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

The article treats the recent development of quasi-steady ELMy high βp H mode discharges with enhanced confinement and high β 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 δ made it possible to extend the performance in long pulses. The DT equivalent fusion gain QeqDT ≈ 0.1 (δ = 0.16) was sustained for ∼9 s (∼50τE, ∼10τ*p) and QDTeq ≈ 0.16 (δ = 0.3) for 4.5 s at Ip = 1.5 MA. In the latter case with higher δ, an H factor ( = τE/τITER89PLE) of ∼2.2, βN ≈ 1.9 and βp ≈ 1.6 were sustained with 60-70% of the non-inductively driven current. In the low q95 (∼3) region, the β limit was improved by the high δ (∼0.46) shape, where βN ≈ 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 α parameter in the ELMy phase increases with δ, which is the main reason behind the improved β limit in a long pulse at high δ. The sustainable value of βNH also increases with δ. Sustainable βN is limited by the onset of low n resistive modes. Direct measurement of island width shows agreement with the neoclassical tearing mode theory.