Sub-microsecond temporal evolution of edge density during edge localized modes in KSTAR tokamak plasmas inferred from ion cyclotron emission

By B. Chapman, R.O. Dendy, K.G. McClements, S.C. Chapman1 ,G.S. Yun, S.G. Thatipamula and M.H. Kim


During edge localised mode (ELM) crashes in KSTAR deuterium plasmas, bursts of spectrally
structured ion cyclotron emission (ICE) are detected. Usually the ICE spectrum chirps
downwards during an ELM crash, on sub-microsecond timescales. For KSTAR ICE where
the separation of spectral peak frequencies is close to the proton cyclotron frequency Ωcp at
the outer plasma edge, we show that the driving population of energetic ions is likely to be
a subset of the 3 MeV fusion protons, born centrally on deeply passing orbits which drift
from the core to the edge plasma. We report first principles modelling of this scenario using
a particle-in-cell code, which evolves the full orbit dynamics of large numbers of energetic
protons, thermal deuterons, and electrons self-consistently with the electric and magnetic
fields. The Fourier transform of the excited fields in the nonlinear saturated regime of the
simulations is the theoretical counterpart to the measured ICE spectra. Multiple simulation
runs for different, adjacent, values of the plasma density under KSTAR edge conditions enable
us to infer the theoretical dependence of ICE spectral structure on the local electron number
density. By matching this density dependence to the observed time-dependence of chirping
ICE spectra in KSTAR, we obtain sub-microsecond time resolution of the evolving local
electron number density during the ELM crash.

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