Improved confinement characteristics of pellet fuelled discharges on JT-60

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Abstract
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) (19 m−3skeV) 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.