Observation of high frequency Alfvén eigenmodes excited by RF-accelerated minority hydrogen ions in the EAST tokamak

The paper presents the original observation of high frequency Alfvén eigenmode excited by fast minority hydrogen ions accelerated by ion cyclotron range of frequency waves in EAST. The frequency of high frequency Alfvén eigenmodes is around $0.75\sim 0.8 * \Omega_H$ , where $\Omega_H$ is the cyclotron frequency of hydrogen ions evaluated at the low-field side plasma edge. It tracks the on-axis magnetic field B _t and the edge plasma electron density n _e via the Alfvénic relation $\omega\sim B_t*{n_e}^{-1/2}$ . The high frequency Alfvén eigenmodes are always accompanied by sub-cyclotron modes in several experiments at EAST, consistent with the high frequency Alfvén eigenmode detected on other conventional tokamaks, namely ASDEX Upgrade and DIII-D. Based on the previous study, these sub-cyclotron modes are consistent with the spectrum splitting produced by toroidal mode numbers (Gorelenkov et al 2003 Nucl. Fusion 43 228). We also found that the location of eigenmode excitation of high frequency Alfvén eigenmode driven by RF-accelerated minority hydrogen ions remains unchanged even though the cyclotron resonance location of hydrogen ions varies with B _t .