New Tokamak Plasma Confinement Regime Realized by Utilizing Small Magnetic Perturbations in the EAST Tokamak

Achieving sustained high plasma confinement both in the core and at the edge, without crashes due to edge instabilities, is highly desirable for efficient fusion energy production in a tokamak device. However, control of edge instabilities usually results in core plasma confinement degradation. Here we demonstrate a new improved core confinement plasma regime in EAST, characterized by a sustainable internal transport barrier (ITB) and effective control of edge instabilities, both achieved by applying small resonant magnetic field perturbations (RMPs) at just 0.1% of the equilibrium field amplitude. The key is to localize the perturbations at the edge, which can be effectively realized by use of RMPs with high toroidal mode numbers. Nonlinear interaction between the perturbed magnetic fields and plasma, especially tungsten impurities, on different scales plays a key role in lowering the threshold heating power by a factor of 2 for triggering the formation of an ITB to improve core confinement. More importantly, the improved core confinement can be actively controlled by tuning the perturbed field profile. This provides an attractive method and a physical basis for an integrated solution of keeping high core confinement during edge stability control for a future fusion reactor.