Nonlinear evolution of ideal pressure-driven MHD modes and their conversion into tearing-type modes

Abstract The nonlinear evolution of ideal MHD instabilities, being unstable in the linear growth phase driven by the plasma pressure gradient in low magnetic shear region, are investigated numerically in a cylindrical geometry using the single helicity approximation. The mode structure and growth rate in the linear phase is not affected by a small plasma resistivity because of the ideal nature of the mode. In the nonlinear mode growth phase, including a small plasma resistivity in numerical simulations, it is found that the local plasma pressure gradient is decreased, resulting in a slow-down of the mode growth and the conversion of the ideal mode into a tearing-type one, and magnetic islands begin to grow. Moreover, it is also found that even when there is originally no resonant surface inside the plasma, the dynamo effect can change the local radial profile of the safety factor such that the resonant surface is moved into the plasma, and the ideal mode can also be converted into a tearing-type one in the nonlinear phase. Our results indicate that the flat q profile with q ≈ 1 in the central region, observed in the hybrid scenario of tokamak experiments, is maintained by the tearing-type modes, even if they are originally ideal interchange-type modes in linear growth phase. The hybrid scenario is usually observed in tokamak experiments with a sufficiently high plasma beta value.