Description
While neoclassical tearing modes (NTMs) are generally considered detrimental to plasma confinement, experiments on LHD [1] and J-TEXT [2] have observed NTM-triggered formation of internal transport barriers (ITBs). Traditional theories attribute turbulence suppression by NTMs to profile flattening or E×B shear flows. However, our previous work has shown that three-dimensional deformation induced by MHD can affect micro-instabilities by modifying the magnetic drift frequency through geometric effects [3-4]. Moreover, we reveal a geometry-dominated pathway for turbulence suppression, highlighting the critical role of three-dimensional magnetic surface deformation induced by magnetic islands (MIs) in regulating electron thermal transport [5].
We establish a theoretical model for the nonlinear interaction between NTMs and collisionless trapped electron mode (CTEM) turbulence. It demonstrates that the magnetic surface deformation caused by an MI can trigger a bifurcation in electron ITB (eITB) formation via a novel positive feedback mechanism. This mechanism stems from the nonlinear coupling between the NTM and CTEM turbulence: the geometric deformation enhances the precession drift frequency of trapped electrons, thereby suppressing CTEM turbulence. The resulting reduction in turbulent electron heat transport steepens the electron temperature gradient, which in turn destabilizes the NTM by increasing the bootstrap current. This further amplifies the MI-induced deformation, closing and reinforcing a self-sustaining feedback loop between the NTM and CTEM turbulence. Numerical results from our theoretical model exhibit characteristic bifurcation behaviors, including power thresholds, multi-stability mode transitions, and hysteresis. These trends are qualitatively consistent with J-TEXT experimental observations [2], where wider MIs correlate with stronger eITBs.
Our findings provide a new physical picture for understanding the self-organized interaction between turbulence and macroscopic MHD structures in electron-dominated transport regimes. The feedback mechanism proposed in this work can be extended to investigate the interaction between MHD and ion-scale instabilities, such as ion temperature gradient mode and kinetic ballooning mode, and explore the formation of ion ITB as well. After further verification and validation, this new paradigm may offer a promising strategy for actively triggering and sustaining high-performance plasmas in future fusion reactors like ITER through the controlled manipulation of NTMs.
References
[1] K. Ida, S. Inagaki, T. Shimozuma, et al. Plasma of Physics, 11 2551 (2004)
[2] F. Mao, N. Wang, K. Ida, et al. Nuclear Fusion, 65 066018 (2025)
[3] Z. Huang, W. Guo and L. Wang Nuclear Fusion 62 066044 (2022)
[4] Z. Huang, W. Guo and L. Wang Nuclear Fusion 65 016021 (2025)
[5] Z. Huang, W. Guo and L. Wang Nuclear Fusion, 2026, accepted