Study of statistical properties of edge turbulence in the National Spherical Torus Experiment with the gas puff imaging diagnostic

An investigation is presented of the edge turbulence in the National Spherical Torus Experiment [M. Ono, M. G. Bell, R. E. Bell et al., Plasma Phys. Control. Fusion, 45, A335 (2003)] based on the optical gas puff imaging (GPI) diagnostic. First of all, the edge fluctuations are characterized for the low confinement mode (L-mode) discharges as a function of the radius. The probability distribution function of the fluctuations is shown to be non-Gaussian for all the radial positions studied, but the deviation from the normal distribution is greater outside the separatrix; in this region the area occupied by the edge structures ("blobs") is greater than inside the separatrix, and this is correlated with the decrease of the logarithmic radial derivative of the pressure gradient. Then the difference between the L- mode and the high confinement mode (H-mode) is studied. With the continuous wavelet transform technique the intense bursts in the GPI signal can be detected; their number decreases in the H-mode with respect to the L-mode, with no significant change in the poloidal velocities. The difference between the two regimes is also observed in the poloidal wavenumber spectra: L-mode and H-mode have two different injection scales for the energy, and different cascades take place. Only in the L- mode the energy flows toward the small wavenumber feeding the bigger blobs. (C) 2007 American Institute of Physics.