Theory of improved confinement in high- beta _p tokamaks

The dissipative self-organization of the plasma pressure and current in tokamaks is investigated through interactions between bootstrap current and the anomalous transport due to self-sustained turbulence. The strongly reduced magnetic shear due to the pressure-driven current and the magnetic axis shift cause a reduction in the anomalous transport, which enhances the plasma pressure and bootstrap current and leads to a transition to a high- beta _p plasma. The threshold is given by beta _p approximately=1. Simulation results are compared with experimental observations.