Observation of Kinetic Alfvén Eigenmodes with RMP Penetration in J-TEXT Tokamak

kinetic Alfvén eigenmodes excited in J-TEXT Ohmic plasmas are observed when the penetration of external applied resonant magnetic perturbations (RMPs) occurs. These modes are identified localized in gaps (20-80kHz) triggered by kinetic thermal ion effect [Chen L and Zonca F 2016 Reviews of Modern Physics] in the Alfvén continuum. One of these modes, which have the highest frequency, is modulated by the RMP intensity, and its frequency converge to the nonlinearly modified beta-induced Alfvén eigenmode (BAE) continuum accumulation point [Biancalani A 2011 PPCF]. The experimental observation of this type of mode agrees with BAE whose frequency is proportional to the magnetic island half width. Another type of mode with lower frequency（~35kHz） well settled down in the thermal ion transit frequency range is found that it has no relation with RMP strength and magnetic island width, as indicated in previous work [Linzi L 2015 PPCF]. The investigation of the third type of modes shows that the mode frequency is in the range of diamagnetic drift frequency (~20kHz) and its damping mechanism should involve the fast particle effect. All of the modes above are observed only in low density (~1×10^{19}) along with slide-away discharge.