Low-frequency coherent fluctuations in the lrsatron

Digital signal processing techniques were used to study low-frequency fluctuations (f < 100 kHz) in the Ohmically heated plasma (n_e ≈ 2 10¹¹cm^-3, T_e ≈ 10 eV) of the Proto-Cleo torsatron. Coherent density fluctuations show a mode structure with a helicity matching that of the vacuum field. The poloidal mode numbers vary from m = 4 at 20 kHz to m = 16 at 100 kHz. The m = 4 mode has the largest amplitude,/n ≈ 15%, and the fluctuation amplitudes decrease roughly linearly with frequency. Fluctuations in plasma current, H emission, and X-ray flux show spectra similar to and coherent with that of the density fluctuations. Comparison with theory and previous experimental results indicates that rippling modes may be responsible for these fluctuations. These results may be relevant to the physics of edge plasmas in larger devices.