Two pairs of high-frequency magnetic probes were installed in the Large Helical Device (LHD).During the injection of a perpendicular neutral beam,ion cyclotron emissions (ICEs) with the fundamental frequency corresponding to the ion cyclotron frequency at the plasma edge were detected,which are the same type of ICE as measured with the former spare ion cyclotron range of frequencies (ICRF) heating antennas.This type of ICE was further investigated with regard to the phase and intensity of signals.Another type of ICE was found in the LHD,and these ICEs were synchronized with bursts of toroidicity induced Alfv’en eigenmodes (TAE) and the rise of intensity of lost ion flux.Therefore the source of these ICEs was thought to be the particles transferred from the core to the outer region of plasma by the TAE bursts.The frequency of ICEs induced by the TAE bursts increases linearly with the magnetic field strength,since the ion cyclotron frequency increases with the magnetic field strength.

Measurement of Ion Cyclotron Emissions by Using High-Frequency Magnetic Probes in the LHD

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

In this work, we present advances in the numerical modeling of the non-linear magnetoacoustic cyclotron instability (MCI) in relation to ion cyclotron emission (ICE) in fusion plasmas. We present the results of first 2D hybrid simulations of the MCI showing that only when excited waves propagate obliquely to the background magnetic field, the characteristic fingerprint of ICE in fusion plasmas is recovered. Different dynamics in 1D and 2D hybrid simulations of the MCI are observed and described. The results in the present work shed some light on the self-consistent dynamics of the non-linear MCI in scenarios where no analytical theory is available.

On the 2D dynamics of the magnetoacoustic cyclotron instability driven by fusion-born ions

Ion cyclotron emission (ICE) driven by perpendicular neutral beam-injected (NBI) deuterons, together with the distinctive ICE driven by tangential NBI, have been observed from heliotron–stellarator plasmas in the large helical device (LHD). Radio frequency radiation in the lower hybrid range has also been observed Saito K. et al (2018 Plasma Fusion Res. 13 3402043), with frequency dependent on plasma density. Here we focus on recent measurements of ICE from deuterium plasmas in LHD, which show substantial variation in spectral character, between otherwise similar plasmas that have different local density in the emitting region. We analyse this variation by means of first principles simulations, carried out using a particle-in-cell (PIC) kinetic approach. We show, first, that this ICE is driven by perpendicular NBI deuterons, freshly ionised near their injection point in the outer midplane edge of LHD. We find that these NBI deuterons undergo collective sub-Alfvénic relaxation, which we follow deep into the nonlinear phase of the magnetoacoustic cyclotron instability (MCI). The frequency and wavenumber dependence of the saturated amplitudes of the excited fields determine our simulated ICE spectra, and these spectra are obtained for different local densities corresponding to the different LHD ICE-emitting plasmas. The variation with density of the spectral character of the simulated ICE corresponds well with that of the observed ICE from LHD. These results from heliotron–stellarator plasmas complement recent studies of density-dependent ICE from tokamak plasmas in KSTAR Thatipamula S.G. et al (2016 Plasma Phys. Control. Fusion 58 065003); Chapman B. et al (2017 Nucl. Fusion 57 124004), where the spectra vary on sub-microsecond timescales after an ELM crash. Taken together, these results confirm the strongly spatially localised character of ICE physics, and reinforce the potential of ICE as a diagnostic of energetic ion populations and of the ambient plasma.

Density dependence of ion cyclotron emission from deuterium plasmas in the large helical device

Edge-localized fast magnetoacoustic eigenmodes (FME) may be responsible for the observed superthermal ion cyclotron emission in recent deuterium/deuterium (DD) and deuterium/tritium (DT) experiments. These modes can be driven unstable by resonant interaction with a small population of energetic ions (e.g., fusion alphas), having an anisotropic distribution in velocity space, provided that the mode frequency is close to the ion cyclotron frequency. In the present paper, the radial and poloidal structure of these eigenmodes is analyzed, by solving the two-dimensional (2D) eigenmode equation both numerically and analytically using a variational approach. In particular, the conditions for the mode localization and the dependence of the mode structure on aspect ratio and ellipticity are investigated. The eigenmode is found to be radially localized near the plasma edge. In a tokamak with finite aspect ratio, the mode is localized also poloidally near the outer midplane edge. The existence of localized solutions...

The radial and poloidal localization of fast magnetoacoustic eigenmodes in tokamaks

https://iopscience.iop.org/article/10.1088/1361-6587/ab7a3b/pdf

Comparing theory and simulation of ion cyclotron emission from energetic ion populations with spherical shell and ring-beam distributions in velocity-space

Lucy Holland

Papers

Comparing theory and simulation of ion cyclotron emission from energetic ion populations with spherical shell and ring-beam distributions in velocity-space