Nonlinear wave coupling within the ion cyclotron emission spectrum from fusion-born protons in KSTAR tokamak plasmas

During ELM crashes in D plasmas in the KSTAR tokamak, the RF emission includes sharp spectral peaks in the frequency range up to ~900MHz. Peaks below ~500MHz that correspond to proton cyclotron harmonics at the outer midplane edge are explained as ion cyclotron emission (ICE) driven by the magnetoacoustic cyclotron instability (MCI) of 3MeV protons born in DD fusion reactions, lying on deeply passing drift orbits. The MCI drives waves on the fast Alfvén-cyclotron harmonic branch, observed as ICE at the antenna. The proton ICE features last a few μs, and exhibit frequency chirping due to rapid changes in the plasma density. Some ICE features below ~500MHz are accompanied, after < 1μs, by a detached “ghost” chirping feature in the range 500-900MHz. This exceeds the local lower hybrid frequency; hence cold plasma waves should be evanescent here. We show that the “ghost” chirping ICE feature is driven by strong nonlinear wave coupling between different spectral peaks within the ICE feature below ~500MHz. This follows from bicoherence analysis of: KSTAR data files for ICE field magnitudes; and the fields generated from a particle-in-cell code, which solves the Maxwell-Lorentz system of equations for the full gyro-orbit kinetics of the plasma particles and their self-consistent fields.