First observation and interpretation of ion cyclotron emission from energetic ions accelerated by lower hybrid waves in a tokamak plasma

Abstract We report the first observation of ion cyclotron emission (ICE) during lower hybrid wave (LHW) and electron cyclotron resonance heating in a tokamak plasma, and subsequently interpret the resulting ICE spectrum using linear kinetic plasma theory to infer characteristics of the bulk plasma and of the fast-ion distribution. ICE spectra from deuterium plasmas in the EAST tokamak exhibit peaks at successive deuterium cyclotron harmonics, consistent with the collective relaxation of energetic deuterium ions through the magnetosonic cyclotron instability (MCI). By carrying out a multi-dimensional parameter scan of MCI growth rates in high-resolution two-dimensional (k//, k⊥) wavenumber space, we obtain best-fit values for the magnetic field, plasma density, background electron and ion temperature, concentration of fast deuterons, and the fast-ion pitch and bulk velocities parallel and perpendicular to the applied magnetic field, which are consistent with the observations within the assumptions of the model. The spectrum is consistent with emission from deuterons with an indicative energy of 770 keV, plausibly accelerated through LHW–ion interactions. Owing to the simplifying assumptions of the model — including an infinite, homogeneous plasma, a ring-beam fast-ion distribution, and the restriction to linear theory — the derived parameters should be regarded as indicative and qualitative rather than as definitive measurements. These results nevertheless suggest that ICE may serve as a passive diagnostic of energetic ion populations and warrant further investigation of the ICE–LHW connection, which may assist alpha-channeling scenarios for ash removal and boosting fusion gain.