Observations of radiation in the ion cyclotron range of frequencies from the KSTAR tokamak and LHD heliotron-stellarator show that energetic neutral beam injected (NBI) ion populations can relax collectively in the edge plasma near the injection point. The resulting radiation is ion cyclotron emission (ICE), whose spectrum has strongly suprathermal peaks at sequential cyclotron harmonics of the energetic ions in the edge region. This ICE is driven by the magnetoacoustic cyclotron instability (MCI), and has been simulated [B Chapman et al., Nucl. Fusion 59, 106021 (2019); B C G Reman et al., Nucl. Fusion 59, 096013 (2019)] from first principles using particle-in-cell (PIC) kinetic codes, which solve the Maxwell-Lorentz system of equations self-consistently for tens of millions of gyro-orbit-resolved particles. These experimental and simulation results relate to plasmas with a single majority thermal ion species, which is identical to the minority energetic NBI ion species. What happens if a second, energetically distinct, minority ion species is also present – specifically, fusion-born alpha-particles?

We report here a new collective process that can rapidly transfer kinetic energy from NBI deuterons to alpha-particles, on cyclotron timescales. Our PIC studies show that the process is a generalisation of the MCI that underlies the observations of ICE from NBI ion populations in the edge plasmas of KSTAR and LHD. The excited electric and magnetic fields mediate the energy transfer between NBI ions and alpha-particles, which is the dominant energy channel. This is in contrast to the related ICE case, where field excitation on the fast Alfvén-cyclotron harmonic wave branch is the dominant energy channel. This new effect is found to be strongest for edge plasma conditions, where the NBI ions approximate to a ring-beam in velocity space, and for characteristic helium ash temperature 0.1MeV. Energy transfer occurs predominantly between NBI ions and alpha-particles that have similar Larmor radii. To our knowledge, this is the first study of direct collective energy transfer from NBI ions to local alpha-particles, on cyclotron timescales, in fusion plasmas. In these demonstration-of-principle simulations, NBI energy of order ten per cent is transferred to the alpha-particles.