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.

Enhanced vertical displacement events (VDEs), which are frequently observed in JT-6OU disruptive discharges, are investigated using the Tokamak Simulation Code (TSC). The rapid plasma current quench can accelerate the vertical displacement, owing to both the up/down asymmetry of the eddy current distribution arising from the asymmetric geometry of the JT-6OU vacuum vessel and the degradation of magnetic field decay index n, leading to high growth rates of positional instability. For a slightly elongated configuration (n = -0.9), the asymmetry of attractive forces on the toroidal plasma plays a dominant role in the VDE mechanism. For a more elongated configuration (n = -1.7), the degradation of field decay index n plays an important role on VDEs, in addition to the effect of asymmetric attractive forces. It is shown that the VDE characteristics of a highly elongated configuration with a rapid plasma current quench can be dominated by the field decay index degradation. It is also pointed out that both the softening of current quenches as was experimentally developed in the JT-6OU tokamak, and the optimization of the allowable elongation of the plasma cross-section are critical issues in the development of a general control strategy of discharge termination.

The heating of toroidal plasmas by resonant absorption of Alfvén waves is considered in the framework of ideal MHD. A theory is developed which closely parallels that of ideal-MHD stability. Computations are performed by using the numerical methods known from stability theory. It is shown that the overall picture of Alfvén wave heating in a torus with circular cross-section does not fundamentally differ from that in a cylinder. This type of plasma may be efficiently heated internally when a collective mode of the plasma response is excited. In contrast, plasmas with elongated cross-sections are shown to suffer from edge heating due to substantial linear mode-coupling. Alfvén wave heating of two specific tokamaks, TCA and JET, is discussed.

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