Nonlinear energy transfer from low frequency electromagnetic fluctuations to broadband turbulence during edge localized mode crashes

Abstract

Long ELMy H-mode KSTAR discharges are used to show that low frequencies (f less than or similar to 50 kHz) of the ion-scale density fluctuations contain electromagnetic components that transfer energy from f less than or similar to 15 kHz to higher frequency (15 kHz less than or similar to f less than or similar to 100 kHz) fluctuations during ELM crashes via three-wave non-linear interactions. Such non-linear interactions are not observed during the inter-ELM phases. These observations suggest that investigations on the enhanced fluctuation and transport levels during ELM crashes require a careful analysis of the non-linear couplings between low frequency electromagnetic and broadband fluctuations in addition to other causes such as open (stochastic) magnetic fields and (E) over right arrow x (B) over right arrow advective transport. Our results are based on conditionally ensemble-averaged second-order and third-order spectral analyses applied to 2, 660 ELMs obtained from three different KSTAR discharges to ensure statistical validity.