Effects of edge collisionality on ELM characteristics in the grassy ELM regime

摘要

Effects of the edge collisionality on ELM characteristics have been investigated in the grassy ELM regime on JT-60U. Both in the high and low q regions (q_(95) > 6 and q_(95) ～ 4.3), ELM amplitude becomes higher at high edge collisionality (density). This collisionality dependence is opposite to the dependence observed in the type I ELM regime, where the ELM amplitude decreases with increasing edge collisionality. The stability analysis suggests that the grassy ELM at q_(95) ～ 4.3 can be triggered by the infinite-n ballooning mode localized inside the pedestal, when peeling-ballooning modes are stable. In the case of higher collisionality, peeling-ballooning modes can also be unstable, and as a result a larger ELM can be triggered. Since both stability boundaries for infinite-n ballooning mode and peeling-ballooning modes are located near the operational point, a small change in the pedestal condition such as a local pressure gradient and/or a bootstrap current can determine the most unstable mode. This mechanism is considered as a possible candidate for the appearance of a mixture ELM of grassy ELMs and large ELMs.