transport barrier is a region of a magnetically confined plasma in which turbulence is suppressed and temperature and density gradients steepen, locally choking off the leakage of heat and particles.
Transport barriers come in two placements. The edge transport barrier is the defining feature of 📝H-mode: a layer a centimeter or so wide at the 📝plasma boundary where turbulent transport collapses and confinement roughly doubles. Internal transport barriers form deeper in the core of a 📝Tokamak under suitably tailored current and pressure profiles, and can raise central temperatures dramatically. In both cases the barrier is not a material structure but a self-organized state of the plasma itself.
The accepted mechanism is sheared flow. Where the plasma's E×B rotation velocity varies steeply across a narrow region, the shear stretches and tears turbulent eddies apart faster than they can grow, cutting the transport they drive; the steepening gradients reinforce the sheared flow, locking the barrier in place. The steep edge gradient that results is the 📝pedestal on which H-mode performance is built, and internal barriers are closely associated with reversed 📝magnetic shear in the current profile.