7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Kinetic-magnetohydrodynamic hybrid simulation study of energetic-particle
driven off-axis fishbone instability in tokamak plasmas
Hanzheng Li1,2, Yasushi Todo1,2, Hao Wang2 ,Jialei Wang2 ,Malik Idouakass2
1
Department of Complexity Science and Engineering, The University of Tokyo
2
National Institute for Fusion Science, National Institutes of Natural Sciencese-mail (speaker):li.hanzheng@nifs.ac.jp
Off-axis fishbone mode (OFM), also known asenergetic-particle-driven wall mode (EWM), is amagnetohydrodynamic (MHD) instability primarily
driven by energetic particles (EPs). This instability has
been observed in various tokamak plasmas such as
JT-60U [1] and DIII-D [2], where the central safety
factor (q) exceeds unity. OFMs are destabilized byprecessional drift motion of trapped EPs, and theirfrequency decreases after the mode amplitude reaches
saturation. These characteristics are similar with classicalfishbones with q = 1. However, OFMs are localized near
the q = 2 magnetic flux surface, whereas classicalfishbones reside within the q = 1 surface. OFMs play a
crucial role in fusion research as they redistribute EPs,
destabilizing resistive wall modes (RWMs) and edge
localized modes (ELMs) [3].
Firstly, we investigated the linear growth of Off-axis
fishbone mode (OFM), destabilized by trapped energetic
ions [4]. The MHD equilibria were constructed using theGrad-Shafranov equation based on experimental datafrom DIII-D [2]. The spatial profile of OFM primarily
consists of the m/n = 2/1 mode inside the q = 2 magnetic
flux surface, while the m/n = 3/1 mode dominates
outside this surface. The spatial profile of OFM exhibitsa strongly shearing shape on the poloidal plane,
indicating the nonperturbative effect of the interaction
with energetic ions. The precession drift resonance is theprimary resonance between energetic particles and OFM.
Next, we focus on the nonlinear evolution of OFM.
We examined the energetic ion distribution function in( P! , E) space. We demonstrated that the gradient of thedistribution function along the constant E' line, where E'
represents a combination of energy and toroidal
canonical momentum conserved during the wave-particle
interaction, drives or stabilizes the instability. In thenonlinear phase, the distribution function flattens along
the constant E' line, resulting in the saturation of theinstability.
Finally, in the experimental observations conducted in
JT-60U and DIII-D devices, both the magnetic probe
signals and δT" -ECE signals exhibit significant
waveform distortion, characterized by non-sinusoidal
oscillations. Analysis of toroidal array data reveals that
this waveform distortion is associated with higher-nharmonics. We expanded our previous simulations [4] tocapture this phenomenon by incorporating higher-n
harmonics in the MHD fluid model. Notably, for the firsttime, the simulation successfully reproduced thewaveform distortion in both magnetic and temperaturefluctuations [5], as illustrated in Figure 1. We found two
types of waveform distortion, rising distortion and falling
distortion, depending on the phase relationship between
the n=1 OFM and n=2 harmonics and the relative
amplitude of the n=2 harmonics to the n=1 OFM.
References[1] G. Matsunaga et al, Phys.Rev.Lett.103,045001(2009)[2] M. Okabayashi et al, Phys. Plasmas 18,056112(2011)[3] G. Matsunaga et al, Nucl. Fusion 53,123022(2013)[4] H. Li et al, Nucl. Fusion 62, 026013 (2022）[5] H. Li et al, Nucl. Fusion 63, 086012 (2023)