The narrow region of steep pressure gradient and suppressed transport at the plasma edge in H-mode, whose height sets a floor for core confinement performance.
The pedestal is the steep-gradient region that forms at the plasma edge when a tokamak transitions from low-confinement mode (L-mode) to high-confinement mode (H-mode). It is characterized by a sharp rise in electron temperature, ion temperature, and density over a narrow radial layer just inside the last closed flux surface, creating a transport barrier that dramatically improves global energy confinement.1
The pedestal forms when edge turbulence is suppressed by sheared E×B flows, which tear apart turbulent eddies faster than they can grow. The resulting reduction in cross-field transport allows pressure gradients to steepen until they are limited by magnetohydrodynamic (MHD) stability boundaries, principally peeling–ballooning modes. The pedestal width and height are therefore set by the interplay between transport suppression and MHD stability.2
The EPED model, developed by Snyder and collaborators, has become the standard predictive framework for the pedestal. It combines a peeling–ballooning MHD stability calculation (using the ELITE code) with a kinetic ballooning mode constraint that sets the pedestal width proportional to the square root of the poloidal beta at the pedestal top. EPED predictions have been validated across multiple tokamaks including DIII-D, JET, and KSTAR.3
The pedestal is inherently dynamic. When the pressure gradient exceeds the peeling–ballooning stability limit, edge-localized modes (ELMs) erupt—violent bursts that expel particles and energy onto plasma-facing components. Controlling ELMs while maintaining a high pedestal is a major engineering and physics challenge for ITER and future reactors. Techniques include resonant magnetic perturbations (RMPs), pellet pacing, and operating in naturally ELM-free regimes such as QH-mode or I-mode.4
Pedestal physics remains an area of intensive research. Improving predictive capability for the pedestal height in reactor conditions, where the pedestal width may be only a few centimeters, is essential for confident extrapolation of fusion performance to burning-plasma devices.