Onset condition for ELMs in JT-60U

For the standard shape of JT-60U at low triangularity delta (similar to 0.1), the onset condition for giant ELMs is clearly correlated with the high-n ballooning limit in the first stability regime over wide ranges of plasma parameters (I-p, B-t, q(95), ...). The limit of the normalized edge pressure gradient (alpha-parameter) increases with elongation kappa (1.5-1.8) and internal inductance l(i). Recently, a new connection of the poloidal field coils enabled us to scan triangularity delta from the original value of similar to 0.1 up to delta similar to 0.4 (currently limited at I-p < 1.2 MA). The delta-scan (-0.05 similar to 0.4) showed that the limit of edge density, edge pressure and the cr-parameter increase with delta. When both delta and beta(p) are high (delta >similar to 0.3, beta(p) >similar to 2), minute-grassy ELMs appear. The edge alpha-parameter during the minute-grassy ELMs can be higher than that for the onset of giant ELMs.