This paper reports results on the progress in steady-state high-βp ELMy H-mode discharges in JT-60U. A fusion triple product, nD(0)τETi(0), of 3.1 × 1020 m−3 s keV under full non-inductive current drive has been achieved at Ip = 1.8 MA, which extends the record value of the fusion triple product under full non-inductive current drive by 50%. A high-beta plasma with βN ∼ 2.7 has been sustained for 7.4 s (∼60τE), with the duration determined only by the facility limits, such as the capability of the poloidal field coils and the upper limit on the duration of injection of neutral beams. Destabilization of neoclassical tearing modes (NTMs) has been avoided with good reproducibility by tailoring the current and pressure profiles. On the other hand, a real-time NTM stabilization system has been developed where detection of the centre of the magnetic island and optimization of the injection angle of the electron cyclotron wave are done in real time. By applying this system, a 3/2 NTM has been completely stabilized in a high-beta region (βp ∼ 1.2, βN ∼ 1.5), and the beta value and confinement enhancement factor have been improved by the stabilization.

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 the first four years of the LHD experiment, several encouraging results have emerged, the most significant of which is that MHD stability and good transport are compatible in the inward shifted axis configuration. The observed energy confinement at this optimal configuration is consistent with ISS95 scaling with an enhancement factor of 1.5. The confinement enhancement over the smaller heliotron devices is attributed to the high edge temperature. We find that the plasma with an average beta of 3% is stable in this configuration, even though the theoretical stability conditions of Mercier modes and pressure driven low-n modes are violated. In the low density discharges heated by NBI and ECR, internal transport barrier (ITB) and an associated high central temperature (>10 keV) are seen. The radial electric field measured in these discharges is positive (electron root) and expected to play a key role in the formation of the ITB. The positive electric field is also found to suppress the ion thermal diffusivity as predicted by neoclassical transport theory. The width of the externally imposed island is found to decrease when the plasma is collisionless with finite beta and increase when the plasma is collisional. The ICRF heating in LHD is successful and a high energy tail (up to 500 keV) has been detected for minority ion heating, demonstrating good confinement of the high energy particles. The magnetic field line structure unique to the heliotron edge configuration is confirmed by measuring the plasma density and temperature profiles on the divertor plate. A long pulse (2 min) discharge with an ICRF power of 0.4 MW has been demonstrated and the energy confinement characteristics are almost the same as those in short pulse discharges.

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 (I p , B t , q 95 , . . . ). The limit of the normalized edge pressure gradient (α-parameter) increases with elongation κ (1.5-1.8) and internal inductance l i . 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 I p < 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 β p are high (δ >∼ 0.3, β p >∼ 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.

In JT-60U lower hybrid current drive (LHCD) experiments, a reversed magnetic shear configuration that was accompanied by the internal transport barriers was successfully maintained by means of LHCD almost in the full current drive quasi-steady state for 4.7 s. The normalized beta was kept near 1 and the neutron emission rate was almost steady as well indicating no accumulation of impurities into the plasma. Diagnostics data showed that all the profiles of the electron and ion temperatures, the electron density and the current profile were almost unchanged during the LHCD phase. Moreover, capability of LHCD in H-mode plasmas has been also investigated. It was found that the lower hybrid waves can be coupled to an H-mode edge plasma even with the plasma wall distance of about 14 cm. The maximum coupling distance was found to depend on the edge recycling.