A tearing mode with m = 3 and n = 2, destabilized in the steady state high-β p H-mode discharges with edge localized mode (ELM), was completely stabilized by local heating and current drive using the 110 GHz first harmonic O-mode electron cyclotron (EC) wave. Here, m and n are poloidal and toroidal mode numbers, respectively. The optimum EC wave injection angle was determined by identifying the mode location from an electron temperature perturbation profile and a safety factor profile. The optimum injection angle was also determined by scanning a steerable mirror during a discharge. In a typical discharge where the tearing mode is completely stabilized, the ratio of the electron cyclotron heating power to the total heating power is 0.17, and the ratio of the EC driven current to the total plasma current is 0.02. Stored energy and neutron emission rate were higher for the case with EC wave injection than that without EC wave injection, which suggests that the reduction of the stored energy and the neutron emission rate was recovered by the tearing mode stabilization.

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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 operation of reversed shear plasmas in JT-60U has been extended to the low-q, high-Ip region keeping a large radius transport barrier, and a high fusion performance has been achieved. Record values of deuterium-tritium (DT)-equivalent power gain in JT-60U have been obtained: QDTeq = 1.05, τE = 0.97 s, nD(0) = 4.9 × 1019 m-3 and Ti(0) = 16.5 keV. A large improvement in confinement resulted from the formation of an internal transport barrier (ITB) with a large radius, which was characterized by steep gradients in electron density, electron temperature and ion temperature just inside the position of qmin. Large negative shear regions, up to 80% of the plasma minor radius in the low-qmin regime (qmin∼2), were obtained by plasma current ramp-up after the formation of the ITB with the pressure and current profiles being controlled by adjustment of plasma volume and beam power. The ITB was established by on-axis beam heating into a low density target plasma with reversed shear that was formed by current ramp-up without beam heating. The confinement time increased with the radius of the ITB and the decrease of qmin at a fixed toroidal field. High H factors, up to 3.3, were achieved with an L mode edge. The effective one fluid thermal diffusivity χeff had its minimum in the ITB. The values of H/q95 and βt increased with the decrease of q95, and the highest performance was achieved at q95 ∼3.1 (2.8 MA). The performance was limited by disruptive beta collapses with βN∼2 at qmin∼2.

Characteristics of the internal transport barrier (ITB) were studied. The region of steep ∇T i and ∇V t , i.e. the ITB front, propagated from the core outwards. The thickness of the ITB front was about 3 cm. The ITB worked as a particle transport barrier as well as a thermal transport barrier for ions. The threshold heating power for ITB formation strongly increased with electron density and was independent of the toroidal magnetic field. ITB with τ E /τ ITER89-P E ≈ 2.6 was sustained for twice the global energy confinement time τ E . A repetitive relaxation phenomenon at ITB was observed, which induced D α spikes like ELMs but had a different poloidal distribution. * This paper is an extended version of a contribution to the 22nd EPS conference on Controlled Fusion and Plasma Physics held in Bournemouth, UK, July 1995.