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

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.