Effect of energetic ions on edge-localized modes in tokamak plasmas

Unmitigated Edge Localized Modes (ELMs) pose a serious threat to the first wall of ITER and future fusion power plants [1]. While extensive efforts are currently focused on the development of ELM control techniques [2], the non-linear physics underlying the ELM crash remains poorly understood. Experimental observations have revealed fast-ion losses and acceleration during ELMs [3, 4], with potential implications for burning plasmas rich in suprathermal particles. We have performed first-of-a-kind hybrid kinetic-MHD simulations of ELMs with the MEGA code [5] for an ASDEX Upgrade plasma. In our previous work [6], we showed that although ELMs are primarily driven by thermal pressure gradients, the resonant interaction between edge fast-ions and the electromagnetic perturbations from the ELMs determines the resulting spatio-temporal structure of ELMs. In the presence of fast-ions, the modes broaden radially and exhibit high frequencies ( f ∼ 200 − 300 kHz) during the ELM. The resulting perturbation structure is strongly sheared, and the fast-ion distribution mirrors the bulk plasma pressure perturbation structure, extending into the far scrape-off layer. These simulations qualitatively reproduced key signatures of large type-I ELMs observed in magnetic diagnostics and fast-ion loss detectors, in low collisionality plasmas with high fast-ion content. Here we extend our previous study and perform systematic scans of fast-ion distribution parameters. We find that the modes undergo significant radial broadening and frequency boost when fast-ion beta and energy reach β_EP ≥ 0.5% and E_birth = 60 − 90 keV, respectively, and that this behavior originates from the imbalance between MHD and energetic particle forces. This research points toward new opportunities to leverage kinetic effects for ELM control and the realization of high-confinement, ELM-free regimes.

[1] A. Loarte, Nat. Phys. 2, 369–370 (2006).

[2] S.K. Kim et al., Nat. Commun. 15, 1275 (2024).

[3] M. Garcia-Munoz et al., Nucl. Fusion 53, 123008 (2013).

[4] J. Galdon-Quiroga el al., Phys. Rev. Lett. 121, 025002 (2018).

[5] Y. Todo et al., Phys. Plasmas 5, 1321 (1998).

[6] J. Dominguez-Palacios et al., Nat. Phys. 21, 43-51 (2025).