Abstract
The radio frequency detection system on the KSTAR tokamak has exceptionally high spectral and temporal resolution. This enables measurement of previously undetected fast plasma phenomena in the ion cyclotron range of frequencies. Here we report and analyse a novel spectrally structured ion cyclotron emission (ICE) feature in the range 500 MHz to 900 MHz, which exhibits chirping on sub-microsecond timescales. Its spectral peaks correspond to harmonics l of the proton cyclotron frequency fcp at the outer midplane edge, where l = 20–36. This frequency range exceeds estimates of the local lower hybrid frequency fLH in the KSTAR deuterium plasma. The new feature is time-shifted with respect to a brighter lower-frequency chirping ICE feature in the range 200 MHz (8fcp) to 500 MHz (20fcp), which is probably driven (Chapman et al 2017 Nucl. Fusion 57 124004) by 3 MeV fusion-born protons undergoing collective relaxation by the magnetoacoustic cyclotron instability (MCI). Here we show that the new, fainter, higher-frequency chirping ICE feature is driven by nonlinear wave coupling between different neighbouring spectral peaks in the lower-frequency ICE feature. This follows from bispectral analysis of the measured KSTAR fields, and of the field amplitudes output from particle-in-cell (PIC) simulations of the KSTAR edge plasma containing fusion-born protons. This reinforces the identification of the MCI as the plasma physics process underlying proton harmonic ICE from KSTAR, while providing a novel instance of nonlinear wave coupling on very fast timescales.
Highlights
During ELM crashes in deuterium plasmas in the KSTAR tokamak, the detected electromagnetic radiation[1] includes features with sharp spectral structure in the frequency range up to ~900MHz
Cases where the spectral peaks below ~500MHz correspond to proton cyclotron harmonics at the outer midplane edge have been successfully explained[2] as ion cyclotron emission (ICE), driven by a collective instability of a subset of the 3MeV protons that are born in deuteron-deuteron fusion reactions in KSTAR plasmas
We demonstrate this[5] by bicoherence analysis of: first, KSTAR data files for ICE field magnitudes; and, second, the fields generated from direct numerical solution, using a particle-in-cell (PIC) code, of the self-consistent Maxwell-Lorentz system of equations for fully kinetic electrons and thermal deuterons, together with a minority ring-beam distribution representing the fusion-born 3MeV protons
Summary
During ELM crashes in deuterium plasmas in the KSTAR tokamak, the detected electromagnetic radiation[1] includes features with sharp spectral structure in the frequency range up to ~900MHz. Cases where the spectral peaks below ~500MHz correspond to proton cyclotron harmonics at the outer midplane edge have been successfully explained[2] as ion cyclotron emission (ICE), driven by a collective instability of a subset of the 3MeV protons that are born in deuteron-deuteron fusion reactions in KSTAR plasmas. Cases where the spectral peaks below ~500MHz correspond to proton cyclotron harmonics at the outer midplane edge have been successfully explained[2] as ion cyclotron emission (ICE), driven by a collective instability of a subset of the 3MeV protons that are born in deuteron-deuteron fusion reactions in KSTAR plasmas This subset is confined because it lies on deeply passing drift orbits which carry the protons from the core to the outer plasma edge and back. We demonstrate this[5] by bicoherence analysis of: first, KSTAR data files for ICE field magnitudes; and, second, the fields generated from direct numerical solution, using a particle-in-cell (PIC) code, of the self-consistent Maxwell-Lorentz system of equations for fully kinetic electrons and thermal deuterons, together with a minority ring-beam distribution representing the fusion-born 3MeV protons
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