Long sustainment of JT-60U plasmas with high integrated performance

The article treats the recent development of quasi-steady ELMy high beta(p) H mode discharges with enhanced confinement and high beta stability, where long sustainment time, an increase in absolute fusion performance and extension of the discharge regime towards low q(95) (similar to 3) are emphasized. After modification to the new W shaped pumped divertor, a long heating time (9 s) with a high total heating energy input of 203 MJ became possible without a harmful increase in impurity and particle recycling. In addition, optimization of the pressure profile characterized by the double transport barriers, optimum electron density and/or high triangularity delta made it possible to extend the performance in long pulses. The DT equivalent fusion gain Q(DT)(eq) approximate to 0.1 (delta = 0.16) was sustained for similar to 9 s (similar to 50 tau(E), similar to 10 tau(p)*) and Q(DT)(eq) approximate to 0.16 (delta = 0.3) for 4.5 s at I-p = 1.5 MA. In the latter case with higher delta, an H factor (=tau(E)/tau(E)(ITER89PL)) of similar to 2.2, beta(N) approximate to 1.9 and beta(p) approximate to 1.6 were sustained with 60-70% of the non-inductively driven current. In the low q(95) (similar to 3) region, the beta limit was improved by the high delta (similar to 0.46) shape, where beta(N) approximate to 2.5-2.7 was sustained for similar to 3.5 s with the collisionality close to that of ITER-FDR plasmas. The limit of the edge alpha parameter in the ELMy phase increases with delta, which is the main reason behind the improved beta limit in a long pulse at high delta. The sustainable value of beta(N)H also increases with delta. Sustainable beta(N) is limited by the onset of low n resistive modes. Direct measurement of island width shows agreement with the neoclassical tearing mode theory.