Disappearance of giant ELMs and appearance of minute grassy ELMs in JT-60U high-triangularity discharges

摘要

In JT-60U H-mode plasmas, giant (type I) ELMs disappear and minute grassy ELMs appear when triangularity delta, edge safety factor q(95) and beta(p) are high enough. Complete suppression of giant ELMs was observed at delta greater than or similar to 0.45, q(95) greater than or similar to 6 and beta(p) greater than or similar to 1.6. At higher delta (0.54), giant ELMs can disappear at a lower q(95) (similar to 4.0) In the grassy ELMy H-mode, edge temperature and pressure can be higher than those in giant ELMy H-mode and a favourable confinement can be sustained without an increase of the impurity concentration. An edge stability analysis suggests that the edge plasma is accessing the second stability regime of the high n ballooning mode in the grassy ELMy discharges. [References: 13]