Bispectral Analysis of the Edge Harmonic Oscillation and Turbulence Measured with BES on DIII-D

An edge harmonic oscillation (EHO) provides continuous particle transport to allow ELM-free operation at good energy confinement, constant density, and radiated power in Quiescent H-mode plasmas (QH). The nonlinear interaction between harmonics of the EHO and turbulence is important to understanding the mechanisms and dynamics of enhanced particle transport in QH-mode. Bispectral analysis has been applied to the localized density fluctuation data in QH-mode plasmas measured using BES in DIII-D. The fundamental frequency of the EHO was typically $\sim$ 10 kHz with long poloidal wavelength ($k_\theta \sim$ 0.02 cm $^{-1}$) and broadband turbulence in the range of 50-300 kHz with correlation lengths of a few cm. The cross-bicoherence among the BES channels showed radially varying magnitude of phase coherence well above the noise floor between the EHO and broadband turbulence in the region of 0.8$ < \rho < $1.0. This indicates that the EHO and turbulence interact with each other, potentially impacting the observed particle transport.