Experimental investigation of ion cyclotron instabilities on HL-2A tokamak*

Dedicated HL-2A experiments have been performed to investigate the physics mechanism of ion cyclotron emission (ICE), including its driven mechanism, frequency dependence, and spectrum feature. We found that the detected ICE frequencies match f_ci in deuterium plasma and scale with Alfvén velocity(v_A), providing the first evidence that ICE is a fast Alfvén wave. Secondly, it has been demonstrated that higher-frequency ICE modes are generated through the coupling of lower-frequency ICE modes, which confirms the related theoretical work [1]. The experiment reveals that the wave-wave coupling processes have the potential to broaden the frequency bandwidth of ICE, resulting in a redistribution of ICE energy and a wider range of wave-particle interactions. Furthermore, the stability boundary of ICE has been directly observed, showing that ICE is stabilized with decreasing pedestal pressure gradient. A theoretical model has been developed to interpret these results, highlighting a new driving mechanism of ICE. This study provides valuable insights for the understanding of ICE and the development of ICE as a reliable diagnostic tool for studying energetic particles in fusion plasmas.

[1]    B. Chapman et al., Plasma Physics and Controlled Fusion 62, 095022 (2020).