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Ben Chapman (UKAEA): Ion cyclotron emission from energetic ion populations in fusion plasmas

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In this talk I present particle in cell (PIC) simulations of ion cyclotron emission (ICE). ICE comprises suprathermal radiation in the ion cyclotron frequency range, whose spectrum peaks at successive local cyclotron harmonics of the emitting energetic ion population. ICE is caused by a collective instability, which in its linear phase corresponds to the magnetoacoustic cyclotron instability (MCI). ICE has previously been observed in all large toroidal magnetically-confined fusion (MCF) plasmas [1, 2]. The passive, non-invasive character of ICE measurements, suggests that it is an attractive way forward for future energetic ion measurements in ITER. In the simulations presented here, we use the EPOCH [3] particle-in-cell code to solve the self-consistent Maxwell-Lorentz system of equations for fully kinetic electrons and thermal background ions, together with the minority energetic ion distribution that drives the primary ICE. We first perform a detailed quantitative comparison between fusion born proton driven chirping ICE observed during edge localised modes (ELM) crashes in the KSTAR tokamak and fully nonlinear direct numerical simulations of the MCI [4]. We find good quantitative agreement between the simulated and observed spectra, to the extent that the simulations can be used to infer fast (∼μs) time scale dynamics of the local electron number density in the emitting region. We then extend this study to determine the origin of a faint, time delayed proton chirping feature observed in one of the KSTAR plasma pulses [5]. We do this using bicoherence analysis of both experimental and simulation data. We then run MCI PIC simulations of the pre ELM crash “steady state” ICE observed on KSTAR, which is believed to be driven by neutral beam injected (NBI) deuterons [6]. PIC simulations of MCI excited ICE in the JET and ASDEX Upgrade (AUG) tokamaks are then discussed, and we show that AUG observations of the fundamental ICE harmonic can only be explained in terms of the MCI if nonlinear wave-wave interactions between higher harmonics are taken into account [7]. Motivated by recent observations of ICE in the core region of several tokamaks, including AUG and DIII-D, we then compare MCI simulations using two types of energetic ion distribution function, a spherical shell of varying thickness, and a ring beam of varying width [8]. It is found that both distribution functions lead to MCI excited waves, and their nonlinear properties are discussed.

 

[1] R. O. Dendy et al., Plasma Phys. Control. Fusion 57, 044002 (2015)
[2] K. G. McClements et al., Nucl. Fusion 55, 043013 (2015)
[3] T. D. Arber et al., Plasma Phys. Control. Fusion 57, 113001 (2015)
[4] B. Chapman et al., Nucl. Fusion 57, 124004 (2017)
[5] B. Chapman et al., Nucl. Fusion 58, 096027 (2018)
[6] B. Chapman et al., Nucl. Fusion 59, 106021 (2019)
[7] B. Chapman et al., Plasma Phys. Control. Fusion 62, 055003 (2020)
[8] B. Chapman et al., Plasma Phys. Control. Fusion 62, 095022 (2020)

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