Bispectral analysis of the L-H transition as seen in the NSTX GPI data*

It was recently reported that the bicoherence of turbulent potential and density fields measured with Langmuir probes increases just prior to and during the L-H mode transition in DIII-D. By calculating the evolving 3 wave-coupling properties between turbulent and large scales with bispectral statistics, we can attempt to identify the formation of possible shear and/or zonal flows at the L- H transition. This poster will present bispectral analysis of the L-H mode transition in NSTX using the data obtained by the Gas Puff Imaging (GPI) diagnostic, with the goal of searching for coupling between modes which may lead to shear and/or zonal flows generated by turbulence. A radial and poloidal array of 13 detectors measures the HeI or D$_\alpha$ light emitted from the plasma and this array can be used to measure profiles of the bicoherence before, during, and after an L-H transition. The bicoherence measured using this collection of 1-D time series data will be compared to the 2-D images of the turbulence obtained from a high-speed camera.