The density limit for a series of gas fuelled and pellet fuelled limiter discharges in JT-60 has been studied. With pellet injection into high current/low q (qcyl = 2.1–2.4) discharges, the Murakami factor reaches (10–13) × 1019 m−2·T−1. The values are factors of 1.5–2.0 higher than those for gas fuelled discharges. For pellet fuelled discharges the central density is high, whereas in the outer region (a/2 < r) the electron density is limited to the same level as that for gas fuelled discharges. The density limit is confirmed to be an edge density limit; this can be explained by the power balance in the outer region of the plasma, for which the plasma purity (Zeff), the heating power (Pabs) and the electron temperature are the key parameters. The onset of a disruptive event can occur when Prad(total) ∼ 12–80% of Pabs(total), and the disruptive limit of the density can be explained by Pabs and ̄ne2 (r = 50 cm) ̄Zeff (r = 50 cm). The thermal stability in the edge region is weaker for lower (<0.4 keV) edge electron temperature.

According to the distribution of the dermal melanocytes, Ota’s nevus was classified into 5 types: superficial (type S), superficial dominant (type SD), diffuse (type Di), deep dominant (type DD), and deep (type De). The histological types were, as expected, relatively well-correlated with the color of the nevus: the most brownish lesions represented type S or type SD while the most bluish lesions showed types Di DD or De. Furthermore these types were strongly correlated with the location of the nevus: types S and SD were more frequent on cheeks whereas types Di DD and De more frequently occurred on eyelid, temple and forehead. When combining both colorations and locations of the nevus, our classification appeared much more significant. Our new histological classification of Ota’s nevus may be very useful in making a therapeutic prognosis of the disease.

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Improved energy confinement for additionally heated JT-60 limiter and lower x-point discharges has been obtained with hydrogen pellet injection. The energy confinement time is enhanced by up to 40% over that of usual gas fuelled discharges at a medium NB or NB + ICRF heating power level of < 15 MW, for which a strongly peaked electron density profile with ne(0)/⟨ne⟩ < 5 and ne(0) < 2.0 × 1020 m−3 is sustained within 0.5 s after a series of pellet injections. The achieved values of the fusion product ne(0)τE(a)Te(0) (<6.8 × 1019 m−3·s·keV) at Ip = 2.5 MA exceed those obtained previously on JT-60 at Ip = 3.1 MA. The improved discharges are characterized by a pressure profile that peaks strongly around the magnetic axis (inside the sawtooth inversion radius) and degrades when a large sawtooth recovers or when the pressure gradient reaches a critical value. The peakedness depends strongly on the penetration length of the pellets. In lower x-point discharges, the improvement is distinct in the low q (q < 3) regime, probably because of suppression of the sawtooth activity.