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

The article treats the recent development of quasi-steady ELMy high bp H mode discharges with enhanced confinement and high b stability, where long sustainment time, an increase in absolute fusion performance and extension of the discharge regime towards low q95 ( ~ 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 d made it possible to extend the performance in long pulses. The DT equivalent fusion gain QeqDT » 0.1 (d = 0.16) was sustained for ~ 9 s ( ~ 50tE, ~ 10t*p) and QDTeq » 0.16 (d = 0.3) for 4.5 s at Ip = 1.5 MA. In the latter case with higher d, an H factor ( = tE/tITER89PLE) of ~ 2.2, bN » 1.9 and bp » 1.6 were sustained with 60-70% of the non-inductively driven current. In the low q95 ( ~ 3) region, the b limit was improved by the high d ( ~ 0.46) shape, where bN » 2.5-2.7 was sustained for ~ 3.5 s with the collisionality close to that of ITER-FDR plasmas. The limit of the edge a parameter in the ELMy phase increases with d, which is the main reason behind the improved b limit in a long pulse at high d. The sustainable value of bNH also increases with d. Sustainable bN is limited by the onset of low n resistive modes. Direct measurement of island width shows agreement with the neoclassical tearing mode theory.