Interpretation of Suprathermal Emission at Deuteron Cyclotron Harmonics from Deuterium Plasmas Heated by Neutral Beam Injection in the KSTAR Tokamak
Journal Article - Open Access
WARWICK UNIV COVENTRY (UNITED KINGDOM) WARWICK United Kingdom
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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, 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 Chapman et al 2017 Nucl. Fusion 57 124004 2018 Nucl. Fusion 58 096027. 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.
- Nuclear Physics and Elementary Particle Physics
- Plasma Physics and Magnetohydrodynamics