Study of the density limit with pellet fuelling in JT-60

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) x 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) approx 12-80% of Pabs(total), and the disruptive limit of the density can be explained by Pabs and n-bar2e (r = 50 cm) Z-bareff (r = 50 cm). The thermal stability in the edge region is weaker for lower (<0.4 keV) edge electron temperature. (author). 15 refs, 10 figs