Onset condition for ELMs in JT-60U

For the standard shape of JT-60U at low triangularity δ (∼ 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 (Ip, Bt, q95, ...). The limit of the normalized edge pressure gradient (α-parameter) increases with elongation κ (1.5 - 1.8) and internal inductance li. Recently, a new connection of the poloidal field coils enabled us to scan triangularity δ from the original value of ∼ 0.1 up to δ ∼ 0.4 (currently limited at Ip < 1.2 MA). The δ-scan (-0.05 ∼ 0.4) showed that the limit of edge density, edge pressure and the α-parameter increase with δ. When both δ and βp are high (δ > ∼ 0.3, βp > ∼ 2), minute-grassy ELMs appear. The edge α-parameter during the minute-grassy ELMs can be higher than that for the onset of giant ELMs. (author)