A localized region of sharply reduced turbulent transport inside the plasma core, enabling peaked temperature and density profiles and enhanced fusion performance.
An Internal Transport Barrier (ITB) is a narrow radial region within the plasma core where anomalous heat and particle transport drops precipitously, allowing steep gradients in temperature, density, or both to develop. Unlike the edge pedestal that forms at the last closed flux surface in H-mode, an ITB forms well inside the plasma, typically near a rational safety-factor surface or at the location of a minimum in the magnetic shear profile.1
The physical mechanism for ITB formation is the local suppression of micro-turbulence—principally ITG and TEM modes—by E×B flow shear. This shear can be generated by several means: strong toroidal rotation driven by neutral beam injection, pressure-gradient-driven flows, or, most robustly, by reversed or very weak magnetic shear. When the local E×B shearing rate exceeds the maximum linear growth rate of the underlying micro-instability, turbulent eddies are torn apart before they can transport energy radially, and a transport barrier self-organizes.2
ITBs have been produced in virtually every major tokamak, including JET, JT-60U, DIII-D, TFTR, and Tore Supra. The strongest barriers feature ion thermal diffusivities approaching neoclassical (collisional) levels, representing a near-complete suppression of turbulent transport. This can yield dramatically peaked ion temperature profiles with central values exceeding 40 keV in optimized discharges.3
However, ITBs present operational challenges. The steep internal gradients can drive MHD instabilities, particularly neoclassical tearing modes (NTMs) at low-order rational surfaces. Impurity accumulation within the barrier is another concern: the inward neoclassical convection of high-Z impurities, no longer flushed outward by turbulence, can lead to radiative collapse if not actively managed.4
For reactor applications, the ability to sustain ITBs in steady state using non-inductive current drive—particularly the self-generated bootstrap current—is a cornerstone of the Advanced Tokamak concept. Active control of the current profile, often via electron cyclotron current drive (ECCD) or lower hybrid current drive (LHCD), is essential for maintaining the reversed-shear equilibrium that underpins ITB sustainment.