Supersaturation and nonlinear crash of tokamak sawtooth oscillations

A full-torus, compressible, resistive MHD simulation reveals that in the slow ramp phase of tokamak sawtooth oscillations an ideal kink mode slowly emerges at the magnetic axis as a result of current peaking and gradually expands radially outwards, forming a crescent-shaped poloidal magnetic structure that allows the central q value to go substantially below 1. Its converging return flows drive nonlinear reconnection to make two magnetic islands. The kink mode turns into a resistive convection type with a slow but larger growth rate. As the flow grows substantially, a large-scale rigid flow arises and suddenly crashes the plasma structure, forming a crescent-shaped pressure distribution.