Optical fluctuation measurements of turbulence using a diagnostic beam on Phaedrus‐T

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.