Nonlinear evolution characteristics of peeling-ballooning mode under negative triangularity

Abstract

Experiments on TCV have achieved high confinement mode (H-mode) operation with negative triangularity. It shows quite different characteristics from the positive triangularity in experiments and simulations. Linear simulations for kinetic ballooning mode and P-B 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.<br />In this work, based on a series of equilibria constructed with different triangularities, the nonlinear characteristics of negative triangularity of peeling-ballooning mode (P-B mode) is investigated. It is found that negative triangularity can destabilize P-B mode by a larger unfavorable curvature region, which would decrease the instability threshold and thus limit the increase of pedestal height. In the nonlinear phase, the pressure perturbation intensity with negative triangularity will extend to the top and bottom areas in the low field side and bring an earlier ELM crash. Meanwhile, modes with different toroidal mode numbers are more likely to be triggered and then grow to replace the initial unstable mode, showing more obvious turbulent transport characteristics, which might play a role in the ELM energy loss.