Nonlinear evolution of neo-classical tearing modes in reversed magnetic shear tokamak plasmas

Abstract

Linear and nonlinear neo-classical double tearing modes (NDTMs) in the reversed magnetic shear configuration with different separations Δrs between two same rational surfaces are numerically studied by means of reduced magnetohydrodynamic simulations, taking into account different bootstrap current fractions fb. It is found that in the case of large Δrs, an explosive burst of fast reconnection, which was previously observed only in the intermediate Δrs case with fb = 0 (Ishii Y. et al 2002 Phys. Rev. Lett. 89 205002), can also be induced if the fraction of bootstrap current fb is sufficiently high. In the case of intermediate Δrs, such explosive bursts can effectively be brought forward, since the bootstrap current significantly destabilizes the tearing mode on the outer rational surface. In the case of small Δrs, higher order harmonics of the NDTMs become dominantly unstable in the linear phase, if fb continues increasing. In its nonlinear phase, the local modification of bootstrap current near the magnetic islands makes the islands move inwards, while the recovery of the Ohm current tends to make them move outwards. The different dynamics of complicated motions of magnetic islands (or rational surfaces) determined by these two effects are analysed in detail in the cases of different fb values.