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

Abstract

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.

First observation and interpretation of spontaneous collective radiation from fusion-born ions in a stellarator plasma

“…In the corresponding PIC and PIC-hybrid computations [42,34,43,35,27,28,14,36,44,40,15], the minority energetic ion population is initialised with a physically motivated non-Maxwellian distribution in velocity space. This population then relaxes collectively under the magnetoacoustic cyclotron instability (MCI) [45,46,47,48,49,50,51,52,53,54,42,34,35], which in these simulations manifests at the level of the Maxwell-Lorentz dynamics of the individual particles and the self-consistent elds.…”

“…For the present application, we develop, as follows, our hypothesis for the velocity space distribution of the emitting sub-population of We rst assume that, as usual, this ICE is dominated by waves propagating close to perpendicular to the local background magnetic eld, and which are excited by the MCI of a highly non-Maxwellian energetic ion population. Specically, these waves are on the fast Alfvén-cyclotron harmonic wave branch; this follows from the analytical theory of the MCI [45,46,47,48,49,50,51,52,53,54,57], and is conrmed by analysis of the oscillations self-consistently excited in rst principles PIC-based computations [42,34,43,35,27,28,58,14,36,40,44,15]. We assume that the ICE is primarily driven by a sub-population of energetic ions, which we can model approximately in terms of a drifting-ring velocity distribution [59]…”

“…This is in contrast to ICE spectra from LHD where the spectral peaks are very close to successive ion cyclotron harmonics, driven by hydrogen (deuterium) NBI in hydrogen (deuterium) plasmas, see Refs. [11,12,13,14,15].…”

Mechanism for Collective Energy Transfer from Neutral Beam-Injected Ions to Fusion-Born Alpha Particles on Cyclotron Timescales in a Plasma

“…A second minority suprathermal ion population is typically created by neutral beam injection (NBI) at energies ∼100 keV, whose primary purpose is usually to heat the thermal ions in the MCF plasma core. Recent studies [2][3][4] show that collective relaxation of a freshly ionized subset of the NBI ions, with an initially delta-function velocity distribution in the edge plasma near the injection point, excites the radiation in the ion cyclotron range of frequencies that is observed in the KSTAR tokamak [2] and LHD heliotron stellarator [3,4]. This is a form of ion cyclotron emission (ICE), whose power spectrum typically exhibits several strongly suprathermal peaks at low integer harmonics of the cyclotron frequency of the injected ions.…”

“…ICE is widely observed from MCF plasmas. In addition to historical observations from the TFR [5] and JET [6,7] tokamaks, and from deuterium-tritium plasmas in JET [8,9] and TFTR [10], ICE has recently been reported and analyzed from the KSTAR [2,[11][12][13], JT-60U [14,15], DIII-D [16,17], ASDEX-Upgrade [18][19][20][21], TUMAN-3M [22,23], NSTX-U [24,25] and EAST [26] and JET [27] tokamaks, and from LHD [3,4,[28][29][30]. ICE is under consideration as a fast-ion diagnostic for ITER [31][32][33]; it is also observed from solar-terrestrial plasmas [34][35][36][37][38], and may be present downstream of supernova remnant shocks [39].…”