Ion cyclotron emission driven by axis-encircling energetic ions

Due to the large gyro-radius, most of the confined fusion products in modern medium-size tokamaks and beam ions in small tokamaks encircle the magnetic axis during the significant part of the drift orbit. As a result, a specific population of particles arises, which is characterized by strong density peaking due to equilibrium distribution in velocity space, as well as strong poloidal flow (current) around the axis. This population drives compressional Alfvén eigenmodes with large radial wave numbers, frequencies matching harmonics of the on-axis fast ion gyro-frequency ωcα (mismatch of the eigenvalue with this frequency determines the instability threshold), and growth rates above the threshold γ α ∼ ω c α f α 1 / 2, with fα being the fast ion fraction. The possible relation of this result with recent observations in medium and small tokamaks is discussed.