Interpretation of suprathermal emission at deuteron cyclotron harmonics from deuterium plasmas heated by neutral beam injection in the KSTAR tokamak
2019
Intense bursts of suprathermal radiation, with spectral peaks at
frequencies corresponding to the deuteron cyclotron frequency in the outer
midplane edge region, are often detected from deuterium plasmas in the KSTAR
tokamak that are heated by tangential neutral beam injection (NBI) of deuterons
at 100 keV. Identifying the physical process by which this deuterium ion cyclotron
emission (ICE) is generated, typically during the crash of edge localised modes
(ELMs), assists the understanding of collective energetic ion behaviour in tokamak
plasmas. In the context of KSTAR deuterium plasmas, it is also important to
distinguish deuterium ICE from the ICE at cyclotron harmonics of fusion-born
protons examined by B. Chapman et al., Nucl. Fusion 57, 124004 (2017) and 58,
096027 (2018). We use particle orbit studies in KSTAR-relevant magnetic field
geometry, combined with a linear analytical treatment of the magnetoacoustic
cyclotron instability (MCI), to identify the sub-population of freshly ionised
NBI deuterons that is likely to excite deuterium ICE. These deuterons are
then represented as an energetic minority, together with the majority thermal
deuteron population and electrons, in first principles kinetic particle-in-cell (PIC)
computational studies. By solving the Maxwell-Lorentz equations directly for
hundreds of millions of interacting particles with resolved gyro-orbits, together
with the self-consistent electric and magnetic fields, the PIC approach enables us
to study the collective relaxation of the energetic deuterons through the linear
phase and deep into the saturated regime. The Fourier transform of the excited
fields displays strong spectral peaks at multiple successive deuteron cyclotron
harmonics, mapping well to the observed KSTAR deuterium ICE spectra. This
outcome, combined with the time-evolution of the energy densities of the different
particle populations and electric and magnetic field components seen in the PIC
computations, supports our identification of the driving sub-population of NBI
deuterons, and the hypothesis that its relaxation through the MCI generates the
observed deuterium ICE signal. We conclude that the physical origin of this
signal in KSTAR is indeed distinct from that of KSTAR proton ICE, and is in
the same category as the NBI-driven ICE seen notably in TFTR tokamak and
LHD heliotron-stellarator plasmas. ICE has been proposed as a potential passive
diagnostic of energetic particle populations in ITER plasmas; this is assisted by
clarifying and extending the physics basis of ICE in contemporary magnetically
confined plasmas.
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