A sawtooth model based on the transport catastrophe

A new model of the sawtooth, which is not triggered by the m=l helical mode, is proposed. A catastrophic change in the transport coefficient is predicted in the central region, where the safety factor q is lower than unity. The magnetic component in fluctuation increases as the plasma pressure increases. If the pressure gradient exceeds a certain threshold, a magnetic stochasticity sets in. The electron thermal conductivity can be enhanced by the factor of the ion-to-electron mass ratio, leading to the rapid flattening of the electron pressure profile. An enhanced transport coefficient also occurs for ions. The increment of the transport associated with the strong magnetic perturbation continues until the ion pressure profile is flattened. Hysteresis behaviour of transport flux to the pressure gradient is obtained. The current profile is influenced by the enhanced current diffusivity from the same mechanism, but the change in the q profile remains small. Simple model equations which follow the dynamics are introduced. The dynamic solution of sawtooth crash is simulated.