Nonlinear evolution of double tearing modes in tokamak plasmas via multiple helicity simulation

Abstract

Nonlinear evolution of multiple double tearing modes (multi-DTMs) with different helicities (DHs) in several reversed magnetic shear configurations is investigated by employing a reduced magnetohydrodynamic model in cylindrical geometry. It is found that the basic characterizations of the nonlinear evolution process of the system depend on the linear unstable spectrum of multi-DTMs with DHs. In the case that the linear growth rates of multi-DTMs with DHs are comparable to each other, continuous bursts occur one after another due to a sequence of magnetic energy release induced by multi-DTMs reconnections on different pairs of resonant surfaces with DHs. Moreover, the overlapping of the magnetic island chains of multi-DTMs during the continuous bursts process results in the strong magnetic stochasticity in the off-axis region. Such unique nonlinear evolution of multi-DTMs with DHs cannot be reproduced by the single helicity simulation. However, in the case that the linear growth rates of DTMs with the dominant helicity are much larger than the counterparts of DTMs with the other helicities, the main nonlinear physical process obtained by the multiple helicity simulation can be reproduced by the single helicity simulation.