Bispectral analysis of low- To high-confinement mode transitions in the National Spherical Torus Experiment

This paper will present an experimental study of the temporal and spatial characteristics of the autobicoherence calculated from light amplitude fluctuations measured in the edge plasma of the National Spherical Torus Experiment (NSTX) [M. Ono et al., Plasma Phys. Controlled Fusion 45, A335 (2003)] using data from the gas puff imaging (GPI) diagnostic [R. J. Maqueda et al., Rev. Sci. Instrum. 74, 2020 (2003); S. J. Zweben et al., Nucl. Fusion 44, 134 (2004)] obtained during a series of thirteen shots in which the NSTX plasma underwent spontaneous low- to high-confinement mode (L-H) transitions. The autobicoherence calculated from the available GPI chord signals in the region near the magnetic separatrix and just above the outer midplane indicates that there is no significant increase, i.e., outside the rms error, in the amount of nonlinear coupling between low frequency fluctuations and high frequency fluctuations during the 10 ms before the transition. Limitations of bicoherence analysis are discussed.