Nonlinear evolution characteristics of peeling-ballooning mode under negative triangularity

Abstract

<sec>Experiments on TCV tokamak have achieved high confinement mode (H-mode) operation with negative triangularity, and this mode shows quite different characteristics from those with the positive triangularity in experiment and simulation. Linear simulations for kinetic ballooning mode and peeling-ballooning(PB) mode without diamagnetic effect show that negative triangularity can enhance the instability of the ballooning mode and close access to the second stable region. However, the understanding of ELM for negative triangularity is not sufficient. Therefore, it is necessary to carry out further research on ELM with negative triangularity.</sec><sec>In this work, based on a series of equilibria with different triangularities in Tokamak, the nonlinear characteristics of negative triangularity of PB mode is investigated. It is found that the negative triangularity can destabilize the PB mode by a larger unfavorable curvature region, which will reduce the instability threshold, and thus limiting the increase of pedestal height. In the nonlinear phase, the pressure perturbation intensity with negative triangularity will extend to the top area and the bottom area in the low field side and bring about an earlier ELM collapse. Meanwhile, modes with different toroidal mode numbers are more likely to be triggered off and then grow and replaces the initial unstable mode, showing more obvious turbulent transport characteristics, which can play a role in the ELM energy loss.</sec>