Access to ELM-free pedestal using RMPs and edge-localized ECCD on the DIII-D tokamak*

Experiments [1] and nonlinear TM1 simulations [2] on the DIII-D tokamak consistently demonstrate that edge localized mode (ELM) stability can be manipulated using edge-localized electron cyclotron current drive (ECCD) in conjunction with n = 3 resonant magnetic perturbations (RMPs). In the presence of RMP-driven ELM suppression, the injection of co- I_p ECCD at the pedestal top stabilizes the RMP-driven magnetic island, recovers the pedestal pressure, and can lead to a back transition to ELMing. Conversely, counter- I_p ECCD further increases the RMP-driven island width, decreases the pedestal pressure, and maintains a stationary ELM-free pedestal. Additionally, injecting counter- I_p ECCD during the ELMing state reduces the RMP current threshold needed to suppress ELMs due to the facilitation of RMP penetration by ECCD, which forms a magnetic island. For approximately 1 MW counter- I_p ECCD in DIII-D, the required RMP current to suppress ELMs decreases by about 30%. Furthermore, after ELM suppression, the pedestal bifurcates from the ballooning stability branch into the super-H branch [3], achieving a high-confinement pedestal regime. These simulations indicate that edge-localized ECCD can be used to facilitate RMP-driven ELM suppression and optimize confinement degradation.

[1] N.C. Logan et al 2024 Nucl.Fusion 64 014003

[2] Q.M. Hu et al 2024 Nucl.Fusion 64 046027

[3] P.B. Snyder et al 2015 Nucl. Fusion 55 083026