Nonlinear wave interactions generate high-harmonic cyclotron emission from fusion-born protons during a KSTAR ELM crash

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 f cp at the outer midplane edge, where l = 20-36. This frequency range exceeds estimates of the local lower hybrid frequency f LH 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 (8f cp ) to 500 MHz (20f cp ), 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 fusionborn 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.

Origin of ion cyclotron emission at the proton cyclotron frequency from the core of deuterium plasmas in the ASDEX-Upgrade tokamak

“…[36]. Previous MCI simulations of KSTAR plasma conditions have similarly shown that MCI excited waves in the same frequency range can also give rise to experimentally observed features above the lower hybrid frequency [41]. This excitation of waves at high and low frequencies due to nonlinear wave-wave coupling is now routinely observed in PIC simulations of the MCI such as those presented in here.…”

“…The simulation is set up in slab geometry, corresponding to the local approximation; it does not incorporate any toroidal effects or eigenmode structure [32,33,34,48,49,50,51,52,53,54,55]. The omission of toroidal effects is expected to have only marginal consequences for modelling the ICE phenomenology addressed here, given the success of calculations and computations that use the local approximation in explaining results from JET and TFTR [1,2,4,3,7,38,36], as well as more recent ICE observations from KSTAR [40,41,42] and LHD [56,44].…”

“…B.2 of Ref. [41]. The value of b 2 c is bounded between 0 and 1, and measures the fraction of the Fourier power of a signal that is due to quadratic nonlinear interactions between three waves that satisfy the frequency and wavenumber matching criteria:…”

“…[62,63,64,65], and has been successfully applied to ICE data and MCI simulations [36,41,43], and to other experimental plasma measurements. For further details see Appendix B of [41]. The squared bicoherence b 2 c of the B z field component is plotted in Fig.…”

On the Effect of Beating during Nonlinear Frequency Chirping

“…In fact, even the enhanced spreading of parti- Note that ghost chirps have also been reported in other studies. For instance, see the recent analysis of ghost features in ion cyclotron emission (ICE) spectra [58].…”