Nitrogen-induced ELM suppression and confinement improvement in the EAST tokamak with a full metal wall

Abstract: This paper reports the achievement of an ELM-free H-mode regime with confinement improvement enabled by nitrogen (N2) seeding on the Experimental Advanced Superconducting Tokamak (EAST) with a full metal wall. Following N2 injection, large Edge-Localized Mode (ELM) bursts are completely suppressed, while global energy confinement is significantly enhanced, with the H98 factor increasing from approximately 0.9 to 1.2. A distinct edge coherent mode (ECM), localized at the pedestal foot (psi_N ~ 0.99), is identified using O-mode Poloidal Correlation Reflectometry and AXUV diagnostics. This mode operates within a frequency range of 20-50 kHz with a poloidal wavenumber of k_theta ~ 0.54 cm^-1. Linear gyrokinetic simulations performed with the CGYRO code reveal a dominant instability that quantitatively matches the experimental measurements. Detailed scans of parameters identify this mode as a Dissipative Trapped Electron Mode (DTEM). The energy and particle transport driven by this pedestal-foot DTEM effectively regulates the edge gradients, preventing the pedestal from crossing the Peeling-Ballooning stability boundary and sustaining a stationary ELM-free state. These findings provide a physical basis for an integrated scenario to maintain high confinement and protect plasma-facing components in future steady-state fusion reactors.