Observation of quasi-coherent edge fluctuations in Ohmic plasmas on National Spherical Torus Experiment

A quasi-coherent edge density mode with frequency f_mode ∼ 40 kHz is observed in Ohmic plasmas in National Spherical Torus Experiment using the gas puff imaging diagnostic. This mode is located predominantly just inside the separatrix, with a maximum fluctuation amplitude significantly higher than that of the broadband turbulence in the same frequency range. The quasi-coherent mode has a poloidal wavelength λ_pol ∼ 16 cm and a poloidal phase velocity of V_pol ∼ 4.9 ± 0.3 km s^-1 in the electron diamagnetic direction, which are similar to the characteristics expected from a linear drift-wave-like mode in the edge. This is the first observation of a quasi-coherent edge mode in an Ohmic diverted tokamak, and so may be useful for validating tokamak edge turbulence codes.