Description
The Internal Transport Barrier (ITB) scenario, characterized by peaked core pressure profiles and a high bootstrap current fraction, is a promising operational scheme for future fusion reactors [1, 2]. Recent experimental studies have demonstrated that magnetic islands can effectively trigger ITBs [3-5]. However, magnetic islands intrinsically degrade plasma confinement and reduce the total stored energy. Recent experiments on the J-TEXT tokamak demonstrate that applying Electron Cyclotron Resonance Heating (ECRH) to the plasma core—to control the current profile and magnetohydrodynamic (MHD) stability—can stabilize the 2/1 NTM. Key experimental observations reveal two synergistic mechanisms responsible for the 2/1 NTM stabilization: on one hand, stabilization is contributed by the coupling between the 1/1 and 2/1 modes, mediated by a change in their phase difference; On the other hand, current profile modification—driven by core ECRH which elevated electron temperature, leading to reduced resistivity as well as an enhanced pressure gradient and bootstrap current fraction from the ITB—caused an outward movement of the magnetic island with its rational surface. This movement increases the local magnetic shear and reduces the local pressure gradient at the resonant surface. Both processes work in concert to stabilize the 2/1 NTM. Consequently, this process resulted in a higher-performance ITB and enhanced core confinement.
Beyond active current profile control, the application of external three-dimensional (3D) magnetic fields has also been shown to improve ITB performance on J-TEXT. Specifically, application of resonant magnetic perturbations (RMP) induces magnetic island locking, and significant core electron temperature elevation is observed under specific RMP phases. The underlying mechanism by which externally applied 3D fields influence ITB performance remains unclear and is the subject of ongoing investigation.
In summary, these findings advance the understanding of MHD–ITB interactions and suggest promising pathways for achieving stable, high-performance ITB operation in future fusion devices.
References
[1] R. C. Wolf et al 2003 Plasma Phys. Control. Fusion 45 R1
[2] J. W. Connor et al 2004 Nucl. Fusion 44 R1
[3] E. Joffrin et al 2002 Nucl. Fusion 42 235
[4] K. Ida et al 2018 Plasma Phys. Control. Fusion 60 033001
[5] F. Y. Mao et al 2025 Nucl. Fusion 65 066018