Investigation of nonlinear three-wave interaction of plasma density fluctuation measured by KSTAR BES during ELM crashes

Abstract

In the thesis, the nonlinear interaction related to the three-wave interaction of enhanced plasma density fluctuation is observed during edge localized mode (ELM) crashes. During ELM crashes, plasma particle and energy abruptly escape from inside the plasma to the outside. The enhanced plasma turbulence during ELM crashes is one of the possible mechanism for enhanced plasma particle and energy transport. The beam emission spectroscopy (BES) observes the plasma density fluctuation of ELMy H-mode plasma experiments at the KSTAR tokamak. The correlation based analysis and spectral analysis are used to investigate statistical properties of plasma density fluctuation. The density fluctuation of broadband frequency($0kHz< f \lesssim \sim 200 kHz$) increases at the ELM-crash phases compared to the inter-ELM phases. The significant level of correlation between plasma density fluctuation and magnetic field fluctuation at low frequency($\sim 10 kHz$) is also observed during ELM crashes. The existence of three-wave interaction induced nonlinear interaction is identified at the enhanced plasma density fluctuation during ELM crashes by using bicoherence analysis. We identify the direction of nonlinear spectral power transfer is from fluctuation at low frequency to fluctuation at broadband frequency range.