Nonlinear parity mixtures controlling the propagation of interchange modes

Abstract

The propagation velocity of a resistive interchange mode is numerically investigated based on a two-fluid model. It is newly found that the nonlinearity mixes the interchange parity and the tearing parity to produce magnetic islands and controls the propagation velocity of the instability in the poloidal direction. The parity of the interchange mode is conserved during the linear growing evolution. However, when the amplitude of the mode becomes large and nonlinear effects are dominant, the pure interchange mode does not satisfy the nonlinear two-fluid equation. Thus, the nonlinear energy transfer occurs from the interchange parity mode to the tearing parity mode, which is called the nonlinear parity mixtures, and the magnetic islands are produced by the interchange mode. The nonlinear magnetic island formation by the interchange mode plays a central role in controlling the interchange mode's propagation velocity, which is equal to the electron fluid velocity. This nonlinear process is essential in quantitatively reproducing the propagation velocity of the interchange mode, which is the same as the electron fluid velocity observed in the large helical device experiment. It is also found that one of the mechanisms of parity mixtures is a modulational instability.