propagation of modes was not seen, but a lab-frame poloidal phase velocity at r/a=0.77 of about 7×105 cm/s in the electron diamagnetic direction was observed, corresponding to m=8–75 kHz.
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Evensen et al. (1992) studied this question.
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