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

Abstract

Ion Cyclotron Emission (ICE) holds great potential as a diagnostic tool for fast ions in fusion devices. The theory of Magnetoacoustic Cyclotron Instability (MCI), as an emission mechanism for ICE, states that MCI is driven by a velocity distribution of fast ions that approximates to a drift ring beam. In this study, the influence of key parameters (velocity spread of the fast ions, number density ratio, and instability propagation angle) on the linear MCI is systematically investigated using the linear kinetic dispersion relation solver BO (Xie 2019 Comput. Phys. Commun. 244 343). The computational spectra region considered extends up to 40 times the ion cyclotron frequency. By examining the influence of these key parameters on MCI, several novel results have been obtained. In the case of MCI excited by super-Alfvénic fast ions (where the unique perpendicular speed of fast ion is greater than the perpendicular phase velocity of the fast Alfvén waves), the parallel velocity spread significantly affects the bandwidth of harmonics and the continuous spectrum, while the perpendicular velocity spread has a decisive effect on the MCI growth rate. As the velocity spread increases, the linear relationship between the MCI growth rate and the square root of the number density ratio transitions to a linear relationship between the MCI growth rate and the number density ratio. This finding provides a linear perspective explanation for the observed linear relation between fast ion number density and ICE intensity in JET. Furthermore, high harmonics are more sensitive to changes in propagation angle than low harmonics because a decrease in the propagation angle alters the dispersion relation of the fast Alfvén wave. In the case of MCI excited by sub-Alfvénic fast ions (where the unique perpendicular speed of fast ion is less than the perpendicular phase velocity of the fast Alfvén waves), a significant growth rate increase occurs at high harmonics due to the transition of sub-Alfvénic fast ions to super-Alfvénic fast ions. Similarly, for MCI excited by greatly sub-Alfvénic fast ions (where the unique perpendicular speed of fast ion is far less than the perpendicular phase velocity of the fast Alfvén waves), the growth rate at high harmonics also experiences a drastic increase compared to the low harmonic, thereby expanding the parameter range of the velocity spread.