Description
Achieving high-performance H-mode operation without Edge Localized Modes (ELMs) is a critical goal for future fusion reactors, particularly for devices with full metal walls like ITER and BEST. In the EAST tokamak equipped with ITER-like tungsten plasma-facing components, we have achieved a stationary ELM-free regime with improved global energy confinement by injecting Nitrogen. This work investigates the physics mechanism behind this transition.
Experimental data indicates that the confinement improvement is linked to a modification of the pedestal structure: the electron temperature gradient steepens, while the density gradient relaxes. This process is accompanied by the excitation of a coherent mode (20-50 kHz). While this resembles the Edge Coherent Mode (ECM) previously reported on EAST, detailed reflectometry measurements localize this new mode strictly at the pedestal foot ($\psi_N \sim 0.98$), distinct from the steep gradient region. Analysis of the mode structure yields a poloidal wavenumber $k_\theta \sim 0.57 \text{ cm}^{-1}$ with mode numbers $m \sim 74$ and $n \sim 14$.
Preliminary analysis indicates that this mode plays a key role in regulating edge transport. To understand its driving mechanism, gyrokinetic simulations using the CGYRO code are currently underway. Initial modeling suggests the instability may be linked to the changes in edge conditions induced by the impurity seeding.
We propose that this pedestal-foot ECM provides a continuous transport channel that prevents the pedestal from reaching the stability limit, thereby sustaining a steady-state, ELM-free H-mode compatible with metal wall conditions.