Doublet splitting of fusion alpha particle driven ion cyclotron emission

Abstract

Ion cyclotron emission (ICE) originating from confined populations of fast ions in toroidal fusion plasmas is an important non-invasive, passive diagnostic for current and next generation devices. The ability to model ICE signals accurately is an essential step towards inferring the characteristics of confined energetic alpha-particles or fast neutral beam ions and background plasma. In this paper, the linear growth rates of the magnetoacoustic cyclotron instability, which is the leading explanation for ICE, are calculated in high resolution 2D (k⊥, k∥) space for parameters corresponding to JET Pulse No. 26148. These calculations shed further light on the origin of doublet- splitting of the cyclotron harmonics as well as the perceived need to invoke nonlinear interactions with 1D3V particle-in-cell studies to account for lower harmonics with substantial amplitudes: the inclusion of signals of ICE from a well resolved range of propagation angles can also account for these effects.

A linear parameters study of ion cyclotron emission using drift ring beam distribution

“…In this work, our focus mainly lies on the locally uniform approximation MCI theory, where γ > τ −1 b . A large number of linear and nonlinear simulations have confirmed the validity of MCI in the locally uniform approximation [14,38,44,85], and these simulations capture most of the key observed features of the ICE measurements in JET and TFTR experiments, including the simultaneous excitation of all cyclotron harmonics, the splitting of spectral peaks, ICE intensity scaling approximately linearly with the fast ion number density, the strong growth rate for nearly perpendicular wave propagation, and the congruence between the linear theory and the observed signal intensities. The experimental phenomenon of the un-captured continuous spectrum for l > 8 in JET has also gained further understanding in this work.…”

“…The line splitting, which is evident in figure 9 due to θ and v d , provides a simple explanation [4,53] for the spectral peak splitting observed in JET experiments. Furthermore, recent computational results reported in [85]. Indicate that the origin of spectral peak splitting is Dopplershifted resonances and the intricate landscape of the MCI growth rate on the dispersion surface in ( k ⊥ , k ∥ ) space.…”

Predicting ion cyclotron emission from neutral beam heated plasmas in Wendelstein7-X stellarator

“…The electrons and majority thermal ion(protons) populations form the maxwell distributed pseudorandom thermal background. The minority fast ions representing the NBI protons are initialised as a drifting ring-beam distribution, originally modelled with a delta function for the perpendicular component in [42,65] and later investigated with a spread in the perpendicular component in [69], given by where v ⊥ and v ∥ are the velocity components perpendicular and parallel to the background magnetic field, u 0 is initial perpendicular velocity and v d is initial drift along the background magnetic field, u r and v r are the perpendicular and parallel velocity spreads respectively. The system is then allowed to relax.…”

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

“…ICE is driven by the magnetoacoustic cyclotron instability (MCI) [2,4,[8][9][10][11][12][13][17][18][19][20][21][34][35][36][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54], which arises when a strongly non-Maxwellian minority energetic ion population enters cyclotron resonance with a fast Alfvén wave propagating nearly perpendicular to the local magnetic field. This resonance can either be wave-wave [41][42][43], between cyclotron harmonic waves supported by the minority ions and fast Alfvén waves supported by the bulk plasma, or wave-particle [45,46], at Dopplershifted cyclotron resonance between the minority ions and the Alfvén wave.…”