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

Nuclear Fushion

article

B. Chapman¹  R.O. Dendy¹  S.C. Chapman¹  K.G. McClements²  G.S. Yun³  S.G. Thatipamula³  M.H. Kim³ 

[1] Centre for Fusion, Space and Astrophysics, University of Warwick;CCFE, Culham Science Centre;Pohang University of Science and Technology

关键词: ion cyclotron emission;    magnetoacoustic cyclotron instability;    ELM;    tokamak;    bispectral analysis;    KSTAR;    particle-in-cell simulation;   

DOI  :  10.1088/1741-4326/aacf47

来源: Institute of Physics Publishing Ltd.

【 摘 要 】

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 harmonicslof the proton cyclotron frequencyf _cp at the outer midplane edge, wherel   =  20–36. This frequency range exceeds estimates of the local lower hybrid frequencyf _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 (8 f _cp) to 500 MHz (20 f _cp), which is probably driven (Chapmanet al2017Nucl. Fusion57124004) 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.

【 授权许可】

Unknown