Speaker
Richard Dendy
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P2.1001.pdf
Nonlinear wave interactions explain high-harmonic cyclotron emission
from fusion-born protons during a KSTAR ELM crash
R O Dendy1,2, B Chapman2, S C Chapman2, K G McClements1, G S Yun3,
S G Thatipamula3 and M H Kim3
1
CCFE, Culham Science Centre, Abingdon, Oxfordshire OX14 3DB, UK
2
Centre for Fusion, Space and Astrophysics, Department of Physics,
Warwick University, Coventry CV4 7AL, UK
3
Department of Physics, Pohang University of Science and Technology, Pohang 37673,
Korea
During ELM crashes in deuterium plasmas in the KSTAR tokamak, the emitted
electromagnetic radiation 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 are explained (B Chapman et al.,
Nucl. Fusion 57 124004 (2017)) as ion cyclotron emission (ICE) driven by a subset of the
3MeV protons 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. Its sharply defined non-Maxwellian
distribution in velocity space means that this energetic proton minority can undergo the
magnetoacoustic cyclotron instability (MCI) in the edge plasma. The MCI drives waves
on the fast Alfvén-cyclotron harmonic wave branch, which are observed as ICE. During
KSTAR ELM crashes, the duration of the proton ICE features is brief, typically a few
microseconds. The chirping results from rapid changes in the density of the ambient
plasma in which the energetic ions are embedded. Some chirping ICE features below
~500 MHz are accompanied, after a time delay < 1μs, by a fainter detached (“ghost”)
chirping feature in the range 500MHz to 900MHz. This frequency range exceeds the local
lower hybrid frequency, and cold plasma waves propagating quasi-perpendicular to the
magnetic field are expected to be evanescent here. Nevertheless, we show that the
“ghost” chirping ICE feature is a real physical phenomenon. It is generated by strong
nonlinear wave-wave coupling between different spectral peaks within the primary
chirping ICE feature below ~500MHz. We demonstrate this 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 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.
This work was supported in part by the RCUK Energy Programme [grant number EP/P012450/1],
NRF Korea grant no. 2014M1A7A1A03029881 and Euratom. The views and opinions expressed
herein do not necessarily reflect those of the European Commission. ROD and GSY acknowledge the
hospitality of Kyushu University during this collaboration. SCC acknowledges a Fulbright-Lloyd’s of
London Scholarship and US AFOSR grant FA9550-17-1-0054.
