With the high-field-side multiple pellet injection into high-triangularity high-β p type I ELMy H-mode discharges, JT-60U has extended the density range with a favourable integrated performance. The pellet injected discharge has a high pedestal pressure compared with the gas fuelled discharges, and the pedestal temperature does not decrease even at high pedestal density. At high triangularity, the pedestal β p and the edge α-parameter increase with increasing total β p . Possible linkage among pedestal and core parameters has been proposed based on a variety of JT-60U experiments. Grassy ELMy discharges have been achieved at the low safety factor regime q 95 < 4 for the first time in JT-60U with high triangularity (∼0.6). The full non-inductive operation has been demonstrated with grassy ELMs.
The operation of JT-60U reversed shear discharges has been
extended to a high plasma current, low q regime keeping a large radius of
the internal transport barrier (ITB), and a record value of equivalent
fusion multiplication factor in JT-60U, QDTeq = 1.25, has been achieved at 2.6 MA. Operational schemes to reach the low q
regime with good reproducibility have been developed. The reduction of
Zeff was obtained in the newly installed W shaped pumped
divertor. The β limit in the low qmin regime, which
limited the performance of L mode edge discharges, has been improved in
H mode edge discharges with a broader pressure profile, which was obtained
by power flow control with ITB degradation. Sustainment of the ITB and
improved confinement for 5.5 s has been demonstrated in an ELMy H
mode reversed shear discharge.
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 Q eq DT ≈ 0.1 (δ = 0.16) was sustained for ∼9 s (∼50τE, ∼10τ * p ) and Q eq DT ≈ 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/τ ITER89PL E ) 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 βN H 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.
In JT-60U H-mode plasmas, giant (type I) ELMs disappear and minute grassy ELMs appear when triangularity δ, edge safety factor q 95 and β p are high enough. Complete suppression of giant ELMs was observed at δ 0.45, q 95 6 and β p 1.6. At higher δ (0.54), giant ELMs can disappear at a lower q 95 (∼4.0). In the grassy ELMy H-mode, edge temperature and pressure can be higher than those in giant ELMy H-mode and a favourable confinement can be sustained without an increase of the impurity concentration. An edge stability analysis suggests that the edge plasma is accessing the second stability regime of the high n ballooning mode in the grassy ELMy discharges.
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