Bispectral Analysis of Low- to High-confinement Mode Transitions in the National Spherical Torus Experiment
Authors: A.E. White, S.J. Zweben, M.J. Burin, T.A. Carter, T.S. Hahm, J.A. Krommes, and R.J. Maqueda
Date of PPPL Report: June 2006
Submitted to: Physics of Plasmas
This paper will present an experimental study of the temporal and spatial characteristics of the auto-bicoherence calculated from light amplitude fluctuations measured in the edge plasma of the National Spherical Torus Experiment (NSTX) using data from the Gas Puff Imaging (GPI) diagnostic obtained during a series of thirteen shots in which the NSTX plasma underwent spontaneous low- to high-confinement mode (L-H) transitions. The auto-bicoherence calculated from the available GPI chord signals in the region near the magnetic separatrix and just above the outer mid-plane indicates that there is no significant increase, i.e. outside the R.M.S. 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.